M90 gearbox
Original article by frpe82 of turbobricks
frpe82
Who is this guide for?:
This guide is made for...
* People who wants to maintain or strengthen their M90 gearbox.
* People who wants to install an M90 gearbox in their RWD car with a redblock.
* People who wants to install an M90 gearbox in their RWD car
In an RWD car, the M90 can be mated with...
* A redblock.
* A diesel.
* A whiteblock, for current 6-cylinder engines or for swapping in a 5 cylinder engine.
What is the M90 gearbox:
First of all: The M90 gearbox came in all manual 900/S90/V90-series cars from early 1995-1998.
The M90 gearbox is the best and latest model of manual gearbox for our RWD 900-series cars. It is the successor of the M46/M47, and the M90 was used because of both durability and comfort demands. Some say that it was actually introduced in the 900-series because the M56 (manual gearbox in the 850, introduced in mid 1994) was so good and successful. The M90 RWD gearbox and the M56 FWD gearbox are considered to be brother and sister because they are built on the same gearbox platform and have many parts in common.
The M90 comes in both a redblock, whiteblock and D24 version.
The redblock versions include M90L, M90H and M90L2.
The whiteblock versions include M90H, M90H2 and M90L2.
The redblock, whiteblock and D24 designed M90 gearboxes are not interchangeable. The bellhousing is not removeable. You can however interchange the internal parts (axles, gears etc.) between all versions.
Since the redblock version is the most common, the info in here will mostly be redblock specific. Most of the info here will also apply to whiteblocks though, and if there are any special circumstances and quirks regarding whiteblocks and M90?s they will be mentioned as well. Redblock and whiteblock specific info will be clearly stated.
The M90 in performance applications:
When I say "performance applications" I don't mean racing or using massive amounts of power. What I mean here is the everyday use in our modded 200/700/900-series cars. It doesn?t matter how much power you make. You can still break a gearbox if you shift it at high rpm without the clutch fully depressed etc.
The M90 is a vast improvement over the old M46/M47. It shifts smooth and makes less noise. It is also a real 3-axle gearbox. No overdrive crap.
The M90 gearboxes can take a lot of torque and HP.
The gearboxes are not equally durable though.
The first versions from early '95 of the M90 can leak from the input and output seals if they are not taken care of and driven on a regular basis. They can also leak if they have many miles on them. The forks, syncros and stop-rings are much better than on the M46/M47 but the 3rd gear stop-ring and syncro is a little bit sensitive compared to the rest of the gears. High revs and quick shifts in combination can destroy it.
The most common versions from '96-'97 are more durable than the early versions and are not so prone to leaking. This may also be due to the fact that they are a little bit newer and has not gone so many miles yet. Most of the M90L boxes from these years have the updated synchros and stop rings as well.
The most durable M90 in stock form is the M90L2 which only came in late '97 and early '98 (or both M90L2 and M90H2 from '97-'98 for whiteblocks). It has the updated syncros and a better stop-ring on 3rd gear and will tolerate more missed shifts etc.
But remember this advice:
* When shifting the gearbox into 3rd gear, let it synchronise on its own. Do not force it.
* Your clutch setup must allow the gearbox to fully disengage from the engine. A "sticky" clutch can make 3rd gear grind.
* After shifting to 3rd, do not hold on to the gear lever while accelerating. This sounds like it is obvious, but for many it is not. Holding on to the gear lever will push the selector and put a big strain on 3rd gear.
Caring for the M90:
Oil:
The M90 takes 1.85 liters of oil. Volvo Oil used to be the best oil to use however the oil has been replaced by a non suitable alternative.
The oil to used to be part number 1161745.
The best oil currently available is https://www.classicswede.org/shop/red-line-synthetic-manual-transmission-lubricant-mtl-75w-80-gl4
When changing the oil in the gearbox, remove the bottom plug at the rear of the gearbox and put it on its end with a little slant. Let it sit for a good amount of time to get all the old oil out. If you want you can flush it with a few deciliters of fresh oil and drain that as well.
Oil specifications:
1161 645:
Quality: API GL-4 / Volvo standard 1273,08 transmission oil 97308
Viscosity: SAE 75W
Density @ 15?C/59?F: 885kg/m?
Oil base: Multigrade, synthetic base
Pourpoint: -48?C/-54?F
Flashpoint: 212?C/414?F
Kinematic viscosity @ 40?C/104?F: 67.8mm?/s
Kinematic viscosity @ 100?C/212?F: 10.5mm?/s
1161 745:
Quality: API GL-4 / Volvo transmission oil MTF 97309 / WSS-M2C200-C3
Viscosity: SAE 75W-80
Density @ 15?C / 59?F: 841kg/m?
Oil base: Fully synthetic
Pourpoint: -60?C / -76?F
Flashpoint: 230?C / 446?F
Kinematic viscosity @ 40?C/104?F: 59.8mm?/s
Kinematic viscosity @ 100?C/212?F: 10.1mm?/s
Seals:
When the gearbox is out of the car or not yet installed, it is a good time to think about replacing the seals (#6 and #12 in the gearbox part list further down the article) and the o-ring (#11).
Reverse light switch:
This is relatively inexpensive and should be replaced if it is old. It will give up after several years and when used several thousand times.
For a new switch https://www.classicswede.org/shop/reverse-light-switch
Flywheels and clutches for the redblock M90:
Flywheels:
The M90 gearbox comes with a dual-mass (double-mass) flywheel in the B200FT, B230FK and B230FT engines.
The dual-mass flywheel consists of two halves that are internally damped/sprung to form a flexible flywheel. It is also very heavy. This is very good for low rpm stability, and since it is so heavy it is easy even for an inexperienced driver to drive the car. It is made mostly for comfort.
Because the dual-mass flywheel is very heavy, it is not good for throttle response or high power. I would not personally trust it to be reliable under heavy power either (250HP+). When approaching 200HP the flywheel will make some low-frequency noise because it is oscillating and it is not dangerous, only unpleasant. Beyond that another flywheel should be (has to be) used.
The stock dual-mass clutch setup is good for close to 250HP and 350NM (260lb/ft), and beyond that point the stock dual-mass flywheel should not be used anyway.
You can replace the flywheel with a direct fit single mass TTV flywheel https://www.classicswede.org/shop/ttv-b200-b230-flywheel-for-use-with-dmf-clutch however it is still fairly heavy and you are limited to the standard clutch
The age old solution is the dog-dish flywheel with LH2.4 drilling.
The dog-dish flywheel is the solid flywheel used in the M46/M47 cars. It is much lighter (but still not especially light), but miles better for throttle response and high HP than the dual-mass counterpart. You should not machine the dog-dish flywheel as they are known to shatter under high stress.
The dog-dish flywheel is an easy replacement for the dual-mass flywheel and no modifications has to be done. You only need to use another clutch setup (a different pressure plate and clutch disc). You also need to fit a longer pivot ball https://www.classicswede.org/shop/m90-extended-pivot-ball
If you don't have an M90 gearbox in your car to start with, there is no reason to use the dual-mass flywheel in your gearbox conversion.
If you are going to use the M90 and dog-dish flywheel on an LH2.4 car, then see to that you get a flywheel from an LH2.4 car since that has the "dimples" for the crank sensor.
Clutches:
First some info.
The M90 has 20 splines on the input shaft.
The M90 uses a 240mm clutch setup with the dual-mass flywheel.
The M46/M47 has 22 splines on the input shaft.
The M46/M47 uses a 228mm clutch setup with the dog-dish flywheel.
Since we want to use the M90 and a dog-dish flywheel, we have to use a clutch setup with 20 splines and a size of 228mm.
There are two options for this.
(All parts are interchangeable between the two options)
Option #1:
Standard 940TDi clutch setup. Good for up to 400NM.
Clutch disc 940TDi, stock, organic, 20 splines, 228mm, Sachs p/n: 18 1862 468 031
Pressure plate, stock, 228mm, Sachs p/n: 04 3082 204 033 and 86 3082 204 033.
https://www.classicswede.org/shop/d24-turbo-940-with-m90-gearbox
Option #2:
Custom clutch setup. Good for a lot over 450NM.
Clutch disc 850GLT, stock, organic, 20 splines, 228mm, Sachs p/n: 18 1862 314 032 and 86 1862 314 032
Pressure plate, Sachs race engineering, strengthened/performance, 228mm, Sachs p/n: 883082 999763
Option #3:
If you want even more clamping force, use the strengthened pressure plate I mentioned above together with a racing clutch disc or sintered disc. Such a clutch setup will hold 500-600NM depending on the disc used.
https://www.classicswede.org/shop/dog-dish-paddle-clutch-for-m90-gearbox
And putting the M90 gearbox through 500-600NM+ of torque may shorten its life conciderably, if it even suvives at all.
Flywheels and clutches for the whiteblock M90:
It doesn't matter if you are putting an M90 in your 960/S90/V90 with a 6-cylinder engine, or if you are swapping in a 5-cylinder engine from an FWD car. This applies to both of the engines.
Flywheels:
The M90 gearbox comes with a dual-mass (double-mass) flywheel in the B6304 and B6254 engines.
The later S/V70's among other FWD/AWD cars, also use a dual-mass flywheel. Many of the earlier 850's and S/V70's with AWD are also beleived to use the dual-mass flywheel.
960 / S90 / V90 dual-mass flywheel:
850 / S70 / V70 dual-mass flywheels:
The dual-mass flywheel consists of two halves that are internally damped/sprung to form a flexible flywheel. It is also very heavy. This is very good for low rpm stability, and since it is so heavy it is easy even for an inexperienced driver to drive the car. It is made mostly for comfort.
Because the dual-mass flywheel is very heavy, it is not good for throttle response or high power. I would not personally trust it to be reliable under heavy power either (250HP+). The whiteblock dual-mass flywheel is different than the redblock one, but when approaching 230-250HP (at the same torque level as a turbo redblock I beleive) the flywheel will make some low-frequency noise because it is oscillating and it is not dangerous, only unpleasant. Beyond that another flywheel should be (has to be) used.
The stock dual-mass clutch setup is good for close to 250HP and 350NM (260lb/ft), and beyond that point the stock dual-mass flywheel should not be used anyway.
This is where the flat flywheel comes in.
There are two original flat flywheels. The one that accepts a 228mm clutch, and the one that accepts a 240mm clutch. The most common one is the 240mm version.
The earliest ones don't have the correct dimples in the flywheel (or maybe they are placed differently), since those cars used LH3.2 engine management systems. You want a flywheel from a car that has Motronic. Motronic 4.3 and 4.4 have the same flywheel pattern.
Even though the flat flywheels from the 850 and S70/V70 may look a little bit different, they both accept the same clutch and are almost identical in weight. They have different ways of triggering the crank sensor (holes vs. machined slots), but they both have the same number of triggers, and works with Motronic 4.3 and 4.4 without a problem.
S70 / V70 flat flywheel:
850 flat flywheel, back:
850 flat flywheel, side:
The flat flywheel is the solid flywheel used in many 850's and early S/V70's with FWD. It is much lighter (but still not especially light), but miles better for throttle response and high HP than the dual-mass counterpart.
The flat flywheel is an easy replacement for the dual-mass flywheel and no modifications has to be done. You only need to use another clutch setup (a different pressure plate and clutch disc).
If you don?t have an M90 gearbox in your car to start with, there is no reason to use the dual-mass flywheel in your gearbox conversion.
Clutches:
First some info.
The M90 has 20 splines on the input shaft.
The M90 uses a 240mm clutch setup with the dual-mass flywheel.
You might remember that I said that there were two versions of the flat flywheel. 228mm and 240mm. There is no reason to use the 228mm at all since there is no real price difference, the 240mm clutch setup is not as heavy to push as the 228mm clutch setup (the 228mm clutch setup requires more spring force to hold the same power), and the 240mm clutch setup grips better and can hold much more power.
(All parts are interchangeable between the options)
Option #1: (parts currently not available)
Standard 850 T5-R (240/250HP) clutch setup. Good for up to almost 420NM.
Clutch disc, organic, 20 splines, 240mm, Sachs p/n: 1862 428 133.
Pressure plate, 240mm, Sachs p/n: 3082 254 232.
Option #2: (parts currently not available)
Custom clutch setup. Good for up to almost 540NM.
Clutch disc, sintered, 20 splines, 240mm, Sachs p/n: 1861 999 843.
Pressure plate, 240mm, Sachs p/n: 3082 254 232.
Option #3:
New flywheel and clutch kit
https://www.classicswede.org/shop/960-dmf-hd-flywheel-conversion-kit-with-clutch
If you want even more clamping force, use the strengthened pressure plate pictured above together with a racing clutch disc or sintered disc. Flywheel modifictions may have to be made to the flat flywheels. Such a clutch setup will hold 600-700NM or more, depending on the disc used.
We have a few options on this
Please take in concideration that most clutch discs like that do not have a sprung hub, and will be pretty vicious to your gearbox and driveline.
And putting the M90 gearbox through 600-700NM+ of torque may shorten its life conciderably, if it even suvives at all.
Adjusting the clutch on the M90:
The M90 uses a hydraulic clutch which is self-adjusting, so you can not alter the biting point of your clutch.
However, sometimes other adjustments needs to be made... and listen carefully because this is important:
Check to see that the flywheel you use has the right depth or else the pressure plate will not excert the correct force on the clutch. It will not bite in the right position either.
Check to see that the clutch disc has the correct thickness (a little difference in thickness doesn't matter, but several millimeters will). Otherwise we are back to the same problem as described above.
When mounting the gearbox to the engine, make it temporary, because you will most likely take it down again after you have read this:
The pivot ball which the clutch fork is sitting on may have to be adjusted, especially if you are using a custom flywheel, lightened flywheel or a custom clutch setup (a custom clutch setup means that you are using any other clutch and flywheel than stock).
Remove the rubber bellow that is sitting on the clutch fork in the hole on the side of the gearbox. Put some force on the clutch fork just like if you were going to push the clutch. You don't need much force, just enough to see where the clutch fork will be staying once the slave cylinder is pushing lightly on it. It should stick straight out, or preferrably leaning a little bit forward (toward the engine). I prefer the later option and recommend it strongly. If the clutch fork is leaning backwards (toward the rear of the car), you need to take the gearbox down and put washers behind the pivot ball. Then repeat the test again.
Why?
Because if the clutch fork is not in the right angle, the following things will happen:
* The clutch fork will touch the gearbox before it is fully disengaged.
* The clutch for will get far away from the slave cylinder, and the piston in the slave cylinder will pop out. If it doesn't, it will still have the wrong angle.
Gearbox parts:
1: Gearbox (versions and part numbers further down the thread).
2: 9442728-1 Reverse light contact, Note? 9442728 (1307086)
3: 960632-2 Plug
4: 11998-2 Gasket
5: 6843173-1 Companion flange
6: 9183891-1 Seal, Note? 9183891 (6843482)
9: 3549685-1 Flange lock nut
10: 3549714-1 Sleeve
11: 978268-1 O-ring
12: 6843480-1 Sealing ring
13: 945444-3 Flange screw
14: 1161138-1 Grease
17: 970972-3 Hexagon screw, 9651872 Flange screw, 9709822 Hexagon screw
19: 940280-3 Spring washer
22: Gearshift and accessories
1: 3549054-1 Gear lever
2: 6814305-1 Gear lever knob
3: 1381967-1 Grommet upper
4: 1381966-1 Grommet lower
5: 966810-1 Spring pin
6: 9143742-1 Rubber bellows, Note? 9143742 (6812566)
7: 949595-1 Retaining ring
8: 3520197-1 Rubber damper
9: 1232173-1 Ball socket
10: 6843606-1 Gaiter
11: 3502966-1 Lock ring
11a: 1232165-1 Rubber bushing
12: 979870-1 Set screw
13: 3549648-1 Bracket
14: 1232457-1 Spindle
15: 3549642-1 Rubber bushing
16: 6812564-1 Seal
17: 3549651-1 Gear selector
18: 1209638-2 Bushing
19: 3549033-1 Bracket
20: 1232595-1 O-ring, Note? 1232595 4DRS CH-200158, 5DRS CH-142495.
21: 381526-1 Gear selector joint
22: 381527-1 Retaining sleeve
23: 381528-2 Pin
24: 947113-1 O-ring
25: 9148623-1 Cable harness, Note? 9148623 (3544703)
26: 948211-1 Cable tie
27: 9143117-1 Sound absorber
28: 9143298-1 Bushing
29: 9143299-2 Rubber bellows
30: 948702-2 Cable tie
31: 3520197-1 Rubber damper
32: 3520209-1 Ball socket
33: 9143368-1 Rim
34: 947542-4 Flange screw
35: 960139-4 Washer
36: 3549663-1 Lock pin
37: 978992-1 Split pin
38: 9143752-1 Seal
1: 8250091-1 Gearbox member, Note? 8250091 (1378417)
2: 946441-4 Flange screw
3: 971098-1 Flange lock nut
14: 1328900-1 Rubber cushion
15: 946440-2 Flange screw
16: 6842485-1 Bracket
17: 946472-4 Flange screw
1: Pressure plate
2: 959219-6 Hex. socket screw
3: Clutch plate
4: 3549391-1 Release bearing, Note? 9163505-1 (New style - recommended)
5: 3549983-1 Release fork
6: 3549638-1 Pivot pin
6a: 9163851-1 Ball seat
7: 7953046-1 Stud
8: 8120265-1 Washer, Note? For M47 (optional spacer for clutch fork on M90)
9: 6843913-1 Control cylinder 4CYL TURBO & B6254, Note? 6843914-1 4CYL EXC TURBO
10: 914463-1 Retaining ring
11: 3520525-1 Clutch line, Note? 9163709-1 6CYL (9143123)
12: 1330129-1 Buffer
13: 11994-1 Gasket
14: 3502937-1 Clamp
15: 946830-1 Hexagon nut (Only used for RHD cars or custom setup)
16: 946814 Tube (Only used for RHD cars or custom setup)
17: 946815-1 Fitting nut (Only used for RHD cars or custom setup)
18: 946816-1 Fitting screw (Only used for RHD cars or custom setup)
19: 8601785-1 Master cylinder 4CYL TURBO & 6CYL (1330248), Note? 6814719-1 4CYL EXC TURBO
20: 965503-2 Press screw
21: 948645-2 Flange nut
22: 3540030-1 Gasket
23: 9183997-1 Bolt
24: 942915-1 Lock brace
25: Bracket for brake pedal
26: 9128669-1 Bracket, Note? (1387553)
27: 949887-1 Flange screw
28: 3530634-1 Clutch pedal
29: 1367312-1 Spring, Note? For cars with gear shift indicator.
30: 942679-2 Blind rivet
31: 1272021-1 Pedal pad
32: 3516125-1 Bushing
33: 1273647-1 Shaft
34: 968893-1 Flange screw
35: 948645-1 Flange nut
36: 944574 Hose, Note? 240 mm
37: 1389647-1 Hose clamp
37a: 1389647-1 Hose clamp
38: Reservoir / Brake master cylinder
39: Bell housing (for illustration only)
40: 3549051-1 Bellows
41: 3530394-1 Seal, Note? 3530396-1 960,S90/V90
42: Clutch kit
Gearbox versions:
1208675: M90H
4DRS CH-145918
5DRS CH-93623
(REPL 1208790-4)
1208790: M90H
4DRS CH 145919-218356
5DRS CH 93624-161856
(B200, REPL 5003925-4)
(B230, REPL 1208812-6)
1208812: M90H
EXC B230FK
4DRS CH 218357-222133
5DRS CH 161857-168179
(REPL 1208884-5)
B230FK
4DRS CH 218357-221970
5DRS CH 161857-167988
(REPL 1208857-1)
1208884: M90H
EXC 230FK
B200FT
(REPL 1208936-3)
1208936: M90H
B200FT
(REPL 5003985-8)
5003985: M90H
B200FT
1208857: M90L
B230FK
4DRS CH 221971-238057
(REPL 1208937-1)
5DRS CH 167989-198076
(REPL 1208947-0)
1208937: M90L
B230FK
4DRS CH 238058-
B230FT
(REPL 5003994-0)
5003994: M90L
B230FK, B230FT
4DRS CH 238058-
1208947: M90L2
B230FK
5DRS CH 198077-
(REPL 8111096-7)
8111096: M90L2
B230FK
5DRS CH 198077-
5003925: M90H
B200/B230FT
(REPL 5003985-8)
5003985: M90H
B200/B230FT
5003994: M90L
B230FK
Gear ratios:
M90L:
1st - 3,91:1
2nd - 2,2:1
3rd - 1,38:1
4th - 1:1
5th - 0,81:1
M90L2:
1st - 3,91:1
2nd - 2,2:1
3rd - 1,38:1
4th - 1:1
5th - 0,7:1
M90H:
1st - 3,54:1
2nd - 2,05:1
3rd - 1,38:1
4th - 1:1
5th - 0,81:1
M90H2 (whiteblock specific):
1st - 3,54:1
2nd - 2,05:1
3rd - 1,38:1
4th - 1:1
5th - 0,7:1
Before mounting the M90 in your car permanently (any car):
Always get new bushings for the gearbox, even for the gear lever. You will regret it otherwise.
Also take good note of what the "Caring for the M90" section has to say.
Don't make a mistake with the clutch setup. Read the "Adjusting the clutch on the M90" section.
Grease all the appropriate places with the right sort of grease (the Volvo grease). Namely the clutch fork pivot ball and the shaft on which the throwout bearing rides on.
Electrics and wires that have to be changed:
Important: If you started out with a car that had an automatic gearbox, you will have to observe the following text. But if you had a manual car (factory built with manual transmission) before the M90 swap, then you don't have to read this section.
Electrics? What more can there be except for the reverse lights?
Well... several other things.
If you had an AW71/AW71L/AW72L gearbox before you decided on the M90, there are several wires that you will have to connect differently. Not all of these wires are present on some cars though.
* Load signal to the ECU. This wire tells the ECU that it should be ready to correct the idle speed when R/D/2/1 is engaged so that it does not stall.
* Starter inhibitor. This wire tells the ECU (or other electronics in some cases) that the gearbox is in gear. Then the starter is not allowed to be used.
* Kickdown inhibitor solenoid and gearbox oil pressure sensor. These wires tells the gearbox that it is not allowed to do a kickdown when the road speed is above ~85-90mph (140-150km/h).
* The wires for the OD button and solenoid.
* Shifter light(s). Wires for the light(s) in the automatic shifter assembly.
* And the reverse lights of course.
What to connect differently when swapping to a manual gearbox:
There is a connector in the car called C166 which many of the wires connects to. This connector may or may not be easily accessible. In most cases you don't have to access it either, and just cut or reconnect the wires instead.
* The wires for the light(s) in the automatic shifter assembly should just be disconnected and removed. Just follow the wires from the light(s) and you will find which ones to remove.
* The wires for the OD button are also easy enough. Just follow the wires from the button on the shifter and you will find which ones to remove. But in case you need it, from connector C166 (pin 1 and 2), the wire colors are both black.
* The wires to the OD solenoid and the kickdown inhibitor on the AW7x transmission should be removed since they are not used on a manual gearbox. Just follow the wires from the solenoids on the transmission and you will find which ones to remove. But in case you need it, from connector C166 (pin 3 and 4), the wire colors are yellow (blue) and brown.
* Then have the reverse light switch wired up. That is also easy. The wires going to the reverse light switch may have to be lengthened though, since some of the AW7x transmissions have the switch in the shifter, and the M90 has a switch on the transmission. Polarity doesn't matter. From connector C166 (pin 7 and 8), the wire colors are (white) black and blue-red (gray).
* The wires for the starter inhibitor and the load signal have to be bridged, or else the starter will not work, and the car will have various driveability issues that are very hard to diagnose if it starts. From connector C166 (pin 5 and 6), the wire colors are blue and pink. Just connect the two wires and you are set.
Mounting the M90 in a 900-series car:
If you had an AW70, AW71 or AW71L automatic gearbox to start with, then you can keep the transmission crossmember since that is the same p/n as the M90 crossmember. You will also need the mounting bracket for the M90 (number 16 in the last picture of gearbox parts) since that is not the same as the AW7x bracket. The crossmember from the AW7x gearbox is then moved forward in the car and mounted in the front threaded holes in the body a few inches in front of the AW7x holes (yes, there are pre-made holes for using the M90).
If you had a M46 or M47 manual gearbox to start with, then you have to get the AW7x/M90 crossmember.
You will of course need the center console trim for a manual gearbox (the M46 trim works although it is not the same as the M90 trim).
Mounting the M90 in a 700-series car:
Same as in a 900-series car.
Depending on which year and model you have, the shifter position may have to be adjusted though.
Mounting the M90 in a 200-series car:
Use support bracket adaptor https://www.classicswede.org/shop/m90-gearbox-to-m47-support-beam
Shorten both gear linkages by 100mm
The 240 prop shaft usually needs to be lengthened by 50mm or a M90 prop can be shortened.
LHD cars will need a hydraulic clutch pedal and master cylinder
Reinforcing the internals of the M90:
You do remember what I said about the M90 internals in "The M90 in performance applications" section, don?t you? Well then. Here we go...
The modification mentioned below is to reinforce the gearbox and to prevent a shot 3rd gear. A shot 3rd gear on the M90 can also be fixed if the gearbox and the other gears are still intact, so don?t worry that the gearbox is totally shot if 3rd gear gives up.
The main reason to a busted stop-ring on 3rd gear is an over-enthusiastic driver who shifts a little too fast between 2nd and 3rd gear, or force 3rd gear to engage. The syncro ring gets a beating and is forced out of place on the gear. In the worst cases it might be sufficient to miss the shift only once, and then 3rd gear is shot. Easy to do in the heat of battle for sure...
Are you ready to take the M90 apart for modification...?
Good! Let?s go!
When the gearbox is disassembled enough for you to get this view, then the axle we should concentrate on is the middle one. This is where the 3rd gear syncro hub is. It is not as hard as it seems. Just remember to put the parts in an order on your workbench that will help you to reverse the process of disassembly, and get it back together and fully assembled again.
The bearing on the axle has to be removed since we are going to weld on the axle and gear, and the casing of the bearing is made of plastic so we don?t want that to melt do we? The bearing is easy to remove since it has an opening (seen in the picture above), and you can just pry it open to remove it.
When looking at the gear, we can see a machined groove. That is where the syncro is supposed to sit. Below the groove the gear is badly beaten. That is what a missed shift does to your gearbox...
The result is that the syncro ring moves to far down on the gear, and that prevents a correct lock-on to the gear when shifting.
Look at the axle above the gear. That is where the bearing in the previous picture is supposed to sit.
This is how the syncro ring is supposed to sit.
Get your welder and welding gear ready.
Fill the entire gearbox with oil (all the way up). This will prevent any molten metal, sludge or other small objects to stick to your gears in the gearbox. The oil also acts as a coolant in this case. And no, the oil will not be flammable so don?t worry about that.
Use a lot of heat on the welder. These things has to be durable, and the metal has to get really hot for the welds to bite.
Weld one cog of the gear to the syncro ring. Rotate the axle 180 degrees and repeat. Then rotate the axle 90 degrees and weld again etc. Well... you get it. Always weld the cog on the opposite side to get it nice and even. Continue until all cogs of the gear are welded to the syncro ring.
This sounds brutal, but it always works.
Now you have repaired your M90, or prevented it from a future breakdown.
Replacing the shifter of the M90:
This is highly recommended if you desire a shorter throw and much better shift quality.
Cravingboost makes an adjustable short-shifter for the M90 gearboxes.
It can be found here: http://www.cravingboost.com/products-short-throw-shifters.htm
The setting on the shifter that I personally use is the top one (shortest throw possible). The throw can be cut in half half compared to the stock shifter. This is the setting I recommend. Any other setting is just a waste of time to try out.
Since the M90 is so easy to shift in stock form, it won't require much more effort to get through the gears with the shifter set to give the shortest throw. The additional force required is neglible.
Because of a much more solid design and the removal of all the plastic bushings, most of the play is taken out.
The gears click in to place with extremely good control and most of the play that the stock shifter had will be gone. Very firm and precise shifts is the result of this modification. You can much more easily feel when the gears are properly engaged, and it is easier not to miss a shift.
The shifter/linkage is far from the drive shaft and all other components and edges when installed, even though it is set at minimum throw.
Installation:
It is much easier to do this on the V90 than on a 940. Special procedures required on the 940/960 are pointed out in red.
* Under the car, where the shifter meets the gear selector linkage, just remove the allen screw (4mm) on the bottom of it and take the bolt/spindle out. Also remove the plastic bushings on the gear selector linkage since they will not be needed anymore.
* Remove the ashtray and the plastic rim and gaiter for the shifter in the car (no tools required).
On the 940 and early 960 with M90: The plastic cover over the e-brake handle and shifter gaiter have to be removed. Two torx screws in the middle of the tray holds the plastic cover down.
Sometimes the metal bracket the ashtray seats on have to be removed as well. One torx screw on each side holds the metal bracket from the inside. Only one screw have to be removed for the bracket to be able to be lifted out of the way.
* Remove the four bolts (10mm head) holding the steel rim and lift up the rubber bellow. (The two bolts towards the e-brake handle may be tricky to get to in some cases though)
On the 940 and early 960 with M90: The bushing bracket that holds the upper linkage of the transmission in place may be fastened to the underside of the bolts towards the e-brake handle. The bracket may have to be removed before you can remove those two bolts (10mm nuts in most cases).
* Remove the retaining ring (with a retaining ring tool pushing inwards). Pull the whole shifter assembly straight up.
* Remove the spring pin holding the two parts of the shifter together and pull them apart (a small drill or screw can be used to push or knock it out). It may require some force both to remove the spring pin and to pull it apart. Vice grips is recommended.
* Put the lower part of the shifter you got from Cravingboost in the upper part of the stock shifter and put the spring pin back. Remember in which direction the lower part should be installed. (Hint: The shifter should point backwards / towards the e-brake handle when installed)
* Clean the hole/seat where the shifter should sit. Put the o-ring at the bottom of the hole. Remove the bolt on the bottom of the Cravingboost shifter.
* Put the shifter in the seat and push it down. You may have to wiggle it sometimes. Put the retaining ring back.
* Under the car, push the bolt through the linkage and the bearing at the bottom of the shifter. The bearing and the bolt have to be clean.
* Try to shift it through the gear and see if it feels ok.
* Install the rubber bellow, steel rim, gaiter and plastic rim (reverse of removal).
On the 940 and early 960 with M90: Don't forget to put the bushing bracket for the upper linkage on the transmission back if you had to remove that.
Enjoy a whole new shifting experience!
