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04-27-2008, 01:05 AM #1
R&R Speedo drive gear. 56k Warning!
The speedometer drive gear drives the speedometer driven gear. The driven gear is easily removed from the side of the transmission, even with the transmission on the vehicle. The driven gear drives an elbow connector that drives the speedometer cable which goes up into the back of the speedometer in the instrument cluster behind the steering wheel.
Drive gear - Driven gear - Elbow - Cable - Speedometer
Any one of these components can go bad or break disabling your speedometer.
On the 84/85 when the speedometer stops working the ecu (computer) will go into open loop because the vss (vehicle speed sensor) is an input into the ecu and without that input the computer can't calculate the proper air/fuel ratio. The check engine light will come on with a code 42 (No vehicle speed input.) Open loop is when your computer does not use your sensors to run your car but a predetermined setting. Not fuel efficient. I had sucky gas mileage and my engine wasn't running very good either.
I removed the speedometer from my dash, unhooking it from the speedometer cable. Then disconnected the other end of the cable from the elbow and removed the elbow also. I hooked up my cordless screw gun to the back of the speedometer to check if the speedometer worked. It did. Then I checked the cable by having someone confirm that it spun or didn't spin when I cranked it with the screw gun. It worked. The elbow is easy to figure out. It fits in your palm so you can see for yourself if it works by sticking a pencil in one side and spinning it with your other hand. To check the drive gear the driven gear has to be installed. Put the rear end up on jack stands making sure it will definitely not fall. Start the car in second gear. Watch out for the spinning rear wheels!! Get under the car and put your finger in the hole where the elbow screws into the driven gear and feel if the shaft is spinning. Mine wasn't. I finally figured it out was an internal problem.
The driven gear is made out of plastic so if anything binds in the system and prevents it from spinning the metal drive gear that's meshed with it in the transmission will actually melt a divot into it and it will no longer mesh. That's one reason this whole thing even started for me. I put in one of those after market neon cluster faces over the black back. After a month it started bowing up and started binding the speedometer needle and prevented it from moving. That transfered back through the system and the metal drive gear melted a divot into the plastic driven gear.
There is a single clip that holds the drive gear in place on the output shaft. After 20+ years of abuse this clip looses strength and wears out allowing the drive gear to slide out of mesh with the driven gear.
Note: *The drive gear is inside the transmission and cannot be replace through the driven gear housing hole. It is impossible!
The reason this problem cannot be fixed through side hole on the transmission the driven gear sits in is because the clip is actually in between drive gear and the output shaft. The gear has to be taken off the shaft to replace the clip.
This is how you replace the speedometer drive gear clip.
The speedometer drive gear and clip.
Viewed through the hole on the side of the tranny where the driven gear is normally installed.
Step 1: remove transmission. To remove the transmission the center console must be taken off and the shifter must be removed. The drive shaft must be removed. Mark it's position before removing and reinstall it in the position it was removed, otherwise it could become off balance and vibrate unnecessarily while driving.
Absence of drive shaft:
Remove all the bolts holding the transmission to the engine. Unbolt the slave cylinder. Remove the cross-member that supports the rear of the transmission. Lower the transmission down.
About to lower the transmission:
Be careful with the throw-out bearing retaining clips. I removed the throw-out bearing because I completely disassembled the transmission to calculate the exact gear ratio since I had it off. I didn't know I broke both of the clips until I had the whole thing back together and ready to put back on the car. Luckily the 3rd dealership I called had them in stock. They were like $4 for both.
These bolts must be removed first. Remove this cover.
Remove the driven gear by pulling/twisting it out. Don't loose the little half oval wedge that the bolt goes through. You must put it back into the gear slot in the side of the driven gear when re-installing. After removing the driven gear remove the bolts circling the case and pull the case up.
When pulling the case off, pay special attention to the shaft because it fits into 3 forks and literally rests on them. The shaft must be 'in' those 3 slots when assembling.
This is what it looks like with that half of the case removed. See those plastic things? Those fell down from behind the first cover when the case was put upright. Those are neoprene shifter bushings. Notorious for falling into the case.
That is the drive gear. The culprit for my non-working speedo.
You can see the clip not fitting into the hole here. This is why is slides out of mesh.
Insert small screwdriver and pull it out.
Slide the gear up.
Remove the other end of the clip from the shaft.
Slide it up the shaft.
The ends that fit into the holes will fit in the grooves and when reinstalling try to evenly place in the grooves to easily slide it down the shaft.
Mark the gear before you fully bring it off the shaft to reinstall it facing the same way, though I don't think it will matter. Do it just to be safe.
These holes on the shaft and are what the clip that holds the drive gear in place fit into. When the clip wears out, it will not be able to stay in the hole resulting in the drive gear sliding on the shaft and not driving the driven gear.
Drive gear without clip:
Drive gear with clip:
Reverse the process to reinstall.
Total time: 1 weekend.
Total cost: $1.67. $7 if you replace the throw-out bearing retaining clips also.
Don't forget to add the cost of 3 quarts of gear oil. I believe I used 75w90. Check the TSRM.
Call around to different dealerships. Some tried to charge me $5 for the gear clip! A 10 minute drive saved me 3 bucks! Woo! Same with the retaining clips.
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04-28-2008, 11:01 PM #2
- Join Date
- Jan 2006
My 82 has been speedoless since the 90's, so I appreciate the writeup! Perhaps someone could link more pictures of the tranny case disassembly. I might add that I don't think the older car has the open loop issues yours did. For future reference....
04-28-2008, 11:04 PM #3
Looks like you spent extra care documenting this - .
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04-29-2008, 12:27 AM #4
- Join Date
- Mar 2006
- Calgary, AB...ex-Newfie
04-29-2008, 01:00 AM #5
04-29-2008, 01:02 AM #6
04-30-2008, 12:16 AM #7
That is very well the required steps.
I just did this, on my way to replace the bearings in the tranny.
You forgot to mention to drain the tranny as the first step.
And did you replace the tranny gasket ?
04-30-2008, 03:15 AM #8
04-30-2008, 11:22 AM #9
04-30-2008, 07:00 PM #10
lucky you if it did not leak after!
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