I have an 86 auto coupe and an 85 auto coupe as a parts car. The parts car has the 3.07 rear end and my driver is a 2.59 car. Of course the obvious and morally correct thing to do swap rears. My question involves the speedometer. Do these cars use a speedometer gear or a signal generator. I couldn't find any reference to a gear in my Haynes manual, but I did find a speedometer signal generator, which I guess is a sensor that gives a voltage to the computer which in turn sends a voltage to the guage cluster and gives the speed display. Am I mistaken in these assumptions? Will swapping the signal generators fix any problems I might have with the speedometer calibrations or is there an actual gear like most cars out there?

Thanks.
Chris

 

There are two nylon gears involved in the speedo system. One is on the tailshaft of the trans; the other is on the input shaft of the 'VSS' (Vehicle Speed Sensor - which is the little electric motor-like thing stuck on the drivers-side of the trans housing toward the rear).

The nylon gears spin the VSS, which sends an AC signal to the ECM. ECM counts pulses and sends them along to the CCM. CCM uses the pulsed input to drive the speedometer display in the dash.

The two nylon gears must be in the right ratio to send a pulsed signal that matches the gearing in the diff. Since you have the 3.07 VSS you are half way there. Now you need to look at the drive gears on the two tail shafts - GM color codes them - if they are the same color you are alll set - I think it will be a red gear in both cases. If not, you need to pull the drive gear off the 3.07 trans and put it on the 2.59. Then the speedo readings will be correct.

Good luck.

drj

 

Yeah, what DOCTOR J said. :yesnod:

 

My parts book lists both drive and driven gears. Why is it necessary to change the VSS too?

 

It's probably not necessary to change the VSS itself, only the driven gear - but as I read the original post, this guy has a spare VSS sitting there in front of him with the correct gear already on it, along with the new diff. Since he has to pull the old VSS off anyway, why not swap the whole thing?

(I say probably because I believe there were at least two models of VSS, which I think had different shaft centers to accommodate different diameter driven gears.)

HTH




[Modified by DOCTOR J, 9:05 PM 2/1/2002]

 

Thanks for the help, I grabbed the VSS and the gear with it today, but I went before checking the post. How is the other gear removed? My driver isn't torn apart yet, so I can't check the color of the gear on my car.

 

When I grabbed the rear, I am going to use its half shafts. Is the color coded part on the rear or is it in the tranny too?
Please bear with me, its my first rear end swap.
Thanks
Chris

 

OK, the VSS drive gear is on the transmission tail shaft. Look in the hole where you pulled the VSS off the donor car. If you see a red plastic gear in the hole, I'm 99% sure you are done. If it's gray you will probably need to take it with you. To do that:

You would need to pull off the trans tail housing - the very last piece of cast aluminum of the transmission, where the long truss bolts to the back of the trans (should be 4 bolts holding it to the trans case, plus the two long bolts that attach the truss that goes to the rear axle - but you better look for yourself, cause I last had mine apart a few years ago so I'm probably not remembering everything right).

There ought to be a plastic gear visible at this point. How do you get it off? Well, you're gonna have to find a manual, cause I never had to pull mine. Sorry, pilgrim. That's as far as I can take you. Maybe somebody else has pulled one recently.

have fun.

drj

 

Check with 18 carfan on what gears, sensor etc. you need. I believe the drive gear is the same for a 2.59 and 3.07. The driven gear is different. (bigger for the 3.07) The reason you have to change the sensor is, the driven gear passes through it and it has to be offset more for the big gear. (hope that makes sense) If you have to change the drive gear, I will tell you how I did mine. Disconnect the speedo wire, unbolt the sensor, and pull it and the driven gear out. Remove the 4 bolts through the tail shaft and pull it off. There is a rubber o ring between the case and the tailshaft. You dont have to replace it, (I didnt but if I had it to do again I would) Also a good time to replace the rear seal in the transmission tailshaft. The drive gear is kinda pressed on the shaft. I had no way to pull it off, so I cut a slice in it and then took a screwdrive and broke it off. To install the new one I used a 1 in dia. piece of pvc pipe and tapped it on the shaft. If you have to change this gear pay attention to the orig. location of the old one. There are 2 small holes in the shaft that hold a retaining clip. One hole is for corvettes and the other is for other cars. Once you pull the tailshaft it will be a lot more clear on what to do. Good luck