It's been a while since I've posted and have been doing tons of work on my seven!(pic below) Its been running way better than when I first bought it and is much more fun to drive now. That said, it still needs more love. I've decided to put it on jack-stands for a few weeks while I sort out some of the more pressing issues. One of those is that my speedo doesn't work at all.

I inspected the cable and it is connected to the cluster correctly. The cluster also works fine except for the speedo. I went under the car and disconnected the speedo cable and ensured the inner cable is intact and in good condition. Then I put the car on jack stands, started it, and put it in gear with the cable off so I could see the drive gear shaft. The wheels were spinning, but the speedometer drive gear shaft was NOT spinning at all. I haven't removed the housing yet to confirm ( I will soon), but it seems like the drive gear is faulty.

Right now I'm trying to source a new speedometer drive gear/shaft unit, but it doesn't look like anybody stocks them for FB's. Reading through some older posts, many others have needed replacements for SA's and FB's, but no clear solution has been found. My car is an 85' GSL manual 5 speed. Do we know if any FC speedo drive gears will work? According to Atkins, the coupe FC part has 20 teeth while the convertible FC part has 19. Cable and cluster connector look identical. Maybe its worth a shot? If not I'll be back in the junkyard trying to find one...

 

I should clarify, I don't know if the issue is with the "drive gear" or the "driven gear" or both. I'm looking for possible replacements for either depending on what I find when I remove them. I don't care if a replacement make the speedo a few mph fast or slow either. I can live with it being slightly off. Just want to have some idea of how fast I'm going without using gps.

 

Isn't it possible to put the cable in a drill and spin it to move the needle? Eliminate the gauge first.
Otherwise, pull the speedometer gear out of the transmission and see if it's truly broken. It's 1 bolt that unscrews easily but the gear unit may put up a little fight to pull out.

 

yeah I could test the speedo that way! Never even thought of that... I'll definitely do that tomorrow and post the results.

 

Okay! I had a little bit of time on lunch (i work close to home) to hook up my drill to the cable and test it. I can 100% confirm the speedo and cable work perfectly. Apparently the fast setting on my drill spins at roughly 1420 RPM since the speedo read ~80mph. Even took a video of it working.

 

Update: I've gotten in touch with someone who has a few transmissions laying around. This weekend I'm meeting with them to see if I can salvage the parts I need. Will update!

 

So I went and pulled the gear... the gear itself is in good shape. Same for the drive gear in the transmission. Clearly, the issue is the broken shaft.



Anyone have any creative solutions for repairing this? I'm still going to buy a replacement when I can find one, but it would be nice to repair this and have a spare.

 

weird, never seen that before. Is the gear shaft seized inside the sleave?

The part number for the gear is 8057-17-441C, but it's the same for all first gens and there's no distinction by color in the parts catalog.

I have a 20 tooth FC speedometer gear, PM me your address and I'll send it.
It exchanges perfectly with my 81-83 sleeve that came with the green gear. According to this page, the 17 tooth green gear has coarser teeth and won't work.

 

Shaft is part of gear. You need a tooth count that matches, IIRC all 12A cars are 17 tooth (5 tooth drive gear, 3
4:1 ratio, 1000rpm speedo drive at 3400 driveshaft RPM, which is your engine RPM in 4th at 60mph) and all GSL-SE are 18 tooth.

I ordered from Mazda a high tooth count gear (from the FC that had 4.30 final drive) to correct my speedometer after a gear change, but it never sealed properly and I had permanent gear oil stains on my socks after the cable Archimedes Screw pumped a little oil up to the speedometer as I drove

 

Wanted to update on things so far. I pulled an intact driven gear assembly from a transmission a friend of mine had laying in his shop. Its a 17 tooth gear and fits perfectly. However, the speedo still doesn't work after installing it, so I went to inspect things.
I pulled off the speedo cable and jacked up the rear of the car, got the tires in the air, and put it in gear with the engine running. The shaft STILL wasn't spinning, so I pulled the driven gear back out to inspect the drive gear instead. Turns out the drive gear is ALSO cracked and loose on whatever shaft it rides on. The problem has been both gears all along. I can't believe I didn't notice before...

Does anyone know if there is a way to fix the drive gear without dropping the transmission? If not, I have to install a shifter return spring and re-tap my clutch slave cylinder mounting points anyway. Might be time to just eat it and do it all at once...

 

To answer your question: No. The shaft was not seized inside the sleeve. My theory is that whatever knocked the drive gear loose laterally along its shaft put an odd torque angle on the driven gear assembly while rotating and cracked it at its thinnest weakest point. Maybe something that happened while installing a driveshaft? Mine DOES look recently refreshed...