JayL

In an attempt to get my speedometer working again I've been tracing wiring and checking connections. If the wiring and connection at the impulse sender for the speedometer are good and it's receiving power, could there be anything other than the impulse sender itself that's causing my speedometer to not work?

mytrplseven

I'd jack up the rear and slowly run the car in gear (flat ground with wheels chocked) and look for changes in voltage with a digital volt meter (DVM). I dont' have the exact connector pin but you should see small increase in volts as the speed increases on the rear wheels. If you see nothing, the sender unit could be bad, the drive shaft from the transaxle could be a problem or the speedo could be faulty. The impulse sender screws onto the speedo output shaft on the tranny and then plugs into a square connector a couple of inches higher up. Under no case try to do this while you're under the car.

Van

The impulse sensors do sometime go bad (or even physically come apart).

JayL

Looks like this one has gone bad, it's not outputting anything from my little bit of testing. Time to find a new one and then pull the old one to see hopefully see that it just stopped working rather than fell apart.

JayL

Got a used sensor in the mail yesterday. Immediately went out to do the install. Found the problem with the old sensor as soon as it was pulled from the car. It was the plastic shaft had twisted and broke. So I ended up having to take the plastic piece with the gear on it out of the transmission and removed the broken piece that was inside it. Put the new sensor in and put it all back together. Went for a drive and everything worked perfectly. It was so nice to have a speedometer that worked. Parked the car and called it a day.

Went for a drive this morning and the speedometer didn't work at all. Got back and took the sensor out and the shaft was broken on this one as well, just in a different place. Anyone have any idea what's causing this? Could the plastic gear that slides in the transmission that the shaft of the sensor slides into be binding and causing this? Something else?

944Ross

It's been a while since I replaced mine; does the impukse unit have a male square drive that slips inside the female end of the shaft driven by the gear? And that short male piece is what is breaking off?

All I can think of is that the impulse unit is bottoming on the shaft, like if the shaft with gear isn't seated all the way. But I wouldn't think the impulse unit would have worked for a while if the gear wasn't seated. When you pulled the gear/shaft out, was the bottom of the shaft, below the gear, in good shape? Are you sure the gear/shaft were fully seated?

JayL

Yes, the impulse unit has a male square drive. That is the piece that is breaking off.

The shaft with the gear seems to float and can move freely. I'm sure there's some pressure on it when it turns while the car is moving. As for its condition, it's probably the original piece with 175000+ miles on it so it's not perfect. Makes me wonder if a small piece of the old sender was still stuck in it.

Since I need to get another impulse sender, I think this time I'll try and get the other plastic gear piece to go with it.

JayL

Seems like this little piece is not easy to find. Anyone have any ideas where I might be able to purchase one? Talked with a few folks and they wanted me to buy a new transmission to get it. Here's a parts diagram from 944online that shows the part. The sender itself is part 8 and I've circled what I'm looking for if anyone has any ideas.

Van

I threw one of those out just the other day... I'll rummage around in the trash and see if I can find it.

JayL

Finally got everything installed and now have a working speedometer. All this was possible due to Van having the hard to find part. Thanks again.

ramius665

Thanks Van!

Van

You're welcome! Glad it worked out.

tomjourdo

hi there - i know this is a late reply to an old post, but looking for some advice

my speedo nor odometer isn't working, however when i take out the transmission sensor and the gear on the shaft that sits beneath, and then spin it the speedo lights up and the odometer starts working.

The gear on the shaft looks in good nic so i'm wondering what drive this gear and if this can be fixed?

thanks

Dimi 944

The gear is driven by another gear that is attached to the differential. I have one available if yours happen to be broken.

tomjourdo

ah i thought that could be the culprit. can you advise the best way of fitting it? is the only way to drop the diff out?