speedo works intermittently

[mti88m3]

Just recently the speedometer in my 325 got to where it only works part of the time. I've apparently lost my manual, so does anyone have any ideas?

Thanks in advance
Jon

[Court M3]

Could be a problem if the gears in the back are strippded out. Just replace it with one from another car. If you need one for an M3 I have one minus the needle.

[atmh]

there is a rubber boot around the connector for the sensor in the final drive, water can get in there and short the circuit. Take it off, dry it off (prefferably with an air compressor) and spray wd-40 on it. It SHOULD work.

The sensor is located at the back of the final drive, you can see it pretty clearly from under the spare tire, its the only wire going to the final drive (differential).

[hotghias]

heh, if your speedo problem is anything like my tach problem, try giving it a flick and see if it pops up. don't know why my tach does it, but it works for me.

[mti88m3]

Tapping it helps for a little bit. Still cuts out though. I'll check that sensor as soon as I get a chance. Thanks for the help guys. I'll let you know how it turns out.

[ian332isport]

I had a problem where my Speedo started to get intermittent. Sometimes it would work, other times it was totally dead. I also noticed that it seemed to come back to life when I had the heater on and the car was nice and toasty inside.

I traced the signal all the way from the diff sensor to the back of the instruments, and this proved to be working fine. I pulled the instruments and stripped them down, and had a good signal path all the way to the speedo head itself.

After thoroughly checking it over, I found what appeared to be a dry solder joint on the circuit board on the speedo head. After re-soldering the joint, everything worked fine. I also checked the same joint on another speedo, and it also showed signs of a bad joint.

If you look at the circuit board on the back of the speedo (talking about VDO here, not sure if Motometer is the same), you will see several components soldered to the board. One of these is a silicon chip of some sort. This is under the big coil of wire on the side of the speedo.

In the centre of the chip, are two very wide pins. The wide pin nearest to the edge of the circuit board is quite close to another component (capacitor) that is a small rectangular shape. If you look at the solder joint for this capacitor, you will hopefully see that it looks slightly cracked around the join. You may need a magnifying glass to see it clearly. Even if you can't see a faulty joint, it is always worth heating all the joints with a bit of fresh solder as dry joints are not always visible to the eye.

Mine has been fine ever since.

Cheers,

Ian.