Faulty Speedo (Not talking swimwear!)

Hi there,

I've just bought my first BMW which is an E30 325i

Amasingly pretty much everything seems to work on it except that the speedometer intermittantly stops working.

Thumping the dash doesn't seem to work and there doesn't seem to be a pattern to the fault as it works from cold (usually) and then if I turn off and on again, it takes a while to come back on.

Can anyone suggest a reason for this irritating problem?!

(Of course it does have the pleasant side effect of shutting off the comedy "economy gauge")

Many thanks!

Alex

Thought it might have been SI board, they can give trouble, but if the lights are coming up then its probably not that. 

Back to mechanical stuff, like Peter suggested, or speedo cable, etc.  Not sure what link there may be between speedo and mpg guage.

GT6-M3 wrote: Hi there, I've just bought my first BMW which is an E30 325i Amasingly pretty much everything seems to work on it except that the speedometer intermittantly stops working. Thumping the dash doesn't seem to work and there doesn't seem to be a pattern to the fault as it works from cold (usually) and then if I turn off and on again, it takes a while to come back on.

Alex,

The most common cause of this problem is broken wiring on the speed sensor on the back of the differential. Have a good look to see if it's okay.

I recently 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.