From digest.v7.n563 Mon Nov 10 01:12:54 1997
From: Jim Cash <j.cash_at_sympatico.ca>
Date: Sun, 09 Nov 1997 23:05:30 -0500
Subject: Re: - Speedo not working

Chris writes:

> My 1984 325e's speedometer is not working. Or, more precisely, it works
> when it wants to. Sometimes it works OK, sometimes it jumps around, but most
> of the time its dead. The mpg gauges and the odometer only work when the
> speedometer decides to work, too. On my 1979 635Csi, I fixed a dead
> speedometer by replacing the cable from the transmission. But, I understand
> that this cars speedometer is 'electronic'. I dont have the Bently manual
> yet, can anyone tell me how to fix it?

Chris

This very condition has been mentioned "numerous" times in the past month.

I see you read the similar item on the E28 in previous digest V7 559 - but I think that may have given some wrong information.

I agree with you - if the speedo is not working the economy guage will not work either. Neither will cruise control. Actually there is one situation where the speedo will not work but the econo guage will, and that is if the speedo guage has a physical fault with the needle mechanism. Otherwise, if the electronics are working then the speedo circuit sends the speed pulses over to the econo guage circuit - this is done internally in the instrument cluster. But there is a seperate lead and connector to get the speed pulses to the cruise control unit.

But I digress.

Yours is electric, and there are 2 likely causes for a fault that comes and goes.

1. It is probably the connection at the speed sensor. This is a toothed wheel inside the differential (yes at the rear axel). You will find an electrical connector on the rear of the differential. Make sure it is well connected. Usually there is corrosion in the connection or the wire itself. Sometimes the movement of the suspension will cause enough flexing of the wire that it will break just before the connector. (this also happens with the brake pad sensor leads if they ae not properly held in their support clamps. Actually the wire could be damaged along its length but it is probably the connection or the sensor.
2. If there has been any work done on the instrument cluster you may have a loose connector on the back of it.