E30 speedometer works intermittently

[Infinityworks]

Hi I've got a 325i and I want to sort out the speedometer it works intermittently as if it's a bad connection but I don't know where the loom for it is or if it needs a new sensor any help thanks.

[Cloggy Saint]

Check the wires coming from the speed sensor on the diff, it's a common problem.

[Infinityworks]

Ok will the sensor need replacing? Or just rewire? Thanks

[Cloggy Saint]

It's usually caused by a chafed wire, the sensor should be ok. If the wire is damaged replace it with something a bit more heavy-duty.

[Brianmoooore]

It's not a chaffed wire; it's a "work hardened" wire. Copper (the core of the wires) has a peculiar property of changing its crystalline structure if it is repeatedly bent, from a soft, flexible metal to a much harder and brittle form.
Over time, the vibration from the diff. has the effect of changing the structure of the copper in the flying wires to the sensor, and eventually a core will break, either right at the plug, or internally in the wire. The outer plastic sheath of the wire is still intact, so the broken ends are held close together, occasionally touching, and giving the intermittent speedo. symptom.
The only way to restore the copper to its original flexible state is to heat it until it glows red, and then let it slowly cool (annealing), which is a bit impractical when it's covered in plastic, so the wire needs to be replaced, NOT repaired.
A repair section of wire, complete with plug is (was?) available from BMW, or you can use an identical plug, complete with a length of (wrong coloured) wire, that can be found going to the headlamps of a scrap E30.

[Cloggy Saint]

It's not a chaffed wire; it's a "work hardened" wire.

Let's not split hairs, it's knackered.

[jmc330i]

It's not a chaffed wire; it's a "work hardened" wire.

Let's not split hairs, it's knackered.

The difference is you can see a chaffed wire, but in this case the wire (or plastic covering) will look fine from the outside and the damage is internal.

[Cloggy Saint]

I stand corrected.

[Brianmoooore]

More importantly, a chaffed wire can be joined and made as good as new. A repaired brittle wire will fail again.

[bigbird76]

If you repair your diff sensor wiring and its still the same the next step is to re flow the soldering in the clocks.

Leigh

[jmc330i]

If you repair your diff sensor wiring and its still the same the next step is to re flow the soldering in the clocks.

Leigh

I was going to mention this Leigh - if the trip meter still turns when the speedo needle drops, the wire from the sensor should be fine. Luckily I noticed the trip meter still turning on mine before I set about replacing the sensor wire!

Have you redone the solder in yours Leigh?

[Brianmoooore]

If you repair your diff sensor wiring and its still the same the next step is to re flow the soldering in the clocks.

Leigh

The bit that needs re flowing is the soldered connections of the pins on the speedo. daughter board that plug in and connect it to the main circuit board.
If thumping a clenched fist onto the top of the dashboard stops the speedo. working when it is, or starts it working when it's acting up, then this is the first place to look.

[miniblob]

A repair section of wire, complete with plug is (was?) available from BMW, or you can use an identical plug, complete with a length of (wrong coloured) wire, that can be found going to the headlamps of a scrap E30.

Anyone got a part number for this please???