Dodgy Speedo

[E30BeemerLad]

Hi all

My speedo has suddenly started jumping all over the place, most notably around 40mph. It does it when maintaining constant speed and also when accelarating. Where should I start looking for the fault, assuming it is some kind of dodgy connection? Is it likely to be the instrument cluster or the sender unit? Where is the speedo sender unit on a 1987 325i?

Just an annoying distraction whilst driving!

Ta muchly.

[orangecurry]

mine just dies very occasionally, then comes back; I am led to believe it is most likely to be the sender on the diff, or the cabling under the car

haven't considered it important enough to get oily just yet

[E30BeemerLad]

thanks mate, I may investigate further now I have a rough idea where I should be looking

Diff is due to come out in near future anyway to replace pinion bearings

[Sukki]

Ive bought a 325i sport cabrio six weeks ago and my speedo and economy gauge has stopped working altogether as well. Havnt got a clue about getting under the yet so who or what could you suggest??

[orangecurry]

I think you are missing a few words there me old china...

the economy (ha!) gauge takes info from the speedo, so don't worry about that until the speedo is working.

A good place to start is the sender unit which is on the differential; if you don't get oily, any mechanic can put a new sender unit in for you (and it shouldn't cost very much), but there may be nothing wrong with it.

Search this forum and some clever chap/chapess will have posted which wires to check for what current to tell you what has broken - good luck

[Brianmoooore]

90% of speedo problems are an internal break in the wire to the sender (on the back of the diff.), just outside the plug.
Pulling gently on the individual wires should reveal a stretchy one.
Cure is to replace the wire and plug. Replacements are available from the dealer apparently, but I use a headlamp connector and a couple of feet of wire cut from a scrap loom, and splice onto the original loom well away from the diff, preferably inside the car under the back seat.
Stick some silicon grease in the new plug and the socket on the diff before you assemble them.

[E30BeemerLad]

yep - found the problem, one of the 2 wires coming from sender unit on the diff had broken, gonna have a better look today, hope it hasn't broken right on the sender

[Brianmoooore]

You need to find a length of wire complete with a plug to repair this.
The copper in the original wire has be work hardened by vibrations, and a repair will not last.
See my post above.

[E30BeemerLad]

Brianmooore - thanks mate, you've been 100% accurate on this. It has split right where one of the wires goes into the plug.

I went to the stealers earlier and they could not find the part on the system. You mentioned using a headlamp connector. Are there any part numbers you are aware of that i can use to help get the connector. Do you know if the connectors come with wire or do you have to crimp wire into the new plug mate?

[Brianmoooore]

I'm talking about using a headlamp connector from a scrap loom.
If the yellow plastic piece on the diff plug is OK, you can use any BMW similar two pin connector and lead as the outer bit just unclips.
The different colour two pin plugs have small differences in their shape, so they can only be plugged into the correct socket.
One of the headlamp connectors is the same as the diff.

[A1BMW325iSport]

top advice from Brian there our resident electrical guru