strange speedo issue

[aaron325ix]

a few weeks ago my speedo/odo just stopped working. I took out the speed sensor from the diff and tested it and was infact bad. So I replaced it with a brand spanking new one.

speedo still doesnt work. My buddy who is an EE looked over my cluster, reflowed some solder here and there, tested the speedo on a frequency generator and got it to work.

We checked the wiring diagram and are getting a signal from the diff where the cluster pluggs in. Weird thing is every now and then I start my car and the speedo jumps up to like 80mph while standing still!! As I drive it will move round between 60 and 105!

We tested 2 known good clusters and have the same results as mine, no speedo.

We are stumped as to where the problem could be. Does anyone with elec./wiring knowledge have any ideas? Thanks.

[kenika65]

Its pretty simple, when you pull your cluster out their is a white connection that plugs in to the left side of the cluster, that is the only one you need plugged in when testing your cluster for problems.

With the cluster plugged in take the speedo out and test the prongs that plug in to your speedo.

The top prong should be pos+ and the 3rd one down will be your reference signal from your diff sensor.

With your tires off of the ground spinning or Wall driving the 3rd prong down will flash ground, so you may need a power probe to check this.

If you have both a constant pos+ and a constant flash of ground, then your problem is only in your speedo.

In your speedo their are a few solders to check but most connections that fail are inside and are not worth my time to fix so just swap in a different speedo until you find a good, old speedo that still works....

[aaron325ix]

We tested 2 known good clusters and have the same results as mine, no speedo

[aaron325ix]

in other words I fear there is some odd wiring issue at hand. we also put in a known good speedo into my cluster and still no workie. pissing me off

[jeer]

have you tried using another sensor, maybe when you are checking it its working fine then while driving something else is happening, could be a short the wires going to the diff under the insulation thats is only acting up occasionally, try driving it in gear with the rear lifted and wiggling the wires around by the diff and see if you get the crazy readings,

just some sugg not sure if it will help or not.

[aaron325ix]

^^ I have tried that and the results are the same:

The speedo/odo doesnt work. Everything else on the cluster works.

I put 2 known good clusters in there and same thing

I have put a known good speedo into my cluster and the same thing.

The weird part is how when sometimes I start up the car the speedo jumps up and hovers around the 60 to 100 area. This damn thing is possesed!!

Any electrical experts out there who have any ideas?

[Keanan]

If two speedos gave the same problem and you have a new sensor chances are it is in your wiring. Check your wiring between the instrument cluster and speedo pick up. Look for continuity and see how much resistance is there, also check to see if there is a short to voltage. If you don't have a good wiring diagram or know how to read one PM me and I will gladly explain.

[george graves]

If two speedos gave the same problem and you have a new sensor chances are it is in your wiring.

That would be my guess as well. Just a best guess. How is that speedo plug looking? See any green-bluish power on the contacts?

[aaron325ix]

Thanks for the reply George. The plug appears to be fine. We actually hooked up an ohmeter to the two pins on the plug and let the car roll back and we were infact getting a signal from the sensor.

However, the car was off when we did this. As mentioned earlier it goes wacky sometimes when the car is on. Do you know of anything that could be causing some kind of interference to the speedo while the car is running?

[george graves]

I don't recall - that a shielded cable right?

Maybe get your friend to hook up his function generator to the plug on the diff and an oscilloscope to the wires going to the cluster and then turn the car on and off? Hard to really guess what's going on.

It's going to get really scary if e30's start to get major electrical problems.