intermittent speedometer

It started out as an occasional zero speed indication when driving. Then it worked reliably for a while. Now it is more inoperative than operative. I have put some dielectric grease on the terminals in the connector on the differential and that seemed to help for a while. Any other suggestions of where to look for the problem.
One other thing, when the speedo is working, the cruise control works and when the speedo drops to zero, the speedo goes off line. Good thing it does not go wide open throttle.

Bob in Lake Havasu

Check the fuses in the engine compartment fuse box. The terminals tend to corrode and one of the fuses (I forget which one, but a diagram will indicate which one is for the speedometer.) terminal is probably corroded. This happened to me and that was my solution.
Salut, Bob P.

That is a good idea. I have wiggled them all so will see tomorrow when I get started on my road trip. Will be a challenge without a speedometer.

Bob in Lake Havasu

It's Fuse-12. But that one also feeds the radio. If it's a bad contact at the fuse, your radio would be cutting in and out at the same time as the speedometer.

I have the same issue, speedo cuts out intermittently. It's something I've been meaning to fix but just haven't gotten around to yet. It shouldn't be difficult to diagnose. See attached wiring diagram.

The most obvious point to check is at the wire connections at the sensor on the back of the differential housing. Those are exposed to the elements.

It's unlikely to be the sensor itself. Those are pretty much bullet-proof. Measuring resistance between the two terrminals on the sensor you should see alternating zero resistance and infinite resistance when the switch in the speedo sensor opens and closes as the rear wheels are slowly turned.

If that looks good, then you can go to the back of the instrument cluster and disconnect C2, the 26-pin White connector. Measuring resistance between Pin-12 (brown/red wire) and Pin-8 (brown) you should see the same alternating zero resistance and infinite resistance as the switch in the speedo sensor opens and closes when the rear wheels are slowly turned. That'll tell you the wires are good to there.

Also check voltage at Pin-13 (Violet) on that C2 connector when the key is Accessory or Run position. Should be 12v coming from Fuse-12.

If that's all good, the most likely culprit is a cracked solder joint on the receiving C2 Pin-13 on the instrument cluster circuit board.

It's awkward seating or removing those big 26-pin connectors. If they don't push/pull on/off straight, you end up putting bending strain on the connecting pins. The worst strain is taken up by the pins at either far end of the connector, i.e. Pin-1 & Pin-14 at one end, and Pin-13 & Pin-26 at the other end.

My guess is the solder joint on Pin-13 is cracked. You'll need to remove and open the instrument cluster to see the circuit board for that. Take a close look at the joints of the four pins at the top and bottom end of the connector row. You might be able to see a thin black crack line. In any case, it can't hurt to take a soldering iron to those pins to re-flow the solder joint.

Hi Ferd and Bob,

Great Info! I too have this problem.

Thanks for posting. :-)

Kelly

Thanks Ferdinand. That is the type of technical info I needed to begin the search. On my two day trip,. the speedo worked most of the time after the first half hour of driving each day. What I need to study is why the cruise control only works when the speedo is working. If it is not working the cruise control drops off too.

To me that says the sensor signal from the differential is not getting through for some reason. I have opened up the connector on the diff and it looks clean inside. Also put some dielectric grease in there to minimize corrosion too. I may have helped some. Not sure.

Bob

Bob in Lake Havasu

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Bob in Everett
What I need to study is why the cruise control only works when the speedo is working. If it is not working the cruise control drops off too

See the wiring diagram attached in my previous post. The speedometer takes a power input, and speed input (from the diff sensor), a ground lead, and generates a speed output signal via pin-1 of C6 (the green connector on the back of the instrument cluster) to the cruise control module.

If the speedometer stops working, then the cruise control module receives no speed signal and shuts down.

Well, that explains the cruise control part. After driving for a while the speedo gets more reliable until the car is parked again for a while. Maybe 20 minutes of driving gets it working again.
Maybe if I drove it every day it would be even better. I had the cluster out of another car and it was difficult to get all the trim pieces back together and look correct. Did not think I wanted too much practice but may have to do it. The light for the tach is apparently burned out as that half of the cluster is dark.

Bob in Lake Havasu

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Ferdinand
I have the same issue, speedo cuts out intermittently. It's something I've been meaning to fix but just haven't gotten around to yet. It shouldn't be difficult to diagnose.

Success! I fixed mine.

Intermittent issues are often difficult to diagnose because it seems every time you try to track down the problem, of course everything is working perfectly that day. grrrrrr.

My speedo has become steadily less reliable until quitting altogether. But I suspected all along that, in my case, the issue is with the connector to the speedo sensor on the back of the differential housing. The speedo would sometimes quit when the car splashed through a deep puddle. So that pointed to an external cause, nothing to do with the instrument cluster itself. Plus, I have had that connector unplugged recently when I changed the clutch disks in my limited-slip diff. It's just that it's been a long winter and I haven't felt like rolling around under the car to check that connector until now that Spring has finally arrived.

Sure enough the plastic end of the wire connector, that plugs into the sensor, is all crumbled and the wires simply weren't plugged solidly onto the pins on the sensor housing. I jammed them on tight and sealed it all up using some plumber's putty. Presto. The speedometer is working beauty fine again.

You others, does your speedo come back to life if you pound on the dash above the instrument cluster? If so, then your problem is not at the diff sensor.

I have that issue with my engine temperature gauge. I'm happily driving along when I get a sudden fright seeing the temp gauge pinned into the red at full hot! Just give the dash a whack, and presto the gauge drops back to mid-scale where it should be. Gotta fix that next, someday soon...

Ferdinand,
Do you have a similar page of circuit diagram for the tach indication? My sedan has an inoperative tach now. It started as intermittent and is now apparently inop along with the back light bulb. Not sure they are related. Speedometer does work full time though. Looks like I need to get more OJT on removing the cluster and re-installing. On the sedan, the most difficult part was the closeout panel under the cluster. Wound up setting the error message on the SRS and not able to find a successful reset tech or tool. Getting the tach and light to work again might be worth the effort to dig into it again.

Bob in Lake Havasu

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Bob in Everett
Do you have a similar page of circuit diagram for the tach indication?

[attachment 246 Tachometer.jpg]

You can download the entire set of wiring diagrams for your car from this link.
http://www.armchair.mb.ca/~dave/BMW/e30/