1996 Volvo 850 R speedometer sometimes stops working

Tzazlaz · Post by **Tzazlaz** » 01 Mar 2015, 03:10

Hi everybody.
My speedometer sometimes stops working and I am very puzzled over why. It can work pefectly for several days, weeks and even months, but it can also stop working several times a day.
When it stops working its always when I come to a complete stop (red lights etc.) and I have never tried that it stops working while the car was driving/rolling. I mean, I can drive along with a working speedometer, brake for a red light and when the light turns green and I start driving again the speedometer does not work. When this happens all you have to do is turn off the car, start it again and then the speedometer will also work again. Regardless if my speedometer is working or not I have no warning lights or error codes. My abs module is also recently rebuild. I have read somewhere that it could be my ignition switch, but I have tried to wiggle the key and lightly knock on the ignition switch and steering column without any luck

So is there anybody with a good idea about what I should do/try next??
Thanks in advance

Post by **rspi** » 01 Mar 2015, 05:55

Who rebuilt the ABS module?

Tzazlaz · Post by **Tzazlaz** » 01 Mar 2015, 08:25

The abs module is rebuild by Swedish Car Parts, so I personally dont think thats where the problem is. But on the other hand I really dont know for sure

Post by **Ozark Lee** » 01 Mar 2015, 10:01

As you have surmised most speedometer problems like you describe are directly attributable to a bad ABS module and the modules themselves have a failure rate that approaches 100%. The fact that you have a re-built module and you do not have ABS / TRACS warning lights would suggest a problem that is actually not a bad module but the chances of that are a bit slim.

The root problem on the ABS modules is bad solder joints at the point where the connectors are soldered onto the PC board. The large 2 pin connector is typically the first to fail and, quite often, just re-soldering the two pin connector that leads to the ABS pump motor will fix the module. The smaller pins on the large hooded connector are also subject to failure and it is through those pins that the speedometer signal is connected and a bad solder joint on that pin wouldn't necessarily trigger a ABS fault.

While my gut says that the module needs some more work you should also try to re-seat the large hooded connector that goes to the module and while it is disconnected give both sides of the connector a shot with electrical contact cleaner. That connector can trick you and you can think it is properly installed when it really isn't. Make sure that the hooks on the hood engage the pins on the socket before you fold the hood down. It is real easy for the hooks to not grab the pins and the hood will fold down anyway but the connector doesn't get drawn down into the socket completely and it causes the connections to be intermittent.

If the module is good and the connector is properly engaged then about the only thing left is the instrument cluster itself going bad. The odometers on the VDO clusters have a failure rate that just about equals the ABS module but the speedometer portion rarely fails. Rarely isn't never and you may have a rare cluster failure. The real test is to look at the signal on the vehicle speed wire going into the instrument cluster but most people don't have an oscilloscope laying around that would let you look at the signal.

...Lee

Tzazlaz · Post by **Tzazlaz** » 01 Mar 2015, 11:48

Ozark Lee wrote: The real test is to look at the signal on the vehicle speed wire going into the instrument cluster but most people don't have an oscilloscope laying around that would let you look at the signal.

...Lee

Thank you for the reply. First I will try to clean with electrical contact cleaner and make sure that the connector is seated properly. So we can rule that out if that doesnt work

I actually have access to an oscilloscope, so if you can explain the procedure I will also like to try that. I dont know which wire(s) that are the vehicle speed wire and I would appreciate some guidance in how I should do to test it correctly

Do you by any chance know where the wire runs from and goes to, so I can check it for breaks and other visible faults?

Post by **Ozark Lee** » 01 Mar 2015, 12:55

The wire is a yellow/brown wire that runs from pin 6 on the large multipin connector on the ABS module to pin A-3 on the instrument cluster. It goes through the ZIF socket (24/13 - pin 1) that is under the dash below the steering wheel. What you should see on the scope is a square wave that varies in frequency with the rotation of a wheel. As I understand the system the ABS module aggregates the signals from each wheel so I think that any wheel can be spun to generate the signal.

...Lee

Post by **rspi** » 01 Mar 2015, 13:14

These clusters are failing at an alarming rate, however, it is rarely the speedo gauge. I once had a few wires at the top of my cluster go bad. The weight of the harness was causing the wire strands inside the singe wires to break and that was causing my issues, which were NOT SO intermittent.

Tzazlaz · Post by **Tzazlaz** » 01 Mar 2015, 14:26

Ozark Lee wrote:The wire is a yellow/brown wire that runs from pin 6 on the large multipin connector on the ABS module to pin A-3 on the instrument cluster. It goes through the ZIF socket (24/13 - pin 1) that is under the dash below the steering wheel. What you should see on the scope is a square wave that varies in frequency with the rotation of a wheel. As I understand the system the ABS module aggregates the signals from each wheel so I think that any wheel can be spun to generate the signal.

...Lee

Thank you so much. I will take a look at it in the near future and post the results here

Tzazlaz · Post by **Tzazlaz** » 01 Mar 2015, 14:31

rspi wrote:These clusters are failing at an alarming rate, however, it is rarely the speedo gauge. I once had a few wires at the top of my cluster go bad. The weight of the harness was causing the wire strands inside the singe wires to break and that was causing my issues, which were NOT SO intermittent.

Ok. Hopefully I will find the error in near future and post it here. Until then ALL suggestions are greatly appreciated

Post by **Matty Moo** » 01 Mar 2015, 18:55

I don't think it's your module. I haven't seen one yet that will not send a speed signal and not throw codes.

I've dealt with a handful of people with cluster issues but it's almost always the 98 clusters.