Speedometer not working on 1991 Bonneville LE
 

I've inherited a 1991 Bonneville LE that is in good shape, except for rough starts after sitting in the cold for a day or 2 and a speedomoter (dial) that does not work. At this point, I'd like to understand the speedometer, then work through the starting issue. How does the speedometer get its information from the car? I can't find a mechnical cable coming through the firewall or attaching to the transmission. From what I've read, the speedometer sounds like it gets its speed reading from the VSS (straight or via PCM). Would anybody be able to give me some direction before I start burning up time and parts money? Thanks in advance.

Well, first thing I would do is check my fuses... does everything else in the interior work? DIC and compass? I think its the bottom right fuse that is hooked to the speedo... (my electrical was "fixed" by the previous owner, so I had to play with fuses to turn things off in my interior...)

From there, I'm not sure about Bonnevilles... but fuses are the easiest and cheapest to check... :)

You will need to know if the ECM is getting a signal from the VSS... If it is than chances are the cluster is shot...

Does the trans hit Lockup while the engine is at operating temps? Say in 3rd( D ) at about 38-40 Mph? Or in 4th ( OD ) at 50-55 mph? If the Torque Converter does lock up and you see the rpms drop to 1500 at 50mph or so than the TCC is functional, and chances are the ECM is seeing a VSS signal or what amounts to roadspeed... If the ECM did not have any idea of the roadspeed the TCC would not lock up.. And the ECM also watches to see what gear the Trans is in, but again its very dependant on roadspeed for the TCC lockup to function..

Best bet may be a scantool so that you will know whats really going on...

Good Luck ;)

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Thanks for the fast feedback. I'll take it out for a test drive and let you know about the TC lockup.

I forgot to mention that the odometer also does not work, both total miles and trip.

Took it out for a spin and found the Torque Converter does lock up, rpms drop to 1500 at 50mph, as predicted. Other gauges (battery, temp, fuel, information center) all work too, just not odometers and speed.

I'll see what I can do to hook up a scan tool to get the computer's view of issues. Thanks.

This maens that the Computer is getting a signal from the VSS or Vehicle Speed Sensor... So there may be a problem with the wiring to the cluster or the cluster its self...

With all of those gages you have the upgraded cluster... It sounds a lot more like the speedo section is dead..

Any junkyards near you?

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One thought would be to pull the cluster apart and check the connections from the board to the speedo needle. Not sure what that little motor like thing is called. You can see them here on this pic of a 95 cluster in pieces. The little tin can has posts sticking out that are connected to the other board by merely pushing through some metal tabs.

Attachment 70770

Just an update:
We hooked up a scantool. The scantool read the ECM as long as the motor wasn't running. Turn over the motor and the scantool blanked out. We replaced the ECM and the scantool now works with the motor running and tells us the miles per hour. But the speedometer in the cluster cannot. So, I'm going to hunt down a replacement cluster.

sounds like you are making progess...keep up the good work.