Question
Expert: Robert
Date: 10/23/2007
Subject: Failed speedo
Question
1991 Buick Lesabre (hopefully similar to Chevy)
Speedometer needle jumps occasionally, inop most of the time. There is no check engine light, the transmission sifts normally, the cruise control works normally. (Eliminates VSS I believe) I have changed the inst. cluster with an identical unit from a salvage yard resulting in no change. I understand the PCM is the "middle man" in the speedo circuit. Is there a shade tree technique to check the PCM short of replacement or dealership diagnosis? Is there a standard location for instrument panel grounds that I might check? What color wires go from the PCM to the speedo-so they can be checked for damage? My entire auto repair experience can pretty well be described as "if it has happened to my car, it has probably happened to MANY others. Thanks for whatever info you might be able to provide.
Art,
I picked up this question out of the pool. It was not sent to me. Before working for Chevy I worked at Oldsmobile for 10 years and Buick for 2 years. My past experience with this complaint most always was a defective ECM. In fact in my own car, a 90 OLDS 88 which is an H-body, same as a LeSabre I have put 2 Ecm's in over the last 10 years for intermittent speedo. I'm keeping the car for my Son to drive and Guess what...the speedo is inop again, only on humid days. I know it's the ECM but have not got around to replace it. The ECM is these vehicles have been updated a few times.
I would suspect the ECM is the cause of your problem. You can try tapping on the ECM with the engine running. If the engine stalls or stumbles or the speedo jumps...replace the ECM for sure.
Replaced the ECM, no change. While fiddling under the dash, we found a small box located just forward of the glove box with six or so wires plugged into it. When those wires were wiggled, the speedo went nuts. I unplugged and the re- plugged the connector and the speedo went to zero. A test drive indicated all was working properly. Reassembled the dash and tried it again. Speedo jumps to 45 when powered, even before engine start. Tried a test drive to see if the speedo would "catch" the speed while driving. The needle stays at 45 until the car is going 45 (estimated) then begins to follow speed while accelerating above 45 and slowing BUT hangs at 45 at all speeds below 45. Our Chilton's book does not identify under dash components.
This is a sealed black, rectangular box about 1/2 an inch thick. The end away from the plug has a white line diagram which looks like it might describe relay logic. It clips to the vertical side of an open plastic box (nothing inside). Fiddling and flexing the wires now has no effect on the speedometer.
I would buy a replacement for this if I knew the name of it and if you could confirm that it has something to do with the speedometer. I am concerned that the erratic speedo indication when the wires were flexed might have had something to do with the other end of some wire in there.
Sorry about the background copies above, I dumped the other answer believing the ECM was THE fix so I couldn't recover your return address for further questioning.
AnswerI'm sorry the ECM did not fix the problem. In my 20 years at GM dealers I never had anything else but the ECM be the cause of the problem you described.
Offhand the only thing I can think of that box being is the chime module. I looked on ALDATA and the chime module has 6 wires. - Light green, 2 blacks, orange, pink/black, & yellow. Are these the colors in that module? If yes I have a hard time correlating a defective chime module to speedo function. You stated that the speedo does do different things with the new ECM, correct? Did you put the original cluster back in or are you using the junk yard one?
I have a 90 Olds 88 manual in my garage somewhere. I can research this further for you. Give me the exact colors of the wires in that module.
I would also recheck that connectors at the ECM just to be sure. Being stuck at 45 mph now seems like it could be a defective cluster. It seems it is now getting the speed signal from the ECM.
Most of the time when the ECM is defective you'll still get a MPH reading on the scanner but not at the Speedometer.
Get back to me if you want me to look it up for you.
