94' Lasabre, Speedometer problems.

[pammey]

I have a 94' Buick Lasabre that was giving to me last year. At first the speedometer worked just fine and the one day when I was at work it just stopped working. No lights or any thing came on. It just went to the 10 mark and stayed. It will work out the blue sometimes for a week or less or maybe just a few miles. But other than that it just stays off. If someone could please tell me what they think that it might be please help!! I will love you for ever and ever! Thanks so much!

[HotZ28]

Pammey, welcome to AF!
You do not have to “love me for ever and ever” just a “long time”!
Your problem could be several things. First, you have a VSS (vehicle speed sensor) in the transmission that rotates and generates an AC pulse that is sent to the PCM to interpret vehicle speed. The VSS could be going bad. Second, you have the instrument cluster that contains the speedometer. That could also be going south! Finally, you have all the associated wiring between the VSS, PCM and instrument cluster that could be at fault. I wish it was as simple as saying, your problem is only one possibility, however, and in this case, I would be in default, if I even suggested that! Love me long time?

[mcqueary]

HotZ28,

I had a question concerning this issue since my mother in law also lost her speedometer indication on her 95 Lesabre. If the speed sensor were bad, would this cause any problems such as the transmission shifting incorrectly? I was under the impression that the computer used the speed sensor input data to help determine the proper shift points. If so, this might be a diagnostic aid. Of course, I realize the speed sensor is used to control the speedometer also.

Thanks,
Tom

[HotZ28]

Transmission shift points are not determined solely, based on the “speed” of the vehicle. You are correct, to assume that the PCM uses input from the VSS, however, that is only one small part of the information needed to determine shift points.

It is very easy to test the VSS output with a live scanner. I use a Matco Tools 9640B professional enhanced scan tool on the 94-95 models. It has a display for mph. Just hook it up and drive around the block to see if it displays speed (mph), if so, the VSS is working. If the VSS is supplying output to the PCM, you have one of the other problems mentioned.

The VSS is simply an electrical pulse generator that sends varying pulse rates (based on speed) to the PCM, and in turn, the PCM uses the pulse count to interrupt vehicle speed. (mph).