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Pontiac Repair/speedometer reads twice as fast and tach reads high


I have asked you questions about this 1990 Pontiac Grand Prix before and you have always been on the money so I have another. Ever since we got it the speedometer has read twice as fast as really going and the tach reads high. We needed to drive 155 miles one way to go to a hospital where our son in law had brain surgery and could not take the car because it was not working. Can you give us some insight into what could be causing it? Thank you. We really appreciate all of your help. The car is running great now. We had to have someone take us to the hospital and we felt like a burden.

Hi Trudy:

no other direability issue other then the IPC issue?? does this have a low coolant light on all the time? any other warning lights on? does the temp and fuel read correctly?  If it was just the speedo I would venture to "guess" it's a bad engine computer that year car had all kinds of engine control module problems. Beinf that the tach is readin hi to would make me think that you could just have an issue with the IPC (speedo) cluster. with out seeing it I don't know for sure Both of those inputs to the IPC need to be tested to see if the induced input matches what it should on the speedo and the tac If the induced input to the cluster results in the proper reading then the issue is with the engine computer. if not then the IPC is the culprit. good luck :-}

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Todd Puccio


General auto repair,electrical issues,driveability issues,anti-lock brakes,SIR systems,HVAC,engine performance,Gm specific. I have worked at my current dealership since the 1990 model year anything much older than that I have limited exposure to. I do not have very much transmission experience so those question you may want to ask of another expert. When you send me a question let me know the exact make/model/engine and year for the car or suv you have it makes it easier to answer the questions that way. If your happy with the answer I gave you please feel free to Donate. This is a free service to you and I am not compensated at all by this web site. I have 22 years of experience to share with you and my knowledge wasn't free to me. I had to pay for my education.


Experience in the area: I have been a technician in GM dealerships for over 22 years and I am a ASE master technician. I work at a Buick and GMC truck store Previously I worked at a Pontiac GMC store for 18 years. I Served a a lead technician when dealership was on a team system. I just recently moved to a Buick,Hummer,Saab Store when my franchise was purchased by this dealership. Organizations: ASE master technician


Education/Credentials:High school diploma,2 year vocational school degree 2 year college degree. Associates degree in applied sciences (auto technology)Awards and Honors: GMC master technician

Awards and Honors
Master GMC truck technician Master ASE technician in 9 categories

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