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Computer Reading "half-speed"

Old 01-18-2010, 10:52 AM
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Default Computer Reading "half-speed"

2004 Convert. 4 cyl.
I had to replace my battery and get a state inspection. I drove for about a week and everything reset in the computer (required for Mo. state inspection) with the exception of the cat and O2 sensors. I took it to a reputable shop and they said the computer only reads half of the vehicle speed. If I drive 60, the computer reads 30. So unless I drive 120mph for 5 minutes, I can't pass my inspection. The shop said both the speed sensors tested OK. They also believe the Cat and O2 sensors are OK. They just don't know why it's reading half-speed. Anyone have any ideas? I have no other history on the vehicle as I bought it used.
Old 01-18-2010, 01:06 PM
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Location: Detroit suburb
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Does your speedometer on the dash read correctly? The signal that generates the information displayed on the dash comes from the same place. If the dash speed reads correctly, then the engine computer is reading and interpreting the information correctly.

For your information, the output speed sensor in the transmission generates a signal based on the rotation speed of the transmission output shaft. This signal is transmitted over a wiring harness to the tranmission control module. The TCM, in turn, transmits it to the engine control module. The engine control module uses the information but also transmits it to the body control module which displays it on the dash.

If the engine control module were reading half the speed, then so should your speedometer and I think you would notice that pretty easily.

If your speedo is reading correctly then it's time for another opinion. Find another shop.
Old 01-18-2010, 02:47 PM
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The speedo works correctly. The first shop didn't have any 'up-to-date' diagnostic equipment. This shop was the one that told me about the 'half-speed' problem.
Thanks anyway....
Old 02-11-2010, 10:02 PM
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Posts: 3

It sounds like they are BS'ing you.
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