dash speedo and ECC speed

I got playing around with the ECC today and noticed that #14 is the vehicle speed and it is a constant 5mph slower than the dash. So I did what any red blooded American would do..... I ran up on one of those "speedometer calibration stations" that the local law enforcement agency sets out for us. Seems that the ECC is DEAD on and the dash is 5 off to the good. SOOOOOO, What controls what? I always thought that the trans sent a signal to the PCM and then it was sent to the dash. Why the difference?

Id be interested to learn about this too. My speedo is about 5-8 mph off according to my gps...

An example of why it can be off is that true speed changes with tire diameter. The tranny puts out a pulse that supposedly is 4000 pulses per mile, and is gear driven. The ECM needs an approximation of speed for some functions, like going into highway mode, but exact speed is not important to it.

GM does have some different sender gear sets for the trannies to correct for different tire sizes and rear end gear ratios. In the long run if the dash says 55 and actual speed is 52, you are better off as the error helps the speeding tickets from happening.

Reality is that it's only important when you've just driven through a radar gun

There is a "Measured Mile" near my house on an expressway. It literally has a white line on the guardrail that says "measured mile starts here" says When I calibrated my Nordskog digital dash over that mile, I registered 4273 pulses per mile, so 4000 would have been about 5% off. The ECM still gets it's 4000 pulses per mile and doesn't' care what's going on with my dashboard. they are independent.

The 96 uses the computer to control the speedo, and odometers, as well as some other functions. To change the speedometer, you have to program your tire size, and gear ratio into the computer (91-93 you have to change the VSS gear). The drive for the VSS uses a set signal. The circuitry for the speedo may be starting to deteriorate (a resistor may be breaking down, and that could reduce or increase the resistance). A contact for the cluster could even be dirty, and have more resistance. There should also be a VSS module under the dash, and it may be on its way out as well. You have some trouble shooting to do.

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I was just curious what controlled the speedo and the ECC. When doing MPG calculations, there is a 10% difference. GPS matches the ECC so I'll go that route, but the odo is now off......

phantom 309 wrote:

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WAKE UP BOY...there is going to be a test!