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Repair Questions and Answers.

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 #35708  by Scotian
 
Recently the transmission range indicator (PRND1234) in my instrument cluster is messed up. It often won't display that I am in Neutral when the others work (it stays stuck in Drive, then when I go to Reverse it lights up N then R), sometimes does not show that I am in Park (it sits at Reverse), it often will display that I am in all of PRND1234 (all squares lit), and once in Neutral it indicated nothing at all. Sometimes it will display P when started (after the startup test of all lit), then it will light them all again after a few seconds. Sometimes it works just fine.As far as I can tell so far, the car's transmission operates just fine, regardless of what the indicators say. The Service Manual states that the Transmission Range Sensor tells the Transmission Control Module what gear it is in, and this is all located in the transmission itself. I mention this because I was just mucking about inside the center console beneath the shift lever, but I didn't touch the shift linkages or other cables and wires down there that were not part of my aftermarket car alarm. If I had screwed it up from inside the cabin, the transmission wouldn't be shifting when I use the lever. I think my recent work and this problem are coincidental.Any ideas? I've tried unplugging the TCM fuse for a while to no avail.
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 #35709  by Bill Putney
 
I just reviewed the FSM schematics on the instrument cluster and its interface with the BCM.Here's my best guess assessment of what you have described: If the source of the error were the Transmission Range Sensor, then you would not *only* be getting erroneous PRNDL readouts - the TCM would be causing the transmission to act according to the PRNDL readouts (i.e., also screwed up). You don't indicate that the tranny is doing anything other than what your shift lever is telling it to (contrary to the PRNDL readouts).Next: The BCM communicates everything to the instrument cluster via the PCI Bus. If this bus (or what was driving it) was the problem, other things in your instrument cluster would be behaving strangely (and they are not).Bottom line, I believe that the problem is in the instrument cluster itself. I could be wrong - but that's my WAG.
 #35710  by Scotian
 
The digital tach in my last car (1990 Buick Regal Gran Sport -- the best looking digital display on a car ever) flaked out. I think it would display a reading for a while and then hold it until it accepted/received a new signal. The problem went away when I got a new computer for another problem. This is acting similarly.It doesn't read P or N. When placed in either position, it will not indicate it, but after about five seconds it will light up all of them. It seems to know that it isn't getting a correct signal and goes to startup test mode.When I start up the car, it will initially display P, but after the few seconds it start tests again, even if I had turned off the car after it went into start test mode.You know, it could have been doing this for months and I probably wouldn't have noticed -- I don't sit with the car on in P or N. Maybe you all want to check yours...