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Rpms drop when I stop excelerating and let off the gas

B

bbfanatic23

New member
I'll be driving down the road going from 0 to 45mph and as soon as I stop accelerating the speedometer will drop to 0 and go haywire going back and forth between my speed and 20 and 0 and 33 and so on. As soon as I let off the gas I can feel what seems like a rough downshifting. My car also has some pretty rough shifting. I've replaced the pcm and that did nothing.


The following is a conversation I had with someone on the chat about my replacement pcm that might get yall up to more up to speed on whats going on. The conversation starts at the bottom of the page...

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[TD="align: left"]25/10/2012 6:21 PM[/TD]
[TD="align: left"]bbfanatic23[/TD]
[TD="width: 100%, align: left"]thats what i need to know... but if thats pretty much true then i think i got a bad pcm[/TD]
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[TD="align: left"] 25/10/2012 6:20 PM[/TD]
[TD="align: left"]bayus2972[/TD]
[TD="width: 100%, align: left"]best bet for you is to post this in a thread...let the whole forum gang voice in....most are WAAAY more knowledgeable than I...[/TD]
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[TD="align: left"] 25/10/2012 6:20 PM[/TD]
[TD="align: left"]bbfanatic23[/TD]
[TD="width: 100%, align: left"]so if your wiring harnest gets shorted out wires then its frys the pcm basically everytime?[/TD]
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[TD="align: left"] 25/10/2012 6:19 PM[/TD]
[TD="align: left"]bbfanatic23[/TD]
[TD="width: 100%, align: left"]i gave him my vin[/TD]
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[TD="align: left"] 25/10/2012 6:19 PM[/TD]
[TD="align: left"]bbfanatic23[/TD]
[TD="width: 100%, align: left"]i didnt 'reset' it because i never got that far because it started up right away.[/TD]
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[TD="align: left"] 25/10/2012 6:18 PM[/TD]
[TD="align: left"]bayus2972[/TD]
[TD="width: 100%, align: left"]someone jump in here..aren't PCM's suppose to be VIN programmed?[/TD]
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[TD="align: left"] 25/10/2012 6:18 PM[/TD]
[TD="align: left"]bbfanatic23[/TD]
[TD="width: 100%, align: left"]but...[/TD]
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[TD="align: left"] 25/10/2012 6:18 PM[/TD]
[TD="align: left"]bbfanatic23[/TD]
[TD="width: 100%, align: left"]ya couse i got instructions on reseting it just like the dealer would[/TD]
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[TD="align: left"] 25/10/2012 6:17 PM[/TD]
[TD="align: left"]bayus2972[/TD]
[TD="width: 100%, align: left"]that's when I found the wire rub...took it back and they fixed the wires...but the PCM was fried[/TD]
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[TD="align: left"] 25/10/2012 6:17 PM[/TD]
[TD="align: left"]bbfanatic23[/TD]
[TD="width: 100%, align: left"]maybe i got a bad one...[/TD]
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[TD="align: left"] 25/10/2012 6:17 PM[/TD]
[TD="align: left"]bbfanatic23[/TD]
[TD="width: 100%, align: left"]thats crazy... its got to be my pcm[/TD]
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[TD="align: left"] 25/10/2012 6:17 PM[/TD]
[TD="align: left"]bbfanatic23[/TD]
[TD="width: 100%, align: left"]wow really?[/TD]
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[TD="align: left"] 25/10/2012 6:16 PM[/TD]
[TD="align: left"]bbfanatic23[/TD]
[TD="width: 100%, align: left"]but people are saying that thats a last resort and its prob not the pcm[/TD]
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[TD="align: left"] 25/10/2012 6:16 PM[/TD]
[TD="align: left"]bayus2972[/TD]
[TD="width: 100%, align: left"]yep..as soon as I picked up the car (from a previous shop job) I test drove it and it did the same thing you described....let off the gas, speedomoieter goes haywire and the car jumps down a gear..[/TD]
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[TD="align: left"] 25/10/2012 6:16 PM[/TD]
[TD="align: left"]bbfanatic23[/TD]
[TD="width: 100%, align: left"]Im just confueed because the issues that ive had/ having make it seem like its the pcm..[/TD]
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[TD="align: left"] 25/10/2012 6:15 PM[/TD]
[TD="align: left"]bbfanatic23[/TD]
[TD="width: 100%, align: left"]so what same problems as me were you having?[/TD]
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[TD="align: left"] 25/10/2012 6:14 PM[/TD]
[TD="align: left"]bbfanatic23[/TD]
[TD="width: 100%, align: left"]I see... but you said you were having the same probs as me?[/TD]
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Anybody know if the PCM has a built in power supply to where I would need to unplug it for a extended amount of time before I can try and reset it?
 
When mine did that it was my VSS, it was badly plugged in. But it caused the same thing, and the transmission did erratic shifting.

It's the VSS for sure, in my opinion / experience. Change the VSS or Check if it's plugged in good.
 
The VSS is on the passenger side of the car, jack the car, remove the wheel, you'll see the transmission differential (where your axle shaft goes in), it's the sensor above. Easy to find. It's held with a bolt or two, don't remember.
 


That was the problem. Bad connection before the sensor at two of the wire's couplers. Thanks for the help guys. I'm good with body work and paint. So let me know if y'all ever need any help in that area.
 
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