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Replaced my PCU twice, car wont start. What am I doing wrong?


HotRod53
04-21-2010, 04:25 PM
I replaced my PCU with a used unit twice, both times with the same results, 1 fire and it quits. I put the old PCU back in and it starts right up. Each of teh three units are the same service number and I have no codes showing.

I am having incorrect oil pressure and volt readings that both trace back to the PCU. Am I doing something wrong?

maxwedge
04-21-2010, 06:28 PM
Post the year and engine for help.

Jrs3800
04-21-2010, 07:24 PM
I believe from what I read on another Forum that this car is a 98 Bonneville..

I have no idea what the issue is so I did not answer...

But I have to ask this question, did you have the PCM flashed ? Some of these PCM's can be very picky about the Theft system..

And iirc oil pressure and volt readings have nothing to do with the PCM in the first place... The PCM does control the fuel gage tho on a 98... But not the Volt, oil pressure or Temp...

HotRod53
04-22-2010, 07:19 AM
it is a '98 3800 and I did nothing with having it flashed. I assumed that if it was the same service number it was plug and play.... I guess not.

I followed the wiring diagram and I seem to recall that the PCU outputs to the oil pressure gauge, I'll have to find that again. Can it be flashed outside of the vehicle? is there a chip that I can swap from the original? I have no SECURITY light thru all of this.

Here is some background... my check gauges light and check washer fluid come on kind of ghosty as RPM increases. Sometimes the check gauges comes on solid for no reason. My oil pressure drops to 0-5 psi but I have already checked it with a manual gauge and replaced the dash and the sender. When the pressure drops the dash dings continuously and drives you nuts. I checked both ground buss bars, I redid all fender grounds, I cleaned all battery terminals, I checked the block ground, and I replaced my alternator.

Jrs3800
04-22-2010, 09:20 AM
What you have is the Brief start from the ICM, the ICM actually starts the car, so what happens is you get a brief shot of fuel so the engine revs up a bit and then dies... This means the PCM is either not taking over or is having security issues... Funny it will start on the other one and not the replacement with the proper service number..

Did the security light even flash when you turned on the ignition?

There is no chip in the 98 PCM that can be removed... I am not 100% certain about the security being a problem, but its a good place to look... When I have too many issues with the security( called VTD or Vehicle Theft Deterrent in the PCM ) I simply go in and turn it off...

maxwedge
04-22-2010, 03:27 PM
How do you turn off the security system on a 98 GM car?

Jrs3800
04-22-2010, 07:13 PM
I use a DHP Tuner for the minor tuning I do on my cars... You can go in and disable the VTD with the tuner...

He most likely has a couple of issues with the PCM... Program and Security...

But we don't think his PCM is the problem with the issues he's been having..

maxwedge
04-23-2010, 08:26 AM
Ok, I knew there was no way to do this without a special scanner/tuner set up.

clutch492
05-19-2010, 05:45 PM
Did you change out the PROM in the PCU before you installed the new one? There is a lot of mistakes made when changing out the PCU. You need to pull the Programmable Read Only Memory "PROM" out and swap it into the new PCU. Then, install it in the car. I hope this helps.

Jrs3800
05-19-2010, 06:38 PM
There is no removable PROM in the 1996 GM PCM, this PCM is flashable... The knock sensor module can be swapped between 96 PCM's but this will not affect starting the the car...

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