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post #1 of 22 (permalink) Old 05-02-16, 05:25 AM Thread Starter
hbugsyc
 
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Question HOW TO TEST PCM

How can I test if PCM is good??These are the symptoms...crank,no start..5V reference voltages at TPS and mass air sensor...Speedo jumps to 80 when turning key to on.slight hesitation on jump...While cranking needle goes down and jumps from 0-30..
Turn key to on and dash lights go on,then turn off except CEL..
Is my PCM good???Any other tests I should perform before declaring PCM bad???Almost forgot,fuel pump priming but,after 5-10 minutes,I hear an intermittent banging from fuel tank(fuel pump?)All suggestions welcome....Thanks..

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post #2 of 22 (permalink) Old 05-05-16, 11:50 AM
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Re: HOW TO TEST PCM

Have you tried disconnecting the battery for a few seconds?

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post #3 of 22 (permalink) Old 05-05-16, 12:32 PM Thread Starter
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Re: HOW TO TEST PCM

Yes,I have...Numerous times....I'm trying to find out how to test if the PCM is good or not...No communication through OBD port..
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post #4 of 22 (permalink) Old 05-05-16, 06:05 PM
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Re: HOW TO TEST PCM

If you're hearing banging from the fuel tank, I'd be looking for fuel rail pressure. What's the year?

Ever notice that there's so many high performance parts available for GM products?

Did you ever think that maybe they NEED them?
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post #5 of 22 (permalink) Old 05-05-16, 06:17 PM
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Re: HOW TO TEST PCM

The gauge sweep says possible power interruption and could be as simple as bad grounds or power terminals or low battery.

You are way too quick to suspect the PCM unless you think you have done something to it. Can't really test anyway, the best way is swapping out a known good PCM as there are literally 500 things to be tested there.

You will get zero through the comm port with just a single bent pin to not make connection. Just went through that at inspection, the loser running the place told me my PCM was 'bad', LOL. Nothing wrong except for the 3 rolls of electrical tape he had wrapped around the OBD connector of his cord, that tells you everything.

Noise from fuel tank may be telling you the pump is bad, they can be heard to spin up and still be bad, the difference is HOW they spin up, you listen for the subtle little difference that says it's not working like usual. Could be partial power from your gauge sweep issue too, pumps get wonky at low volts. You may not get PCM communication if the voltage to it is off as well.

Need to be going after battery WAY before PCM, it being low can blow your mind with various oddball errors. You need to verify both battery and alt charging, they could be fooling you................need at least 12.3 volt or more at battery.
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post #6 of 22 (permalink) Old 05-06-16, 07:00 AM Thread Starter
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Re: HOW TO TEST PCM

The battery and alternator are both good...Battery is at 12.5V 14.3 when charging...The car died while running....I tested every fuse in the car,all good..I first suspected a short somewhere..If there is one,I can't find it....The thing that makes me wonder is that there is a minute delay with the needle sweep..I'm talking like 1/10 of a second..I was thinking maybe the time it takes for a relay to open or close...She cranks strong..Getting fuel..I'm checking for spark today..If all 3 are present.air,fuel,spark..I'm thinking PCM...
I've spoken to PCM/ECU techs and they say that bad or burnt coils are the most common causes of the PCM frying...
I really appreciate you trying to help....I encourage anyone with constructive comments...
As I said,a short was my first thought...I checked under the BJB,the main harness from the the BJB (under hood fuse box)all the way to the pcm and cannot find a bad wire.Ground or voltage..
What's left to check..???Your comment that the only way to check if it's the PCM is by trying a new one or taking it out to be bench tested was what I needed to hear...No real way to test it on the car..

Last edited by hbugsyc; 05-06-16 at 07:02 AM. Reason: additional info.
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post #7 of 22 (permalink) Old 05-06-16, 09:23 AM Thread Starter
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Re: HOW TO TEST PCM

OK,so I just tested and I am getting spark...The only thing that I noticed was that there is 11.8V at the coil connector while there is 12.3V at the battery....Is this a clue????
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post #8 of 22 (permalink) Old 05-06-16, 10:04 AM
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Re: HOW TO TEST PCM

Probably not. Albeit a half a volt drop is greater than I'd expect, you should still be able to fire. You had mentioned an intermittent banging from the gas tank, ever figure out what that was, and if that might be taking a pull on the system? You also stated that she's getting fuel, what's your fuel rail pressure?

Ever notice that there's so many high performance parts available for GM products?

Did you ever think that maybe they NEED them?
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post #9 of 22 (permalink) Old 05-07-16, 10:17 AM Thread Starter
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Re: HOW TO TEST PCM

I don't have a fitting that will work with either my vacuum tester or my compression tester..I just know that there is pressure..Pressed in Schrader valve and fuel shot out.I'm not sure that the actual PSI is important at this point...Only that there is fuel pressure....I can hear the fuel pump priming when I turn the key..The banging has stopped...for now...I'm thinking a shorted wire in the starter motor area????Anyway...I have the big 3...Fuel,Air,Spark+fully charged battery..Should start..I'm stuck...
As far as testing PCM...Sort of a dead issue...From everything I've read,they can ONLY be bench tested by a qualified elec. tech or switched out...The fact that the fuel pump is working and it's getting spark,would indicate that the PCM is NOT the problem....
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post #10 of 22 (permalink) Old 05-07-16, 09:47 PM
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Re: HOW TO TEST PCM

'I'm not sure that the actual PSI is important at this point...'

I do not agree, you can spray fuel everywhere with only half pressure and for sure MPFI likes having all it is supposed to have, in fact pretty bitchy about it. Really need to measure it. Often you can hear the pump priming but if you haven't trained yourself to what a normal pump spinup sounds like you won't have a clue as to what an abnormal one sounds like. Often that's a clear clue. It drags, or spins slower than normal, or hesitates even though you hear it work. All are bad signs.
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