Possible Bad PCM?

[max13077]

I was sitting in the parking lot at the grocery Wednesday afternoon after getting some BBQ fixins', scratching off a lottery ticket(first mistake). My 04 envoy with her 4.2 was running with the ac blasting cause it was super hot. All the sudden she just stalled and "service brake system" comes up on the display. Hmmm, not good. I tried starting it again. Turned over like a champ, but no start. Gas gauge has never worked properly, so first thing I went and got some gas, poured it in, same. Turn over, but no start.

Swapped out relays for #41 fuel pump. Nothing. Opened the gas cap and had somebody turn the key...silencio. Hmmm, thought maybe it was the fuel pump. Had it towed home, thank god for that $1.35 a month roadside charge on my cell. I've been farting around with it playing detective. Came across a Fuel Pump Test video on youtube(thank you MAY03LT). Followed the test for battery voltage and ground which both passed. Then tested for PCM control... I got the reading in the picture...

What does that mean? Bad PCM?

 

[Mooseman]

So basically a crank but no start condition.

https://www.google.ca/search?newwin...2i30k1j33i22i29i30k1j33i160k1.102._aDpP4EvelQ

This one seems to be the most relevant:
https://gmtnation.com/forums/threads/crank-but-no-start-service-brake-system.9602/

 

[max13077]

Thank you Mooseman I will have a look.

Another thing I just tried is bypassing the PCM I think and sending power directly back to the pump by putting a wire in between the bottom right hole in the relay and the top left. And it sounded to me like the fuel pump turned on... IDK, it defiantly made a sound in the engine bay, but I'm by myself presently and can't have somebody listen by the fuel cap to see if it's coming on.

Thanks again for the reply, I'll look that stuff over!

 

[limequat]

As a rule of thumb, PCMs very rarely go bad.
The first step is checking the DTCs.

 

[max13077]

I checked for codes, but unfortunately it says there are none. The only weird thing is it says service brake system in the display. Started that just when it stalled in the parking lot.

 

[meeceman]

had a code p0014. changed camshaft solenoid. ran for about 3 minutes and died. changed camshaft sensor and it still wouldn't start. getting code P0230. checked the relay slots and i have about the same voltage from PCM (.43v). good ground and good hot. 55 psi on fuel gauge when jumped, otherwise the fuel pump doesn't hum. bought a new crankshaft position sensor, took out the old one and the two sensors ohmed the same .07. put the old one back in and it started and ran for a few seconds. will crank but not start any more

i have exchanged the relay with the windshield wiper relay and the relays are good. something isnt right coming out of the PCM, i am getting low voltage to give power to the fuel pump. i dont know what will tell the PCM not to give the pump power.

For you both... pull all three PCM Connectors and examine the interior Male Pins areas for moisture and eyeball the Hell out of the harnesses for any cuts or pulled wires. If you have electrical contact cleaner like the CRC flavor from Autozone... spray out the inside of the PCM Pins and also the female side of the three connectors and allow everything to air dry before re installing them.

Don’t use any Pentrating Solvent Oil like WD 40 in order to loosen the three 10.9 small Fasteners in the connectors as the stuff will go deep inside and you’ve never get it out of the harnesses and it will interfere with the male to female contacts.[/QUOT does the battery have to be disconnected first?

You haven't done a scan for fault codes. That would be the first thing to do before replacing a bunch of parts. If you don't have a scantool then autozone will do it for you.

No I did not do that. I had my scanner but somehow I can not find it. So I'm going to buy another tomorrow. Thank you.

Codes tell all!

Got it. I'm getting another today.

 

[mrrsm]

Not certain if you are near Souix Falls... but with the annual flooding happening in that area... has your vehicle been sitting in standing water at all for any length of time? There may be something shorting intermittently in the circuit... and bad grounds act weird anyway ...most of the time. It also would not hurt to work the Ignition Key differently ...jiggling the insert point to see if perhaps the Ignition Switch is involved as well, since nothing says that there cannot be two electrical problems becoming coincidental... ( ..and as the OP mentioned...especially when scratching off Lottery Tickets and tempting the fates). Your OEM Fuel Pump Relay could be eliminated if sketchy with a brand new one too if that seems an inexpensive replacement.

 

[max13077]

I hadn't but begun scratch, but as soon as it quit, the following thought ran through my mind: "OMG, would this not be the most poetic time ever to win a crapload of money on a lottery ticket" HAHA!!

 

[max13077]

Something interesting...

I was backtracking to see if I'd missed anything. Looking at fuses etc. My neighbor came home and listened through the gas cap while I bypassed the fuel pump relay, and sure enough, the fuel pump primed!!!

Ok, now what. I was looking at fuses and came across #28 Powertrain Control Module 1, a 15amp fuse. It was blown... I pulled another 15 amp out and replaced it. Turned the key, the fuel pump primed!!! I went to start it, it fired!!! But for a second and then stalled. I again pulled the #28 fuse...blown! So does that mean the PCM has a short in it or something?

Incidentally, there is another PCM fuse, PCM B 20 amp. That one is not blown.

 

[mrrsm]

For you both... pull all three PCM Connectors and examine the interior Male Pins areas for moisture and eyeball the Hell out of the harnesses for any cuts or pulled wires. If you have electrical contact cleaner like the CRC flavor from Autozone... spray out the inside of the PCM Pins and also the female side of the three connectors and allow everything to air dry before re installing them.

Don’t use any Penetrating Solvent Oil like WD 40 in order to loosen the three 10.9 small Fasteners in the connectors as the stuff will go deep inside and you will never get it out of the harnesses and it will interfere with the male to female contacts.

Review the links posted by @Mooseman in Post #2 ...as the secrets are all in the diagrams-videos covered by @MAY03LT in running down what could be Dead Shorting enough to draw so much Amperage and kill the Fuses.

 

[max13077]

" I just got through repairing a customer's 04 Trailblazer I-6 that was towed to the shop. It died while driving. It would crank but not run, fuel pump would not run, and fuel pump relay would not energize. Investigated the fuel pump relay circuit to discover the PCM fuse #28 in the underhood fuse box was blown. Replaced fuse, started truck for 5 seconds, and it blew again.

The PCM fuse circuit feeds many components under the hood, including the injectors, and the ignition coils. Starting with the most likely, the ignition coils were tested first. The number 1 cylinder coil was aftermarket, so I tested it first. Measuring pin 1 and 3 on the coil (12V and Ground) or (black and pink) I read 6-7 ohms. BINGO! These coils should show NO resistance across these pins.

Replaced faulty ignition coil, pcm fuse, and started it up.

Found the above on trailvoy. Aside from the brake messages occurring that the same time it sounded too similar. I pulled the coils and tested them.... Sure enough, #4 shows resistance. Waiting for a ride to go get one. Hoping it's that.

 

[meeceman]

I pulled all three PCM Connectors and the middle one has 3 to 5 frayed wires. Is there a way to get that wire bundle out to have more room to repair the wires? thanks for all the ideas.

 

[Mooseman]

@limequat has lots of experience with these connectors. Maybe he'll chime in.

 

[meeceman]

Any suggestions for the best way to tackle this? Thanks.

 

[limequat]

Well, usually when I'm working on a harness, its out of the vehicle and on the bench...plenty of room to work. I will say, that just about every harness I've had come in has some sort of damage: rotted wires, aftermarket splices, worn wires, etc.
I don't see any bare wires in the above pics. So while not pretty, it could be a red herring. I suggest to follow a similar procedure as copied from the other thread. Start measuring resistance from any of the Fuse 28 wires to ground. IIRC, the PCM, all coils, and all injectors are on this fuse.
The good news is each device has a separate wire coming from the fuse box, except for the injectors which are spliced together in the injector harness. So you could pull the white connector out of the electrical center and then check resistance between ground and the 8 pink wires on Fuse 28 (one PCM, 6 coils, 1 injectors).

 

[max13077]

Solved!!

I tested the #4 coil and got like 7.8 ohms of resistance. Replace that and she fired up!! Ran real rough though. Took it for a short test drive. Immediatly popped a po301. Came home, pulled #1 coil, it was cold. Another trip to town and the parts store for a second coil. Just got home and put it in. Runs like a champ!! Well till the next time I scratch off the lotto ticket and tempt fate. lol

Thanks so much for everybody's help and guidance. Truly appreciate it.

 

[Mooseman]

@meeceman , clean off all the wires and check actual condition of those wires. Resistance check as @limequat explained.

It would have been preferable if you would started your own thread. Although this one was similar to your issue, it's actually a different. We can stay in this thread for now.

 

[meeceman]

i wasn't sure where the wires were to chase, so i took my TB to a shop and they changed the ignition and it is starting fine now.