Wiring issue - Crank, No Start

BoostedC230 said:

Hello everyone, I'm new here. I've got an interesting issue here. I got ahold of an 04 gmc yukon that had been sitting for four years with no engine in it. I got an engine went through it and got it ready to go. I threw it in but all it does is crank no start. Fuel pump primes, I have rail pressure. I also have spark. I have a relay marked as " IGN 1 " that clicks a lot when turned to position 2. To see if the relay was bad I used two new relays that I had in its place. They both clicked like crazy. I have checked both grounds behind the intake. Those grounds are good, I've also checked the two block grounds and they're also good. I've unclipped the injectors and checked the power wire and my test light flickers the entire time. ( Not the grounding wire to the ecu ) the red power wires. In the fuse block there is an ecu and tcm fuse which never have power. I'm not great at electrical diagnosis. I've been trying to improve with this project. I'd like to see if someone could assist me as I feel like an outside person looking in would be very helpful. I appreciate you taking the time to read my write up.

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dadwagon95 said:

View attachment 439869

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rockola1971 said:

You're in luck....former aviation electrician USN and been an electronic tech and electrician for decades now. Need to know what trim package (SLE, SLT) and 2wd/4wd/AWD and what are the 2 fuses exact name in the fuse box cover table? (ECU1? or ?) Yours has a ECU and TCM in 2004 instead of just a PCM (which has both the ECU and TCM internally)?
Once I get trim and chassis along with fuse names I can pull up the correct electrical diagrams and I work with you from there on checks to see where your voltage stop is.

My preliminary guess is that you have corrosion underneath your fuse box. The green crusties so it wouldnt hurt to pull that up and have a look. Lots of youtube vids on how to do

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rockola1971 said:

You're in luck....former aviation electrician USN and been an electronic tech and electrician for decades now. Need to know what trim package (SLE, SLT) and 2wd/4wd/AWD and what are the 2 fuses exact name in the fuse box cover table? (ECU1? or ?) Yours has a ECU and TCM in 2004 instead of just a PCM (which has both the ECU and TCM internally)?
Once I get trim and chassis along with fuse names I can pull up the correct electrical diagrams and I work with you from there on checks to see where your voltage stop is.

My preliminary guess is that you have corrosion underneath your fuse box. The green crusties so it wouldnt hurt to pull that up and have a look. Lots of youtube vids on how to do.

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Fless said:

I don't know about others, but I can't read any of this even when it's blown up. Too fuzzy.

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BoostedC230 said:

This is a breath of fresh air to read. I was worried I wasn't going to hear from anyone about this matter. I've got an slt 4wd, fuses are pcm 1 & etc/ecm. Both 15a fuses under the " PARK LP " relay

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rockola1971 said:

OK now things are starting to make some sense here. Your PCM 1 fuse ONLY gets 12v when you are in Run and STart position with your key via the ignition switch. The actual electrical ignition switch which is down low and right side of your steering column shroud provides that 12v to your PCM 1 fuse. Without 12v to that fuse which ultimately ends up at your PCM as what appears to be a input signal (Not actually powering the PCM itself). You can see this on the attached schematic for your 04 Yukon. You are going to have to put a voltmeter on the PCM 1 fuse while someone is cranking the engine and see if 12v is there. Check your PCM B 20a fuse which should be hot all the time.

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rockola1971 said:

1.1v at PCM1 aint gonna cut it. Thats a problem. So there should be battery voltage there when cranking and its source is IGN B 40a fuse (Hot all the time) then it goes to your ignition switch and out terminal C (Pink wire) when in start or run position then to your IGN 1 relay (THE COIL of it), NOT the contacts. Now PCM 1 fuse gets its power from the contact closure of IGN 1 relay. The question here is do you have a control problem (Coil relay) or an actual power problem (Coil contacts) and since you are hearing what sounds to be IGN 1 relay clicking but then reading 1.1v at the PCM1 fuse which got through the contacts of IGN 1 relay, I would check for good solid battery voltage at the relay coil while ignition is in the start or run position first. You will have to pull relay and measure at the socket. It will be the 2 smaller terminals of the relay. One will be ground and the other 12v+ when ignition is in start or run position.

Since you already replaced the ignition relay as mentioned in the first post I suspect you have burnt contacts in your ignition switch OR a bad associated ground for your coil in the ignition relay or the green crusties underneath the fuse box and its corroding something associated but its entirely possible that you have 2 problems and one is the control side and the other is in the power side.

You could just jumper the 2 bigger terminals of the ignition relay socket while someone is cranking engine and that could help rule out the power side of the ignition relay.

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