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Need some wiring help. Pulled a running 4.8 out of a 2004 Sierra.
Stock harness, fuse block, and PCM. Everything is wired except evap, maf and o2. I have no 12v at coils, and no spark at coils. I grounded a plug against the engine, and checked the 4pin connector on an individual coil for 12v. I do however have continuity to ground. The coil grounds are directly to battery.
I don't want to cut open the harness without starting it. I did some searching and found that most people gave direct power to the coils via the 7 pin connector, and had success.
Does anyone have other advice? This is measure twice, cut once situation. The two 40amp ignition fuses have continuity throughout, so are okay. Injectors are firing, I can smell fuel coming through the pipes during crank.
Thanks guys, anyhting helps.
To do:
Re check the crank and cam position sensors.
Somehow get 12v to the coils
Wire in obd?
Cam/Crank/OBD shouldn't be causing this, if that is what you were thinking. Did you check just one coil or all of them? If you just checked one, I'd just go ahead and check the pink wire on both 7 pin connectors for 12v.
If you don't have power on just one bank, I'd start looking for an open in the pink wire going to that bank.
If you don't have power on both banks, I'd look into verifying that the coil circuits are wired to a circuit in the car that actually is getting 12v in the start and run positions.
Hope this helps. Let me know if anything I said doesn't make sense. I seem to have a way of making things way more confusing than they need to be at times.
Hey, thanks a tonne. I'll do exactly this, and update tomorrow afternoon.
So far it's on the factory fuse block, and I just ran pin 'A9' to 12v. This should have gave it power. I'm going to check what you mentioned tomorrow, and get back.
Thanks again
Originally Posted by bigryry_s10
Cam/Crank/OBD shouldn't be causing this, if that is what you were thinking. Did you check just one coil or all of them? If you just checked one, I'd just go ahead and check the pink wire on both 7 pin connectors for 12v.
If you don't have power on just one bank, I'd start looking for an open in the pink wire going to that bank.
If you don't have power on both banks, I'd look into verifying that the coil circuits are wired to a circuit in the car that actually is getting 12v in the start and run positions.
Hope this helps. Let me know if anything I said doesn't make sense. I seem to have a way of making things way more confusing than they need to be at times.
I've got 12v at the coils, seems like I have no spark
So I just unplugged the fuse box, and plugged it all back up again. I have 12v at the coils. I don't have spark when I crank with the plug against the head. I have 12v at the 7 pin and the 4 pin connector. Ground on both as well.
So I just unplugged the fuse box, and plugged it all back up again. I have 12v at the coils. I don't have spark when I crank with the plug against the head. I have 12v at the 7 pin and the 4 pin connector. Ground on both as well.
I'm going to wire in Obd. Any suggestions?
So you have power and ground at all the coils? You're just not getting spark at any of the plugs?
Just no spark. I've got ground and 12v constant on the 7 pin and 4 pins. I can smell fuel coming through the pipes during cranking, but no hint of firing.
I connected/disconnected the maf. Connected/disconnected the pedal (drive by wire). I'm just wiring in the serial data now.
I checked the crank sensor wires, and have 12v and ground. I gave a direct 12v to ignition pink.
I don't seem to have a low reference. From what im reading, low reference should be a ground from pcm. I don't have that in the connectors. I removed pcm to check all the 12v and ground, all are perfect.
Does the low reference only happen when 12v is attached? Instead of low reference I'm getting something lower than 12v. Confused .
This is weird. I think the whole time, I only had low reference. Not ground. And where ground is supposed to be, I'm getting 10-11 volts....
On the 7 pin, the ground I thought I was getting was actually low reference. In the earlier pic I posted, those were on pink-11.x volts, and black, 10.x volts. That black wire should have ground.. the ground I thought I was getting was actually low reference. This is getting so confusing. Maybe I have a short somewhere. Completely stock harness.
The coil ground wire was bad, and didn't have continuity. Somehow rust seeped up. Jesus Christ.
Thanks a tonne for asking about the low reference. I just never even thought about it. I'm going to give battery (-) to this ground wire, put everything back together and crank. Fingers crossed this is it. Something tells me there are more gremlins though let's see.
04-15-2019, 09:25 AM
Mercedes Benz 300sel 4.8 swap - no 12v at coils 
let's see.