6.7 Stall, No Start, 5V Reference Low

JoeR · April 9, 2022

I've got an interesting one. 2012 F450, 6.7. This truck is deleted, I'm the 3rd guy looking at this, and the guy is $5k lighter with the same problem.

The truck has the following codes:

P06A6, 5V reference low

P0335, 5V Ref to crank sensor low.

P0193, Fuel Rail Pressure Sensor

P0532, AC Pressure Sensor 5V Ref low.

P2138, Throttle D/E correlation

I confirm that when those codes appear, the 5V reference on Pin 21 of the center PCM connector reads .5V. Also, APP 1 voltage reads 0V on the scanner. I didn't confirm it at the sensor.

When it does run, it runs poorly, then shuts off hard. When it's running poorly, the 5V reference at pin 21 looks like the first pic.

Previous repairs include a new PCM and an complete engine wiring harness. From the looks of it, they did a nice job.

Here's where it gets interesting. I wiggle the B+ battery cable where it heads down hill. This is the harness that runs under the engine. If I touch that harness I can kill the truck or bring it back to life. When it dies that one 5V reference drops to .5V and it's dead. Touch it again and the 5V comes back, and it runs. As far as I can see, that harness has NOTHING to do with me losing that 5V signal. That wire goes to the crank sensor, fuel rail pressure sensor and the crank sensor. All of that was replaced with the new harness, so there's just no way it can be grounding out. Unplugging all the sensors doesn't change anything. Only touching that harness does. I opened that harness up, like you do when you have alternator problems, and I see nothing wrong, however, now the truck won't die. I know I'm close to seeing the problem, but after what this guy has been through, I really want to find a problem. Any chance that high amp fuse setup near the right battery could cause trouble? the center fuse has corrosion on it, but it passes a wiggle test.

cbriggs · April 10, 2022

The AC pressure sensor is down in that area. I have seen more than one of them rub through and short to ground down there. I think the AC pressure sensor wiring is in the body harness so it likely wasn't replaced with the engine harness.

mchan68 · April 10, 2022

3 hours ago, cbriggs said:

The AC pressure sensor is down in that area. I have seen more than one of them rub through and short to ground down there. I think the AC pressure sensor wiring is in the body harness so it likely wasn't replaced with the engine harness.

NOPE. The A/C pressure switch is part of the main battery cable/harness assembly referenced by the original poster, as are the alternator connections. I have replaced quite a few of my share of these.

JoeR · April 11, 2022

Thanks for the replies. Here's a progress report. The circuit giving me my P06A6 was fine from end to end. So were the sensors it connected to. However, there was a different 5V Reference (PCM Pin 44 if memory serves) and THAT one does run under the engine. That circuit was shorting to ground, and it took the other 5V reference with it. Is that a common thing with Fords, to run multiple 5V references from the same voltage source? I don't believe I've ever seen it before. If I had to guess, and I do here, we would have had a bunch more codes for the other 5V circuits, but maybe with it being deleted those codes weren't reported any more. This is why deleted trucks suck to work on sometimes.

I'm still not done with it. I fixed one problem and created another. With a bunch of harnesses apart I accidentally touched B+ to the AC sensor ground. It popped the PCM hard. That ground circuit now has 5V on it and the 5V reference now has 12V on it. There's also an oil can displayed on the dash with no codes stored anywhere. I'm thinking it is for the oil temp.

Thanks for the help. I can't believe I killed a PCM.... I need a drink.

mbhan68: That harness under the engine looks like a real bear to replace. Better you than me.

Joe

Fordracer · May 9, 2022

I've had deleted truck mess up the vref because when they unhook the exhaust sensors the open connectors can corrode and short wires together.

JoeR · May 10, 2022

Fordracer, thanks for the tip. I bet that info will be helpful before too long.

To follow up on this, I'm going to call this one fixed. I ran an overlay harness for the 5V reference that runs under the engine and it is fixed. Interestingly, the wire to the oil pressure sensor was corroded right at the sensor. It fell off in my hand when I touched it. It is unnerving that this truck had a second, unrelated electrical problem in the middle of me working on it, but I'm glad it's gone. I'm also glad it broke before I returned it to the customer. They don't need any more reason to distrust the truck, or the guy repairing it.

Joe