Howdy. I have a 1999 F250 Super Duty 7.3L Powerstroke, manual transmission. It was manufacturing March 1998. I am the second owner; first owner was a family member, so I know the history of this truck since it left the lot.

It is throwing a P1391 Glow Plug Circuit Low Input Bank No. 1 (right). No other codes.

1. Glow plug relay = brand new
2. Glow plug shunt = brand new
3. Glow plugs = brand new
4. Wiring Harness = brand new
5. Pigtail from wiring harness to 42-pin connector = brand new and all testing with no resistance (just 0.2 ohms for length of wire). I repinned many of the pins that had corrosion or high resistance across the connector. No resistance across the 42-pin connector.

Using AutoEnginuity Scan Tool, I'm reading:

1. Normal voltage from battery
2. Engine oil temperature ~127°F

On GPR, I have source voltage between ground and black/orange wire. Between terminals A and B on GPR I have less than 1 ohm. Nothing wrong there.

I have checked for short to power on PCM pin 101 and ground. No voltage present. I have also checked resistance between purple/orange wire at terminal A of PCM and pin 101 and have <2 ohms.

I have checked for short to ground on purple/orange wire (terminal A). No problems.

104 pin connector is just fine. No damaged pins, no corrosion, no loose wires, no resistance across the connector at any of the pins (went ahead and checked the whole danged thing while I was there).

Literally, everything in the system that touches the P1391 code is either new or it has been tested upside down and sideways. Everything is bright and tight. There are no high resistance faults anywhere along the way. There are no shorts to ground.

THE TRUCK RUNS PERFECT! No roughness. No pulsing. No hard starts. No badness of anything. The truck is a gem.

Is the PCM the only thing left that could be wrong? Am I seriously looking at a new PCM for a truck that runs just fine so I can clear this damned code?

California smog is due in 7 weeks.... I need this code cleared (permanently). I'm out of ideas. Help a sister out.

For the record: putting all the wire back into new conduit IS. NOT. FUN. Especially at the 42 pin connector where so many things intersect. That sucked. My electrical taping skills leave much to be desired.

Thanks in advance... I'm getting twitchy about this.

 

I hope you used OEM glow plugs, others have a history of swelling in the tips making them impossible to remove without removing the head(s). SkySkiJason also has a recent thread here regarding counterfeit glow plugs from sellers on fleabay, if you got bargain glow plugs there, check his thread for info on that. Have you ohmed your glow plugs through the UVCH? The two outer pins on each end of the plug are glow plugs. While it's apart, look for burnt pins in the connector.

 

I did use OEM glow plugs. Most of what I've done in this project has been motorcraft or recommended replacements. I didn't take shortcuts or go cheap.

The connector is brand new (on both the harness side and pigtail side) and none of the pins are burnt or damaged.

Thank you!

 

Have the UVCH's been replaced? I can't quite tell from the terms used above.

Can you clear P1391 and see if it comes back? Maybe just an old code? I'm not sure if these codes clear themselves when the issue is fixed, or if they have to be cleared.

What if you temporarily put both sense wires on the good side of the shunt. You are not getting P1393, so put both sense wires on the left bank (driver's) side of the shunt. If P1391 does not come back, that could mean the PCM is fine, and the monitoring circuit is fine. If P1391 comes back, maybe that monitoring circuit has an intermittent open, or the PCM (least likely) has an issue with that pin.

 

The UVCH is brand new, so yes....

I like your idea of checking with the shunt. I will try this and post back. You give me hope.....

Also, I have cleared the code and it does come back every single time.

 

Woodnthings has a nice writeup on how this system works:
https://sites.google.com/site/woodnt...ow-plug-system

Are the sense wires going to the right spots on the shunt? From this pic - it's not clear to me if the wires are color coded - they all look blue, but appear to have a color code after the fusible link for each.

 

Hmmm. This is my top-down BEFORE I redid the pigtails from the valve cover harnesses. My left bank glow plug (yellow) is mounted on Center.

 

That's likely the issue. The PCM can't see the voltage drop it expects through the shunt for the left bank, because the left bank GP wire is in the wrong spot.

 

Darn, looks like all of the wires are in the wrong spots on the shunt. I just uploaded a better schematic in post #4. Check all three locations.

 

Well, I owe you a beer! That fixed it right up! Hot dang. WOW. Thank you so much for chiming in on this post. You saved me a lot of heartache and $$$$!!!!

Amazing. THANK YOU!

 

You are very welcome, sir! Glad it's working. You have a rare rig there with a GPR/shunt instead of a GPCM on that Cali truck. I enjoyed learning about it.

 

It's ma'am. :-) Yup. This girl has a big old truck (and does her own work on it).

This truck has been a beast to get exact info on being an early '99. Quite a few oddball things that don't quite match service manuals and wiring diagrams.....

Seriously, you saved the day.

ROCK ON!

 

It is good to hear a success story, congrats on getting it conquered. I have an E99 as well and there's not that many differences. But it is good to know what they are when time comes to work on it. The folks around here offer a wealth of knowledge and great help.

 

Sorry about that! Thank you for describing the issue and steps you've taken in detail - it got us to the solution quickly.

 

glad you got that sorted these guys know how to help everyone

Always love it it when a gal don’t mind getting her hands dirty! Way to go!