Chevy and GMC Duramax Diesel Forum banner
  • Hey Everyone! Enter your ride HERE to be a part of this months Ride of the Month Challenge!
21 - 34 of 34 Posts

·
jokerfabworx.com
Joined
·
3,662 Posts
ChevyTech77, 9.7 volts between the driver's side positive terminal and the block during cranking, when it is 21°C outside.
Like I said earlier, it behaves like a poor ground. That said, it is likely more than just a partially corroded ground terminal, since both Ken's LML and mine show the same voltage at the plug. There must be a hard fault somewhere.

My Tech II says that 3.4 volts are being sent to the GPs when commanded "on".

The dealership's diesel techs had the truck for ten days. They tested the GPCM and ECM, both of which were deemed "good". The techs installed all firmware patches on the GPCM and ECM, with no changes. They tried to blame the Diesel Rx glow plugs, so I let them install an ACDelco plug, and they got the same .95 volts at the plug.

All scan tools will determine that the GP system is functioning correctly, even though the proper voltage is either not being sent to, or not reaching the GPs. Probing a GP lead between the GPCM and a GP is the next step, to see if the GPCM is actually sending the voltage the Tech II is reporting.
 

·
Premium Member
Joined
·
3,450 Posts
9.7 volts? What is static battery voltage?
 

·
Premium Member
Joined
·
3,450 Posts
12.46 volts at rest.
So first off, 12.5 volts is 75% state of charge.

Secondly, if you’re dropping almost 3 volts, you’ve got issues. Keep doing the same test, section by section of all of the main grounds and power cables until you find the section causing the drop. Charge the batteries first.
 
  • Like
Reactions: nashguy207

·
jokerfabworx.com
Joined
·
3,662 Posts
So first off, 12.5 volts is 75% state of charge.

Secondly, if you’re dropping almost 3 volts, you’ve got issues. Keep doing the same test, section by section of all of the main grounds and power cables until you find the section causing the drop. Charge the batteries first.
How should we test the individual grounds and cables?
 

·
Premium Member
Joined
·
3,450 Posts
How should we test the individual grounds and cables?
DVOM connected to each end of each cable you want to check, set DVOM for dc voltage, crank engine for a load. Nothing over 300 mv. A DVOM with min/max/average record, like a Fluke 87 works best to catch this.
 

·
jokerfabworx.com
Joined
·
3,662 Posts
DVOM connected to each end of each cable you want to check, set DVOM for dc voltage, crank engine for a load. Nothing over 300 mv. A DVOM with min/max/average record, like a Fluke 87 works best to catch this.
Perfect. I can get to this when I get back from work. Do you know the locations of all the engine ground locations by chance?
 

·
Premium Member
Joined
·
3,450 Posts
Perfect. I can get to this when I get back from work. Do you know the locations of all the engine ground locations by chance?
Depends on model year (GMT900 vs K2). Easiest to just start with the cables at the battery and work your way in. IIRC, GMT900's grounded one batt to the block and one to the frame horn under the rad core support. All dmaxs usually have all of their important grounds (including the one for the gpcm) on both sides of the block, down near the upper oil pan rail.
 

·
Registered
Joined
·
2 Posts
Did anyone ever figure out there problem?? I have been dealing with this issue for a few years. No one seems to figure it out.... Plug the pickup in for a few hours and always starts..
 

·
Registered
Joined
·
4,185 Posts
Lots of information here if you want to use it and find out what your problem is. There were major changes in the glow plug system over the years and we don't even know what year truck you have, battery voltage at great, battery voltage while cranking. what you have done to find the problem etc. It's not a one solution fits all.
 

·
Registered
Joined
·
2 Posts
Lots of information here if you want to use it and find out what your problem is. There were major changes in the glow plug system over the years and we don't even know what year truck you have, battery voltage at great, battery voltage while cranking. what you have done to find the problem etc. It's not a one solution fits all.
I was hoping that one of the original guys found their problem. Deleted 2012 LML.. Bought the Pickup in the summer of 2017. Of course it was the summer and never knew the glow plugs didn't work... Come late fall/winter no start when it got below 25-30 degrees out. Cranked like a champ but no fire making. Assumed GPCM since no DTC's. New GPCM and programmed twice - still not starting... No one can figure it out. Its the exact same as the original posters'.. Just wondering if JOKER or Ken from MN figured their issue out...
 

·
Registered
Joined
·
13 Posts
Discussion Starter · #32 ·
I have not looked any further into mine since this post. Seems odd that we have 4 LML's here with the same issue. Frustrating for sure.
 

·
Registered
Joined
·
4,185 Posts
The first thing to do when you are having electrical problems such as these is to make sure both of your batteries are fully charged, 12.7 volts. Make sure BOTH ENDS of the battery cables are free from corrosion and shiny clean, tightly attached. Amazing how many people think their battery cables are tight when you can remove them without a wrench. Have the batteries load tested to make sure they are good. Test your mega fuses to make sure they are not blown. In really cold temps, any weakness shows up quickly. Get a wiring diagram for your truck here https://www.gmupfitter.com/body-builder-manuals/ and do your best to understand how the truck starts and how electricity is fed to the glow plugs.
 

·
Registered
Joined
·
13 Posts
Discussion Starter · #34 ·
Good advice Ron.
In my case I started with the basics (batteries, grounds, connections, fuses, etc), working my way to individually load testing each wire from PCM to GPCM and GPCM to each glow plug lead with my homemade load tester. I use a variety of resistors to equal, or slightly exceed the actual load the wire carries. Unless I missed something, my system up to spec which leads me to a programming issue.
Beginning with the LML's, each glow plug is controlled individually by the PCM & GPCM with varying voltage to each one based on multiple factors, including air temp, coolant temp, fuel temp, etc. Also, the glow plugs operate at a max of 4.8V and are not interchangeable with the true 12v units.
In my case, I have a heated shop to keep it in & if it doesn't sit out in the cold (30F or colder) more than a couple hours between starts, it's fine. My daily schedule works with that.
 
21 - 34 of 34 Posts
Top