How to diagnose GP Controler on LBZ

[a5150nut]

Searched without any luck.
Replaced #4 GP twice and still get the SES light back after a few starts. GP checks good and voltage to plug checks.
So I am thinking its the controller but I cant find any test procedures and at $130 for the part I don't just want to try swapping without knowing its the problem.Hope its an easy test with volt meter I don't have a fancy scanner and the ones at the parts store just give fault codes and its #4 GP

[DmaxMaverick]

How to test glow plug controllers (not unlike testing 6.5 PMD's):

Remove the part from the vehicle. Relocate yourself, with the removed part, near a large body of water (lake, large pond, ocean, etc.). Lob said part in to large body of water. Height and distance is important. If it floats, it's good.

Seriously though, I'd suspect the harness or a connector before the controller if it's only one plug. No-load voltage won't tell you much, if anything. What's the DTC(s)?

It's also possible you're replacing that plug with a brand/lot that's inferior to the rest of them. Maybe switch it with one from another cylinder and see if the problem follows the plug or the cylinder.

Fortunately, a GP problem only annoys you with the SES lamp. It doesn't cause any issues, unless you have a smog test to pass.

[a5150nut]

Replaced with two new AC Delco glow plugs. Both worked for a couple starts then code back to GP #4 being bad. Guess Ill order a new controller and try that.

[More Power]

Both worked for a couple starts then code back to GP #4 being bad.

Do they ohm check and light up OK?

[a5150nut]

Yes, and what are the chances of two Delco GPs being bad out of the box. After changing they work for a few starts then engine light comes back on. Checked voltage at GP 11.3 IIRC. #2 & #6 both test same. Keeps coding #4 bad.

[rapidoxidationman]

Are they low or battery voltage glow plugs? IIRC the proper plugs are 5V...

[a5150nut]

Ok I finally got fed up with every cold start having to plug in my code reader to clear the damn SES light. Pulled wheel well liner out to get in to the glow plugs. Checked wire for resistance back to Controller end. 2, 4, and 6 all ohm out 1.3. so I tried checking voltage on each one. Key OFF I'm getting 0.93 volts. Key on 2.5/2.59 on all three. Too warm for a glow cycle until after I start it. Was hoping to find wire to #4 to be bad but its same as other two. Harness is main engine harness no glow plug harness $900 + so I'm not going to try that. Controller is new a year ago and glow plug was replaced before that but has not changed anything.
Guess I'll build a mount for my code scanner and just permanently mount it to the dash.

[DmaxMaverick]

The truck needs a mask. Maybe 2 or 3. It's the only effective method. Forget logic and science.