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Fault code 29

DMT

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Hi. My 1994 Suburban with a 6.5 turbo diesel threw a 29 code which appears to be related to the glow plug system. My glow plugs are Bosch replaced about 8 years ago, with a new GP controller. The vehicle has 235,000 miles on it, has a relocated PMD, and a high capacity Powermaster alternator. Other than that pretty much a stock engine. It has been pretty reliable. I'm wondering if my GP cables might be the culprit and am looking to hear what other people have had to fix with a 29 code.
 

Will L.

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8 years is a long run on glowplugs.
A couple years after you got that set, the bosch went to crap and many have experienced failures from them. Myself included- ruining a piston. Sooo, the only one to get nowdays is the AC Delco 60G.
You could remove them and bench test if you’r rather not spend the cash before knowing.
There is ohms testing procedure, but I’ve seen good plugs test bad & bad plugs test good. So don’t waste the time.

You can check for voltage at each plug to ensure you are getting power to them all first.

If you are not getting power to all 8, remove the harness where the 8 leads attach and check there for correct operation.
It is possible the relay is bad- however the plugs can cause the fault and are far more likely.

if relay is good and plugs are good- that leaves the harnesses. Note the 8 leads to the plugs are not normal wire, They are made of fuseable link.
 

WarWagon

Well it hits on 7 of 8...
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Code 29 means the ECM isn't seeing +12 volts from the glow plug side of the relay(controller). Tapped, I believe, the same place as the glow plug lamp in the dash. It doesn't care if all 8 plugs are burned out. Do you get a glow plug lamp anymore?

Check the controller for loose nuts, melting, 12V on one big terminal, and the wiring in the area for a burned out fusible link. As always clean the grounds esp. the ones by the auto trans dipstick passenger rear of engine as the relay is grounded there. No ground - no worky. With a helper or volt meter on the windshield you can check for 12v on the other side of the terminal immediately after key on.

The ECM tells the controller to turn on/off. It's a dumb relay on 1994+

Glow plugs do not have a specific lifetime. Change with injectors is recommended.

And Photobucket sucks for images that disappear from old threads or have enough of their ad garbage graffitied all over your pics.
 

FellowTraveler

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Hi. My 1994 Suburban with a 6.5 turbo diesel threw a 29 code which appears to be related to the glow plug system. My glow plugs are Bosch replaced about 8 years ago, with a new GP controller. The vehicle has 235,000 miles on it, has a relocated PMD, and a high capacity Powermaster alternator. Other than that pretty much a stock engine. It has been pretty reliable. I'm wondering if my GP cables might be the culprit and am looking to hear what other people have had to fix with a 29 code.
WOW, It's been a long time, how are you and yours?
 

DMT

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Thanks, Fellow. Yeah, mechanically my Suburban has been running fairly well. There was that incident with the 16 year old. She was on her phone going through an intersection and failed to stop. She slammed into my driver's side, severely daming both doors rendering them unrepairable. I replaced them with two white doors from a 96 Suburban but they're not the same, the doors don't hang like they used to. So yeah, that was a bummer. But she is still a workhorse. Used her on Saturday to take about 8 trailer loads of tree branches from an acre we are clearing. A handful of hunting trips, including going over two Colorado mountain passes in a blizzard, probably the worst winter driving I've ever experienced. But she came through with flying colors. So yeah, thanks for the warm welcome back.
 

DMT

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The connectors to the GP controller may get dirty?
Be careful, the GP controller studs are very delicate, it cannot be tightened too tightly.
Yeah, I've checked, cleaned and tightened the GP relay connectors. That was my first hunch. The relay seems okay. My hunch is that it could be the cables, but I'm still looking into it. The glow plug dash light goes on still. The engine warning light goes on and off intermittently. I disconnected the battery cables, waited a minute and put them back on. The warning light went back on as soon as I started the engine.
 

DMT

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Code 29 means the ECM isn't seeing +12 volts from the glow plug side of the relay(controller). Tapped, I believe, the same place as the glow plug lamp in the dash. It doesn't care if all 8 plugs are burned out. Do you get a glow plug lamp anymore?

Check the controller for loose nuts, melting, 12V on one big terminal, and the wiring in the area for a burned out fusible link. As always clean the grounds esp. the ones by the auto trans dipstick passenger rear of engine as the relay is grounded there. No ground - no worky. With a helper or volt meter on the windshield you can check for 12v on the other side of the terminal immediately after key on.

The ECM tells the controller to turn on/off. It's a dumb relay on 1994+

Glow plugs do not have a specific lifetime. Change with injectors is recommended.

And Photobucket sucks for images that disappear from old threads or have enough of their ad garbage graffitied all over your pics.
WarWagon, thanks for the suggestions. Yes, I get a glow plug lamp still. My grounds seem good. I'll use a volt meter and go through more troubleshooting steps.
 

DMT

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8 years is a long run on glowplugs.
A couple years after you got that set, the bosch went to crap and many have experienced failures from them. Myself included- ruining a piston. Sooo, the only one to get nowdays is the AC Delco 60G.
You could remove them and bench test if you’r rather not spend the cash before knowing.
There is ohms testing procedure, but I’ve seen good plugs test bad & bad plugs test good. So don’t waste the time.

You can check for voltage at each plug to ensure you are getting power to them all first.

If you are not getting power to all 8, remove the harness where the 8 leads attach and check there for correct operation.
It is possible the relay is bad- however the plugs can cause the fault and are far more likely.

if relay is good and plugs are good- that leaves the harnesses. Note the 8 leads to the plugs are not normal wire, They are made of fuseable link.
Good to know about Bosch glowplugs. I'll go with AC Delco 60Gs next. I'll probably just go ahead and replace them. I'm wondering about the harness too. Thanks for your suggestions.
 

WarWagon

Well it hits on 7 of 8...
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You need to check on the wiring diagram and make sure the ECM is getting the glow plug signal. Wouldn't hurt to spot check a few glow plugs, one on each side as they are banked that way, that they also get 12v or have continuity in the harness back to the relay.
 

FellowTraveler

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Thanks, Fellow. Yeah, mechanically my Suburban has been running fairly well. There was that incident with the 16 year old. She was on her phone going through an intersection and failed to stop. She slammed into my driver's side, severely daming both doors rendering them unrepairable. I replaced them with two white doors from a 96 Suburban but they're not the same, the doors don't hang like they used to. So yeah, that was a bummer. But she is still a workhorse. Used her on Saturday to take about 8 trailer loads of tree branches from an acre we are clearing. A handful of hunting trips, including going over two Colorado mountain passes in a blizzard, probably the worst winter driving I've ever experienced. But she came through with flying colors. So yeah, thanks for the warm welcome back.
Shamefully, a great many of young ones have to learn from their mistakes...and others take the brunt of the damage done and hopefully you are okay.
It becomes problem trying to R&R when the hinges are welded.
 

DMT

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Yes, those welded hinges made things more difficult than they needed to be. Another thing I noticed, the cabling and attachment points of the door interiors are different, even though they are GMT400 doors, which meant for quite a bit of improvising to fit my old door handles, upholstery, switches, etc. I live a couple of hours away from what is claimed to be the largest bone yard in the Rockies. I thought this was a good thing. They had five white GMT400s to choose from, and even a couple pristine examples, but alas, no white 94s. So I had to make do with slightly different 96s doors. At least I didn't have to paint them. And yes, the poor teenage girl was quite traumatized by her first collision. She had gotten herself lost and was on the phone trying to get directions (all of this she admitted to a police officer who was on the scene immediately having witnessed her mistake). She had just turned 16 and had her license only 6 weeks. Her birthday present from her parents, a shiny and new crossover SUV, was a complete loss. It was all so unfortunate but avoidable. Talking on the phone while driving is never a good idea.
 
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