# '07 2500HD Check engine light/trans failure



## 9400 ft. (Sep 30, 2009)

This is about the craziest thing I've ever come across, and I'm wondering if anyone has any ideas about what is going on. Here is the background:

I have a 2007 2500HD that was purchased new in October '08 from a dealer in Texas and brought to Colorado to be used as a plow truck for a condominium project. When it arrived here it had about 850 miles on it, and today it has just over 3000 miles on it. During this time it's been in the local dealer's shop no less than nine times, and they were unable to pinpoint the problem.

The truck is a 3/4 ton with a 6 liter engine, a 6L90 transmission, a trailer package and a 7 1/2 foot Western Pro Plus plow. It's a great setup, except for one thing.

The problem is this: low speed plowing causes the check engine light to come on while simultaneously defaulting the transmission to 4th gear. With continued use, or after being parked with the check engine light on, reverse is lost.

Through trial & error I found that removing & reinstalling the TCM fuse temporarily fixes the problem, but it doesn't last very long. The dealer, under direction of GM engineers-including the guy who designed the transmission- replaced the TCM, some wiring and the manual shift shaft position switch. Still no luck.

Last winter they installed a fancy data recorder, and were still unable to figure out what was causing the failure. In May, engineers threw up their hands and ordered a new transmission be installed hoping that would solve the problem. 

I haven't done any plowing since then, and the truck has been working fine. Then, on Saturday night I put the plow on in anticipation of a storm that never materialized. The truck sat until this morning (Tuesday), when after warming it up for a couple minutes I drove less than 100' and removed the plow. Then I drove it about 1/4 mile at low speed and parked it. After about an hour I started it, backed up, switched gears and drove forward about 75', put the truck back into reverse, and that is when the check engine light came on and the trans defaulted to 4th gear again.

Last winter the mechanics told me they looked over the plow, that it was installed correctly and the truck should carry it with no problem. They also said there are no sensors or anything that could cause an error code due to the weight of the plow. I have to wonder though, as it ran great all summer until I mounted the plow again.

After today, it now seems to me that the weight of the plow is a factor. And with snow season just about here, something has to be done soon.

Until then, I'll be plowing with my 1978 Chevy half ton with the ancient Western plow.

Sorry for the long post, but I wanted to provide as much background as possible. And thanks in advance for any advice you might have.


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## B&B (Nov 4, 2006)

Certainly isn't due to the plows weight. It's an electrical issue for sure. Now as to what specifically; that could be a little more tough to determine. But for starters I'd be checking the red power wire from the plows control harness to see where the plow installer tied it into a 12V ignition source from the truck. And second would be a low voltage condition caused by the plow, possibly form a loose ground or power lead on the truck itself and the plow use is amplifying the issue.

I'd be shocked if the dealer tech's overlooked these before condemning it to the point that they ok'd the replacement of the entire transmission but stranger things have been known to happen there.


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## ondagawood (Sep 19, 2007)

Yeah I would agree with B&B about electrical issues. Especially a ground and/or the power source. Good luck !


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## 9400 ft. (Sep 30, 2009)

Thanks for the tip about the electrical being suspect. I'll look over the plow installation today and see if I can find anything unusual. It would make sense, seeing how it seems to be a software issue more than a mechanical one. The GM mechanics already looked over the plow installation and said it was fine, but maybe I'll find something they overlooked.


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## 9400 ft. (Sep 30, 2009)

*update...maybe found the problem?*

after reading the two posts pointing towards an electrical problem, i went out this morning to investigate. i started started the truck and it had no reverse. turned off the engine and pulled the tcm fuse, resetting the check engine light and fixing the no reverse problem. checked underhood wiring for obvious problems and found none. checked in cab wiring and found harness routed through metal dash support just before the connector for the controller. removed harness from behind the metal dash support and found that the protective harness ended short of the plug, allowing wiring to rest directly on dash support. found where insulation was rubbing on dash support. found one wire with insulation damaged to the point where i could just barely see the copper wire inside. for the heck of it, traced wire back to terminal under the hood to confirm it is hot. this would explain why the problem was intermittent, as it would be caused by use of the hand held controller and not the weight of the plow etc. it might also explain why GM couldn't put a finger on it with their codes.

i'm not ready to call you guys wizards until i can test it out, but i don't see how this couldn't be the problem.

6" expected by tomorrow night, so i'll be back to let you know how it worked out.

thanks for pointing me in the right direction!


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## ondagawood (Sep 19, 2007)

Glad to hear you probably found the culprit. Any small amount of wire insulation and metal exposed, especially a "hot" one would most likely cause your headaches.:realmad:

Good luck tomorrow night push'in the white stuff ! xysport

Scott


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## 9400 ft. (Sep 30, 2009)

my two other plow trucks are a '78 and an '86, so computers on trucks are new to me. i had no idea that a short would freak out the computer before blowing a fuse, but thanks to you guys, now i know.

at least i have an excuse, unlike all the gm mechanics and engineers that looked at it. nine warranty invoices and a new transmission later... i found it pretty quick once i knew what to look for. it's almost like none of them even bothered to look under the dash.


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## ondagawood (Sep 19, 2007)

Yeah ya know for what they get paid an hour, it should be by process of elimination, etc...

Are you going to call and tell them what is was ?  

Have fun push'in tomorrow. payup

I was in Aspen once about 24 years ago for an anniversary trip, where are you ?

Scott


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## millsaps118 (Aug 13, 2006)

9400 ft.;815607 said:


> after reading the two posts pointing towards an electrical problem, i went out this morning to investigate. i started started the truck and it had no reverse. turned off the engine and pulled the tcm fuse, resetting the check engine light and fixing the no reverse problem. checked underhood wiring for obvious problems and found none. checked in cab wiring and found harness routed through metal dash support just before the connector for the controller. removed harness from behind the metal dash support and found that the protective harness ended short of the plug, allowing wiring to rest directly on dash support. found where insulation was rubbing on dash support. found one wire with insulation damaged to the point where i could just barely see the copper wire inside. for the heck of it, traced wire back to terminal under the hood to confirm it is hot. this would explain why the problem was intermittent, as it would be caused by use of the hand held controller and not the weight of the plow etc. it might also explain why GM couldn't put a finger on it with their codes.
> 
> i'm not ready to call you guys wizards until i can test it out, but i don't see how this couldn't be the problem.
> 
> ...


That's really good news! This is why I love this site so much. You can come on here and post an issue and get great feed back form some experienced ppl. You get guys like B&B who is like the G.O.A.T. (Greatest Of All Trades) to chime in and give you some great honest advice, as well as many others on here.

Good luck, hope it all works out for you!


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## B&B (Nov 4, 2006)

It's a bit of a doubled edge sword in a way. As a tech the first thing you're instructed to do when dealing with any type of electrical issue is to isolate any add on accessories. In fact right in the diagnostic procedures it states and I quote: "Always check for aftermarket accessories (non-OEM) as the first step in diagnosing electrical problems. If the vehicle is so equipped, disconnect the system to verify that these add-on accessories are not the cause of the problems."

Now, the issue with dealing with a snowplow as an add on accessory is that the vehicle technician performing the diagnostics may not know the first thing about a snowplow or it's interface on a vehicle. So in such they may or may not understand all the various wiring that goes along with it thus may not have the knowledge to check all the related wiring that may be causing an issue on the truck, so they have know way to know that they didn't in fact isolate it all so they're forced to move on to the next testing procedure. In this case it should have been a person familiar with snowplows that should have been at the head of the diagnostics, not a vehicle tech. Since the issue was directly related to the plow that was quite clear from the beginning.


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## 9400 ft. (Sep 30, 2009)

The mechanics had brushed aside the plow installation, saying they checked it out and everything was fine. Everything under the hood looked nice and clean, and under the dash too from what I could see. So, a faulty plow installation was the furthest thing from my mind. 

I was very bummed out when the 'check engine' light came on again last week. The last thing I want to do is have to deal with lemon-lawing a truck that was otherwise fine, and already had upgraded tires and a nice plow setup.

I figured as a last resort, I'd maybe run it down to Denver and have the plow installers look it over. Then I thought, "heck, why not first run it past those guys at plowsite.com ?" I'd searched your threads many times for transmission problems, but it never occurred to me to search for electrical problems. Duh.

So if this is it, and I can't see how it's not, then I owe you a huge debt of gratitude. 

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Scott,

I probably won't go back to the dealer to let them know what I found, as who knows what can of worms that would open? It wouldn't do much good, and it looks their service dept might have shut down anyway.

I'm in Summit County, about 75 miles west of Denver. Aspen is a good 1 1/2 hours from here. It's still ski country though, and we average about 250"-300" per year.


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## THEGOLDPRO (Jun 18, 2006)

so when do you find out if this was infact the problem???


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## 9400 ft. (Sep 30, 2009)

*Next storm I'll know for sure*

But hindsight being 20/20, everything seems to make sense. The damaged insulation was only a problem while or shortly after using the plow, because even the slightest pressure on the cord for the plow controller would cause the damaged wiring harness to catch and dig in on the dash support. All summer long it wasn't a problem because the controller wasn't being used. But the first time I put the plow on this season, and exerted pressure on the cord, it happened again.

If there's a way to post pictures in my album, I'll put a couple in there tomorrow.


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## RichG53 (Sep 16, 2008)

I think you should go back to the dealer ..... Let them know how incompatent they are .... It might educate them & It might not .... I love to show the so called experts they don't know every thing !!


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## lilweeds (Aug 3, 2007)

Couple other things you can do, to make the truck electrical system work a bit better too. Of course make sure the plow is grounded to the battery, and use some heavier gauge wire from the ALT to the battery. It's funny how just one small little wire can cause such a mess!


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## 9400 ft. (Sep 30, 2009)

yep, experts know more & more about less & less. i probably should go back and tell them what the deal was, especially since the dealer had the nerve to tell me the problem was that i was "shifting too fast between drive & reverse". yes, it's all there in writing on the invoice. the more i think about it, the more i'm appalled that they had no answers, they were just making stuff up and trying to blame me. i'm a total DIY-er, so i'm kicking my own butt for not figuring this out myself, but even more, i'm shaking my head in disbelief at this one time that i tried to enlist experts to fix a problem for me. a new transmission for some damaged insulation...wow.

i posted some pictures of the damaged wires in a photo album so you could see what was going on.


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## RichG53 (Sep 16, 2008)

That is a CRAPPY install.. (no thought on ruoting the wires) That's why I don't like to take my vehicle in to have work or install done I don't trust any one to do quality work ... I prefer to do it my self .... I know it will be done right .... Not baby sit the problem down the road


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