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2003 duramax problems

rick88blaze

rick88blaze

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I've got a 2003 GMC w/ the Duramax/Allison in it. Ran fine yesterday but this morning I fired it up to go get some breakfast and on my way to the store I notice my gear indicator isn't working. I move the truck into neutral and back to drive at a stop light and then my brake and ABS lite come on and the dash reads "SERVICE BRAKE SYSTEM". BTW, the brakes work fine.

Then I notice that my truck won't shift and the speedo quit working.

When I shift from Park or Neutral to Reverse or Drive the tranny slams hard into gear.

The truck runs and stops fine otherwise. All the fuses in under the hood check out fine.

There are no trouble codes on the OBDII computer. The only lights on are my brake and ABS.

I changed my speed sensor and ignition switch today and neither helped. Some of the other forums talk about changing the neutral safety.

What do you all think?

Thanks,
Rick
 
Your neutral safety / backup switch on the transmission is finished. It's on the side of the transmission and has two plugs in it. You'll need to remove the shift cable arm to take it off.
 
My 01 did that same type of thing. Instrument cluster had to be changed. I asked what was with the shifting issues? It was screwing up the computer is what I was told. Worked fine until we sold it at 200k miles.

Not sure if that's what's going on with yours.
 
That is possible... In 03 they switched to a cluster that communicates on the serial BUS. If the cluster is sending confusing / screwed up messages it may put the PCM into limp mode.

But the fact that he has lost his reverse lights tells me it is probably just the switch on the trans. They don't have anything to do with the BCM or instrument cluster.
 
I have owned a 01', 03', 05', and now my 08'.

My 03' had the same issue as yours and Russell described exactly what was wrong with mine. I really loved my 01 but had injector issues at 97K that the dealer couldn't seem to fix so bought the 03'. That truck never had one issue, not even a light bulb out in 117k when I traded it for my 05'. Had the 05' for 228k until I blew a head gasket. Bought the 08' used and love everything about the truck except the fuel mileage. It sucks compared to all my others. The cat and fuel reburn system totally screwed up the Duramax. They should have left it alone.
 
Deletes are still easy to get for the older LMM engines! You'll wind up with better fuel economy than the old LB7s and LLYs after the deletes are in.

My 2012 LML still averages around 22 mpg on the highway even with the DPF. Well, after a fresh service regen anyways. Once it gets plugged up that drops to about 13 mpg haha!
 
Russell said:
Your neutral safety / backup switch on the transmission is finished. It's on the side of the transmission and has two plugs in it. You'll need to remove the shift cable arm to take it off.
Click to expand...

yup, crazy things can happen when that thing is messed up. Make sure you get the Allison replacement.
 
It's at Certified Transmission right now here in Bellevue.

They ran some tests on it while I was there and found out the TCM isn't communicating. They don't know if the computer went out or if there's just a bad wire or ground. The mechanic's exact words are 2 of my least favorite words: "Electrical Gremlins". Hopefully they can figure it out tomorrow. I'll keep you guys updated.

What really sucks is I'm supposed to be moving down to Missouri this weekend hauling my 20' trailer full of household goods. My trip might get delayed now.
 
My bet is the switch on the trans like Russell said.
My truck went through 3 of them before I got one from the Ally dealer!

That was when it was new...around 40 thousand k got over 300 now it has been good!

Good luck! :thumb:
 
Is the TCM's sheild still in place? Is there any chance you may have grounded it's heat shield with the ignition on by accident?

Usually the truck will go immediately into limp mode and flash all kinds of codes if the TCM has failed to communicate with the PCM or BCM. How did they determine that the TCM is failing to communicate?
 
They hooked up their scanners and he said that it wasn't reading the TCM at all. He also pulled a couple engine codes that didn't trip the check engine light which I didn't quite understand.
 
I got my truck back today. It ended up being just about the only fuse I didn't check. It was one of the ignition fuses in the cab. If I remember right he said it was the fuse that feeds ignition power to the tranny computer. I installed a backup camera a few months ago and it said to tie the monitor power into an ignition feed. Whoops.

I'm just glad it only cost me $80 instead of the $1200 they said a new computer would be.

Now I'll be wiring the screen straight to the battery feed under the hood with an inline fuse.

Thanks for the advice everyone!!
 
That'd do it! The TCM shares the same ignition power feed as your transmission switch and reverse lights.

Glad you got it figured!!
 
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