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boondockdad 08-14-2009 07:37 PM

'burb 'bucks' and slips under load
2005 Suburban 2500, 4x4, 8.1, LT, 60k miles... Around town, no problem... drives fine... About a month ago, started hearing a 'buzzing' from undercarriage. Sounded a lot like a loose heat shield on muffler, so I disregarded for the time...

Last week hitched up for an extended trip east... 20 minutes on secondary roads under 45mph was fine... but after getting on freeway, and cruising at 65mph (speed control), truck started to 'buck', then RPM's spun freely up to redline... I switched off cruise, and coasted for 4-5 seconds, then gassed it, but engine reved like it was in neutral... shifted into 'N', then back to 'D', tranny seemed to engage OK... 55mph to the next exit about 5 minutes up... drove home... again, no problems on secondary roads...

My mechanic looked at it... said it sounded like clutch plates were gone... swapped out remanufactured tranny. Brought home... hitched up... drove secondary roads- no problem... no rattle... get on highway- same thing... identical problem... buck... buck... reevvvvvvvv...


Mechanic is stupefied.

Is my trust in my mechanic misplaced?
Could there be a tranny 'computer' problem?

RedSHED 08-14-2009 07:45 PM

I'd guess - and it is a guess - transfer case. Are there any codes?

Reasoning is this:
If it was the harness, the system should be able to self diagnose that there was an intermittent connection to the PRNDL (or gear lever if you'd rather) and you'd get a code. Also, that shouldn't make any noise.

However, if the transfer case was acting up, the trans controller might go into a self-preservation mode and disengage. I'd think there'd still be a code, though, but maybe not.

boondockdad 08-14-2009 07:47 PM

no codes.. no engine light

boondockdad 08-14-2009 07:59 PM

I got Bender troubleshooting... he he
j/k, I really appreciate the help

RedSHED 08-14-2009 08:03 PM


Originally Posted by finalcutjoe (Post 735125)
no codes.. no engine light

I still wouldn't rule it out. The check engine light is only required for emissions purposes, and technically the trans doesn't affect combustion.

Thinking about it, I can imagine a scenario where there are no codes. I don't know how the transfer case is engaged or disengaged, but I'm guessing there's a wire harness that winds up down there. Possibilities would include a flaky connection, which could be diagnosed and resolved by disconnecting devices from the harness, looking for corrosion or deformation, and reconnecting. It's been 15 years since I've messed with modules on a GM product, and that was during development, so I can't offer too much help, I'm afraid.

Maybe call Click and Clack in the morning ;)

[edit: Bender doesn't mind. He consults with the knights who until very recently said "ni."]

ibgl6 08-14-2009 08:17 PM

I had a problem like that with my 99 1/2 ton 2 wd it turned out to be a crank position sensor no check engine light just alot of bucking and jumping around rpm gauge going crazy too...SAM

boondockdad 09-02-2009 01:22 PM

update: transfer case actuator was 'binding'... it was not fully engaging the 2HI gearset, and it would bounce out and in. actuator won't throw any codes.

RedSHED- right on the money. next time I'm PM'ing u first...

RedSHED 09-02-2009 05:30 PM

Bender said something about his shiny metal...


Thanks or you're welcome. Glad you got it figured out.

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