Allison Trans problems.. Need help!

[1QuickZ]

Ok guys and gals.. Let me first say that I don't know jack about these transmissions. I am looking at a 91k mile 2500HD GMC with the 8.1L gas burner not the diesel. (unfortunately) Anyway this truck is CHEAP but it has a problem.

The Allison transmission will not shift out of third gear. It it VERY firm when you put it in reverse or forward gear and when shifting. The current owner took it to a local dealership that did a diagnostic check and they say that it needs a Trans Control Module aka TCM. The cheapest I have found a TCM is around $1,000.

If it really does need a TCM is there a place that sells reman TCMs or new ones for a better price than the dealerships? I know I have to get it programmed by the dealer once I have replaced it, but is there anything else that I need to know? The dealer also said to flush the trans at the same time.

I need all the help I can get with this thing.. Thanks in advance.

--Matt

 

[Cougar281]

Well, this really should be in the trans section, not the trans tunes section.

Sounds like the TCM is going into limp mode. See if you can get the codes and post them up. They would give a much better idea what might be wrong. Without them, it's pure speculation.

 

[racinmike77]

I picked up a TCM at a junk yard for $150. I bought a truck with a blown trans and the dealer said it needed a TCM. When I pulled the codes it clutch overpower codes and when I tore it apart the clutches were burnt up. This truck could be the same way, read the codes and you will find out.

 

[1QuickZ]

I can get the truck for $2500.. It looks good and everything seems to be fine except for the trans issue. If I buy it and it ends up needing a trans I'm just gonna flip it..

He says the truck was running perfectly until one day he comes out and the trans was acting up. It still runs and drives but only has 3 gears.

 

[1QuickZ]

I picked up a TCM at a junk yard for $150. I bought a truck with a blown trans and the dealer said it needed a TCM. When I pulled the codes it clutch overpower codes and when I tore it apart the clutches were burnt up. This truck could be the same way, read the codes and you will find out.

If I get a TCM from a junk yard does it have to be from the same year model and engine, etc?

And I "thought" I was going into TRANS TALK.. Didn't even notice. Move it if need be.

 

[wes06dmax]

You limped it time to rebuild dont understand how a 8.1 limped a allison though

 

[1QuickZ]

So you are telling me the trans is shot with only 91k miles and a bone stock 8.1 driven by a 50 year old dude? He pulls a horse trailer with it but damn I thought the Allisons were a lot tougher than that.

I'm not gonna buy it if it is gonna need a new trans...

 

[x MadMAX DIESEL]

is the trans is limped, that doesnt mean the trans is shot at all necessarily. its a "safe mode" for it to protect the engine and trans from catestrofic failure.

it can limp for numerous reasons, i.e tcm senses slippage in the clutches or TC, or actual fuel rail pressure is not what the desired pressure should be... and anything in between... try either clearing any dtc codes u might have, or shut the truck off for 20 and restart it, if all is well after that it was just limped and nothing really to worry about, just a sign something may need to be reaplaced or serviced soon.

if that doesnt fix it, than its prolly a mechanical failure in the transmission

 

[1QuickZ]

The current owner drove the truck to the dealership which is about a 15 minute drive. If its in "limp mode" would that even be possible? I thought Limp Mode would only let the truck pull just enough to get off the road.

The dealership scanned it and said it needed a TCM.

 

[racinmike77]

Sent you a pm, sounds like the tranny is burnt up. Probibly needs clutch packs, looking at a couple thousand to fix.

 

[d-max joe]

the dealership will tell you anything...

 

[bonescarolfi]

I thought when an allison limps i gets stuck in third and will not shift period. At least thats what mine did before it totally gave up. If its shifting 1st through third and then stopping and still doing it after the truck has been shutoff and restarted then there is something else going on and might not necisarily need a full rebuild. It doesnt seem to have failed the way we are used to from too much power.

 

[dmaxvaz]

I would change the neutral safty switch first and go from there

 

[Mike L.]

I would get some codes first instead of throwing money at it.

 

[x MadMAX DIESEL]

i did know where a 6 speed tranny was with 16,000 on it for $650:spit: had a broken bellhousing but thats it. not sure if its still available thought. what tranny is in it, 5 or 6 speed?