# Transmission help 400turbo



## Banshee (Apr 11, 2009)

I'm thinking about buying this 91 Chevy dually. It's the square style with the solid front axle. It's pretty rough and has transmission problems.
The transmission been rebuilt about a 1000 ago. But one day the own got into it and put in reverse and it didn't do anything. Hasn't worked since. 
All the other gears work fine including 1st, but has no reverse. 
My question is. Could this be a simple fix? What should I try? Or is it ready for a rebuild. If so what does the average price of a rebuild run?

The truck is a 4 door dually, He put a 6.2 non-turbl diesel in it. It had a 350 at one time. The body is rough, needs a bed and passenger fender, and has bad rockers, Inside is rough too. 
I think I can get it for 1500. Is this too high for the condition?

Thanks.


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## clearance (Apr 11, 2009)

The last good year of Chevy 1 tons. It has a Dana 60 front axle, that often sells for over $1000, used. Body parts are cheap for these trucks, and easy to change. I would get a flatdeck for it, more usefull than a box.

Bargain him down a bit. Can't advise you about the tranny that much, I am sure you could find out more on a truck forum.


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## PasoRoblesJimmy (Apr 11, 2009)

The THM-400 has a reputation for being reasonably bullet proof. Checkout your options for repair with a good tranny shop. I might just be something minor. Used/rebuilt THM-400s are plentiful. 

THM-400s rarely fail on the open highway. Most THM-400 failures are caused by transmission overheating which can occur during repeated back-and-forth backing with heavy trailers. 

Overheating of the THM-400 is caused mainly by abuse from impatient/stupid drivers.

Caveat Emptor!
(Let the buyer beware)


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## LD1 (Apr 22, 2009)

It sounds to me like a low/reverse band problem.

In them trannys, the low gear (not drive but actually low) and reverse use the same band clutch.

When you put it in drive, your low range is driven by the typical clutches and steels, however when you put it in LOW the band also engages, which is why a lot of people will tell you when you're pulling a tree or something to put it in low, because you add the band as well as the clutches and steels so you don't slip the clutches and burn a tranny up. The band also serves to give you engine breaking where as the clutches and steels are linked to a one way roller clutch with no breaking.

Good news is, it is easy to diagnose. Drive the truck to about 20mph and then shift it into low and see if you have engine breaking, if you don't then you have a problem with the low/reverse band or actuator. Iy you do have engine breaking, then it is probabally in the valve body, which is easy to work on b/c you dont have to pull the tranny out.

It may be something simple like the band is out of adjustment.

Try the engine breaking and let us know.


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## treemandan (Apr 22, 2009)

Its the 6.2 that scares me. Sounds like a good bit of time and money has to go into it and you still have a 6.2


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## PasoRoblesJimmy (Apr 22, 2009)

treemandan said:


> Its the 6.2 that scares me. Sounds like a good bit of time and money has to go into it and you still have a 6.2



IMO, the 6.2 is crap. The 350 passenger car diesel was a total disaster. Both can be maintenance nightmares. Price out an injector pump for a 6.2 *before* you even think of buying one.


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## funky sawman (Apr 25, 2009)

The best thing to do is pull the engine and tranny, Rebuild the tranny and install a fresh 350 gas engine. 1500 dollars for a truck like that is a steal. Those older chevs have tons of potential.


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## GASoline71 (Apr 25, 2009)

Better check what that is... I had a 1991 3/4 Chevy Suburban with a small block 350, and the tranny is not a TH-400... it's a 4L80E... A great tranny based off of the TH-400... it's elecronically shifted, and has a massive torque converter. I had to have mine rebuilt to the tune of $1,600 bucks. That included the converter.

That was back in 1997... 

Gary


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## PasoRoblesJimmy (Apr 25, 2009)

GASoline71 said:


> Better check what that is... I had a 1991 3/4 Chevy Suburban with a small block 350, and the tranny is not a TH-400... it's a 4L80E... A great tranny based off of the TH-400... it's elecronically shifted, and has a massive torque converter. I had to have mine rebuilt to the tune of $1,600 bucks. That included the converter.
> 
> That was back in 1997...
> Gary



Thats a good idea. If its the old body style, it can have a combo of new and/or old parts in it. The THM-400 is 3-speed. The 4L80E is 4-speed with overdrive. There should be an ID tag/plate on the side of the AT.


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## funky sawman (Apr 26, 2009)

One ton chevys from 1977 to 1991 all used the turbo 400 if it was an auto. The 4l80e requires the use of a computer to control the shifting and torque converter lock up. If the truck has a 205 t-case (it should) then it a 99.9% chance it is a turbo 400, GM never mated the 4l80e to a gear drive 205 t-case.


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## GASoline71 (Apr 26, 2009)

Mine had a NP208 t-case, and was a 3/4 ton. Plus it was a 'burb, and not a truck. 

Are you sure the NP205 was still around in '91? I thought GM stopped usin' them in '88.

Gary


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## brokenbudget (Apr 26, 2009)

GASoline71 said:


> Are you sure the NP205 was still around in '91? I thought GM stopped usin' them in '88.
> 
> they did sorta, you could get it as a special order until 1990, up here anyway.


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## PasoRoblesJimmy (Apr 26, 2009)

The only way to tell for sure is to first read the ID tags on the XFER case and AT. Second, decode them with the GM factory service manual.

I owned a 1989 GMC Burb 4WD with the old body style. It had an odd collection of metric and SAE fasteners and other misc. parts in it. The rubber spring bushings, spring hangers and spring shackles were of non-standard sizes. It looked like GM could have been using up its inventory of old body style parts then gradually switching over to the new body style parts.

The new body style Burb came out in the 1992 models.


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## funky sawman (Apr 26, 2009)

I once had a 1988 gmc k30 that was the old body style and a 1988 k3500 that was the newer body style, The weard thing was, the older body style 
k30 had a later in the year build date than the newer body styled truck, They were both regular cab 1 ton 4x4 dually's with turbo 400's and 205's


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## 046 (Apr 26, 2009)

pass.. not worth the trouble... 

fully functional early chevy diesel duallys come up all the time on craigslist for $2500 range. 

6.2L Chevy diesel engine is prone to head gasket failures

if you want a good inexpensive diesel dually ... look for a first gen Dodge Cummins turbo diesel 88-93. they come up all the time for under $3500. they hardly ever go for below $2500. for the cummins 12v is worth $2500 alone.


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## funky sawman (Apr 26, 2009)

I dont like the 6.2 diesel but I have seen many go fo 300,000 miles with no major repairs other than glow plugs and injection pumps that cost a third of what cummings parts cost.


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## 046 (Apr 26, 2009)

yup the folks that like 6.2 ... really like em. guess if you get a good one, they are pretty reliable. trouble is finding one. 

cummins parts may be expensive, but odds are you will not be needing many parts. other than normal filters. note this goes for 12v cummins... not the dreaded 24v cummins, which ate fuel injection pumps at $1200 a pop. stay away from 98.5 to 2002 dodge cummins trucks. 



funky sawman said:


> I dont like the 6.2 diesel but I have seen many go fo 300,000 miles with no major repairs other than glow plugs and injection pumps that cost a third of what cummings parts cost.


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## Peacock (Apr 26, 2009)

TH400 and 4L80 are easy to tell apart with one foolproof thing. The 400 has a vacuum modulator and the 4L80 doesn't.

If the transfer case is aluminum it's NOT the 205.


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## gilraine (May 5, 2009)

PasoRoblesJimmy said:


> IMO, the 6.2 is crap. The 350 passenger car diesel was a total disaster. Both can be maintenance nightmares. Price out an injector pump for a 6.2 *before* you even think of buying one.


run it it dies and then drop in a TBI 454, or a Cummins 



funky sawman said:


> The best thing to do is pull the engine and tranny, Rebuild the tranny and install a fresh 350 gas engine. 1500 dollars for a truck like that is a steal. Those older chevs have tons of potential.





GASoline71 said:


> Better check what that is... I had a 1991 3/4 Chevy Suburban with a small block 350, and the tranny is not a TH-400... it's a 4L80E... A great tranny based off of the TH-400... it's elecronically shifted, and has a massive torque converter. I had to have mine rebuilt to the tune of $1,600 bucks. That included the converter.
> 
> That was back in 1997...
> 
> Gary



If it had the 4L80e, it would not shift into first at all.. the TPS for the TBI is gone and the trans would be in limp mode, second and third only...

personally, assuming it is a 400 I would get it flushed, could be a small piece of friction material stuck in the valve body... then pull the 6.2 for either a TBI 350, if you want cheap or a cummins if you want a project..


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## Peacock (May 6, 2009)

gilraine said:


> then pull the 6.2 for either a TBI 350, if you want cheap or a cummins if you want a project..



It may be tough to shift the 400 with a Cummins in front of it....


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## mercer_me (May 6, 2009)

I wouldn't put no money in it. I had a 1989 Chevy 2500 and I loved it, but it rusted out and it wasn't worth fixing. 1988 to 1993 Chevy trucks are known to rust out.


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## gilraine (May 6, 2009)

Peacock said:


> It may be tough to shift the 400 with a Cummins in front of it....



run the modulator off the vacuum pump.. Had a P30 deleivery van with that exact combo..


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## Peacock (May 6, 2009)

gilraine said:


> run the modulator off the vacuum pump.. Had a P30 deleivery van with that exact combo..



It was sarcasm....

People say all the time you can't run a 400 behind a diesel.


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## funky sawman (May 6, 2009)

mercer_me said:


> I wouldn't put no money in it. I had a 1989 Chevy 2500 and I loved it, but it rusted out and it wasn't worth fixing. 1988 to 1993 Chevy trucks are known to rust out.



The 3/4 ton (2500) of those years are a totally different body style. 1977 to 1991 chevy 1 tons are worth fixing, lots of parts available and very cheap and easy to work on.


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