86 GMC K20 TH400 - 700R4

Okay so here is where I'm at. I have a 1986 GMC Sierra 2500/K20 with the TH400. The truck has a 350 in it and is 4WD and stock with the exception of a snow plow. I am having a problem with the Trans not shifting into second gear. I checked the fluid and it was somewhat burnt. I changed the fluid and checked the throttle valve cable and it was functioning properly. Still the Trans will not shift into second. Even when i select second in the gear change. Upon looking at the used ATF I took from the Trans I found metal in it. So the Trans is toast. I guess I’m looking at three options. 1) Rebuild the Trans, could be costly and would need to be done by a professional. 2) replace the trans with a used unit, I found two on Craig’s list one from a 85 Chevy 4x4 for $200, and one from a olds for $100. They are both TH400's. Also I can look in junkyards; I just don’t know what GM's applications for the TH400 were. Will the one from the olds work if I can change out the tail housing, or is that not possible? And the Third option, I have access to a cheap if not free 700R4, would it be a wise idea to down grade to this Trans. The truck will most likely not be plowing much snow anymore, and plans do not include anything larger than 35's on the truck and the motor will stay pretty mild.
Any help is appreciated,
Thanks,
Tim  

Well a THM400 does not have a throttle valve cable to it. A THM 350 does have a detent cable. You might check vacuum modulator on it as if it is bad it will tend not to shift out of first. BTW, I have been plowing snow for over 20 years and I have yet to loose my first tranny. If it was burnt it was overheated because it did not have a aux cooler on it. As far as using a 700R4, there is nothing wrong with tranny other than it does not like big tires and tall gears because of tall OD gear. If you plan to upsize tires and not regear axles, a 700R4 is not the best choice for you here. Finally, as far as metal particles in fluid, this is kinda normal as long as they are not nasty looking burnt pieces and metal chips.  

Okay so your saying that I ought to check the vacuum modulator first. Where exactaly is that located then. I looked all over the truck and only saw one cable coming out of the trans. Did i miss it?  

Oh and the truck has a carb. if that makes a difference.  

There is a cable to carb from gas pedal but none to tranny if it is a THM400. A THM350 and a 700R4 has a cable going to it from throttle plates. The vacuum modulator is on passenger side of tranny and has a vacuum line for engine feeding it. The THM 350 and 400 both have a vacuum modulator, the 700R4 does not have one.  

Maybe I'm a dummy and its already a TH700 thats in it now? is that possible. its a 2500  

If it had a 700R4 it would have 4 drive positions (D,3,2,1) on shifter column. It could be a THM350 if it is a 3 speed as both those and THM400's saw usage in 2500's. If you got truck used someone could have already done some tranny trading in it and it might not be original.  

it wouldnt be a THM350 they stopped using those around 1978 or 79 if i remember correctly.  

heavymetalk20:
it wouldnt be a THM350 they stopped using those around 1978 or 79 if i remember correctly.

No, they used them in trucks up till about 86 or so. After that GM used either a 700R4 or a THM400.  

oh well thats just what i saw on this site called chucks chevy truck pages but ok. wasnt to sure. is a THM400 really that much stronger than a 700r4?  

Not a lot stronger but stronger but is a lot of ways it is inferior to a 700R4. The 700R4 has a much deeper first gear and a deeper reverse too and locking converter and OD too. The 4L80 is a THM400 with OD. back in 70 and 80's after market offered a deeper first and second gear kit for a THM 400 and made first about 2.75 vs 2.48 which would be nice in a 4x4 but rather than GM using this concept with 4L80 they keep first 3 gears the same. I would have rather seen GM regear 4L80 and keep using it than new 6L80 which is not a smooth tranny and hunts around a lot and really does not do anything well.  

ah ok. i have heard that an sm465 is the ultimate in tranny strength. any truth in that?  

It is not a bad tranny. I think it is a toss up between a SM465 and a NP435 for best in its class. Both are outclassed by newer NV4500 though.  

oh. i have heard that np235's are really good as well. im a firm believer that you have more control with a stick and i know i do but an auto is sure nuff nice in 5 o'clock traffic in the middle of town lol.  

Well in my "book" 4 speed and better automatics together with deep axle ratios made possible because of OD has mostly obsoleted the manual gear box. When it was a 3 speed automatic, sticks kinda ruled off road but today it is different.