Need Help on THM400 Diagnostic Steps
 

My son borrowed my 77 C20 big block with the THM400 transmission. He brought it home and parked it and a day later was going to take it again but no power to the wheels in any gear. He said he had not noticed anything unusual during the last outing.

1) Oil level shows a little high but the truck is parked facing downhill on a slight slope so I think it is OK.
2) Manual linkage seems OK. My son moved the column sift lever through the gears while I was under the truck and I could seek the manual shaft rotating at the transmission.
3) As I understand it, the detent switch only affects kick-down and even if defective would not stop power to the drive shaft. Am I wrong here?
4) I don't think the vacuum modulator can cause this behavior either but I am less certain of this than I am of the dentent switch. Do I need to take off the vacuum modulator and check it?
5) If I am right so far, then my only guess is the problem is clutch related and I need a new transmission or have someone open up this one and repair it - either case I need to have it towed to a shop. How many say this is my only option if I don't want to drop the transmission myself.

Thank you very much for your help.
I love that rusted truck.
--jd

Re: Need Help on THM400 Diagnostic Steps
 

Pump out ?

Re: Need Help on THM400 Diagnostic Steps
 

whats the fluid look and smell like?

Re: Need Help on THM400 Diagnostic Steps
 

Pump out? - Don't know. How would it tell?

Fluid looks black on the stick but very dark grey/brown when I drip it on white paper. It smells like the old fashion insect repellent I used as a kid in the MN lake country.

Does this sound like it could be the modulator? Or, should I just call AAA and get it into a shop?

Thanks.
--jd

Re: Need Help on THM400 Diagnostic Steps
 

Transmission Fluid that looks like that indicates that it is either contaminated with dirt or more likely that the clutches are burned out.

Pull that pan and see what it looks like - If there are lots of little chunks of stuff in there then it needs a rebuild.

Otherwise, you could try and drain it and replace the filter - but this will probably just be a waste of money and time.

Sounds like its time to rebuild based on the info.

Re: Need Help on THM400 Diagnostic Steps
 

Thanks, 83burban.
Rebuilding a transmission is out of my league so I'll take it to a shop.

Any advice between the choices of having a shop rebuild what I have and buying one complete "new" rebuilt unit and just have the shop put it in for me?

How about any upgrades at the same time - bigger capacity pan maybe (don't know what else is possible)?

Thanks again for your response.
--jd