Need Help on THM400 Diagnostic Steps

[jdpavek]

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