Automatic transmition does not down shift

I have a 1983 chevy cut-away motorhome that does not downshift. It has been stored indoors for the last 15 years. I’ve checked the gas pedal switch and there is voltage. I then checked the other end of the switch wire that is attached to the small module on the side of the transmition; and yes there’s 12 volts when pressed. There is only 35,000 miles on this detroit diesel, and it runs like new. The transmission upshifts perfectly when starting out… but going up hills it slows down to a crawl in third gear. I’ve had the fluid, filter and gasket replaced… Can anyone tell me the next place to look or test to try?

Is this transmission the TH-400 or the TH-700R4? Can you hear the solenoid click when activated? Do you get a 10-20 ohm resistance to ground on the solenoid coil? On the TH-400 it looks like the solenoid bleeds fluid once activated and then the detent spool allows line pressure to increase the modulator pressure to the shift valves. If the solenoid is working, the problem could be a stuck detent spool.

Keep us appraised of your progress on this matter.

These are all good questions. and it sounds like you know your transmissions… I on the other hand am not worthy… I’ll have to do some digging to find out which model transmission is in the old girl… is this an easy find? serial numbers are located where?

As for the module on the side of the transmission clicking when activated?? I having listened… but it will be my next task… I’m assuming this is the solenoid you’re referring to… just a stuck plunger? is it an easily replaced item? Will it bleed if taken off? I don’t recall seeing any mounting clip or holding device. how is it secured in?

Do you know of a web site that would have exploded view of this type of transmission and module? I’m eager to learn. I’ll try to get the info you requested this evening. thanks for your help.

Your right I have no click at the solenoid when power is sent from the switch!
Is this an easy fix or do I have to have to take it in to have a new one installed?

Thanx for helping to narrow it down!

Oh by the way I have 11.4 ohms at the solenoid to ground!
Good guess!

If you have a switch under the accelerator then you are working with a TH400. If you have 12vdc when the switch is pressed then the wiring is good and I would be working towards the solenoid.


Yes it sounds like the solenoid plunger is struck. The solenoid will be inside the transmission so you will have to remove the transmission pan to change it. It is fairly easy to replace. At the same time you should clean the pan; change the filter; get a new gasket; reinstall the pan; and refill, i.e. do a transmission service if this has not been done recently.

The Th-400 is a three speed, heavy duty automatic with a locking torque converter. The TH-700R4 is a four speed overdrive, heavy duty automatic with a locking torque converter. Take a picture of the pan and/or count the number of pan bolts. Show this to a auto parts person and they can match it up to your transmission. Sometimes the transmission model is stamped into the bottom of the pan.

Hope this helps.