Hi everyone,
Is anyone here familiar with GM automatic temperature control (ATC) programmers and the special diagnosis tools? I do admit my current problem is in a 1979 Cadillac Coupe de Ville, but some Oldsmobiles had similar systems (luckily not including mine) - and this forum has proven to be so much more responsive that the Cadillac Forum :-)

My ATC has been going in and out of operation while driving, being stuck at max A/C intermittently. Battery voltage and ground are present at the programmer, so is vacuum. So I removed and dismantled the programmer. There are no obvious faults. I even got myself a mint-condition J-23678 testing device as the shop manual suggests, as well as a (not-at-all-mint-condition) used programmer off ebay. Now everything is set up on the workbench, and here`s the thing: The programmer will pass all the tests described in the shop manual, the vacuum motor will move. Then, on the next try, seconds later, with no change to the setup, it won`t. Then I noticed that tapping the check valve will get the motor to move. So I swapped the check valve for the one out of the spare programmer. I was very enthusiastic, thinking I had found the issue. It even made sense as slight tapping, as in hitting a bump while driving, had been switching the ATC in and out of operation! However, I was wrong. No movement on the next try. Still: Amplifier electric signal is fine, as are vacuum readings, but the motor will move fine at one try, veeeery slowly at the next - and sometimes not at all.

The replacement programmer off ebay also shows proper signals from transducer and amplifier, but the motor never moves...

Do you have any ideas where to look? Any suggestions?

 

Is the in car temp sensor plugged in? If I recall it's in the upper dash pad behind the Cadillac name plate.

 

Thanks, but that can be ruled out. Measured resistance from the sensor string is in fact a little too high, which would drive the system hotter, not colder. Also, I have the programmer on the workbench, acting up on its own.

 

Is the Cad unit fully automatic or semi-automatic (driver has control of compressor and outlets)?

My experience with Tempmatic like 74-later Olds used is that majority of the time that vacuum check valve/relay on the programmer body is at fault.

After there was no more Oldsmobile, I got them from the Buick/Pontiac dealer- who kept several on the shelf at all times, if that tells you what the failure rate was.

 

That is the fully automatic one. The system is a fascinating piece of engineering for its day, unfortunately it does not age too well....