***This is an update to an older article as I had a very similar problem in the past already.***

I finally found the time to take the 1978 Eldorado out of winter storage. It started right up after many months of inactivity and I checked all accessories and options for operability.
Everything checked out fine but the air-condition… It only blew warm air out of the heater outlets and would not change modes, blower speed or temperature.

I immediately suspected a problem in the ATC programmer again. I already fixed it last year when the system would only work in full AC mode.
Last time I had a bad contact at the transducer which is the most important part in the programmer as it regulates the vacuum supply to the system.

The shop manual mentions that when the system is stuck in full heater mode, most probably an electrical problem is the main cause.
So I tried to push the transducer in again but the system did not come back to life like it did last time when I did this.

The MKII programmer which you can access through the glove box had to come out again for inspection.
This is how it looks with the cover removed:

The 1978 MKII programmer with the cover removed.

I decided to take the transducer/amplifier board out of the programmer this time to see where the problem could be. It can be removed very easily - it's only held in place by 4 screws. Be careful not to turn the gear wheel with the potentiometer though as you would have to recalibrate it again afterwards.
After the board came out, I quickly found out where the problem was. Below is a picture of the removed board:

The transducer/amplifier board.

The transducer/amplifier unit was sitting pretty loosely on the circuit board, and one of the very thin and fragile wires going to the coil was broken, as you can see on the picture below:

A broken wire at the coil of the transducer.

When I turned the circuit board around I could see that the soldering points on the circuit going to the transducer and the amplifier were broken and that they no longer were making good contact. That was the reason why the system did no longer work.

Broken soldering points at the rear of the circuit board.

The pretty simple solution was to re-solder the 3 soldering points and fix the broken wire going to the coil. I then put everything back together and now have a perfectly working ATC again. These old circuit boards can be very troublesome and I think that most of the time when a programmer goes bad on these 70s cars its probably a fixable problem like this. I was very worried that I would need an expensive rebuilt programmer and I am very glad that I could fix it by myself.
I have no idea though why everything worked when I put the car into storage last fall and how it broke while the car was inoperative…

Check my older post of the MKII programmer to also see the electrical diagram for the ATC system.

Tags:1978 Cadillac Eldorado Biarritz, 1978, 1978 Cadillac Repair Tip, Cadillac, MK II, Aircondition, Restoration, ATC, Programmer, Transducer, AC