***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.


Fixing the 1978 Cadillac Eldorado´s ATC programmer. The AC works again.


An update to the article below can be found here as I had to repair the programmer again in May 2015:

The air-condition in my 1978 Biarritz did not work correctly. For some reason the system always stayed in full AC mode and no matter to where I put the temperature dial on the control head I could not get the heat on, no matter what I tried.
The shop manual is very helpful in trouble shooting the system. Its covered in the 1977 factory shop manual (FSM) as there were no changes made to the system in 1978.
It took me a while to fully understand how the system works - so I had to read the FSM multiple times to get the grip what is going on in the system.

First I inspected all the vacuum lines in the car to make sure that there was no leak anywhere, as the heating mode requires a very high vacuum at the vacuum motor in the controller.
All lines were in very good condition. I also checked the control head connections in the dash which were good as well.
I then suspected the vacuum motor in the programmer as it did not move. I removed the programmer from the car. You can access it from underneath the dash on the passenger side or even better if you remove the glove box lining through the glove box.
From there you only have to unscrew 5 screws, unplug the wires and the vacuum lines to get it out.

When I had it out of the car I checked the vacuum motor. I just sucked air through the hose and it moved and held vacuum perfectly.
The transducer or the amplifier in the programmer were most likely the problem.
When I checked the transducer which regulates the vacuum supply to the vacuum motor, I noticed that it seemed to not be connected to the electrical board as it should. I pressed it in, so that it made good contact with the board again and put the programmer back into the car and connected everything again.

***UPDATE 2015 - its not connected with pins but soldered to the circuit board - 3 of this soldering points were broken - the “repair“ I made here only temporarily worked. You have to check the soldering points for a permanent fix***

I started the car and the AC came back to life again immediately. I could see how the vacuum motor went through all the modes as it should according to the position of the temperature dial on the control head.
The build quality of the programmer obviously is not very good and the transducer somehow came loose causing the ATC to not work as it should. ***Update 2015 - it did no come loose but the soldering points broke***
After I tested everything I put the parts back together and can now enjoy a full working ATC in my 78 Biarritz again ;-)

The programmer as seen through the glove box with the connectors and the plastic case removed

The 1977 / 1978 Eldorado MK II Automatic Temperature Control Programmer

The transducer which caused the problem (the golden part with the single vacuum line going into it)

The transducer is connected to the electrical board through a couple of pins - it was not connected properly and somehow became loose over the years - that was why the system did not work.
***UPDATE 2015 - its not connected with pins but soldered to the circuit board - 3 of this soldering points were broken***

The vacuum checking relay and the electric connectors.

The vacuum power motor
from the 1977 FSM

Trouble shooting guide for when the system works in full AC mode only.

A color coordinated electrical schematic with annotations to describe the various functions of the system.