climate control switch in 67 DeVille

[L Langlois]

My '67 DeVile AC compressor runs when the climate control dial is up to 85, blower on, hot air blowing. The AC works fine.  Any suggestions?  Thanks, L Langlois

[Julien Abrahams]

Start with the easy things first. Check all electrical connections and and vacuum lines. These are relatively complicated systems which rely most heavily on vacuum. First I would check all the vacuum lines. The blower motor will only come on if that main switch receives vacuum.
The power servo has a sliding rotary valve inside which can warp and lead to vacuum leaks. Check your shop manual on how to remove the power servo and dismantle it. The discs can carefully sanded flat again to make a good seal.
Iam sure others will chime in and Cadillac Tim has a book that describes the system and can help with diagnosing the problem.

["Cadillac Kid" Greg Surfas 15364]

As Julien says, look for the simple things first.  IF the Automatic Temperature Control (ATC) system is wired correctly , and the fact that it DID work correctly points to that, I would look for a wiring issue in the blower circuit.  These old contacts and connectors tend to fail, especially at the blower relay. The blower relay itself should be checked. If the compressor is turning on, the vacuum circuits to and from the Master switch are most likely OK, and since this seems to be intermittent a bad connector or blower relay might be the cause of the problem.
Greg Surfas

[L Langlois]

Thanks for the responses. The CC switch seems to be functioning normally now but the compressor runs when I have the dial turned up for heat.  (whenever the blower is on the compressor is on)

["Cadillac Kid" Greg Surfas 15364]

That is correct. The compressor operates in both cooling and heating modes. ATC dehumidifies the air and the. Re heats it as required.
Greg Surfas

[www.eldorado-seville.com]

Greg is 100% correct! The shop manual mentions this. Only when the outside temperature is very low the AC compressor will shut off.

[35-709]

Note that the blower motor is designed to run at all times (at low speed) when the ignition is on, even with the ATC is switched OFF or in the VENT position.

["Cadillac Kid" Greg Surfas 15364]

Not on a 67 Geoff
Greg  surfas

[35-709]

Wasn't '67 the first year for the VENT position?  Or are you saying the blower motor doesn't blow all the time the ignition is on in '67?

["Cadillac Kid" Greg Surfas 15364]

“Flow through” ventilation with constant fan started (I believe) in 73
Greg Surfas

[cadillactim]

Vacuum to the vacuum master switch is cut off on 67 in the Off position . When in Vent vacuum is again sent to master switch allowing blower to run.

Tim

[cadillacmike68]

Thanks for the responses. The CC switch seems to be functioning normally now but the compressor runs when I have the dial turned up for heat.  (whenever the blower is on the compressor is on)

The compressor is Supposed to run unless the outside temp is below 32F.

The blower fan will not run in OFF. This changes sometime  1971-73 or so.

[chrisntam]

Clearly the designers (of this era car - '70 included) did not care about fuel mileage / compressor wear or else the compressor would not be engaged when heat is needed.

["Cadillac Kid" Greg Surfas 15364]

The compressor operates to provide dehumidification. If the temperature outside is say 60’ degrees and raining, all you would get if the cooling/dehumidification was not engaged would be a swamp by adding just heat.
Greg Surfas

[chrisntam]

Not all cars back in the day had a/c, I don't recall that (the swamp) being the case when the heat was on.  Additionally, I don't think the a/c is engaged in late model cars when in heat mode.

What am I missing?

["Cadillac Kid" Greg Surfas 15364]

Remember steamed up windows?
Greg Surfas

[cadillacmike68]

Exactly correct, Greg.

When it rained a few weeks ago and my compressor would not engage, I got treated to fog city driving home.


Chris,  even current model Cadillacs cool and de-himidify the air before it goes to the heater (if needed), however as of the early 90s the temp cutoff for the compressor has raised to 40F from 32F. The main purpose is to prevent the formation of fog on the windows. Even if it is 40 F out, the cooled air is easily heated by the 200 F temp coolant in the heater core so it's not counterproductive.

That's why, even in AUTO a small amount of air goes to the defroster vents. I can attest to the fact that de-himidified air is necessary.

When it's that cold (below 40), windshield fogging is not a problem.

[chrisntam]

Remember steamed up windows?
Greg Surfas

Back in the day, that wasn't from the car's heater.



Back to the point at hand, I guess I don't recall fogged windows (while using the heater).

Greg, you leave me low hanging fruit, I'm gonna pick it...

[Scot Minesinger]

Greg is right the compressor is on when the heat is down to 32'F or so to dehumidify.  If removing moisture from the air was not a good idea, Cadillac would have more cheaply added a lock out so when heat came on, compressor turned off.  Correct operation is compressor operates during heat mode except when it is below freezing outside.

1969 and 1970 operated that when the ignition is on, the blower fan is too.  This may be the case for 1967, read FSM

[cadillacmike68]

For 1967 and 68 if the control is set to OFF, there is no blower fan operation. The 1969s and later had it running to purge the system.