76 cadillac a/c

[caddymanmarc]

my climate control blower motor on my 76 cadillac sedan de ville will shut off unexpectively during operation. it will also take about ten minutes to come on when first starting the car. i have replaced the blower relay. i also have converted to the 134 coolant, but i still have a coolant leak and must add coolant every two weeks or so. when the blower shuts off, all funtions stop except for the defroster. after about ten minutes, the a/c will come back on, but after a short while, it will again shut off with just the defroster working. i have read the trouble shooting guide in the reapir manual, but being a novice, i'm not sure what to do. any ideas? thanks for your feedback.

["Cadillac Kid" Greg Surfas 15364]

Marc,
I'm going to guess that when whoever you had do it, replaced the r-12, they put a compressor cycling low pressure switch on the system and since you are continually adding refrigerant, the system is most likely always low on charge with the switch cycling the compressor.
Regarding the start up delay, there is a thermal delay switch at the front of the passenger side cylinder head that delays the fan start up until the engine is warm.  You might check the connection at this switch, and if it is good, perhaps the switch might need replacement.
Greg Surfas

[cadillactim]

Since you have to wait about ten minutes until the blower comes on, very likely the thermal switch in the dash control is not working. Once the engine warms up the other thermal sensor in the cylinder head Greg mentioned is finally completing the ground circuit and allowing the fan to come on. But it sounds like this is also not quite right. It shouldn't take ten minutes, only about five or so for the fan to come on with this thermal switch. Could be this switch too is going bad since the fan shuts off and starts again.

A simple test is to unplug that wire from the cylinder head switch and connect it to a good ground. Now, the blower should run fine and not shut off. If the blower stays running this way, then that thermal switch is bad.

The reason the blower works in Defrost is because in that setting there is a direct ground circuit. All other settings have to go through the either of the other two thermal switches.

Tim

[TJ Hopland]

I dont have a 76 manual but in the earlier ones I thought the switch on the head was only used for heat modes.  In AC mode it did not use the switch.  If it did that would be a pretty stupid design making you wait for the engine to warm up before you could run the AC.

Is it possible he is just sometimes loosing vacuum to the system?   I seem to recall that you need vacuum for even the blower to operate. 

[R Schroeder]

TJ. My 78 comes on after a short delay in both heat and cooling modes. It only takes me a few blocks in summer, because it is so warm out. Cooler days it may take about a mile to warm up.
Freaked me out when I first got the car. Came on full blast ,and I didn't touch anything. Thought the car was haunted.

[The Tassie Devil(le)]

Doesn't the sensors work when they sense that there is sufficient heat, or cold to warrant the complete A/C to operate?

The A/C in Cadillacs is  not only a Cold A/C, but a temperature controlled Climate Control system, and therefore until the sensors "talk" to the control unit, nothing works till the parameters have been reached, for whatever the control wheel has been set to.

Bruce.

[cadillactim]

There are three ground circuits in the system. Any one of the three ground circuits will turn to blower on. One ground is a thermal switch in the dash control. When the temperature inside the car is above 75 degrees, that thermal switch closes and completes the ground allowing the blower to turn on (the sensors will cause the programmer to go to full a/c but the sensors do not turn the fan on).

The thermal switch in the cylinder head is for cold weather. Since the temperature inside the car is below 75 degrees, the dash control thermal switch is open. Now, when the coolant temperature warms up, the cylinder head thermal switch closes and allows the blower to turn on blowing hot air (sensors will cause programmer to be in heat mode).

The third ground is an internal ground in the dash control. When the lever is in Defrost, the blower will come on regardless of temperature inside of car or coolant temperature.

Again, any ONE of these three ground circuits will turn the blower on. Often, the dash thermal switch goes bad, and that's why even in hot weather, the blower won't turn on until after a few minutes - when the cylinder head thermal switch closes and grounds the blower circuit.

Tim

[TJ Hopland]

So it would seem the easiest way to see if the rest of the system is working correctly would be to ground the sensor on the head (the one behind the alternator) since its likely going to me much easier to find than the interior sensor?  If the system then seems to work correctly you know you just have some sensor issues.  If not then you will likely have to do some digging under the dash?

[cadillactim]

The thermal switches are SWITCHES, not sensors. The sensors have varied readings. The thermal switches are either
OPEN (off) or CLOSED (on).  It's easy to use the term "sensors" loosely, but they are different than the thermal switches.

The dash control thermal switch is made into the wiring harness of the control, and is not replaceable by itself. The cylinder head thermal switch is replaceable.

As stated in an earlier post, simply ground the wire going to the thermal switch behind the alternator and see if the blower runs and does not shut off. If it runs okay, then the problem is in the thermal switches.

Tim

["Cadillac Kid" Greg Surfas 15364]

Just to add my 3 cents worth here, there seems to be a couple of issues, and  we need some more information help him determine what the actual problem is.  As Tim says, there is also a thermal switch built into the control head that is supposed to close when the temperature is over 78 degrees in the cabin.  This was in the control heads beginning in 1971 (I believe)). This switch  allows the blower to come on without having to wait for the thermal switch on the right cylinder head to reach 120 degrees.
That being said, for some reason I have found that only about one car in about 10 where this actually works, but that doesn't seem to be an issue with Marc's car.
The blower seems to stop when the system has been in operation for a while, and then comes back on.  Since the blower will operate in "Defrost" it doesn't sound like the blower motor or the wiring directly to it is in question, but I would strongly suspect either the blower relay or the wiring connection at the relay might be the culprit.  I would suggest checking them first.  You should carefully unplug the blower relay (on the blower /heater/evaporator box near the heater water valve), and look at both the male and female connections as well as the actual wires to female spade connectors.
The issue of refrigerant leaks and compressor short cycling is another issue, but we need to isolate and solve one problem at a time.
Greg Surfas

[caddymanmarc]

from what i have read, i think i have 2 problems. one, the climate control switch maybe faulty. the blower does not come on when first starting up and the temp inside the car is well above 85 degrees. second is the blower relay and wiring to it. i have replaced the relay. the wiring coupler to the relay was shot. i could not find a wiring coupler anywhere, so i hook up each individual wires to the correct post on the relay. the blower motor still shuts off during operation. so I'm thinking i will try to locate a climate control switch to replace the one in the caddy now, and see if that cures both problems. if not, i will take the caddy to a pro next spring and see if they can figure it out. thanks for all your help!

[cadillactim]

I have a rebuilt Climate Control switch. You can email me directly at tagroves@wctel.net

Tim