A/c blower problem

[albjerryg]

I just read the thread on A/c Heater blower problem. I have the same problem the blower only works on defrost but also my compressor is not kicking on the magnetic clutch is not activating.  I have a 1974 Cadillac Deville .Is there a way to apply 12 volts to the clutch to see if the A/C is working. Thanks for any help Jerry.

["Cadillac Kid" Greg Surfas 15364]

First step, check the fuse under the dash.  The one labled AC/Heater.
Greg

[albjerryg]

Checked all the fuses and  they are ok. jerry

[Tim Groves (Cadillac Tim)]

Check your low blower relay and connections.  Also sounds like your heater turn on switch is bad.  This switch is located in the cylinder head directly behind the alternator.  To test it, set the dash control on Auto, then unplug the black wire from the heater turn on switch and ground it to the engine.  See if the blower runs.  If so, that switch is bad. 

You can put battery voltage directly to the a/c compressor to see if the clutch works.  Check the thermal fuse and the compressor switch located on the evaporator case.  If either is bad, no power can get to the compressor.  Tim

["Cadillac Kid" Greg Surfas 15364]

Jerry,
The compressor on your system should run regardless of whether the engine temperature switch is "made" or not.  It is on a direct circuit and is energized (or supposed to be) whenever the control head is calling for AC operation.  It does seem like you have a couple of issues to deal with.
I am going to suggest that you first of all, check the electrical connections at blower relay, the programmer (under the dash, below the love box, and the control head (drivers side control).  I suggest you gently separate the connections, check the male and female ends for poor surfaces due to corrosion, dirt, or damage, and then re connect them.
This is the first step.
If the AC system has or had been run while short of refrigerant you may have "popped" the superheat fuse, but we will get to that after you check the connections and get back to us.
Greg

[Tim Groves]

The a/c compressor is designed to run in any position except Off and Vent (or Economy depending what dash control is in your 74).  However, the compressor WILL NOT ENGAGE if the outside air temperature is below 32 degrees.  This is what the compressor switch does.  Contacts in this switch are open below freezing, and close above freezing, allowing power to go to the thermal fuse.  Sometimes this switch goes bad, then power can't get to the thermal fuse or the compressor regardless of the temperature. 

Whether the compressor engages or not, has no bearing on whether the blower fan comes on.  What controls the fan are three grounds.  These grounds are the heater turn on switch, the in car switch (mounted in the dash control), and a direct body ground when the dash control is in Vent or Defrost. 

Since your fan won't come on in Vent (or Economy), I suspect you have a low blower relay problem (or connection at the relay).  The ground circuit in Vent is through the dash control, so there is apparantly an absence of battery voltage.  The Defrost blower gets power from the auto relay, not the low blower relay.  Of course you still may have a bad heater turn on switch, which will prevent fan operation in Auto settings.  Tim

[albjerryg]

The wire to the Heater switch(behind the alternator) was disconnected so I connected it and now the fan works. The compressoe clutch would engage when I put 12 volts across it. I had to quit because my battery went dead. the next day I will check the connecters under the dash. Thanks for your help..Jerry Guillotte

["Cadillac Kid" Greg Surfas 15364]

Jerry,
Good for you.  I have found that loose or disconnected, connections can account for 90% of the problems with an older ATC.  Usually people change parts first without checking and in the very act of changing the parts they "fix" what was wrong, a bad connection.
If the refrigerant in the system is low, I would check to see that the thermal limiter fuse( on the heater box, next to the relays on the passenger side of the engine side of the firewall.  You can pick one up for a couple of bucks at almost any auto parts store. BUT, check the connections first.
Greg

[albjerryg]

Checked the connectors could not find any obvious problem. I jumped the battery across the compressor clutch and activated it , checked for cold air but it was not cool. Thanks for your help. jerry Guillotte

["Cadillac Kid" Greg Surfas 15364]

Jerry,
It sounds like your refrigeration system needs to be checked.  First of all, check to see if there is ANY presure in the system. If not, you will need tyo find the leak, repair it, rep[lace the dryer, evacuaate and recharge the system.

If there is 25 to 30 pounds of pressure in the system with the system off, you might be able to just bring the refrigerant charge up AFTER you replace the (probably) blown superheat fuse/switch.
\Greg

[albjerryg]

Caddillac Kid Greg you were right on the system was out of Freon and blew the thermal fuse and the dryer was leaking. Thanks for your Help Jerry Guillotte