1974 Coupe deVille A/C problems

[gcasamayor]

I have an issue with the a/c compressor, it does not engage when I turn on the a/c.  I noticed that there is a thermal fuse or thermal limiter on the passenger side just in front of the firewall.  It connects to a harness with 3 prongs.  When I replace this fuse the compressor engages and the a/c works fine, however, it blows this fuse after a few minutes of operation.  The wiring looks good.  What could be causing the thermal limiter to blow?

["Cadillac Kid" Greg Surfas 15364]

Thermal fuse is activated by superheat switch on compressor when the refrigerant charge is low. Check the refrigerant charge by looking at the site glass either on the dryer up next to the condenser or on the  VIR next to the evaporator depending on whether you have an early or late production 74.
Greg Surfas

[gcasamayor]

Thank you, will check there next.

Greg Casamayor

[TJ Hopland]

On these is the superheat switch a single terminal that gets grounded or a 2 terminal?    Either way I would think you could connect a volt meter set to volts or maybe even a test light across the switch to monitor its state. 

Under normal (correct) conditions you would see the light light or show voltage on the meter.   If the switch is activated due to a low pressure condition the light will go out and or there will not be a voltage showing because the now closed switch will be conducting all the voltage.  I don't know how much current it takes to heat the fuse so its possible a test light could be too much.   A digital volt meter should be safe because they barely put any load on.

This doesn't do anything to correct the problem but should save you from blowing those fuses. 

I didn't understand why they did this at first but its basically a time delay buffer damper sort of thing.   Its normal for there to be short pressure drops in the system just due to all the varying conditions and for many reasons they didn't want the compressor clutch dropping out for every little blip.

Most electrical fuses work and get 'heated' by the power going through them.  Too much power and they basically melt and break the circuit.   This is different in that its not the power for the clutch in this case that is doing the heating.  There is actually an electrical heater in there hooked to the super heat (low pressure) switch.    So low pressure switch closes and it starts heating.  If its closed too long it heats the fuse to the point it 'blows' but instead of cutting the power to the low pressure / super heat circuit its cutting the clutch power.         

[gcasamayor]

Thanks TJ - that makes a lot of sense.

Greg Casamayor

["Cadillac Kid" Greg Surfas 15364]

FWIW the super heat switch is exactly that.  A thermal element that senses the temperature.  It is located in the high pressure side of the compressor and when the temperature of the sensor is "X" degrees hotter than saturation temperature at the pressure the switch finds itself it grounds, giving the thermal limiter a completed 12 volt circuit along its time delay fuse.  If the condition exists for "X" length of time, the fuse blows and power to the compressor clutch is cut off.
Greg Surfas

[TJ Hopland]

Interesting.  I thought it was just pressure.  I guess if it was just pressure there would be no reason to locate it in the compressor.