Air con guy says my compressor is on all the time in my 12 cylinder xjs. He said it should not turn on without the fan working. How is the clutch turned off

 

There are different compressor circuits depending on year and model.

Until '89 or so the compressor clutch was, be design, energized whenever the climate control system was in use, period. Only when the mode switch was turned 'off' was the clutch de-powered.

Convertibles, in 1989 (maybe 1990?), had a different scheme. The compressor clutch stays engaged even with the mode control switch turned 'off'.

Later cars (maybe 1993 or so?) had a revised scheme with a vent feature that allowed the compressor to be disengaged from the control panel, I think.

Cheers
DD

 

ON Delanaire mark II, Doug, the compressor certainly starts as soon as the system is switched on and the water is up to a certain temp in the unit. The compressor then kicks out once the required cold temp is reached in the interior unit cold side. Kicks in when it climbs etc etc. The compressor does not actually run all the time. I expect you knew this, but the above is a bit ambiguous.
Greg

 

Actually, I beg to differ !

The system is designed so that all incoming air is refrigerated and dehumidified through the evaporator first and then heated as needed, thus the compressor is engaged at all times....even when full heating is called for. This is why you never see a Jag with fogged-up windows

Cabin temp is regulated entirely by blending refrigerated air with heated air. And, to help facilitate this blending, the heater valve remains open at all times except when max cooling is selected.

Any cycling of the compressor would be due the Ranco thermostat opening and closing the circuit to the clutch to prevent evaporator icing....not uncommon if the system is refrigerating really well !

The early Mk III systems functioned the same way as far a compressor engagement goes. Later versions were different, as mentioned.

Cheers
DD

 

Yes - and no! Perhaps fair to say the climate control system is on at all times, but that doesn't mean the compressor is engaged at all times.


Engaged to me means the clutch is powered and the compressor is turning all time, which isn't true. It will cycle on and off as needed to maintain the evaporator temperature, but that doen't mean the compressor needs to be turning continuously to maintain evaporator temp in all conditions. If I drove the car in -20C the compressor might never engage, as the incoming air is already cold.

 

Quite so Doug. I was referring to the fact that the unit in the car stops the compressor engagement when the correct temp in the cold box is reached
Also that is what I meant to express above. My only point is that, as Jagboi says below, the compressor is not actually engaged at all times when the system is switched on, it cycles so as to maintain the temp in the cold side.

Also, that is what I was trying to get at, Doug. The thermostatic switch in the cold box of the unit actually cycles the compressor on and off pretty frequently, at least on my car.
So we are agreed it seems! ?

 

Just what I meant to express, but you have said it much more clearly JB!
Exactly.

 

The clutch is switched on and off via the Delanair unit utilizing the 'sensor' on the rear of the compressor. This part does fail and does need service on a regular base. If it fails, it will tell the Delanair unit that the pressure isn't high enoung, possibly overpressurizing the system.

 

The distinction being evaporator temperature, not cabin temperature!

Cheers
DD

 

We're veering off course a bit.

The switch at the back of the compressor functions as a protection device. It isn't designed to cycle the compressor on/off in normal operation.

Until 1987-ish it was a "Superheat Switch" which worked in conjunction with a thermal fuse. If the freon level dropped the switch is triggered and melts the fuse. The circuit is then dead until the fuse is replaced....hopefully in conjunction with correcting the freon problem.

The Superheat Switch was replaced by a "HSLP Switch". High Side, Low Pressure. If pressure drops to xxx-level the switch opens and disables the circuit. This scheme doesn't use the thermal fuse. After rectifying whatever caused the low pressure condition the switch will remain closed at all times...until/unless the pressure drops too low gain, typically from loss of freon.

In either case this switch does not communicate with the Delanair unit

Cheers
DD

 

There are three temp sensors: cabin temp ( inlet in the tube above the glove box), ambient temp (in the RH blower case), and evaporator temp (capillary tube into the evaporator itself)....the last being the "Ranco" thermostat.

The first two are used to give the amplifier comparative readings for cabin temperature regulation. That is, mixing of refrigerated air with heated air. No control of the compressor takes place as a result of these readings.

However, the Ranco thermostat, which measures evaporator temperature (not cabin temperature), will open and close the compressor circuit. There is a single point (33.8ºF) when this occurs. Under the right conditions it is very possible for evaporator temp to hover right at the cusp of 33.8ºF and the Ranco thermostat will open-n-close the compressor circuit fairly frequently. I strongly suspect this is what's happening in your case. If for any reason the evaporator temp stays above 33.8ºF, the compressor circuit remains closed.

Cheers
DD

 

It dawns on me that I shouldn't have been so absolute in my wording only to contradict myself later. When painting with a too-wide brush you always end up splattering on yourself


Cheers
DD

 

Quite so Doug, just differences of description, the thing we are talking about is the same. By the "cold box", I was referring to the cold side inside the unit.