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I have a 78 Lincoln 5 that the a/c runs constantly on. Air…

Customer Question
I have a 78...

I have a 78 Lincoln Mark 5 that the a/c runs constantly on. Air is cold but I never see the compressor cycle. What controls the cycling of the a/c clutch?

Mechanic's Assistant: Has the clutch been slipping? Are you having any trouble shifting?

A/C clutch on the compressor

Mechanic's Assistant: What is the model of your '78 Lincoln?

1978 Mark V

Mechanic's Assistant: Are you fixing your vehicle yourself? What have you tried so far?

Yes, I just replaced the compressor assembly and charged the system. It's almost working too good. I can turn off the compressor by switching off the system so I know the a/c clutch is not dragging. I also evacuated the system after that and the compressor still runs. Does this system have a low and high pressure cutoff switch?

Mechanic's Assistant: Anything else you want the mechanic to know before I connect you?

I don't see where these switches are if this system has them

Submitted: 7 months ago.Category: Classic Car
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Answered in 10 minutes by:
1/15/2018
Classic Car Mechanic: George H., Technician replied 7 months ago
George H.
George H., Technician
Category: Classic Car
Satisfied Customers: 19,362
Experience: Old enough to have worked on them when they were new
Verified

Hello I will help,

You do not have pressure switches on this system. It uses a TXV (Thermostatic expansion valve) which opens and closes according to the evaporator outlet temperature to throttle the flow of refrigerant.

Was this system converted to R134a ?

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Customer reply replied 7 months ago
the system has been idle for some time so I only added 2.5 lbs of 134 to a system that asks for 4+ lbs of r12 just to run the system and listen and watch. I guess I thought I would see lots of cycling because of the lower charge.
Classic Car Mechanic: George H., Technician replied 7 months ago

No cycling with this system, if the engine overheats it will turn off the power to the compressor but otherwise the compressor runs when the controls are on.

To convert a system it should be recovered, flushed, 80% of the original charge and PAG oil installed after the installation of a high pressure safety switch.

Let me know if you have further questions, if nothing further please complete the rating

Thanks

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Customer reply replied 7 months ago
All good information, thank you. What are the chances you might know who would offer such a switch kit?
Classic Car Mechanic: George H., Technician replied 7 months ago

Click here for procedure

Click here for wiring diagram

Click here for switch

That is one source. Depending on you plumbing there are others. You will cut the wire to the compressor and put one end to one side of the switch and the other to the other side. The switch should open between 300 and 450 psi and they come in different plumbing fittings so you have to work out what will fit on your high pressure line.

You did install new valves?

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Classic Car Mechanic: George H., Technician replied 7 months ago
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Classic Car Mechanic: George H., Technician replied 7 months ago

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