Automotive Air Conditioning Information Forum

I am the third shop working with this beast. It has an A6 compressor with a compressor mount pressure switch. It has a single txv on the high side and some kind of remote sensing valve on the outlet of the evaporator. I think they keep a balance of some sort for the evaporator.

Some one broke the switch Lincoln calls an "A/C low pressure switch". The schematic show it being a switch on the ground side of the clutch coil. The first shop replaced it with a 4 Seasons low pressure switch rated at ON 43 & OFF 37. I have no idea what the original one was rated at. There is no other pressure switch on this system. It allows the clutch to come on initially but then starts cycling off and on at about 43psi or so.

If I ground the coil, the system stays working normally with the low side dropping down to 32-34 psi or so and staying there. High side around 115-125psi. It does fluctuate a bit which I put down to the two valves at the evaporator core.

I found how they broke the switch, they replaced the suction/pressure line with one that hit the wire connector and I think this is how the original switch was broken. It is a new compressor. It has been flushed by me, the o-rings all replaced, fresh oil in the compressor and the liquid line to the evaporator is new with a nice R134a high side fitting.

Any one got some advice for me to get this monster to work properly? Thanks.

The first thing that jumped out at me is the high-side readings, which seem very low. In that regard, what were the engine RPM and ambient temperature when testing the system? Was this converted to R-134a or is it still R-12? Does it have a full refrigerant charge?

Second thing is the LP switch limits: On 43/Off 37. In particular, the off set-point seems too high to me.

Third thing is the valve on the outlet of the evaporator. Does this system not have Ford's version of a GM POA valve?

Fourth thing is the A6 compressor isn't supposed to cycle. But when you grounded the clutch coil, it ran (cooled?) normally with good low-side pressures and very low high-side pressures? Doesn't seem like it would cool at all with only 125 PSI on the high side. If it's charged correctly, it could be a faulty compressor.

Sounds like the POA was changed to a POA eliminator set up. Do not forget you have in car temperature sensors. As mentioned, A6 compressors were not meant to be cycled. GM cycled them for one year and they found out what happens. Then they went to the R4 with larger clutch surfaces. This was around 1977. Tim knows where to get POA replacements.

This 79 LIncoln Continental is now is setup for R134a and that is what I charged it with after the flushing and o-ring replacements. It got 2.75lb R134a for the testing and 8oz of PAG46 oil. There is no sticker on this car for charge amount, I am guessing at 3 lb or so R12. It does not have rear A/C.

Ambient temperature is around 68F or 18C when I was testing. Super heat goes from 34F to 50F at evap core. Then there is some remote valve on the outlet with a sense bulb inside the box. There is a restriction of some kind there as the temp downstream from it drops to close to 15F and builds frost. Outlet temp. about 44F or 7C. The high side pressure bothered me a bit but since I do not have good information on this system, I don't know why the pressure did not go higher when left on continuous operation.

I have not looked under the dash for any other molesting that may have taken place over the years. This car is show room condition with only 15000km (9000mi) on it. The original complaint, as told to me, was a leaking compressor seal. I think this is where the new hose, compressor, oil contamination, and broken pressure switch happened.

send some photos of the under hood items near the evaporator. We need to see if there was a system change over.

This is the valve. I released the car to the customer until I learn what to do here.

Yes, the part in the photo appears to be a stock Ford POA valve (or Suction Throttling Valve, as we used to call them) from that vintage and not a POA eliminator. I wonder, though, whether it has been recalibrated from the original R-12 spec to that for R-134a. The pressures are different.

Although my sources don't list the original R-12 charge amount, saying only to refer to the under-hood decal (which in this instance is missing), my recollection is that these big, V8 equipped, 1970's-era Lincolns took as much as 4-1/2 pounds (72 ounces) of R-12 without rear A/C. If so, your recharge of 2.75 pounds (44 ounces) of R-134a is on the low side and should be more like 3.5 pounds (56 ounces). But 1979 was quite a while ago, and my memory might be off.

A low charge could be the reason for the low high-side pressures. At a 68-degree ambient in a system having a full charge, I would expect the high side to be around 150-160 PSI. A low charge could also be why the LPCO keeps cycling the A-6, as could an incorrect LPCO with the wrong set-points. But unfortunately, I wasn't able to find any specs for a LPCO for that vehicle.

Thanks John. I may try to see if one of the repair/rebuilders of A/C parts might tackle this one. I think the customer would prefer to keep this original. I learned a lot about this old critter so far and am not finished yet!

This A6 compressor has its pressure switch located toward or just under the HP port and I have some misgivings that it is not correct to have a low pressure switch located there. It seems uncanny to be right beside the outlet. I need to see the inside of one of these to verify the actual port is connected to the suction side.