A/C Experts, I've got a real stumper!

[The_Eric]

Hi all, the boss and I have been working on a 1995 Mercury Tracer off and on for a while now. The customer complaint was that in cold weather the windshield would fog over and he didn't think the compressor was running.

When it was just my boss doing the work, he replaced many things without checking, instead going for the "low hanging fruit" - the common things. He also consulted tech boards like IATN and Identifix.

Things he did/replaced:
Checked fuses
Checked A/C pressure in shop
Replaced low pressure switch
Replaced high pressure switch
Replaced cooling fan relay/switch
Replaced A/C relay
Replaced WOT cutout relay

He also realized that the problem is temp related. Works fine in warm temps (35*-40*+) but not cold temps.

It was at this point that I wanted a crack at it. With schematic in hand, I started checking it out. I determined that the negative signal from the control head to the low pressure switch wasn't passing through the switch. Why? A/C system pressure was too low. Boss checked outside and it was near zero. Added almost 2 cans of freon. Problem still existed. Adjusted the low pressure switch (cycling switch) so it would come on at lower pressures... Then it wouldn't cycle.

And that's where we're pretty much at. Either the A/C and defrost works at normal temps and cycles, and doesn't work at low temps, or you adjust the low pressure switch, then it engages at low temps but won't cycle.

After my wire chasing and finding that the low pressure switch was not closing due to low pressure, he replaced the receiver, high pressure line and expansion valve. Problem still persists..

Now admittedly I know very little of A/C theory and how it operates, but I can follow a schematic and chase wires fine. It appears that wiring and related components are correctly functioning - either because they were tested, or randomly replaced. Everything that I see points to the A/C system pressure being too low to close the contacts on the low pressure switch, thus the PCM doesn't see the input it needs to tell the WOT relay to kick in the A/C clutch. Boss says that pressure are correct, and at room temp, I'm inclined to believe him. But every time the compressor won't kick in I can pull the wiring connector off the low pressure switch and the contacts will be open. That to me says that the pressure is too low when cold.

I know that I'm probably jumping around a bit and I'm sure there is some info left out - probably because I've been over it so many times and am taking certain things for granted. Feel free to pick my brain. If need be, I can scratch out the schematic... It's been permanently burned into my brain!

 

[tightwad]

just a shot in the dark. is this a temp-setting system or manual? If temp-controlled, is there a bypass circuit to kick in the compressor for defrost duty when it's cold?

 

[The_Eric]

Yes, critical info I omitted. This is a manual system. If there is an ambient temp or other temp related circuit, it is not shown on any schematic thus far...

 

[mechanicx]

Quote:
He also realized that the problem is temp related. Works fine in warm temps (35*-40*+) but not cold temps.



I'm probably missing something, but it sounds like the A/C is working correctly

. The compressor is not suppose to run at temperatures below ~40F. You most likely already know that, but I don't see the malfunction here

. At lower temps the car owner should run the heat and defroster to defog the windshield. He may have have leaves in the air intake holding moisture or maybe a slightly leaky heater core or something like that condensing up the windshield.

 

[Texan4Life]

agreed. all A/C service literature I have seen says to perform A/C work in 50F or higher ambient temp.

low pressure switch is a safety for the compressor. Both if the ambient temp is to low or if it is low on refrigerant.

No matter how much you add to the system it wont help because at those temps it is mostly all a liquid. And forcing the compressor to run could damage it from lack of oil or worse have it draw in liquid refrigerant. not to mention it would be overcharged.

 

[Chris142]

First. R134a systems are very picky when messing with the charge amount. Adding "2cans" is not the correct way to charge the system. It needs to be evacuated, vacuumed, oil and dye added if needed and charged With an accurate scale. If you continue to do AC work this way you will just keep beating your head on a wall.

Sounds like you still have a leak. If you had a charging and recovery station (and a required refrigerant identifier) you could remove the refrigrant and immediately know if it has lost any or not.

 

[Chris142]

Ps. Think of ac work like doing alignments. if you have a customers car with a pull you can replace things move shims whatever you want to do to try to correct the problem or you can put it on the alignment rack and find the bent frame.

Same with doing ac work. You would not do alignments w/o the proper tools so why try to do ac work w/o the proper tools.

That car will have the same static pressure if it has 1oz in it or 2 lbs.

 

[The_Eric]

Sadly no, I really don't know much of A/C systems, other than some topical knowledge and their controls. A/C is something that I've yet to delve into.

The thought had occurred to me that everything was normal, but not based on any real knowledge, just gut feeling based on everything being more or less normal. I have been going off of the complaint and the input from my boss and coworker.

 

[The_Eric]

Chris, the boss added two cans based on reading cold pressures. I'm guessing this is incorrect, but I think he was grasping?

He does have a machine and has vac'ed and added the correct amount twice. You can add the listed amount and set it outside and get the same result. Only stating to clarify...

 

[mechanicx]

That's understandable. Sometimes you get handed off something by others and involved with it and don't see the forest for the trees.

 

[Chris142]

Once the underhood temps go up the pressures will go up and the system will work as designed if everything is working right.

Sorry for being so abrupt. I get calls all summer long
From people with no ac knowledge asking for my help. Usually they have already added 3 or more cans and wonder why it still wont work.

 

[TFB1]

This thread is a good example of why I do all my own trouble shooting...

 

[The_Eric]

Well like I said, I only involved myself thinking that this was an electrical/controls issue. After some basic trouble shooting, it was determined that it wasn't. Now I'm pretty much out of my league and only going off the boss's input which I don't trust entirely... Hence the reason I'm here.

So, I'm guessing that by the sounds of things, we're chasing our tails here? Guess it's time for me to bone up on my A/C knowledge...