a/c won't turn off

[Rhett]

I need some help from you guys who know a little about a/c. The a/c won't turn off on my truck. The compressor runs all the time now. It all started when I used a little defrost the other night, and as you probably know, the a/c compressor is wired in to activate with the defroster. Well, evidently the a/c compressor never shut off when I turned off the defroster, and now it's on all the time, as soon as I start the truck, even though I have it set off on the controls. I mean I can turn it to 'off' and the compressor is still running. At least the blower shuts off when I switch it off.

91-94's use a cable in the dash to connect the switch to the control I believe. 95+ is different.
Is this a/c control cable out of whack or is it some sort of switch that is out?

 

[dogfriend]

You might not like this reply...

I'm not an a/c expert but I know that the a/c compressor clutch is controlled by the computer. It sounds like your compressor clutch is staying engaged all of the time. Could be a computer issue.

You could try to verify by unplugging the connector. If that helps, I would try resetting the computer by removing the battery cable.

Another thought is that it could be a frozen a/c clutch.

 

[Crankcase]

The compressor is supposed to shut off in "Floor" mode also. You should try that and see if it is indeed a control issue.

 

[Rhett]

It won't shut off in 'floor' mode either. No matter where I put the lever, the compressor never shuts off.

I am going to reset the computer tonight to see what happens.

Isn't there a solenoid on the compressor? If so, maybe this is stuck?

 

[dogfriend]

uh, check that

I started looking at my factory manual (97, but the basic a/c system is much the same).

I was only partially right, the computer controls the a/c clutch but only to turn it off by grounding the WOT relay. It does this in two situations: wide open throttle (WOT) or if it detects an overheating problem thru the engine control sensors.

The a/c clutch cycling switch is what turns the compressor on. It is located on top of the accumulator/dryer. I would check for a short across the terminals of this switch. It should only be on when the a/c is on and the system pressure at the accumulator/dryer is in the correct range(between 21 - 47 psi for my system)

You could pull the connector off the a/c clutch to test. If the compressor goes off, then it is most likely the cycling switch. If it stays on then your a/c clutch is frozen.

Hopefully it is the switch. If it is, you can replace it without evacuating the system because it is mounted on a Schrader valve.

 

[Rhett]

Just for kicks I did the computer reset and it still stays on. I know the major diff between 94 and 97 a/c systems is in the electronics. Mine uses crude cables on the controllers whereas on the 95+ it's electronic. Luckily 94 was the first year to use R-134a.

Thanks for the tip about the a/c clutch and the switch. I know about the switch on top of the dryer. I will pull it's electrical connector and see if that shuts off the compressor. From what you say, I assume that if the compressor keeps running after I pull the switch, then the compressor itself is stuck on. That would be bad for me as you know $$$$$$$
If its the switch then hopefully it will be a $10 or $20 part.

 

[Rhett]

I just got home from work and pulled the power from the a/c clutch switch. Sure enough, when I pulled its power, the compressor shut off. When I put the plug back in, the compressor clicked in and started running. It looks like it is the a/c clutch cycling switch and not a frozen compressor. Now I gotta go to AutoZone and try to find this switch.

I noticed some blue liquid inside the plug to the a/c compressor cycling switch. Is this leaking R-134a or just condensation? It looked blue to me and I thought R-134a was colorless. It looked like windshield washer fluid but it can't be since that reservoir is on the other side of the engine compartment. Condensation on the a/c pipes is one thing, but I think it should be dry inside where the switch's plug plugs in.

 

[Rhett]

Ok, I just replaced the a/c compressor cycling switch ($17), and the compressor still cycles off and on like it was before. At least I have a shiny new switch. hehe

Since the compressor can turn off and on, the compressor must be ok.

Could this just mean the system needs a recharge, or could the control cable be stuck under the dash?

 

[dogfriend]

If it is cycling on and off, I think that means that your system pressure is either too high or too low (most likely low) because there is a switch for each of these. The purpose for the low pressure switch is to protect the compressor. I think that the one you replaced controls the high pressure.

One thing you could try as a long shot...Once a had a vacuum leak on my Ford Ranger, and after I fixed the leak the a/c compressor started cycling rapidly. I reset the computer and everything went back to normal. It probably won't work, but it won't cost anything to try

 

[dogfriend]

I reread your last post a couple of times, and I think that I misunderstood it.

Are you saying that the compressor is staying on all of the time(when connected) like at first? Or is it cycling on and off now?

If it is still staying on all of the time ignore my last post because I misunderstood. Is the A/C indicator light on? (on the heater control) The schematic in the Haynes manual shows that there are two switches that can send power to the a/c cycling clutch. They are both built into the heater control. One is the a/c switch that turns on the a/c when you push the knob, the other is labeled the defrost switch and I am assuming that it turns on when you move the mode lever over to defrost. If one of these two switches is closed then power is sent to the compressor via the cycling switch. I would check this next.

Sorry that the switch didn't fix it. At least it isn't the compressor clutch

 

[jimbo74]

is the blue stuff in the connector thick like a dielectric grease or runny like water?

 

[DaleG]

If you replaced the low pressure switch and the compressor is still staying on all the time then check your WOT relay in the main fuse box under the hood. This relay also tells the compressor clutch to turn off and and on, sounds like yours might be stuck. There are 3 or 4 relays in there that are identical to it so you may be able to just swap relays around to test to see if that is the problem.

Dale

 

[taxx]

Check your pressure. It will run all the time if your pressure is low. If it is way too low it won't run at all. If the pressure is fine you can just put a switch on the sensor that connects to the compressor. If that is open then the compressor will not run, same as if the sensor was removed. That is what I did, I put a switch right below my A/C selector, I know it isn't right but atleast I can turn my compressor on and off when I want.

But seriously check your pressure.

 

[Rhett]

thx for this info, I appreciate it bigtime

J. Abena: it was runny like water. It wasn't like any dielectric I've ever seen. Maybe it was a mix of dielectric that was put on the switch and condensation water. ??

Dogf. : Correct me if I'm wrong, but doesn't the a/c compressor always cycle on/off periodically? I thought it was only if it was cycling excessively that you had low pressure. Anyway I will have to do a pressure check.

Basically, what I meant was, after I replaced the a/c clutch switch, there was no change in the way the compressor was operating. The problem is, the controls don't do anything.

The a/c switches you mention..the one that turns on the a/c when you move the lever to defrost is stuck, I'm bettin money on it. I say this because it all started when I moved the lever to defrost the other night, and the a/c compressor kicked in like it should, but since then, it's never turned off, no matter where i move the lever. I can press the a/c button switch and the light turns on and off like it's supposed to but it doesn't turn off or on the a/c. I need to locate the defroster a/c switch. I have a haynes but I'm not sure if its in there. Is it on the firewall or will I have to tear into the dash (ugh).

Right now I'm driving around with the power to the clutch switch disconnected until I figure this thing out. If I don't it will run all the time.

daleG: i reset the computer hoping that, if it was a relay, it would flip it over. That didn't work. I will test the relays too.

 

[dogfriend]

Yes the a/c should cycle on and off, that is the purpose of the a/c cycling switch..

I misread your previous post and I thought you were saying that it was cycling on and off after you replaced the switch, but you were actually saying that the switch made no difference. If your pressure is too low I believe that it will cycle on and off much more rapidly than normal because the low pressure switch will turn off the clutch when the pressure is too low, then back on as pressure rises etc.

I am almost willing to bet money that the switch that turns on the a/c when in defrost mode is stuck closed. I think that it is probably part of the heater control switch assembly; the schematic in Haynes implies that it is part of the control switch. I would look at that assembly and look for a switch that is somehow attached to the mode lever.

 

[Rhett]

I agree, I think it's that switch on the defroster. For 2 reasons 1) it all started after I moved the lever to defrost that time, and 2) see below

I just tested pressure of the system: 30 psi right on.

then I swapped identical relays: ABS relay (# 620) and the A/C WOT relay ( also a #620). Guess what? The ABS light came on. So I know something is not right with the relay, orthat circuit anyways, since the relay is stuck on.

Or is it possible for a bad switch (like the defroster a/c switch) to trip the relay? Maybe the relay is ok but it will remain open until I replace the defective a/c switch?

 

[Rhett]

Oh yeah, I forgot to mention, after I switched the relays around, it really didn't change anything. The a/c controls still do nothing, no matter where I move them, the a/c runs all the time. Gotta be that defroster a/c switch.

 

[dogfriend]

this is how i think it works

I looked at the schematic in my Haynes manual which is generic for the 4.0L OHV. My interpretation is as follows:

Power for the A/C comes from a 10A fuse which is on when the key is in the on position (engine running). The circuit supplies power to two switches: the a/c switch (push knob) and the defrost switch (it is not clear how this switch is activated, but it appears to be part of heater control panel). Also, if the a/c knob switch is on, the light on the panel should be on.

If either the a/c or def switch is closed, power is supplied to the a/c cycling switch. (The one you replaced) If the pressure is in the correct range, the cycling switch will close. If the pressure is either too low or too high the cycling switch should open.

When the a/c cycling switch is closed (and the a/c sw or def sw), power is supplied to the WOT relay. The relay should be closed unless the PCM (computer) grounds the relay to open it. This should only happen during WOT or if overheating (the overheating may only be true for newer models, I'm not sure about yours). The relay then supplies power to the A/C clutch which then turns the compressor. (Ever heard of Rube Goldberg? )

I'm now willing to bet $5 that either the a/c switch or defrost switch is stuck closed, because your a/c clutch is getting power when it should be off. One of those two switches is where the power must be supplied from.

 

[Rhett]

Yes, that's the conclusion I have too. The stuck switch between the control panel and relay is causing all this.

Looks like this weekend I am going to have to tear apart the dash and pull the heat/ac console and trace the connectors to the switch. I hope I can find a replacement switch at some auto parts store.

 

[dogfriend]

alternate source

On my Ford Ranger, the switch for the fan went out. I tried to get it at the dealer and they told me something like 6 wks. I went to the wrecking yard and got it cheap.

The Ranger had the same dash as yours. I don't recall it being too much of an ordeal to pull that control, but it was a few years ago so I don't remember any of the details.

The Haynes manual has pictures of the control assy removal.