AC cutoff relay won't let compressor engage

[CAW]

Hi, My wide open throttle relay won't let the compressor engage. Occasionally the compressor will engage for a instant but won't stay on. I have checked the system pressure and it is in the normal range. The AC has worked intermittently with this problem in the past. So, i think that the AC system it's self is ok and this is probably an electrical problem. When i turn on the AC it just blows hot air ,the compressor will almost never engage, the engine will repeatedly start slightly revving and the WOT relay clicks as the engine revs.

With the AC on i will pull out the WOT relay or disconnect the wires to the compressor clutch. The engine stops repeatedly revving and the WOT relay stops clicking. But as soon as i put the relay back in or reconnect the AC clutch wires, the symptoms return and the clutch will almost never engage. When i disconnected the clutch wires i tested for voltage and it was present. But with the clutch wires in place i loose voltage after the WOT relay. Every time the relay clicks my volt meter will jump and then return to zero.

I can't figure out why the computer would be activating the WOT relay. I don't think that it is a problem with the throttle position sensor. Because I've not had any problems accelerating or driving. It has been hot and humid lately and i miss my AC. Any thoughts what the problem might be?

thanks

1997 4.0 4x4

 

[tbirdsps]

It seems to me that you have solved the problem but just didn't replace the part.

I could be wrong.

 

[Jason]

Hi, My wide open throttle relay won't let the compressor engage. Occasionally the compressor will engage for a instant but won't stay on. I have checked the system pressure and it is in the normal range. The AC has worked intermittently with this problem in the past. So, i think that the AC system it's self is ok and this is probably an electrical problem. When i turn on the AC it just blows hot air ,the compressor will almost never engage, the engine will repeatedly start slightly revving and the WOT relay clicks as the engine revs.

With the AC on i will pull out the WOT relay or disconnect the wires to the compressor clutch. The engine stops repeatedly revving and the WOT relay stops clicking. But as soon as i put the relay back in or reconnect the AC clutch wires, the symptoms return and the clutch will almost never engage. When i disconnected the clutch wires i tested for voltage and it was present. But with the clutch wires in place i loose voltage after the WOT relay. Every time the relay clicks my volt meter will jump and then return to zero.

I can't figure out why the computer would be activating the WOT relay. I don't think that it is a problem with the throttle position sensor. Because I've not had any problems accelerating or driving. It has been hot and humid lately and i miss my AC. Any thoughts what the problem might be?

thanks

1997 4.0 4x4

Go to Autozone and see if they use the bigger Actron scanner. You can use that one to view the data the ECM sees from your sensors. It's really easy to spot a TPS problem that way.

 

[CAW]

Are there any conditions, other than the TPS showing wide open throttle, that will cause the AC cutoff relay to activate? Just curious. I thought i read someplace that a bad engine temp sensor could cause it to activate. I will go by the parts store sometime and see if they have a code scanner that will show the TPS readings. I may just replace the TPS if they are inexpensive. Will report back with what i find.

 

[CAW]

just got done replacing the TPS. No luck, the WOT relay still clicks repeatedly when the AC is turned on and the compressor won't engage.

 

[gw33gp]

Have you tried replacing the WOT relay?

 

[Earl43P]

Sounds like it's low on refrigerant. As soon as the compressor engages, the low side pressure drops and the cycling switch opens (removing power to the clutch).

Disconnect the cycling switch and jumper its connector's pins. That will make it operate even if low. See if it quits cycling = add refrigerant.

If it really does have good pressure, the cycling switch may be bad. It can be replaced without deservicing.

 

[CAW]

i haven't replaced the WOT relay. But i did test it with a multimeter. The normally closed contacts read less than an ohm of resistance and the relay coil read about 50 ohms. I'm pretty sure that the relay is good. I even took out the relay and jumpered a paperclip in it's place (the normally closed contacts). The compressor still wouldn't come on. After that, I cheched again to make sure that the pressure switches were closed. so far, this has me stumped...

 

[CAW]

It seems that i have fixed it. It was a bad clutch cycling pressure switch. I measured the switch and it had 7 ohms of resistance. Probably corroded contacts. This caused a voltage drop in the circuit. But apparently not enough of a drop to keep the computer from sensing that the AC was on. It is my understanding that the computer activates the WOT relay for a few seconds after the AC is turned on to allow the engine to speed up to compensate for the added load. This would explain why i could feel the WOT relay click. When the relay was closed again the computer would not be sensing full voltage and think that the AC was off? The computer senses the voltage right before the WOT relay. I think that the computer was expecting to sense either zero volts or full voltage. Any voltage in between ,as i have experianced, can make the computer do strange things.

Thanks for the replies.

 

[gw33gp]

I had an A/C clutch relay go bad and the A/C would not kick in at all. That was a pretty simple diagnosis.

You had a more difficult one and it is good you found the problem. I will keep that in mind for the future. Sounds like you have a lot of determination and take the time to think out the diagnosis process.

 

[CAW]

I almost didn't catch it. I initally assumed that the switch read a few ohms because i wasn't making good contact with the test leads on my voltmeter. In a 12V circuit a few ohms of resistance can have a large effect on the current the circuit will draw.

 

[XLTsplash]

My 1997 Ranger was doing the same thing. The wide open throttle relay would open and close, the idle speed would fluctuate and the A/C clutch wouldn't engage. Mine was also caused by the cycle switch.