Air Conditioning will not turn on. WOT Relay related

[Nobody]

[Resolved] Air Conditioning will not turn on. WOT Relay related

The a/c will not turn on in my 1997 5.0L Explorer. I've narrowed the issue down to the WOT relay being activated by the computer all the time. If I unplug the computer, the clutch cycles with the dash switch just fine.

The problem is not the WOT relay or the a/c wiring. It seems to either be the computer itself or something telling the computer to activate the WOT relay.

When you first turn on the key with the a/c switched on, the clutched does briefly engage, then immediately disengages when the ECM activates the WOT relay. I popped the cover off the WOT relay and I can manually deactivate the relay and a/c clutch engages, so I think that pretty much confirms the rest of the circuit is fine.

I did try unplugging the TPS sensor and resetting the computer but that didn't change anything. I have not had the chance to test the TPS yet, but I suspect that's not the issue.

Anyway, before I throw money at a new ECM, I'm just curious if anyone else has experienced this issue and what the resolution was.

Does anyone have a schematic handy?

edit: and no check engine light or codes on DashCommand

 

[SoHK_alumni]

The PCM gets it's input from the A/C low pressure switch and A/C high pressure switch and TPS to control the WOT relay.

The relay is wired normally closed and the PCM will open the relay if either of the pressure switches is open or if the TPS signals a wide open throttle.

So unplugging the PCM from the body disables the WOT relay and the a/c cltuch will cycle with the dash switch.

The first thing I'd try is to jump both the high and low pressure switches. I think it's a far better bet one of those switches has failed than the PCM has.

Both of the pressure switches can be changed with the refrigerant still in the a/c system. No need to go to a specialist or worry about blowing the charge when replacing them.

 

[my98nnj]

Did you lose the refrigerent charge? The switches might be doing their job...

 

[SoHK_alumni]

Did you lose the refrigerent charge? The switches might be doing their job...

Good point!

 

[Nobody]

The PCM gets it's input from the A/C low pressure switch and A/C high pressure switch and TPS to control the WOT relay.

The relay is wired normally closed and the PCM will open the relay if either of the pressure switches is open or if the TPS signals a wide open throttle.

So unplugging the PCM from the body disables the WOT relay and the a/c cltuch will cycle with the dash switch.

The first thing I'd try is to jump both the high and low pressure switches. I think it's a far better bet one of those switches has failed than the PCM has.

Both of the pressure switches can be changed with the refrigerant still in the a/c system. No need to go to a specialist or worry about blowing the charge when replacing them.

This morning I bypassed both the hi and low switches individually and at the same time. Symptoms did not change.

If I understand correctly, the WOT relay is also used to cycle the compressor in normal operation?

Now that I thinking about this, I didn't bypass the switches with the engine running, so I guess it would make sense that the ecm disengages for startup. I'll have to check again this evening.

And for further info, I did take the explorer to the shop when the a/c quit working a few months ago. I had assumed there was a leak and didn't even try to diagnose the problem. They diagnosed a bad clutch and said the compressor needed to be changed, but pressures checked out OK. I took it home and adjusted the clutch air gap and the a/c worked great up until last week. When it quit again, I assumed it was the clutch again and replaced the clutch. The old one was in really bad shape!

This time, the a/c quit just as we were returning home from a long trip. It started blowing warm/cold, then just warm a few trips later. Just as if the clutch was failing, which I assumed it was.

The a/c does blow cold when I manually toggle that WOT relay. Hopefully this evening I'll have some more time to look at this and test the TPS.

Can someone confirm what is supposed to happen when you have the a/c switched on and then turn the key on without starting. Does the clutch stay engaged, or engage and immediately disengage?

 

[Nobody]

By passed both switches and it didn't change anything. TPS seems to check out. I'm really stumped here. It really seems the ECM is the culprit. I'm generally the last one to suspect the ECM for most issues.

 

[SoHK_alumni]

I think in your position, I'd consider simply bypassing the PCM for the a/c clutch relay control.

Use the original pressure switches to control ground to the relay, rewire the relay to be normally open and the power from the dash switch for both the relay coil and switched power to the clutch.

A number of years ago a Dodge Ramcharger I had displayed similar issue, the ECU shut down the a/c. Bypassed the ECU and got through the summer before the ECU itself failed. Replaced the ECU, backed out my bypass and the a/c worked perfectly until I sold the truck. I'm pretty sure the A/C shutdown was simply the warning shot of the impending ECU failure.

Still, I'd bypass the PCM a/c control rather than replace an otherwise working box.

 

[Nobody]

I'd really rather not hack it. I want to find the problem and fix it properly. Up here in the PNW we can survive without a/c for short durations since it doesn't generally stay hot for too long.

Anyway, I found some schematics in this thread
http://www.explorerforum.com/forums/showthread.php?t=243407

In the schematic below, I've got 12v at WOT relay pin 85 whether the a/c is switched on or off. WOT relay pin 86 is being grounded somewhere in "Engine Controls". I need EVTM pages 23-5, 24-5, and 25-2. Anyone have those handy?

For WOT relay operation, I'm not sure if it's being switched by input voltage or the ground side in "engine controls", or both...

 

[Nobody]

Dug up my t-bird 5.0L EVTM. It's slightly different, but it appears both sides of the WOT relay go to the ECM. I'm guessing the ECM is switching the ground side of the relay. So that puts me back where I started, either the ECM is defective, or some other input is telling it to disable the a/c.

Since there are no codes or symptoms of another problem, I have to assume its a bad ECM.

 

[Turdle]

Might be as simple as unplugging the pcm connector and plugging it back in to reseat it?

Earlier it was suggested to jumper the ac compressor clutch directly. This isn't permanent, only troubleshooting tip to see if the clutch even works.

 

[Nobody]

As I mentioned in an earlier post, I did unplug the PCM. I can manually bypass the WOT relay by jumping pins 87A/30 and the a/c cycles correctly and blows cold.

In the research I've been doing, the PCM energizes the WOT relay for various reasons such as the brake being pressed to wide open throttle. In all situations, the relay should only be energized for a short time ~5-30 seconds I think.

 

[2000StreetRod]

normally closed WOT A/C relay

Your WOT A/C relay is normally closed (mine is normally open). The PCM energizes the relay by "grounding" pin 85. Power is supplied to pin 86 whenever the PCM power relay is closed. When you measure pin 85 voltage to be +12 that means there is no voltage drop across the relay coil and the relay is not energized. If you measure 0 volts on pin 85 then the PCM is energizing the relay by grounding the pin and the A/C compressor clutch solenoid is disabled.

When there is +12 volts on pin 85, and + 12 volts on pin 30, there should be +12 volts on pin 87A. If not, the relay is bad and replace it.

 

[Nobody]

In my post above I had mixed up pin 85/86. I had 12v at 86, and 85 was being grounded full time by ECM. I verified there were no shorts on the pk/y wire to the ECM which confirmed it was the ECM causing the WOT to be energized full time. Not having anywhere else to turn, I ordered an OEM ECM on ebay for $50. Received it today and it resolved the issue!

 

[SoHK_alumni]

Getting your A/C back in August is a GOOD THING!

Yea, the PCM is not the likely suspect with this problem, but that doesn't mean it can't be the culprit!

 

[JonV]

Nobody, I have the same vehicle and the same problem with my vehicle. Did putting in your new PCM solve this problem permanently or did it eventually fail again after the PCM adjusted to the vehicle? I just don't want to put the money into it if it will fail again.

 

[mhersh2356]

Where is the WOT relay located? Mine shuts down at WOT. Seems like it would be an easier replacement than the whole ECM as a first try. Thanks!

 

[2000StreetRod]

The A/C relay is located in the Battery Junction Box (relay position 2).

 

[fastpakr]

Where is the WOT relay located? Mine shuts down at WOT. Seems like it would be an easier replacement than the whole ECM as a first try. Thanks!

It sounds like it's doing exactly what it's supposed to in your case.