A/C Compressor clutch not getting power

[OG Gearhound]

You should have 12 volts when checking between a good ground and both/either wires at the pressure switch if the switch is working correctly. It's a switch, so 12v+ in and 12v+ out when the system has sufficient pressure. Unless the contacts are actually burned out (unlikely) in the WOT relay, it will have nothing to do with the AC not working, as it is a normally closed relay.

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Thanks for this link! I get what you are saying, but shouldn't me jumping/bypassing the pressure switch socket give me voltage to the a/c clutch through the WOT?

 

[RobbieD]

Thanks for this link! I get what you are saying, but shouldn't me jumping/bypassing the pressure switch socket give me voltage to the a/c clutch through the WOT?

Yes, it should, unless you have an open circuit between the pressure switch and the AC clutch coil (i.e.- a wire break or bad connection).

Look carefully at the 2-pin connector that plugs into the AC, as I've seen the connections fail here where the wires are crimped into the terminal (it's not hard to unlock and pull out the terminals from the connector body). Another place to check is the back of the terminals under fuse/relay box where the WOT relay plugs in. It's not uncommon for wires at these terminals to corrode there on these old trucks.

Otherwise, you're looking at following the AC clutch coil wire from the point that you see +12 volt (key on, AC on) and testing for voltage as you go. You an use a straight pin to pierce the plastic insulation, for testing if needed, but tape up the prick hole afterward if you do.

 

[OG Gearhound]

UPDATE: So here's what I just found out. I couldn't have gotten this far without you guys!
BOTH old and new WOT relays are working fine.. the problem is, they are actuating as soon as I connect them. I have verified this 100% both by hearing it "click" and I popped the top off of one and Visually saw it actuate as soon as I connected it.
This happens when the "A/C" knob on the panel is lit and when OFF. Further more, my power line from the relay to the a/c clutch is good as I tested this at the relay socket with a jumper.. clutch engaged fine.
So my guess now is that it's whatever sensor on the throttle that activates the WOT is stuck or whatever. I just need to locate that throttle sensor now

 

[Uncle Gump]

Just doing a bit of forum clean up... moved this to the Heating and Air Conditioning Forum.

Good information being share and much easier for someone else to search answers.

Carry on gents....

 

[Paulos]

The ECM/computer controls the WOT relay by the signal received from the TPS. So, I can't see what could cause the problem you're having other than a malfunction in the ECM. Maybe someone else will chime in better info.
A quick fix would be to:
1) Remove the WOT relay connector from the relay and place a good jumper between the Black/Yellow and Tan/Yellow wires in the connector (WOT relay remains disconnected).
or
2) Cut either the Red or Purple wires on the WOT relay connector (and tape up the ends) leaving the WOT relay connected.
The Black/Yellow and Tan/Yellow wires (they should be larger gauge wires) are the 12 volt + circuit going to the AC clutch, so they need to stay connected. The Red and Purple wires (they should be smaller gauge wires) are positive and negative control wires from the ECM that energize the coil inside the relay. Disconnecting/cutting either wire will prevent the ECM from energizing the relay, which causes the AC clutch to disengage.
Personally, I would cut one of the relay's coil wires (Red or Purple) and tape up the ends. They can always be reconnected later. The only down side to this fix is that your AC will not shut off during WOT. But that's better than no AC, right? And you can always turn off the AC if you really need to get on the throttle.

Oh, and a "good" jumper would be (in my opinion) crimping a male spade terminal on each end of a small piece of the same or larger size wire as the Black/Yellow and Tan/Yellow, and then plugging them into the same positions that the Black/Yellow and Tan/Yellow wires occupy in the connector. Cutting the Red or Purple wire would be a lot easier.

 

[OG Gearhound]

The ECM/computer controls the WOT relay by the signal received from the TPS. So, I can't see what could cause the problem you're having other than a malfunction in the ECM. Maybe someone else will chime in better info.
A quick fix would be to:
1) Remove the WOT relay connector from the relay and place a good jumper between the Black/Yellow and Tan/Yellow wires in the connector (WOT relay remains disconnected).
or
2) Cut either the Red or Purple wires on the WOT relay connector (and tape up the ends) leaving the WOT relay connected.
The Black/Yellow and Tan/Yellow wires (they should be larger gauge wires) are the 12 volt + circuit going to the AC clutch, so they need to stay connected. The Red and Purple wires (they should be smaller gauge wires) are positive and negative control wires from the ECM that energize the coil inside the relay. Disconnecting/cutting either wire will prevent the ECM from energizing the relay, which causes the AC clutch to disengage.
Personally, I would cut one of the relay's coil wires (Red or Purple) and tape up the ends. They can always be reconnected later. The only down side to this fix is that your AC will not shut off during WOT. But that's better than no AC, right? And you can always turn off the AC if you really need to get on the throttle.

Oh, and a "good" jumper would be (in my opinion) crimping a male spade terminal on each end of a small piece of the same or larger size wire as the Black/Yellow and Tan/Yellow, and then plugging them into the same positions that the Black/Yellow and Tan/Yellow wires occupy in the connector. Cutting the Red or Purple wire would be a lot easier.

Man, I'm with you on this.. my TPS is not bad as it turns out. If the ECM is malfunctioning, I can't understand why. This truck literally has less than 30k original miles. But that (what you said about the ECM) is the only analysis, at this point, which makes sense.

I did discover one other thing that might be related... on the plug to the pressure switch there is a green wire and a brown/yellow wire. The green is hot with 12v.. the brown/yellow is reading .12+v and that wire reads the same at the relay socket.
EDIT: The Red wire at the WOT relay socket is hot w/ +12v.. the Purple wire is showing +.02v.. You advised to snip the Red OR Purple.. which one?
Thanks for the information!

 

[Paulos]

Man, I'm with you on this.. my TPS is not bad as it turns out. If the ECM is malfunctioning, I can't understand why. This truck literally has less than 30k original miles. But that is the only analysis, at this point, which makes sense.

I did discover one other thing that might be related... on the plug to the pressure switch there is a green wire and a brown/yellow wire. The green is hot with 12v.. the brown/yellow is reading .12+v and that wire reads the same at the relay socket.
EDIT: The Red wire at the WOT relay socket is hot w/ +12v.. the Purple wire is showing +.02v.. You advised to snip the Red OR Purple.. which one?
Thanks for the information!

True, it may have less than 30K on it, but if it's the original ECM it also has 32 years on it. Since I've been on this forum I've heard of several things that happen to ECMs. Like corrosion, bad capacitors, damage from over voltage from a bad alternator, etc. It doesn't make sense that the ECM would just cause that issue, but I agree it doesn't sound like it would be the TPS either. I found no evidence of a physical switch that attaches to the accelerator pedal/cable assembly either, so the ECM must be using the signal from the TPS to engage the WOT relay.

Either wire will work. Disconnecting one or the other will prevent the relay's coil from energizing and opening the normally closed contacts (preventing your compressor from engaging).

Try cutting either the red or purple wire (then tape the ends), start the truck, and turn on the AC. If the pressure switch is working and the system has a sufficient charge, the AC compressor should engage.

I have a bad connector on my pressure switch that I have not gotten around to changing. I have to pull it out just a little bit for it to get a good connection, but it will stay working like that. Sometimes it's something simple, but a total pain to diagnose.

And I agree with @RonD, you don't want to bypass the pressure switch for the exact reasons he gave.

 

[RonD]

Good work on finding the issue with the WOT relay activating with key on, but there may be a reason

Yes, red wire should have 12v key on, 12v is from EEC relay, just FYI
Purple wire goes to computer pin 54, its the Ground that would activate the WOT relay at full throttle

BUT......................With RPMs at 0 computer SHOULD ground pin 54 to activate WOT relay so AC is OFF for engine startup

So I would test that again with engine running, purple wire should not be grounded with engine running

If so then could be computer issue or purple wire has a bare spot and is grounded to another wire or the body

 

[OG Gearhound]

Good work on finding the issue with the WOT relay activating with key on, but there may be a reason

Yes, red wire should have 12v key on, 12v is from EEC relay, just FYI
Purple wire goes to computer pin 54, its the Ground that would activate the WOT relay at full throttle

BUT......................With RPMs at 0 computer SHOULD ground pin 54 to activate WOT relay so AC is OFF for engine startup

So I would test that again with engine running, purple wire should not be grounded with engine running

If so then could be computer issue or purple wire has a bare spot and is grounded to another wire or the body

Ok, so this is a new variable to consider (the RPM/Engine running). Some of the testing I did was with Engine on. Most not.

First, I want to highlight 2 things:
1. Ya'll are fantastic!
2. I forgot to mention this.. After removing the TPS, I tested the TPS connector. With Key ON, I jumped the 5v to the furthest-out, black wire (it has 3 pins/wires) which I think is the ground. Will edit this later if needed: I believe the wires are, Red (hot w/ +5v), Blue (Signal?), Black (ground).
This caused the WOT relay to flip back. remember, it actuates immediately with key on. I did not try this while engine on.

Here's my next step based on what info you guys just gave me:
1. Disconnect pressure switch and jump it to eliminate bad switch/low pressure variable.
2. Observe the WOT relay (cap is removed on the Chyna version) to see if it goes to deactivated position with engine on.
3. Remove WOT relay and test volts on Purple wire with engine on for Volts and Ohm.
4. If nothing changes, I will snip the Red wire (at the WOT socket) and tape the ends (leaving enough wire to add connectors just in case).
5. If frustration persists and time allows, I will then carefully place 3 high velocity, 9mm, lead components straight through the ECM.

~Last minute idea just before posting... Since I have 2 Relays that work, I think I will just snip the spade that connects to the Red wire instead of snipping the wire.

 

[RobbieD]

Yes, you can snip one of the WOT relay's spades (that go to the relay's coil).

If it was my truck, and it was needing the WOT relay bypassed, my preferences would be that; or making a good jumper wire with spades (as Paulos suggested); or, unlocking and slipping the Purple (or Red) WOT trigger wire terminal out of the relay socket, and taping it up.

Let's say, later on you (or the next owner) wants to put things back the way they were. It'd be easier to put in a good relay, or unplug a jumper, or reinsert a terminal, versus repairing a cut wire.

 

[Paulos]

BUT......................With RPMs at 0 computer SHOULD ground pin 54 to activate WOT relay so AC is OFF for engine startup

I knew there was something I was overlooking. Thanks @RonD.

 

[OG Gearhound]

UPDATE: Before I get started, I just want to thank everyone on this thread for the help and the education. Ya'll are amazing and I hope this thread will help someone else who finds themselves in a similar situation.
That said, here comes the embarrassing part. I did all the tests I listed earlier. The Purple wire is not at full ground according to my multimeter, key on/engine on and it shows much less than when off.. but that's not the issue.
When I disconnected the Purple wire, the Relay still actuated. So I reconnected it. Next, I was about to disconnect the Red wire but instead decided to jam a piece of heavy plastic in gap between the coil and the point of the Relay to prevent it from opening the circuit. Makes sense, this should solve it. Nope, still no power to the clutch. So I then thought to myself, surely I tested the pressure switch properly.. right? So I made the jumper that @Paulos suggested and used it to bypass the pressure switch.. Lo and behold! The dang thing is alive!!
Now I feel like an idiot. This entire time my other 'jumper' wasn't reaching down far enough into the socket
Thanks again, at least I got a very good education! @RonD @RobbieD @Jazzer @Paulos

 

[RobbieD]

Good on you, for figuring it out, and especially for staying with it.

It's REAL easy to get led astray when troubleshooting electrical, all the more so in these old trucks. And thanks for updating your progress; it's the "whole story" posts which will most help others in the future.

 

[SCOTTY]

Do you have 12volts at the pressure switch?

Unplug it and test, with AC switch ON in the cab, one wire at the pressure switch should have 12volts
If not then that's the problem, wire from AC switch to the pressure switch

If it has 12volts then you can do a jumper from the OTHER WIRE on the pressure switch to the AC clutch, by passing the WOT relay wiring

DO NOT by pass the pressure switch, you will either burn up compressor or blowout an AC hose

Can't you temporarily bypass the pressure switch if you know you have a good enough charge of freon in system until you get another switch ?
& the WOT relay should still work, correct ? (To me the WOT relay thinks there's enough pressure so it will work.)

 

[RonD]

Can't you temporarily bypass the pressure switch if you know you have a good enough charge of freon in system until you get another switch ?
& the WOT relay should still work, correct ? (To me the WOT relay thinks there's enough pressure so it will work.)

You can jumper(bypass) a pressure switch but risk burning out compressor if fluid is too low, or blow out a hose if pressure is too high

WOT = wide open throttle
WOT relay is 5 pin, so when its off it passes 12v to compressor clutch if AC is turned on in the cab
If you press the gas pedal to the floor, so wide open throttle, the computer turns on WOT relay which cuts the 12v going to compressor clutch, shutting AC off, so engine can make best power for passing or ???
When foot comes off the gas pedal AC comes back on