Howdy. I just bought this 89 truck not too long ago. I've read TONS on these forums and others about this problem, but either the posts never have solutions or the tests they suggested haven't worked for me...

Symptoms:

Truck starts up fine (I've been told that older vehicles in general would idle high for about 30 - 50 seconds before dropping to appropriate idle - my truck does this, so I'm saying it starts up fine assuming this is normal, if not, maybe this will give more insight into the problem).
If I turn the A/C on it will immediately drop down and stall. I can turn the A/C off quickly and it will pop back up. I can also maintain it running if I give it gas.

Troubleshooting I've done so far:
1. Replaced IAC - no effect
2. Replaced Clutch Pressure Cycle Switch - no effect
3. Replaced TPS - no effect
4. Cleaned Throttle Body (with toothbrush and Seafoam) - no effect

This next step is key and I have not seen anyone else that did this and had these symptoms:
5. Removed A/C compressor/clutch power - turned A/C on - truck still dies. This I think clearly indicates that the compressor is not to blame and is probably an electrical issue... (the truck also has a slight electrical issue with the locks - the passenger door lock sometimes doesn't work or will unlock but not lock... may not be associated, but just FYI).

6. Unplugged Clutch Pressure Cycle Switch (w/ compressor power plugged in) - turned A/C on - truck continues to run without problems, air blows through vents but no A/C. (This is the main reason I replaced the Cycle Switch, but to no avail)

I have been trying to find A/C wiring diagrams for this truck. I've found this, but I can't tell if it's for my year and model - it doesn't even show the compressor clutch anywhere in the circuit...

The issue, I surmise, is somewhere between the A/C switch in the dash and whatever circuitry goes up to, but does not include the compressor.

Anyone have any ideas? Also, if you have diagrams or at least know what the sequence of sensors/plugs goes from the A/C on switch to the compressor that would probably help. If you know any of the voltages or tests for the sensors too, please let me know.

I have both a Haynes and Chilton manual but they don't provide any of the A/C circuitry in there...

Thanks all for you help in advance.

 

I would guess a c clutch or something isn't telling the computer there is an additional load on the engine and it doesn't add fuel so it dies

 

By "A/C Clutch" do you mean the clutch on the compressor? If so, I've already ruled it out since I disconnected it for a test. If that's not what you mean, please do tell!

 

Yes that's what I meant, when you start it next time while it is high idling try it than to see if it bogs out or dies

 

Yeah, I've already ruled it out though. With the compressor/clutch power disconnected, the truck still dies... the problem cannot be the compressor clutch... It's either a sensor or some form of circuitry that is malfunctioning... I just need to find the full circuit path from the AC on switch in the dash all the way to the compressor.

 

Ok what did it do

 

Check the post again, I edited the message I just posted...

 

If it were wiring In the ac system the ac wouldn't work. I am thinking that the engine isn't getting more fuel when the compressor kicks on, it should click the engine should bog a little than pick back up right away

 

No. I know it's long, but did you read everything in the main post? The point is, even with the compressor/clutch unplugged (as in it has no power and does not turn on), if I turn the A/C on, the truck will die.

If the compressor/clutch IS plugged in, and the compressor turns on (as in I turn the A/C on), the truck dies also... it does not "pick back up."

 

Check all grounds, check wires for ac and blower motor

 

Well, that's what I came on here for. I don't know the entire circuit path... I was wondering if anyone on here knew, or has diagrams...

 

I think you need this diagram from SeattleFSB:


Note the A/C clutch power goes through the HVAC control panel then onto the Clutch Cycling Pressure Switch. If the truck had a 4.9L engine then the same supply voltage is fed to the PCM on Pin 43. This does not apply to your 5.0L equipped truck.

When the A/C is commanded on the Clutch Cycling Switch is normally closed, this sends +12 VDC to the compressor clutch and a resistor diode network labeled A/C Clutch Resistor Diode. Notice the supply voltage is buffered with a current limiting resistor and "suppression" diode. This A/C On signal is sent to the PCM on Pin 10. I suspect this is causing your stalling problem.

I am not familiar with the location or voltage/current values on the output side of this Resistor/Diode network, but your symptoms lead me to believe something is amiss in this part of the circuitry. I do not have access to a full 1989 EVTM so I cannot point you to where this component network is located.

This Resistor Diode Network is using Connector C106. According to this diagram C106 is located on a bundle of connectors located on the driver side fender well.

 

Oh, what a glorious reply! I'll check all this out as soon as I get a chance. I'll let you know what I find. (I'm also going to do a vacuum leak test with a smoke machine when I get the chance)

 

Updates: I tried to rig a smoke test up today but it was a no go... my chemistry experiments never seem to work out in the end ... (I need to get some nice equipment one day). So, that's postponed until I can find something or someone that's suitable to do the smoke test with.

I began tracing the electrical system for this A/C stuff. It's been great poking through the engine (I always find things that need readjusting or could be done a little nicer). So, I've traced everything except the resistor diode... I found this page dealing with that: https://www.ford-trucks.com/forums/2...t-is-this.html.Am I to assume that the diode is really that "thriftily" soldered into the circuit and that it's just wrapped into the wiring somewhere??? If so, does anyone know it's location? (the post with the picture I listed there says 6 inches, but I've gone that far and don't see it yet...)

 

For a vacuum leak, you can use an unlit propane torch. Open the gas on the torch just a little with the engine running, and trace around all potential vacuum leak points. When that gas hits the vacuum leak, the idle will drop (vac leaks should cause high idle).

Jason