I have a 97' F-150 Extended Cab with the 5.4L engine. My A/C has stopped working. I checked the voltage at the compressor and get nothing. So I took a look at the wiring diagram in my Haynes manual. Here's where I'm at right now...

I have jumpered both the "A/C Clutch Cycling Pressure Switch" and the "A/C Pressure cutoff switch". With the A/C off I get no voltage on either, as expected. With A/C on I get 12+ volts on both. According to the diagram the next stop on our tour seems to be the PCM and then on to a A/C Clutch Diode.

So, my questions are

a) Where is this diode and how do I test it? Is it a simple continuity test or what?

b) What am I looking for in the PCM other than a loose/broken wire? Seems that if it isn't a wire the PCM much be "choosing" not to allow the voltage to the clutch. But if that were the case, wouldn't I likely be getting a code on the scanner?

Any other suggestions would be greatly appreciated. I read another post that was very similar to mine but in the end, the guy just hotwired from the low pressure sensor to the A/C clutch. Didn't find anything else quite like this.

Thanks, all.

 

First, welcome to FTE!!!

There is also a WOT relay that shuts the AC compressor off when at WOT. The "normally closed" contacts are used, which open when the relay is picked (at WOT). Make sure this relay is ok. The diode should be at the AC compressor clutch connector. Unplug the connector from the clutch, and with an ohmmeter, you should measure infinite resistance with one polarity ( + on cathode, - on anode), and a low resistance with the opposite polarity (+ on anode, - on cathode).

 

Thanks a lot for the reply, Bob. I looked through my Haynes manual as well as the Fuse/Relay charts in my Ford book. I don't see any mention of the WOT relay. Is it a typical relay, small black square, etc. Or is this a different type of relay buried somewhere in the engine compartment?

 

What happens if you "jump" the low pressure switch? Short it and see if the comp turns on. If it does, see if it blows cold.

 

I'm already jumpering two switches that I found in the circuit diagram. One is on the a canisor just behind the main mattery fuse assembly. The other is near the front of the truck on a standalone port. If you are referring to a third switch, where is it?

Thanks,
LEJ

 

P.S. I'll try to post some pics when I get back home in a bit to make things clearer. Thanks again, everyone.

 

Yes, the WOT relay is a standard "black square" relay, and it should be in your relay box in the engine compartment.

 

Tested all relays both under hood and dash, all checked out. Started "fiddling" with the conduit that the black/yellow wire to the compressor runs through. Probably not the brightest thing to do but I was curious if that wire was getting voltage so I cut off a tiny piece of the insulation... just enough for a voltmeter probe. I figured I would tape it with electrical tape after testing. Cranked the truck so that I could go back and read the voltage and much to my surprise cold air blew from the vents. This led me to the conclusion that the black/yellow wire has a short somewhere and I must have moved it around just right. Anyway, I took a test drive of about 5 miles to my parents' house and chatted for about 10 minutes. When I got back and cranked the truck I had cold air for only a minute, then slightly warmer with an "ozone-ish" smell. Then completely warm.

After getting back home I let the truck cool completely and re-started, no air. Since I still had the small nick in the black/yellow I tested the voltage... 12+ but the compressor isn't running.

So... Looks like I'm getting voltage almost all the way (I nicked the wire about midway down the fuel rail.) The only thing I can think of (assuming the compressor is good, since it was running only an hour before) that is left is this magic diode. Where is it? Would a faulty diode cause such an intermittent problem? Sorry for the long post but I can't help but think that I'm really close. I would like to put this thing to bed.

 

It is sounding like you have a bad ground on the AC compressor clutch connection. One wire should have +12V, and the other wire at GROUND.

 

I have a similar problem. A/C was intermittent at first and now is dead. Checked current into the compressor through the two switches. About 4 amps. If I lift the plug on the compressor and messor the resistance I get about 70 ohms. Ehh. The diode? One drawing I have says its on the firewall. Haynes had nothing. Anybody replaced or checked it so they can tell me where it really is and what it looks like. Half the problem with this truck is finding stuff. My 4WD hi lo switch failed and the parts guy @ ford said they never fail. Works now after replacing it. Now for the A/c diode

 

Before you nick into any more wires and get to chasing your tail with this, try this test:
With the truck runnibg abd the AC turned on, tap the face of the clutch with the handle of a long screwdriver or a broomstick. If it engages, the clutch gap is too wide. It's a very common problem.
The gap is adjusted by removing the clutch center bolt, pulling the clutch off and removing a shim. No special tools needed.

lackey91: This sounds like your problem also.
The clutch diodes very rarely fail. If the diode failed open, you probably wouldn't notice intil a voltage spike took out some unrelated electronic component. It wouldn't effect the AC operation one bit. If it failed shorted, it would blow the AC fuse.

 

First off, here's the schematic: 1997 F-150 HVAC Schematic.pdf

Download it for reference.

As Bob mentioned, the diode CANNOT cause this problem. It's there for the sole purpose of shunting the voltage spike that is created by the collapsing magnetic clutch field to ground. The diode is physically located taped into the harness near the compressor clutch.

4 amps is about right for the current draw. The coil's resistance is around 3.2-3.6 ohms. If you're pulling 4 amps and the clutch didn't engage, you have an air-gap problem. If you measured 70 ohms, you may need a new meter...

The 97 doesn't use a WOT relay. The clutch is operated directly by the cycling switch. The high pressure cutoff switch is a normally-closed switch that should only open when pressure exceeds 445 psi.

Steve

 

Isn't the diode plugged into the under hood fuse panel on this truck, or am i thinking of a diode for something else?

 

Thanks for the drawing. I have 3 now. All have P for the wire from the switch. Mine is Brown??? Okay Now for the clutch. Yup whacked it and it spins for a while, gets things cold and then drops out. How do you take the clutch apart to clean and remove a shim as I am a believer of the clutch needs help and will also review DVM calibrations with known good resistors.

 

Something else....

Steve