Ok, I feel like im missing something., Lots of you have posted on this and Ive read them all i think. I have the typical no compressor clutch engagement. System has been converted to 134 and was working fine. Took my console out and installed Alan's flush console adaptation . It looks great and I will post pics with comments for those who want to do that modification. ONce reassembled, the compressor is no longer engaging. Having said that, I don't know when it stopped working because it was apart over the winter. All the connections are fine. My fans engage etc when the AC button is pushed. Controls all work, just the clutch doesn't engage. I have gone through the testing listed in all the great posts by Dwayne et al. I have power the the freeze switch and power at the low pressure cut off with the key on (not running) and the AC on with the fan on 3. If I jump the pressure switch, the clutch engages. If I do it a couple of times, I lose power at the low pressure connector. If I shut off the ignition and turn it on again, I can repeat the process again losing power after a couple of engagements. If I start the car, I have no power at that same connector.

Everything seems to point to the relay but before I unsolder it from the board I just want to make sure im not missing something.

 

Have you replaced the relay in the CE panel?

what year is your car?

 

If you are getting power to the freeze switch and the low pressure switch then that tends to suggest that the relay must be working. However it is one thing seeing voltage - it is another whehter the circuit is carrying enough power to fire the solenoid- takes something like 5 amps if my memory is correct.

Sad to say the HVAC relay is just another of those things that goes with age on the 928 and sooner or later [mostly sooner now] the things need refreshing or replacing. My a/c was having dodgy and/or intermittent operation- swapped out the control head for that from my late S4 and the a/c has been nice a cold since.

 

As the author of the original diagnosis/relay replacement write-up...

First thing to check is the system pressures at rest, then running. Your test by jumpering the pressure safety switch tells us that the switch is [correctly] interrupting current flow to the AC clutch, almost undoubtedly due to loss of refrigerant charge. Translated: You have a leak(s). Find the leak(s). Fix the leak(s). Recharge the system.

The symptom that has the clutch engage a few times and then fail to engage points to a failing relay. Fred is spot on with his analysis. Your clutch is likely drawing more current than will pass through the relay as the contacts heat up. You see that when the clutch fails to engage, and with your DMM when the current drain at the clutch exceeds the contact capability of the relay -- voltage between the relay and the clutch drops in proportion to the resistance and the current demand. Recommendation: Go ahead and do the relay "fix" now, regardless of the exact symptoms you see. The original relay is less than marginal at best, so you'll need to replace it at some point. Just Do It, so you can eliminate it permanently from your possible-faults list. Be sure to add a 3A fuse to the clutch circuit to protect the traces on the control head circuit board. Thanks go to Greg Brown for that fuse tip.

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I was a relatively "new" owner when my AC started to lose reliability. Car had low miles (~20k) on it when it came to me, yet within a year the availability was dropping. I lived in the Los Angels area, so AC is needed almost the whole year. Black car, no tint, driven regularly, no AC... not a good mix! Finding the relay problem was an exercise in logical problem solving. The original version of the fix document started out as documentation for the modification I made to my own car. As others started reporting similar symptoms, the original version was improved to include the diagnostic steps. Since then, others have added info on using Radio Shack relays, and now more recently we see other available relays used now that RS has failed. Regardless of the history though, all the control head relays will fail sooner or later. They are not rated for the DC current that the clutch coil draws, and for sure are not protected from the inductive "kick" that happens when the relay opens. So the relay replacement is inevitable.

 

A/C clutch requires 3 Amps.

 

Thanks guys for everything. The system is fully charged. I was just worried about the difference between the ignition on and the car not ruining and with the car running. I agree that my troubleshooting points to the relay and I have it apart ready to desolder. Just wanted to see if im looking for anything else as well.

The pressure switch is working and tests fine and has continuity. It does appear to be a current draw issue.

And yes, I have the fuse ready. I am also using ford micro relays that are rated at 30a so the relay should be good when im done. If it doesn't fix, ill post for sure.

Thanks again, the help from this forum is indispensable.

Ian

 

Ian --

You don't share the year of your car. There's a second set of contacts in the factory relay in my S4 that's used to tell the cooling-fan controller when the AC is running. A similar configuration is used in earlier cars. Regardless, do make sure the little Ford micro-relay you choose is a DPDT configuration. Most are not.

 

To be certain check the voltage at the freeze switch while running with AC on - if that is substantially below 12v (like a few volts) it indeed proves the AC relay contact are damaged and can't keep up with demand.

If the relay can't drive the current needed the voltage will drop (just as long as the load is still in the circuit).

Note that in the S4+ cars the suppressor "Relay" on the CE panel is protecting this AC Head/Clutch relay from switching transients - if you take this out - the AC head relay will get fried much faster. But Dr Bob is also quite right that it is ~over-loaded anyway all the time - so it will likely eventually die on every car.

Alan