928NOOBIE

Also, try bypassing the pressure switch on the drier (connect the 2 wires together that plug into the switch). If you have enough refrigerant there's a chance the pressure switch is kaput but be careful because you can damage things if you are running the system without enough refrigerant..I know above was mentioned it looks like plenty.

Personally I am putting my money on the freeze switch...it'll also be over-worked like the on-board AC relay on the climate module when the engine raises resistance in the wire to the point that more current is needed to keep the compressor running...

FredR

If the outer part of the pulley is not rotating your ac compressor is not doing anything. Grab hold of this and see if your can turn it by hand- you should feel some resistance. If you cannot move it at all then the compressor has seized- possible but not likely I would suggest.

Possible causes for our problem have pretty much been covered but the possibilities are:
1. Compressor clutch solenoid fried- apply 12 volts from hot post to black cable leaving the 14 pin connector and see if you can hear it click on/off.
You can also try this with the engine running but if you do stick an in line fuse in the cable rated for 5 or 10 amps max- in this condition the a/c should run OK assuming the blower is running etc etc
2. Dash panel a/c control head unit relay gone south.
3. Freeze switch failure- check for continuity across the terminals
4. Low pressure switch mounted on the filter dryer failure- check for continuity across the terminals
5. Electrical discontinuity from the relay through items 3 & 4 above, through to the black wire on the 14 pin connector and finally through to the clutch itself.

If items 1,3,4 & 5 are positive as I suspect they most likely will be, then the relay has taken a dump and I suspect that is the most likely fault scenario based on your description.

Ramp

Dr. Bob - Can you explain this ? I'm diagnosing similiar/same issue. I had no power at the freeze switch. Disconnected the clutch wire and tested the clutch with 12v. Clutch good. When I reconnected the wire, all of a sudden I had power from the freeze switch when I didn't before ? How come ? Is removing power draw from the relay somehow resetting the relay ? What does this say about the relay ? Thanks

worf928

A typical failure of the relay in the HVAC head is that you will get 12v to the clutch circuit but not be able to sink 3A for closing the A/C clutch. (I call this “HVAC Head Zombie Mode”.)

A similar phenomenon can occur with heavily oxidized connections in the clutch circuit.

You can test for this mode by putting a 0.5-3A load in the circuit in series. I use a tail light bulb, in a socket with pigtails, between the low-pressure switch connector and a ground.

If no ‘bulb light’, check the connectors for anti-freeze switch under the drip shield and single-pin connector on the front main harness as these three connectors are the usuals culprits if the HVAC head isn’t in Zombie mode.

dr bob

The relay failure symptoms process start out with intermittent clutch function, progressing to less and less function as the contacts in the little relay continue to deteriorate. Be aware that a system that's actually low on refrigerant may offer similar symptoms, as the pressure for the safety switch varies with ambient conditions.

The root causes of the relay failure are that the relay itself is not designed for DC use on the contacts, and the contact current rating is marginal at best. Relays have small silver-plated contacts that open and close mechanically based on a magnetic coil and a switch. In our use, the AC clutch has another coil of wire to make a magnetic field strong enough to draw two metal pieces together, so the drive belt cab spin the compressor. Specifically, when the relay contact opens, the remaining magnetic field at the clutch coil collapses. That generates a voltage spike similar to the way an ignition coil works. In this case the spike generates a spark at the relay contact. Each of those tiny sparks is like an arc-welding flash. A tiny amount of metal moves from one contact to the other. Over time, actual contact area shrinks thanks to a little spike of metal that forms on one side. With the smaller contact area comes more resistance to current flow, so there's local heating at the contact in the relay. The heat can warp the little arm on the moving contact, so the contacts don't meet correctly. More resistance, more heat. At some point the resistance is high enough that there isn't sufficient magnetism generated at the clutch coil to engage the clutch.

Relay coils rated for AC duty depend on the change in current flow direction to break the arc as the relay opens. The spacing between contact faces can be smaller too. DC rated relays typically have larger contacts and spacing, especially when there's an inductive (clutch coil) load attached. Porsche included a diode "suppressor" as a CE panel plug-in to help some with relay contact arc suppression, but it's not enough. It really needs a 5-amp Zener diode rated at maybe 16 volts to capture and shunt the excess voltage to ground. Even then, the current rating on the original relay contacts isn't really sufficient.

Solve the problem by installing a better relay, using any of the many recommended methods. I have an industrial-strength relay mounted on the back face of the control head. Others have placed mini ice-cube relays inside the housing, some near the jump post or the freeze switch. Greg B adds a fuse in the control head to protect some circuit board traces that self-destruct when there's a fault in the clutch coil circuit.


Back to Van's situation and discussion...

Hey_Allen

This is why some domestic ac compressor plugs are built with a diode in the connector to the compressor, allowing snubbing the reverse polarity surge right there, as close to the source as possible, and before it can damage anything upstream.

I don't know the coil specifics to calculate an appropriate diode, but it is something that could be easily added at the compressor, by tying the cathode (banded, negative) end of the diode to the AC coil positive wire, and the anode (positive) end to a good ground point.

vanster

Can I jump the relay and maybe learn if this is the issue?I have checked most everything else.
Belt is tight
R12 is topped off ( it wasn't that low before it quit)
Fuse OK
Outer pulley spins freely
When the AC button is pushed, nothing is clicking on

Do I need to pull the head unit out of the dash and look behind it?

bureau13

I'm still confused about your description of the inner vs outer pulley. They way I think of it, there is an outer pulley, which is what the belt drives, and an inner part that is connected to the outer via a clutch. All the talk about the AC head unit relay, freeze switch, etc turning on the AC clutch simply means that causes the inner and outer pulleys to be connected, which means the compressor is turning when the belt is driving it. If any of these things don't pass current, the clutch won't engage and the outer pulley will just spin freely via the belt, but won't spin the inner part ie the compressor isn't turning...just like if the AC were off. Unless I'm mistaken, you mentioned that the outer pulley isn't spinning, but does when you turn it, and the belt is attached/tight? These things don't make sense to me, which says we're probably not using terms to mean the same thing.

Sorry if this is obvious and I'm misinterpreting you...

StratfordShark

Instead of jumping the relay, you can find the 12V connector to the compressor clutch (follow it back from compressor to a connector coming out of loom hanging in front of engine). Pull that connector out, and apply 12V to it directly (use a lead or power probe from jump post, but be very careful not to short to ground). With a light shining on compressor clutch, and engine running, touch 12V to the clutch lead and see if it runs. If it's ok, track back as detailed in posts above to see where you're losing power in the compressor chain (the head unit relay is a very likely suspect, so you do need to check that it will power a load, not just a multimeter).

vanster

Thanks,
Back to the head unit relay...it seems to me that if I pull out the head unit and replace the relay this would be somewhat proactive regardless of just trying to chase it.

monkez

I would also check the pressure switch that monitors the pressure of the system. No pressure, the condenser will not work. If those wires become disconnected or damaged, the compressor will not engage. It's just one more "maybe it's this" kind of thing, but you never know and it's easy enough to bypass and see if the compressor engages.

Good luck!

gbgastowers

Mine wouldn’t engage and one of the problems was the brass feelers that touch the slider contact points on the head unit weren’t making contact. Bent them down and compressor engaged

vanster

I just read Dwayne's write up. I understand it. Now to fix it

vanster

Freeze Switch question
I am getting between 9.5 and 11 volts at the freeze switch. Is this enough to run the compressor?
If it is it means that the relay in the head is faulty. I assume I can just replace the relay or do I have to send the entire head unit in to have it rebuilt?

dr bob

Van --

The relay is a solder-in piece on the circuit board in the head unit. Greg refurb's them if you aren't comfortable doing the surgery yourself.

The original diagnostic/relay upgrade was a documentation I did for my DIY modification to the head unit. Dwayne grabbed that and did his magic on it for "public consumption". As such it's a pretty darn good guide for you to do the work yourself if you're comfortable doing a little soldering.