A/C Blowing Ambient Air

The air went out on my car about a week ago. It was blowing ice cold, then out of nowhere was blowing ambient air. Both fans are working, but the clutch is not engaging. I’ve checked all of the fuses and none are blown. I’m thinking it might be the FCM, but am looking for input. I also, around the same time, got DTC’s 16512 and 16502. I thought I read somewhere that the coolant temp sensor was wired into one of the same harnesses as the A/C. Could these two issues be connected? I have the upgraded green coolant temp sensor also. TIA.

Disconnect the clutch connector at the compressor and measure the resistance of the coil. If it's open the coil is bad. The clutch coil can be replaced without discharging the system or replacing the entire compressor.

BEWARE putting 12V on that connector; it does NOT have a full 12V on it when running normally and you WILL burn it up if you do that for more than a couple of seconds.

There's a rather complete troubleshooting guide here on the site, but the first place to start if the clutch doesn't pull in is with the electrical resistance of the clutch itself, as the coils do fail.

I had the headgasket replaced about 8k miles ago, and had them switch over to the green updated CTS. Could a faulty sensor cause the DTC's and ultimately the fault with the AC?

What are your pressures on the high and low side? You have a pressure switch in the system that will not let the compressor run if the pressures drop low. The connector that the pressure gauges attach to has a little rubber ball as the system gets old will let some of the charge escape. There are replacement valves if that is your problem.

I just traced an ac problem down on my Golf. There is a wiring harness connector fastened to a bracket that is directly on top of the starter. There is a four pin connector on top and the wires are even labeled on the connector, 1 thru 4. Check the wires labeled 3 and 4 both of those go directly to the ac and supply it with power.

My #4 wire was broken right at the connector. Spliced in a new wire and its all good now, system is working again.

vdubwannab said:

I had the headgasket replaced about 8k miles ago, and had them switch over to the green updated CTS. Could a faulty sensor cause the DTC's and ultimately the fault with the AC?

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Unlikely. There's an ambient temperature sensor input, but to the best of my knowledge the CTS is not involved in gating the A/C.

I second tdijarhead. Same thing happened to me, I had 2 broken cables. The 2 end ones if I’m remembering correctly, just jumpered past the plug and was good to go.

Thanks guys - no clue on pressures as of yet. I was going to go get a gauge to check. I am thinking pressures are fine since it was blowing ice cold and then wasn't, which leads me to think electrical. Also one of the DTC's says it could be a short somewhere.

I will check that plug by the starter. It could be the culprit because I replaced the starter over the winter, and maybe I frayed that cable. Unlikely that I did, but I will check regardless.

Genesis said:

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BEWARE putting 12V on that connector; it does NOT have a full 12V on it when running normally and you WILL burn it up if you do that for more than a couple of seconds.

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That is a false statement. I have several customers who are semi- DIYrs that have been driving around for years with toggle switches to power up the compressor clutch directly because they are unable or unwilling to properly fix the system.

And the older cars, with essentially the same clutch, always applied full 12v. The reason the newer cars are like that is to lessen the current through the solid state fan controller that carries the current, nothing more. That was in the "new model" self study guide way back in the middle '90s.

Ah.... I assumed the reason for the roughly 9V was that the coil was designed for that, not the full 12V power....

Current ramping, lessens the load on the driver. Solenoid operated fuel injectors do the same thing... that's why they did away with series resistors in the '80s, because those acted as a shock absorber on the ground side to keep the driver circuit in the ECU from cooking.

This is sorta the same thing.

Especially if the coil shorts...

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The Temp Switch/Sensor for the fans on the Radiator is wired separately from the AC wiring!

There is an Ambient Air Temp sensor. It is located on the left side behind the cowl just below the corner of the windshield (on A4 Jettas for sure). If critters chew one or both of the wires, AC will not function (don't ask me how I know).

Yes, the coil and clutch can be replaced without dumping the refrigerant.

Yes, the pressure switch will not allow the Clutch on the Compressor to kick-in if the refrigerant pressure is too low.

I've been running straight 12 volts on the AC coil with the ALH in my Vanagon since August, 2012.

Fuse #16 is for the AC compressor clutch.

Yes, check voltage at the compressor.

Yes, check the ohms resistance of the clutch coil.

You need a set of Manifold Gauges if you are going to do diagnostics on the AC system.

Typically, the amount of refrigerant (R134a) is labeled somewhere under the hood. It may show as much as a 10% deviation (plus or minus 5%). Thus, you do not have to be "perfect" in charging it. R134a goes in slow. Ambient temp, max AC setting vs normal, out front fans, etc., all have an affect on the pressures you will see on both low and high sides. Switching from Max to normal will show an increase in both low side and high side pressures (all contingent upon temperatures). But, generally about 35 psi to 45 psi low side and 150 to 200 psi high side is what you need. Always check dash vent temps as you add refrigerant... something in the low 40s is what you want. Again, ambient temp, direct sunshine, etc., all affect pressures, temp, etc.

Hope this helps!

EDIT: In reference to the work order. In all the work experience with VW AC, research, studying wiring schematics, etc., I've never ran across anything that indicates that the Fan Control Module "senses" anything! In my opinion, that statement is absurd!

AndyBees said:

In reference to the work order. In all the work experience with VW AC, research, studying wiring schematics, etc., I've never ran across anything that indicates that the Fan Control Module "senses" anything! In my opinion, that statement is absurd!

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There isn’t a way I can “hotwire” my clutch coil to make the clutch engage is there? The way he explained it was that the coil didn’t have continuity, so it would not engage.

If the coil ITSELF does not have continuity (checked at the plug) then it is burned out and must be replaced. While you probably COULD shim it so it never disengages this is an EXTREMELY bad idea as there is a pressure sensor on both the high and low side of the system that the ECU can and does use to command the unit "off" should it detect a fault (e.g. either too high or too low pressures, respectively.)

If you shim the compressor clutch to force it on then you've disabled those safeties, and the sole remaining safety preventing a "boom" if something bad goes wrong is what amounts to a "freeze plug" sort of assembly on the compressor itself. If that goes while in theory it will close in practice you're buying a new compressor as it will leak.

So while you COULD do this, DON'T.

The clutch coil can be changed without unhooking the compressor from the refrigerant lines.

Shimming the clutch actually has the reverse affect. Too many shims will keep the face away from the contact area of the pulley. Removing shims will allow the two pieces to make contact ..... not sure if there would be enough contact pressure to provide a solid engagement.

There is only one sensor on the AC system of the OPs 2002 Golf. I feel certain it handles both high and low pressure. I just changed AC Compressors today on my 2000 Jetta which has the exact same set-up .......... only one sensor and it is on the high side of the system, but senses both low and high pressures (Clutch and Fans kicked on as I was adding refrigerant). Thus, if the pressure is too low, neither will the fans nor the clutch on the compressor kick on. When the AC is off, there is an "equalized" pressure on both the high and low side. It must be high enough for the AC Compressor and Fans to kick-in when the AC is turned on!

There are two snap rings that must be removed in the work order to remove the coil. Also, depending on the circumstances, the use of two or three jaw puller may be needed to remove the clutch face and then the pulley!

Yes, OP, you can hot-wire the AC compressor clutch! (Test purposes only). You do not want it to stay engaged without knowing whether or not there is adequate refrigerant (R134a) as well as oil (total of about 8 ounces) in the system.

Considering the AC Compressor has a manual Refrigerant Control Valve, with everything working properly, it will never cycle off while in the ON position. If it does it's because pressure is too high or too low, thus, everything isn't working properly.