A/C intermittent on 1998 Jetta

[chrisgt]

I have a very weird and annoying intermittent issue with my HVAC. Sometimes it works perfectly fine, AC is cold, all blower speeds work, etc.
And sometimes it stops working correctly and does this...

Fan speed 1 lights up the AC button and engages the AC compressor, but does not run the fan motor.
Fan speeds 2, 3, and 4 work, but the light on the AC button goes out and the compressor does not run.
Randomly, this behavior will stop and everything works correctly until next time I shut off and start the car.

It acts up on cold as well as hot starts, leading me to believe it isn't a sensor malfunction making the AC system "think" it's overheating, but maybe this assumption is leading me astray. Am I correct that the two-wire sensor in the middle of the upper radiator hose is for the AC system?

 

[ToddA1]

I’d probably delve into the wiring diagrams. There are several safeties built in, which could simply be a bad sensor.

With scraps of wire, these safeties can be temporarily bypassed, until you find your issue.

-Todd

 

[chrisgt]

The thing that is odd to me (and maybe this is throwing me off), is that fan speed 1 doesn't work when it's acting up, regardless of the AC state. I thought maybe the fan switch, but I can take a look at the Bentley wiring diagrams., too.

 

[ToddA1]

Take a peek at the radiator fan, when it isn’t working. That fan module in the engine bay is known to go bad and it’s directly under the coolant reservoir, on the A3s.

-Todd

 

[chrisgt]

I looked at that, fan is not running when the AC is not running. That kind of makes sense. When I put the fan switch on one, the compressor clicks on and the fan starts to run, but the blower motor does not run.

When it is acting up, the blower motor does not run on fan speed one regardless of AC State.

When everything is working correctly, the fans run exactly like you would expect. Could the fan module under the coolant reservoir cause the issue with blower motor not working on speed one?

 

[Steve Addy]

I looked at that, fan is not running when the AC is not running. That kind of makes sense. When I put the fan switch on one, the compressor clicks on and the fan starts to run, but the blower motor does not run.

When it is acting up, the blower motor does not run on fan speed one regardless of AC State.

When everything is working correctly, the fans run exactly like you would expect. Could the fan module under the coolant reservoir cause the issue with blower motor not working on speed one?

The FCM does not control hvac blower in the cabin. Typically if you have only high for the blower motor it's the resistor, but in this case you have more than just high speed, so we can rule that out.

I would venture a guess it's the control switch for the hvac blower. The control switch also controls AC (when you turn the blower to off the AC goes off too). And if the switch turns off the AC that will cause the cooling fans to shut down as well.

This is my best guess without being there to investigate...

Steve

 

[chrisgt]

I have not had a whole lot of time, but i spent some tinkering tonight, this is what i found...

The fan switch is operating correctly. It's pretty simple, one common and four switched outputs. I took it apart, cleaned the wiper and contacts, and no change.

Then I found that if I turn on the key and leave it in fan speed 2 for a couple minutes, everything starts to work correctly. If i let it sit for a few minutes it acts up again, but leaving it on speed 2 for a few minutes "fixes" the problem...
I'm honestly baffled. Maybe the resistor is warming up and making contact again? I still haven't had the time to look through all the wiring diagrams. However I did find that relay 94 seems to be the AC clutch relay, there's also a signal to the FCM, so I should be able to start at those and work backwards.

What I really want to know is, what triggers the AC system to kick in? That trigger is obviously missing/acting up. When I find that, I should be well on my way to understanding what's going on here. Car wiring drives me literally insane (I write firmware on medical life support devices, I should be used to frustrating stuff.....)

 

[Steve Addy]

Your Bentley will have a good diagram for the AC system.

You said before that fan isn't working on lowest speed...that is typically a faulty fan switch. That is unless the blower is so bound up that it can't power through what ever is blocking it. If the fan works on 2-3-4 without screeching or making other noise then I would guess it's not blocked, and I'd be going back to the switch as a problem.

The AC switch controls compressor operation, but fan switch must be in any position other than 0.

The FCM is rarely the faulty party although it gets blamed a lot. The FCM we have in Mk3 / B4 cars is the consequence of the past methods to control the cooling fans being particularly prone to failure.

Good luck!

Steve

 

[chrisgt]

I finally had a free minute to read the wiring manual. Turns out the AC system gets its trigger off the low speed end of the blower resistor.
That means if the low resistor dies, it will not work on low (duh), but the trigger will still be given to the AC system.
If the switch is put onto 2, the fan will run on 2, but the signal through the resistor to the AC system gets lost.

After looking at the manual, my symptoms can be perfectly explained by a failing resistor pack. My guess is that running it on 2 heats up the resistor and allows the broken element to once again make contact.

I was at the junkyard for something else today so I grabbed a resistor pack. It doesn't look like it can be changed without removing the airbag, but i think i'll be able to plug the wire into the "new" one to test this theory. If that's the problem, i'll just buy a new one. Getting that thing out is kind of a pain ....

I'm the lead mechanic for a car running Mt. Washington hillclimb, so I'll be tied up with that until next week, then hopefully i can get to the bottom of this for good.

 

[ToddA1]

i usually only run the AC on high, to avoid this issue.

Is that resistor pack located in the HVAC box? On my earlier VWs, they are. I’ve read they need to have a steady flow of air moving across it, to help keep them from burning out.

-Todd

 

[chrisgt]

The resistor is in the air box behind the airbag. In the junkyard I just took the airbag out, not sure if it's possible to remove with the airbag in place.

Running it on 2, 3, or 4 will run the fan, but not the AC... The AC is triggered off the far end of the low side resistor. So if the low resistor is broken, then the trigger signal won't make it to the AC system. According to the manual, backfeeds the signal to the AC system through whichever parts of the resistor are inactive (or all of them in the case of 4).
This works because voltage drop across a resistor is proportional to the current flowing through it, all it's driving in the AC system is a small relay. A small relay takes milliamps, so it'll see no appreciable drop. It looks like the indicator light on the AC button itself is driven in the same fashion, so it all adds up.

I was kind of suspecting this is how it was wired based on the symptoms, but looking at the actual wiring diagram drives the point home. As I said, i'll experiment with it next week, but it HAS to be this.

 

[chrisgt]

I checked all the elements of the junkyard resistor, it was fine.
I figured out how to remove the one from the car without taking out the airbag.

Put the junkyard blower resistor in the car and all is well.

The ceramic housing of the old one was all cracked and I confirmed the first resistor element in the resistor pack was bad.
I put the thing on the bench and heated it with a heat gun, after it got warm, the broken resistor element started to work. I suspected this was the case, since running on fan speed 2 heats up the resistor pack, expanding the windings just enough to contact again. Once that connection was re-established, the signal to the AC system made it though and the AC started to work.

The way this system is wired made a simple problem have a very strange result, glad that's behind me.