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fortec March 7th, 2018 10:13

1998 VW Jetta TDI ahu a/c not coming on

My 1998 VW Jetta TDI a/c unit clutch do not engage when I push the ac button on the dash. The AUX fan is not coming on as well.

What could it be? I am thinking of the faulty a/c relay, but I don't know which one it is. Could it also be lo/hi pressure switch?

Thanks for the help.

oilhammer March 7th, 2018 10:19

Could be a lot of things. Fan control module (the "box" near the coolant bottle), pressure sensor (on drier assembly), wiring, all kinds of stuff really.

No silver bullet here, sorry.

And there is only one fan with a motor. The second one is run via a little belt off the first one.

Steve Addy March 7th, 2018 12:12

Ditto what OH said, but I will say that it would be rare for the fan control module to fail. As much as people have said "it's the FCM" in the end they've been wrong. That module is very reliable. The cooling fans won't bother coming on if the AC isn't coming on so there is that clue. Alternatively I'm not sure if the AC will come on if the fans are inop so that's also something that needs looked at.

You should take it to an AC shop first and have the refrigerant levels checked. If it's leaking they can ad dye to locate the leak.

Further, was it making any odd noises before it quit working? If you can get a socket on the AC comp clutch with the car off you can try and turn the compressor. If the compressor refuses to move with a 3/8" drive ratchet attached then I would say your clutch (or compressor) is shot.

You might also check the pressure switch that's on the bottom of the dryer, tucked into the front right corner ahead of the front wheel. If you see any residue on that switch or oil then it might be leaking.

If you have a Mk3 Bentley I would first take a look at it to see what it says. Relay locations are identified in the manual in the first few pages of the electrical section.


oilhammer March 7th, 2018 12:15

Yes, the fan controllers in the A3s do not ever seem to have been a high failure item. Pretty sure the refrigerant has to show some high side pressure before the fans will come on too (unlike the later cars, where the fans come on regardless of pressure).

It may just have a leak somewhere. Condensers were common to get holes punched in them, especially on early A3 cars until they started putting that little plastic guard on the bottom portion.

Mongler98 March 7th, 2018 16:53

1st step is to put a set of proper gauges on it, for your use, a set from harbor freight will do. Report back with pressures, 99% chance your low on 134a, dont and i mean DO NOT by any means go buy that cheep can of 134a with stop leak with that cheep POS low pressure gauge and try and dump in more 134a. there is only 1 way to property fix a low system and that is with a super cool/supper heat method for your high and low pressures. you can hillbilly hotdog your way though it with just bringing up your refrigerant levels but your shortening the life significantly.

TDLR, get gauges, report back with pressure. dont do anyting else until notified.

FasterXR4ti March 7th, 2018 17:46

I'm not sure if your Jetta has a fan resistor module, but my B4 Passat AC would not function at all with a bad one. The tell was the cabin ventilation fan was not running at the slowest speed. Replaced the resistor pack with one from my wrecked car and viola, the AC came back to life. Oh yea, I lived without AC for 2 years before I fixed it. I thought it was something more serious. Good luck.

FasterXR4ti March 7th, 2018 17:53

I guess they do have a fan resistor pack. Here is the part I'm speaking of.

Mongler98 March 7th, 2018 18:10

If its not coming on, it could be MANY things, relay, low charge, high low switch, you can pump the connections on the high low switch to check the fans work proper, i cant remember how but you need to look that up or someone can chime in what pins to jump on the switch on the dryer.

was it working perfectly and plenty cold or just barley cold, it is winter and its usually the time things show issues the next summer.

put a set of gauges on it.

RoseBud68 March 8th, 2018 16:55

Best advice i can give, is to take it to someone that knows more about AC.....

williambill March 8th, 2018 18:08

OP, I had a similar situation with my old '98 Jetta with the AHU. If your diagnostics confirm that you have enough freon in the system with no leaks, here are a few things you can check easily yourself.

The power for the AC compressor comes from the supply, through the blower fan/AC switch system in your dash, through the fan control module, through the ambient air temp sensor in the rain tray, through the temp sensor that's located in the middle of the radiator hose on the top right of the engine, and then through the hi/low pressure switch on the dryer before going to the plug on the back of the compressor.

The ambient air temp sensor and the sensor in the radiator hose can be jumpered by simply unplugging the wires and inserting a paper clip bent into the shape of a "U" to bypass the sensor itself. You're just trying to rule out an open sensor. Take care not to let you jumpers touch anything.

The hi/low pressure switch can be jumpered the same way, but it has 4 wires, not just 2 like the other sensors. Google how to jumper this one and it'll tell which ports to jumper as the are numbered on the plug socket itself.

If any of these sensors are bad (open), your compressor should engage once you bypass the bad one. Again, this process assumes everything else is ok. However, if you are sure everything else is ok, and the AC still doesn't come on, take a look at the main coolant temp sensor. If I recall, it's in the same radiator hose as the other on you jumpered already, but it's closer to the engine and has 4 wires.

This turned out to be my problem. It was reading temp according to the gauge fine, but after giving up for a few days trying to diagnose the AC, I got a MIL for the CTS. When I replaced it, the AC started working again. The circuit for the AC compressor has a wire coming from the ECU, and it's my theory that the AC was being disabled by the ECU due to the failing CTS. Maybe there's a way to ohm this sensor to check it, or maybe there's a soft code stored in the ECU that you can check for.

I found a schematic for all of this by searching "ambient temperature switch" in this forum, but the schematic is gone now because of the photo bucket issues.

Sorry for being long winded, but maybe this will help. Also, I had to rely on my failing memory for all this, so I apologize for any errors in my recollection.

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