grabo

1998 A4 B5 2.8L 30V (AHA)

In the summer months, with the AC on, you can hear the car from half a block away - seems that the supplementary electric fan is on high. Always.

Looking at the schematics, it appears that this car has two relays that drive this fan. One supplies it directly with 12V, and the other supplies it with 12V through a series resistor. The series resistor is an unusually expensive ($120) part with a massive heatsink.

Since the resistor is easy to get to - I measured its resistance. It's 0.5 Ohms.

0.5 Ohms seems to me like it's essentially a short. Which could be an explanation as to why the fan sounds like it's always running at full power all the time.

My question - what's the resistance of this thing supposed to be? Does anyone know, or can someone who happens to be underneath their car bust out a meter and check? I've scoured the interwebs and haven't found anything that indicates what the resistance of this thing should be.

Victorymike18

Does the 30v have the primary fan driven by the engine with a clutch?

I am familiar with the 12v and it has two fans: One that's engine-driven by a clutch and a secondary fan that supposedly will run whenever the AC is activated. In my case, the previous owner deleted the engine fan and replaced it with a Hayden electric fan. His wiring was a bit of a hack-job so I cleaned it up and run them off two relays triggered by the radiator thermo-switch.

Just an idea for consideration: Most modern electric fans have two-speeds built into their windings and you may be able to get away cheaper by purchasing one (or two) new fans and running them off relays and use a combination of the AC wire and the radiator thermo-switch (two speed) to trigger them as desired.

grabo

Yes, that's exactly the configuration. I had the clutched fan removed when I did the timing belt a couple years ago.

According to the wiring diagram, the secondary electric fan is apparently driven by two relays - one which provides a full 12V to the fan, and the other which provides voltage through the series resistor. I haven't bothered (yet) to check to see the inputs and outputs of those relays to see if the fan is being commanded appropriately.

Unfortunately, VCDS-Lite doesn't see the climate control module - it just spits out some junk characters for the part number and then nothing else but an error message, so I can't query it.

I'm really not interested in turning this into a bigger project than it needs to be... just want things to work as they're supposed to.

Victorymike18

Hmm, I hear you on not wanting to make it a bigger project.

After I replied I saw some "Related Topics" listed below this thread that had similar discussion. One pointed out that if your primary fan is shot, then the secondary fan will kick on at a higher temperature threshold and it will sound very jet-like when it operates. Is your primary fan operating normally?

4Driver4

Something is wrong. Either the clutch on the mechanical fan is stuck (most likely) or these is an issue with the low speed fan.

I think VAG-COM output tests can check electric fan operation.

desertsage

grabo, is the resistor that you're speaking of the horizontal silver plate with wire connections mounted on the underside of the left body bumper/impact frame member? If so, I have that fully accessible right now and can take a resistance reading for you. That reading might not be reliable as I have yet to determine why the AC is not working on this '96 project car that I've newly purchased.

grabo

Primary fan (mounted to the engine with a variable-speed clutch) seems to work fine. Engine never heats up past the middle of the temp gauge.

grabo

I'm sure it can, except that I'm unable to scan the HVAC unit - it just errors out with a bunch of garbage characters listed for the module part number.

I suppose the problem could be the HVAC module itself...

grabo

That's exactly the part!

desertsage

I'll isolate and measure the resistance.