Electric fans on S4's

mmolto · #1 (**permalink**) 06-08-2008, 03:08 PM

Hi gang,

I have questions reated to the two electric fans in my 1991 Spider: I tried search other threads but was getting anything conclusive.

1) Are both fans supposed to come on at the same time (i.e 175 Degrees)? or is one for AC cooling and the other for normal cooling? Any clarity would be appreciated

2) Does anyone know what people typically do to make their fans run all the time? The PO wired my fans to run all the time as soon as the ignition is turned on. This car used to be in the south US where temperatures were hotter... this is why this was done. I'm thinking about going back to the way the fans are supposed to run.

Thx

**Gubi** · #2 (**permalink**) 06-08-2008, 04:38 PM

If you have the Cardisc, the wiring diagram is on page 40-16.

From that, both fans are supposed to turn on together. They are triggered either by the radiator thermal switch or by the A/C pressure switch. Either one of those switches will actuate the cooling fan relay (fuse box, top row, second from left per the wiring diagram) which then actuates both fans.

The easy way to make both fans run all the time would be to pull the relay and then hotwire the 30 and 87 connectors with an appropriate fuse, or internally modify the relay. Considering that the PO wasn't bright enough to realize that this mod really wouldn't make the engine run any cooler (the fans come on ANYWAY when the engine gets hot) my guess is to look under the hood for some horrific wiring bodge.

mmolto · #3 (**permalink**) 06-10-2008, 05:43 AM

Gubi,

Watch what you say buddy!!!.. The previous owner was my dad!!!! Kidding!

Thanks for the info. I'll do some investigation.

Since I currently have two S4 spiders (temporarly), Do you think a good method to test your relay mod theory would be to simply switch the relays between the vehicles?

Thx again!

**Gubi** · #4 (**permalink**) 06-10-2008, 07:57 AM

Sure, that's easy enough to try. If that doesn't work, compare the wiring on the two cars and see if you can figure out what was modified. It's a pretty simple system to run the fans, so it shouldn't be too hard.

**Gubi** · #5 (**permalink**) 06-11-2008, 11:41 PM

You know, I was lying in bed tonight thinking about this and I realized...I'm a freaking idiot. The easy way the PO would have done this would just be to jumper the thermal switch at the radiator.

Duh. Sorry to overthink this. In my defense it's been a *really* long week, and my brain is tapioca about now.

mmolto · #6 (**permalink**) 06-12-2008, 04:50 AM

Gubi,

No problem at all. it happens to all of us.
*
I did a bit of investigation and here is what I found out so far:

1. I swapped the Fan Relays and the fans still came on with the ignition

2. Removed the Fan Relay and the fans did not turn on

Based on the above it look like nothing was done to the fuse box or the relay itself.
*
I started thinking about the thermal switch also. I took pictures of the thermal switch on both vehicles and from what I can see they are the same. 2 red wires on one side and 1 black on the other side.

I'm attaching pictures from the vehicle with the constantly running fans so you can see what is there. Does anything seem "out of whack"?

How would someone typically jumper the thermal switch? Could simply reversing the wires on the termal switch make a difference?

Thanks for you help

**Gubi** · #7 (**permalink**) 06-12-2008, 07:38 AM

Okay, the relay tests tell you that the problem is under the hood.

Those connectors look normal to me. To jumper it, you'd pull off the connectors and directly connect the black wires to the brown, and that hasn't happened.

One of those brown wires runs back to the relay, the other goes to the AC trinary switch. The way the system works is if that brown wire grounds (either by the radiator thermal switch closing or the AC trinary switch closing) the fans run.

I would start by pulling the wires off the thermal switch. If that fixes the problem, your thermal switch is likely stuck closed which can be verified with a voltmeter. If it still runs, the wires may be jumpered at the trinary switch.

The trinary switch is at the top of the A/C filter-drier unit, which is under the very right front of the car - you can only see it from underneath I think. It's a black coke-can thing with two A/C hoses coming into the top and an electrical sensor with two connectors. The brown wire should run from the radiator thermal switch directly to the trinary switch...see if it's jumpered there.

If you unplug both the trinary switch and the thermal switch and the fans STILL run, then I need to think about it some more.

mmolto · #8 (**permalink**) 06-22-2008, 07:35 AM

Quote:

Originally Posted by Gubi

Okay, the relay tests tell you that the problem is under the hood.

Those connectors look normal to me. To jumper it, you'd pull off the connectors and directly connect the black wires to the brown, and that hasn't happened.

One of those brown wires runs back to the relay, the other goes to the AC trinary switch. The way the system works is if that brown wire grounds (either by the radiator thermal switch closing or the AC trinary switch closing) the fans run.

I would start by pulling the wires off the thermal switch. If that fixes the problem, your thermal switch is likely stuck closed which can be verified with a voltmeter. If it still runs, the wires may be jumpered at the trinary switch.

The trinary switch is at the top of the A/C filter-drier unit, which is under the very right front of the car - you can only see it from underneath I think. It's a black coke-can thing with two A/C hoses coming into the top and an electrical sensor with two connectors. The brown wire should run from the radiator thermal switch directly to the trinary switch...see if it's jumpered there.

If you unplug both the trinary switch and the thermal switch and the fans STILL run, then I need to think about it some more.

**Gubi** · #9 (**permalink**) 06-22-2008, 02:49 PM

Well, it looks like you found your problem. It's going to be tough to diagnose remotely...I think you'll need to trace the wires and use a multimeter and figure out what was done. But it sounds like they maybe screwed something up when they did the retrofit. That, or you've got a bad trinary switch.

The reason for two switches is that the radiator switch turns on the fan when the engine hits a certain temperature. The trinary switch is a pressure switch that turns on the fans when the A/C is turned on. This maintains airflow over the condenser when you're stopped and keeps the A/C blowing cold.

The trinary switch has another pressure switch in it that disconnects the electromagnetic coupling at the compressor if the pressure in the system gets too high or too low, which is why it should have four wires going into it. I'd suggest you get the wiring diagram from the Cardisc and try to figure out exactly what was done to the A/C system.