deliriousga

I want to confirm this before I order the unit since it's so expensive. I'm talking about the control unit under the hatch switch on the passenger side of an '87 S4.

My fans aren't working at all so I did the diagnostics on the control unit. Everything checked out except two of the tests.

1) Testing Pin 7 in Plug I with pin 6 on the unit's plug II connection w/ ignition & A/C on says it should read 7V but I'm getting .59V.

2) Testing Pin 7 in Plug I with pin 8 on the unit's plug II connection w/ ignition & A/C on says it should read 7V but I'm getting .59V.

My A/C compressor won't kick on either which I thought was just low freon since it sat for so long while I worked on it, but it seems this might be from the controller as well as the fans. Is this a definite sign the control unit is bad?

Also, is there a way for me to connect the fans around the control unit temporarily so I can drive the car until the unit gets here if I need the new one?

Thanks for the help!

tveltman

I suggested this once before for a person with an aftermarket fan, but for you, it may work. I had trouble with my fan not running when it should, and I determined that it was not receiving the correct signal from the temperature sender when the ignition was on, although it was working properly with the ignition off. This didn't make sense to me and in a fit of frustration, I jumpered the two points together (with the plugs still connected, you have to pull off the black caps on the plugs to get at the backside of the connectors). IIRC it was pin 1 on plug 2 and pin 7 on plug 1, though you might want to check that. When I jumpered these points (for just a couple seconds) with the car running, the fan turned on and has been running exactly as it should ever since. Maybe it reboots the system, idk. (what it is actually doing is telling the fan control unit that the engine is at something like 600,000 degrees C, and thus the fan turns on). When you remove the jumper, the fan control works just as it should and follows the correct voltage profile in the FWSM. I don't know how safe this is for your control unit, but I thought mine was shot and when I did this, it started working again and has been for a couple months.

If you look through teh wiring diagram (I don't have mine as I'm at work), you should be able to find the voltage input to the control unit, voltage out of the unit to the fan, and the ground. Jumper where appropriate and hook up to the fan wiring, and possibly a switch. That will keep your fan running while you wait for the new part. Be sure you use sturdy enough wire and heat insulation, as you definitely don't want a fire. If you use a switch, get one that is rated for plenty of current, although I doubt the fan draws all that much.

Also, if what I just described is a bad idea, someone please point it out and explain why, as I have only my personal experience to go on, and that has been 100% positive

EDIT: Did you check the continuity of all the wires? It may be that you have a load of resistance in one of the wires to/from the fan or the A/C compressor that is causing you trouble. In that case, the control unit may be working properly but receiving improper signals due to the high resistance in the wire. Check the coolant water temperature sender very carefully (the one in the bottom front of the radiator) as the sender itself provides variable resistance, with high resistance corresponding to low coolant water temperature, which would result in the fans not turning on (i.e. the control unit thinks the engine is not hot enough to need cooling)

dr bob

That signal from the AC comes through the second relay contact from the relay that engages the clutch. It's unlikely that the fan controller signal contact is burned but it could happen. Coincidence with the AC not working? No such thing in a 928...


Does the light in the AC button work when you push it? If not, have you done work in the cenbter console recently, like pull the radio out? There's a little plug on the back of the AC button panel, right behind the AC button. There's no retainer on that plug so it very (too!) easily pulls off if the little wire harness gets even a gentle tug on it during console work.


On the fans, follow the basic troubleshooting procedures in the WSM, but add a few more just to be sure. Fuses OK? And are the fuse holders still OK? They melt if the fuse connections come loose.

The fans get power through dedicated feeder wires that run directly from the battery plus to those two fuse holders. Do a full battery service, disassemble and clean all the little red wires that attach to the positive terminal. All but a couple fan-problem cars I've worked on have had connection problems at the battery terminal ends. the other two needed the underhood part of the controller pair rebuilt or replaced.

I have a spare set of controllers, and because of that I've never needed them. They are available for troubleshooting help if you exhaust all the common failure possibilities.

deliriousga

Thanks guys!

I checked the fuses and fuse panel and all is well there. I also cleaned the connections at the battery and the battery terminals.

Dr. Bob, the A/C light comes on and the interior fans work at all speeds.

Should I try swapping the A/C relay and see what happens?

dr bob

It's unlilkey that the AC relay is keeping the fans from working at all, but it will interfere with teh fans coming on when the AC is supposed to be runnng. There's a separate troubleshooting procedure for the relay described by many including me in the regular tips sites.

Did you try powering each fan motor separately and directly from the jump-start post? Proves that the motors are OK. It's a challenge testing the power supply lines to the front module with the round connector still attached. What else did you find in the primary module testing (the one under the pass side cover)?

Mrmerlin

have you tried disconnecting the connector on the front of the radiator its on the driver side of a USA car . With this done thew fans should come on with the key on

mark kibort

I had problems with mine and took it apart and found loose connections. then, later it failed again. being on a race car, i found it safer to just wire up a redundant pair of fan relays . one fan controled manually, and the other controled via the old style temp switch in the radiator to turn on the fans. Now, they are on or off, not like the cool variable speed system of the S4.

deliriousga

I jumped the fans to the jump start post and both fans work. I took the controller out and opened it up looking for loose connections but I don't see any. Also don't see any loose connections on the control unit connectors. Same readings from the control unit as before, everything is as the WSMs say they should be except for pins 6 & 8 on connector 2. Both fan fuses and fuse connections are fine, no melting, looseness, etc.

I've followed Wally's write up on the 928 Specialists site & the WSMs and both say it's likely a bad control unit. Anything else I should try before I fork over the $$$ for a new control unit?

blown 87

Come get the one out of my car and see if it fixes it, I know mine works.

WallyP

"Come get the one out of my car and see if it fixes it, I know mine works."

Often the best possible diagnostic technique.

blown 87

"Substitute with a know good unit"

deliriousga

Great idea. Thanks for the offer blown 87, I'm gonna see if one of the guys close to here can do the same. If he can't I'll make the 2 hour drive down on a weekend and try it out.

I ran a straight switch to each fan with a 30amp fuse on each switch so I can manually turn the fans on and drive it so it will make the trip down there if needed.

Thanks again!!

blown 87

Bring me a Chili Cheese Dog from the Varsity if you come down.

I have been wanting a couple all day for some reason.

mcholdfast

John, you're welcome to try the controller from the green car. If it works, you can keep it until you get a new one since the green car is hibernating until I get the TB/WP done.

Whata ya have? Whata ya have? Whata ya have?
I'll bring you a couple dogs next weekend.

blown 87