Radiator Fan stuck on (A100)

ovalking · 08-04-23, 08:25 PM

Connected up the battery after winter hibernation, and the radiator fan started running - made me jump! It won't stop, so have unplugged the motor connector for the moment.

1988 Audi 100 with KU engine.

I checked the resistance across the terminals of the thermo switch screwed into the radiator - that is correctly open circuit.
However, I measured about 1K* across the terminals of the disconnected thermo switch plug (so not a clean short, but I was expecting open circuit). I measured 60* between one pin and ground (probably as expected). What is correct?
I removed relay no. 2 and confirmed it is open circuit between pins 30 & 87, so the relay is not stuck.

Ideas on what to check next appreciated.

Isimmo · 08-04-23, 10:45 PM

Our Ur radiator fan occasionally gets stuck on, swift firm but gentle 'tap' of the thermostatic switch at the bottom right of the radiator sorts it out for a few months.

tractor Dave · 09-04-23, 06:00 AM

Is that the turbo engine? If so, has after run relay (324) which controls cooling after ignition is switched off. Small zender diode in that relay fails causing constant fan. Common on S2.

Sandro · 09-04-23, 07:46 PM

Sure it does not have 2 fan relays? One for speed 2 and one for speed 3?

ovalking · 09-04-23, 08:30 PM

Thanks for responses.

Quote:

Our Ur radiator fan occasionally gets stuck on, swift firm but gentle 'tap' of the thermostatic switch at the bottom right of the radiator sorts it out for a few months.

Not sure I understand the purpose of the tap. I confirm the switch is open circuit, which I thought was correct?

Quote:

Is that the turbo engine? If so, has after run relay (324) which controls cooling after ignition is switched off.

No turbo on KU. There seems to be more info on the Turbo for some reason!

Quote:

Sure it does not have 2 fan relays? One for speed 2 and one for speed 3?

No not sure. I'm only seeing one relay in the (rather confusing) Haynes wiring diagram. Where/what is the second relay?
I've got '214' in slot 2.

ovalking · 15-04-23, 01:35 PM

Quote:

Originally Posted by ovalking

I'm only seeing one relay in the (rather confusing) Haynes wiring diagram. Where/what is the second relay?
I've got '214' in slot 2.

It turns out there is a second circuit / relay - '272' in slot 3.

I found out by accident as had to remove slot relay 3 to get access to slot 2.
And discovered that removal of relay 3 stops the fan runing!

Now to work out how this circuit is supposed to operate. The Haynes gives few clues...

Sandro · 15-04-23, 09:57 PM

Could be relay itself, or thermo switch on engine. With relay at the place, disconnect the thermo switch at the engine. If fan is still on, then it's relay at fault.
Dismantle, some WD40 and manual movement might bring it back to life.

ovalking · 17-04-23, 08:02 PM

The problem has been resolved by replacement of the slot 3 relay. But I never did suss out how it is supposed to operate!

The old relay gave an unstable resistance across pins 30 & 87 (few hundred K*), and the new one was a few M*. So neither a clear open or short circuit; this device is not a straightforward relay.

Hanuman · 17-04-23, 09:01 PM

The few Meg-Ohms reading will be the open circuit, and what you are actually reading is probably just the total circuit impedance itself. There’s more than just coil and contacts inside some of these.

The few hundred K-Ohms is quite possibly damaged/welded but dirty contacts. Very high resistance. If this is one of those relays with a metal case, slightly crimped on, for fun you could open it, and have a look. Some fine emery or wet-dry could clean the contacts.

I say could. It’s also probable that a circuit component such as a capacitor has actually failed. Arcing across contacts will recur.

08-04-23, 10:45 PM	#2
Isimmo Super Moderator Classic Audi Club Member Join Date: Jul 2007 Location: Sissix-by-the-sea Posts: 15,218	Our Ur radiator fan occasionally gets stuck on, swift firm but gentle 'tap' of the thermostatic switch at the bottom right of the radiator sorts it out for a few months. __________________ 1985 WR quattro, 1985 20vT RallyRep, 1993 MTM ABY S2 Avant. 2010 S3 Sportback.

17-04-23, 09:01 PM	#9
Hanuman Trickster Classic Audi Club Member Join Date: May 2009 Location: Heckling from the cheap seats, Phnom Penh, KoW Posts: 7,016	The few Meg-Ohms reading will be the open circuit, and what you are actually reading is probably just the total circuit impedance itself. There’s more than just coil and contacts inside some of these. The few hundred K-Ohms is quite possibly damaged/welded but dirty contacts. Very high resistance. If this is one of those relays with a metal case, slightly crimped on, for fun you could open it, and have a look. Some fine emery or wet-dry could clean the contacts. I say could. It’s also probable that a circuit component such as a capacitor has actually failed. Arcing across contacts will recur. __________________ I wish they would keep the damned Chinese away now that I can go home, so that I can enjoy Fish amok and a draught Angkor

09-04-23, 06:00 AM	#3
tractor Dave Senior Member Classic Audi Club Member Join Date: Apr 2018 Location: Wales Posts: 1,174	Is that the turbo engine? If so, has after run relay (324) which controls cooling after ignition is switched off. Small zender diode in that relay fails causing constant fan. Common on S2.

09-04-23, 07:46 PM	#4
Sandro Grown up member Join Date: Feb 2011 Location: Chelmsford Posts: 651	Sure it does not have 2 fan relays? One for speed 2 and one for speed 3?

15-04-23, 09:57 PM	#7
Sandro Grown up member Join Date: Feb 2011 Location: Chelmsford Posts: 651	Could be relay itself, or thermo switch on engine. With relay at the place, disconnect the thermo switch at the engine. If fan is still on, then it's relay at fault. Dismantle, some WD40 and manual movement might bring it back to life.

17-04-23, 08:02 PM	#8
ovalking Grown up member Classic Audi Club Member Join Date: May 2009 Location: Ipswich Posts: 271	The problem has been resolved by replacement of the slot 3 relay. But I never did suss out how it is supposed to operate! The old relay gave an unstable resistance across pins 30 & 87 (few hundred K), and the new one was a few M. So neither a clear open or short circuit; this device is not a straightforward relay.