A3 Cooling Fan Problem

[MOTOW]

Hello people.
When I start the engine from hot or cold both fans kick in and out all the time and temp only reaches 70.
Now temp reaches 90 with no fans which is normal but when temp rises both fans kick in not the small one then large one.
But somtimes neither kick in and temp rises to 120 but if the air con is switched on both fans start and temp comes down but only sometimes.
I think the problem is with the fan switch or the fan control unit.
Its an AUDI A3 1999 1.8 SPORT WITH AIR CON.
I would like any input that will help . THANKS.

[kow]

Sounds like a faulty switch or possibly electrical connection on the Radiator.
Best thing to do is find the switch, usually part way down the side of Rad., pull the connector off and start the car, confirm that none of your fans start as they are doing now. Stop engine.
Then turn ignition on and check you are getting 12v to the switch using tester, then try shorting between the pos and other wires the fans should come on as this is simulating the switch operating with temperature increase at Radiator. If they do all start and run OK then I would change the switch. Also check the wiring and connections in this area, give them a clean etc.

[MOTOW]

When switch is disconnected and live is red, shorting to one with ignition switched off starts the fans at low speed. The other terminal nothing happens until ignition is switched on then fans come on at full speed.
On the cylinder head there is another sensor which I think is to do with the air con how does this figure in the configuration.

[kow]

Think you have confirmed that the wiring and relays etc are all OK, I assume the slow speed fan is there to come on if necessary when you stop, switch off etc. and this keeps the Rad at N W Temp. The higher speed is for when idling hot in traffic etc.
Don't see why there should be another switch on the engine to affect the Rad, they are normally for indication on dash and the ECU for cold/normal starting / running etc.
I would change the switch on the Rad, if your quick with the changeover you can do it without draining the system. Have it all slackened off and the new one ready and hey presto, just top the header tank up when things have settled.

[MOTOW]

Hi its me again.
Tested thermal fan switch - ok.
Ran diagnostic with VAGCOM to module but after scan says comunication problem with HVAC.
Just doing that and fans came on going up and down again on high speed.but engine cold.
No aircon switched on.
Pulled connecter off thermal switch on bottom of rad made no difference,pulled sensor plug from cylinder head still no difference fans still droning up and down.

[kow]

Yep, sounds a right b-gger,at this stage I would be looking to get my hands on a wiring diagram. As I remember from looking at an A3 there will be a cooling fan associated with the air con system which should run once a/c selected to condense the refridgerent. This fan is only to look after the air con. The other larger fan is to cool the radiator and engine.
So it's a case of trying to figure out from the wiring diagram and your indications what it might be.
Assume you are miles from Kendal, so a look see is out.
As a start on a modern car, all the wiring will be going to and from the right place unless someone's been at it.
So have a wiggle at all the connections. With the ignition off it's safe to pull all of them appart and reconnect just in case you have a duff contact/connection, all you can find in engine bay. Then check all your fuses are OK, do the same for the relays.
It might be and probably is something simple, giving the wrong or missing info to a relay etc.
Good luck

[MOTOW]

Thanks for info.
have tried all that.
It is all coming down to the heating ventilation aircon control unit.
Of course this is the dearest bit that you just have to replace.

[kow]

Sorry it's not a simple fix, let us know how you get on.

[MOTOW]

Well, after much searching and testing there are two faults.
The water pump was the original audi plastic impeller, so it now has the modified brass one, works great.
The other fault was with the fan control unit under the battery.
After I fitted those and a new thermostat all is well.