time666

I have a problem which is intermittent with the heater controller. The front fans behind the dash seem to just run all time ramping up and down from medium to high speed. Regardless of fan speed selector position.

If I adjust the the fan speed to max then back to off, then the fans do turn off for about 10 secs then slowly come back on. It's driving me mad!

I had a spare heater controller with the car and it happens with both. Also tried swapping the front of the controller via disconnecting the ribbon cable and cleaned temp sensor too, both cabin temp sampling fans work do and clean.

For the moment I was going to disconnect the heater controller from the back which does kill the fans, but knowing that the heating system is not straight forward, would this be a problem for cooling of the engine or the oil??

Not sure if I am ok to do this, or any ideas on why the fans would ramp up and down like this would be appreciated.

rollingjack

Yes, disconnecting plugs behind controller leave you without front oil cooler fan.
Of coarse, this will be problem only in slow city traffic.

Maybe you can check temp sensor in rear? By rear fan.

time666

Thanks Rollingjack for confirming that. I did wonder if the the controller had some influence on the oil cooling side of the car.

I think I know the temperature sensor your talking about, is that the little sensor on the fan tube near the dizzy?

Looks like a helical resistor type with 2 wires connected. I will try searching some more for the correct ohms value to see if it's within normal range.

Wonder if this was faulty it would result in the front fans running.

Thanks for the advice too

boxsey911

No control over the speed of the front fans usually means that your final stage amplifier has failed. It's under the bonnet under the plastic cover near the windscreen. You'll see a plug in the middle (near the petrol tank) and underneath what looks like a triangular piece of aluminum. It takes about 10 minutes to swap it. Albeit being a bit fiddly. Used ones are usually about £60 off ebay.

In the mean, you can stop the whole of the heating system working by pulling fuse number 1 in the fuse box. I've run without that fuse with no problems (except for it being cold!).

time666

Thanks Steve
I did not know about this final stage amplifier unit, I can see now how the both LHS and RHS fan units seem to be wired to this device so would make sense.

I think I know the part you mean; is this the part:
http://cgi.ebay.co.uk/PORSCHE-964-HE...7#ht_500wt_731

I will take a look today to see if I can find it, can this be tested with a multimeter? Looks like there's a few wires and signals from the main heater controller within the car so could be a difficult one to test. May just be easier to take a chance and get a replacement from ebay (could always stick it back on if not!)

Be nice to know what function this amplifier plays in controlling the fans, fair play the heater system does seem overly complicated to me!

Thanks for your help

boxsey911

Yep that's the one. Watch you don't drop the two nuts and bolts while swapping over as you'll never get them back. I don't think anyone has had much success repairing one, hence they are regularly in demand.

Swapping it is the easiest test - you don't even have to remove the old one to test it. Just plug it in and try it out.

p.s. if you want a good summary of the 964 HVAC system, look here:

http://www.964uk.com/html/Tech%20-%20Heating.htm

time666

I have found the signal amplifier near the petrol tank firewall, but just realised that one of the two cabin blower fans is not running, and thinking could this upset the rhs fan causing it to ramp up and down in speed.

The car is RHDrive and the RH fan blows inside through the left hand side of the upper windscreen demise (nothing on rhs) and then also blows on the rhs drivers footwell. So I assume that if the LHS fan was to function, then the rhs upper windscreen demist and the LHS footwell would be operating too?

Also the wire to the LHS fan is just ever so slightly warm (not hot in anyway) so adds to my suspicion of failure.

Should both fans LHS and rhs run at the same time?

I checked the rear blower works fine when the ccu is set to heating.

I was going to just disconnect the cabin blower fans via the signal amplifier so I only lose these fans for the short term fix as my thinking is that I will retain control over the oil cooling fan and the rear blower fan this way, rather that pulling fuse 1 for the whole system. What do you think?

Help much appreciated as usual!

boxsey911

While trying to find a fault on my HVAC system I ran around with that amplifier disconnected for a while with no adverse effects on the oil cooling.