AUXILIARY COOLING FAN PROBLEM?

[Mears]

Unless my senses deceive me my auxiliary engine cooling fan is acting very oddly recently! It sometimes comes on immediately with the ignition on a cold engine, it has NOT been coming on as expected after warm shutdown on a hot day but on one occasion DID kick in and switched to full speed when I switched the ignition back on again 5 minutes later. Any ideas please before I fry my engine? Perhaps there is more than one thermostat, one fed from the ignition and one not? []

 

[dommorton]

When the temp probe connected to my rear blower was shot the blower in the engine bay was all over the shop although mostly just stuck on max regardless of temp. Replaced this and the resistor and all is well again. Part Number for the temp probe is 964.659.120.00 and the Resistor is Part Number is 964.616.550.02. Both fit in the air duct off the fan housing of the engine. The Sensor is £30+vat and the Resistor £20.91+vat

 

[Mears]

Many thanks for your advice. I had seen the earlier post but could not see how it was the same problem. Now that I've read your reply and pulled it all apart, I can see that the resistor is also a secondary thermostat and can understand how it might well be playing up! Will get the parts on order asap! Actually my upper hose appears very prone to coming apart from the lower pipe so perhaps a good job I was looking around there today Thanks Again []

 

[Mears]

Well I've changed the sensor and resistor as advised but it's still doing it! On a hot engine the auxiliary blower will only come on with the ignition (and I think, turn off if I restart the engine). No ignition, no auxiliary cooling fan - odd! It certainly used to start running after the car was locked up and left as one might expect. Anybody any ideas please? []

 

[D Ward]

ORIGINAL: Mears Many thanks for your advice.  I had seen the earlier post but could not see how it was the same problem.  Now that I've read your reply and pulled it all apart, I can see that the resistor is also a secondary thermostat and can understand how it might well be playing up!  Will get the parts on order asap!  Actually my upper hose appears very prone to coming apart from the lower pipe so perhaps a good job I was looking around there today  Thanks Again []

hi, it could be the rear fan blower relay( located in the near side fuse box it the engine bay) it controls the high and low speed running and the low speed only running with the ignition off,the reason it only runs at low speed with ignition off is to save the battery & more importantly to protect the car from risk of electrical fire as the ballast resistor has build in bi metal fuse for protection if it over heats.as way of infomation the ballest resistor does not have a thermostat in it,it is only there to reduce the voltage down to around 9 volts,which is how the fan is able to run at a slower speed. Cheers

 

[Mears]

Thanks for your reply. Having read it and also pored over Streather's book (regrettably not 100% clear) I too believe the relay might be the problem but if not, it could be the A/C & Heating control unit behind the dashboard (which sounds horribly expensive). There has to be a timer somewhere that limits 'ignition off' fan operation to 20 minutes (or zero in my current case). Actually the little fan behind the dash control unit runs for about 20 minutes after switch-off although I cannot imagine why............

 

[D Ward]

as you rightly say its all controled by the heat and a/c control unit in the dash.so hopefully it will just be the relay.i think the timer for the small fan is the same that controls the blower fan and i belive thats the only reason the fan in the dash continues to run,as it seems to serve no real purpouse when the ignition is off,except that both fans share the same timer signal so it has to run on

 

[dommorton]

ORIGINAL: Mears Well I've changed the sensor and resistor as advised but it's still doing it!

Oh that's a pain! fingers crossed for the relay then. How did your old resistor look when you removed it? mine was very tired looking and it was clear it had been getting stupidly hot.

 

[Mears]

Thanks for your comments above. Old resistor was dusty but not obviously burnt at all. Relay helpfully had a circuit diagram on the side and it all tested fine on the bench (which doesn't surprise me actually; if relay was duff then fan would never work). So I'm possibly back to the 20 minute ignition-off timer but again I can scarcely believe that the blower does not share the same timer as the dashboard 'air sampler' fan (which for sure lasts 20 minutes after switch-off). The fact that the blower comes on with the ignition confirms that the engine is hot enough to switch the sensors on. Hmmm; getting perplexed here......I wonder if anyone knows the inner workngs of the dashboard a/c and heating control unit please? I do have the recent Porsche Post article on how to remove it but am cautious of doing so!

 

[szklarek]

Hi Removing it is relatively easy as identified in the article. Perhaps you could find someone with the same model (year and type) who'd be prepared to do a quick swap of the unit to check if that's what's causing the problem. Failing that maybe try your local indy who might be prepared to do the same for you. At least that way you might be able to quickly identify the cause of the problem. Regards Anton

 

[D Ward]

there is a guy from norway (i think thats where hes from) seems to be something of an expert on the heater/ac control unit,has done a great piece on his website,tells u all about it how it works,how to fix it and detailed pictures of the circuts and what it all does. I'll have a look later see if i can find the link and post it, or u can probly find it if u do a search on the various porsche forums or maybe someone on here know the site.your other option could be if u know someone with a bosch hammer or durmetic software and lead to read the fault codes, although it may not show a code if its a fault on a circut board

 

[D Ward]

there is a guy from norway (i think thats where hes from) seems to be something of an expert on the heater/ac control unit,has done a great piece on his website,tells u all about it how it works,how to fix it and detailed pictures of the circuts and what it all does. I'll have a look later see if i can find the link and post it, or u can probly find it if u do a search on the various porsche forums or maybe someone on here know the site.your other option could be if u know someone with a bosch hammer or durmetic software and lead to read the fault codes, although it may not show a code if its a fault on a circut board

 

[dommorton]

Having read your expectations of it's workings again these don't match up with the explanations I was given when mine played up. I was told the Aux blower comes on with the ignition to assist with the heat circ inside the car. With the temp to off it doesn't come on with the ignition. With it on mid heat e.g. 20 it will come on gently. Anything higher and it comes on full blast. This is how mine now operates having replaced the temp sensor and resistor. Here's a link to the site D Ward is referring to www.ccu.tore.bergvill.com

 

[Porker993]

There is a pretty good write up on this fan and it's expected parameters and working conditions in the factory workshop manual. It's quite complex, and depends on engine temp, time after switch off, and whether the ignition key is in the on position. When you factor in internal cabin temp settings, things can get quite complicated ! The first thing, as has been said above, is to replace the ballast resistor, fan relay and temperature sensor in the engine bay, which has been done. After that, it's time to plug in the diagnostic tester, I think, before you start wasting money on trial and error substitution of components. That will highlight any faults in the system and prevent guesswork. I speak from experience !

 

[Mears]

Thanks all. Yes there comes a point where a diagnostic test might help. In fact after speaking to a professional 993 mechanic I decided to be scientific and test the new temperature sensor accurately myself with a thermometer in a cup of hot water (protecting the sensor in a polybag of course!). And indeed it switched the blower on at 75 degrees during the 20 minute ignition off period as per spec. I think the slightly cooler weather after I swapped the components had been fooling me and it is probably working OK now. For the record it only switches to low speed when ignition is off. With engine running it will kick in at 40 degrees and then switch to high speed at 75. It is also useful to know that disconnecting the sensor will cause the HVAC unit to switch the fan ON. The Nordic article is great too but also a little scary!