UrbanSasquatch

Hey, this is my first 964, and after I drive it, with the engine off I notice a couple of things:

1. A lot of hot air circulating from the area of the rear wheel wells. My '88 didn't do this, nor does my '71.

2. The heater blower fans continue to run with the engine off.

My guess: Porsche decided to cool the engine post-shutdown, so designed some type of thermostat that will run the blower fans and open the heater boxes, so that air blows across the cylinders and out under the car at the sides, where I feel it in the wheel wells. Anybody know whether this system "evolved" from the earlier cars?

Thanks in advance.

John

'91 C2 Cabriolet
'71 911E Club Racer #806

Jeff Curtis

John, this is normal, the engine compartment heater blower motor cycles for awhile after engine shutoff...depending on the temperature of the ducting. It does this to assist in cooldown as your exhaust is nice and hot when you shut the motor off...and a lot of heat gathers in that ducting and your heat exchangers.

The hot air is expelled just ahead of your rear shocks...almost towards the brake rotors.

But this is EXACTLY what you're experiencing, hope this helps.

Bill Wagner

How hot does the engine need to be for the fan to come on after shutoff. I've never had mine come on and it makes me wonder if it's working properly.
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Adrian

Dear Bill,
VERY HOT. It is very unusual for the blower fan to come on whilst the engine is off. The most common reason for this is a failed NTC temp sensor in the pipe plumbed into the blower fan itself. It is not unusual on really hot days for the fan to crank up after a hot start for a few seconds. The other cause for the protection system to activate is if the oil cooler thermostat fails to open.
There is only one rear blower fan John. I do suspect a fault unless you are running terribly hot. The key issue which keeps coming up, "What is the engine temp indicator reading"? when you shut down or whilst you are driving.
Unless the engine is super hot and I mean super hot when you shut down the protection circuit for the rear blower fan should not activate.
Really hot fenders is actually quite normal at this time of the year in the Northern Hemisphere.
Ciao,
Adrian
911C4

PS: A little trick John which you can try especially seeing you have a cab. First monitor the engine temp. When driving, especially if stuck in traffic, crank the temp control **** up to the red dot position. Are you getting max heat inside the car and does the engine temp indication drop.

91C2wrencher

Well this is an interesting thread! On my '91 C2 the fan does not run-on after I shut it off, but if I turn the ignition back to the "on" position(with engine warm/hot) the fan will run and then seemingly never shut off! I've replaced the little sensor things(wound wire & probe) on the tube some years back with no change in operation, I just figured it wasn't bad. However, I begining to think that the fan should run-on, on its own. shouldn't it? I mean if its hot enough to come on "after the fact" shouldn't it be hot enough to come on automatically? could a relay be at fault here? Adrian? PS. I believe there was technical bulliten on this sensor(wound wire) and possibly a recall?

Andy Roe

91C2Wrencher,

I had the same problem with mine. The rear heater blower would run, even if I set the CCU temp to minimum. It could be a relay. Try swapping the Heater Blower Relay(Rear Fuse box) with the Oil Cooler Relay (Front Fuse Box) The relay part number is 811 951 253. If after swapping these, the problem is cured then you know it was the relay at fault.

In my case it turned out to be a faulty SWITCH on the temperature sensor in the engine bay.

I was lucky, my mechanic had a spare one & fitted it at no charge!!

I hope this helps - I was worried that it was the CCU at fault and thus $$$$ !

Andy
<img src="graemlins/jumper.gif" border="0" alt="[jumper]" />

91C2wrencher

thanx, Andy. I'll give it a try. Still looking for more info on how this circuit is supposed to operate.

Bill Wagner

Adrian:

The hottest my engine has ever been was when I was stuck last summer in stop and go traffic for about 30 minutes on a sunny day with an outside temperature of about 95 deg F. The needle on the temperature gauge hit the 10:00 marker (248 deg F). Nothing came on...not the rear heater blower while stopped, and out of curiousity I checked the front oil cooler when I restarted the car. That fan didn't come on either, but then again it had been sitting for about 10 minutes before I restarted the car, so maybe the oil cooler had cooled the oil (at least in that area) enough that the oil cooler area was cooler than the engine temp.

Thoughts anyone?

Adrian

Dear Bill,
The point with the rear blower coming on is that it is not looking at engine temp directly. It is looking at the air temp in the plumbing. It is looking at the air temp of the air supply to the blower fan. Even with your engine super hot you will find the top of the engine bay to be still quite cool. I have had ice on the top of my engine still there even though the engine temp was up to normal. This is the primre reason the fan does not come on.
Now to 91C2wrenchers question. Wrong part. The part you are talking about is called the blower resistor. The recall was to replace the blower resistor with one that had a heat related protection device installed to detect a seized fan motor and cut off the current supply to the motor to avoid a fire. A failed blower resistor will stop the fan coming on. The blower resistor is the low speed fan control. When we are talking the blower fan for addtional engine cooling with engine running we are talking about the air temp sensor in the plumbing not the blower resistor.
This sensor is a NTC or negative temp co-eff resistor. As the temp goes up its resistance drops. These things tend to short out with age and this tells the controller that you have too high temp air in the pipes and it switches the fan on. It never gets told to switch fan off.
Ciao,
Adrian
911C4

91C2wrencher

Adrian, thank you. I now understand the function of the blower motor resistor. Thus the NCT is the "thermistor" probe on top of the duct controls this cooling function. I have replaced this in the past and this morning checked the resistance of both NCT's at room temp. @10K ohms and under hot tap water its @5K ohms, so it would seem that both are funtioning properly. This does not however answer my concern that the fan does not seem to come on automatically, only after I've shut off the hot engine and move the key back to the "on" position does this cooling fan seem to operate. I've used this sequence to cool down the motor at DE events, but often thought it should be an "automatic" sequence since at most of these DE's the motor really was quite hot as I came into the paddock. Any other devices to look at that control this operation? PS Adrian you are an invaluable source of 964 info thank you.

Randall G.

Hey Bill W.,

I think your oil cooler fan should have came on before your temperature reached 10:00. At a minimum, I would verify the thermostat is opening, and jump the slow-speed fan to see if it runs.

BTW, feel free to add my oil cooler fan DIY to the webpage you're building. Or, perhaps just provide a link (easier, I think).

Randall G.

Hi 91C2 Wrencher,

As your car is running hot at the track, you might seriously consider disconnecting the oil cooler temperature sensor while at the track. This will "force" your oil cooler fan to run in fast-speed--very effective.

Better yet, you can add a switch to run the fan in fast speed. This is down by splicing the switch into wire G12 at the back of the CCU. G12 runs between the oil cooler temp. sensor and the CCU. Opening the switch makes the CCU think it's lost continuity, and it starts the fan in fast-speed. Closing the switch restores normal operation.

Bill Wagner

Adrian:

Thanks for the info...it makes sense.

Randall:

The weather here tomorrow may just get hot enough for the oil cooler to come on..at least I may be able to check it. I'll use the site you have under your profile as a guide (nice start by the way )

Thanks for the link offer. I WILL include your site in my list of links. It is the site you have listed under your profile, right??

<img src="graemlins/beerchug.gif" border="0" alt="[cheers]" />

Randall G.

[quote]Originally posted by Bill Wagner:
<strong>Adrian:

***snip***

Thanks for the link offer. I WILL include your site in my list of links. It is the site you have listed under your profile, right??

<img src="graemlins/beerchug.gif" border="0" alt="[cheers]" /> </strong><hr></blockquote>

That's the one!

Bill Wagner

Randall,

Just to let you know, I let the car idle today and the oil cooler fan kicked on with the car running when it got to about 9:30 on the gauge. I have to assume it was on high because it was blowing air out almost as much as the AC was.

I'll use your site as link. Thanks.

<img src="graemlins/beerchug.gif" border="0" alt="[cheers]" />