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Heater Blower

robwright

robwright

New member
Hi. I am posting this on behalf of a friend who's internet is up the swanny at the moment. He has just purchased a 1990 964 carrera 2. After finding that the heater didn't work we began the investiagtion. The blower in the back wasn't working. Did all the usual fuses, relays - all ok. Removed the blower and low an behold it was disconnected inside. Reconnected it and it does run but only at full speed - all the time!!! Question is - is there a resistor matrix to control the blower speed like in my 944 and if so where is it? If anyone has any other things to try all suggestions would be gratefully received. Sorry to be a front engined man invading your forum. But it is all for a good cause lol. Come on give a virgin 964 and indeed Porsche owner a helping hand.
 
Rob,

There is, indeed, a ballast resistor which drops the voltage to the blower to enable slow speed running. It's clipped into the left side of the black plastic blower plenum - right opposite the fuse/relay panel in the engine compartment. There are 2 wires going to it. The later design incorporated a bi-metal safety cut-out which can be reset by pressing it but the early types didn't have this. If the resistor is open circuit it's likely that the blower won't run at all in slow speed. When full speed is called for (by the CCU) the resistor is short circuited and the full voltage applied to the blower motor.

Hope that helps.

Regards

Dave
 
Feel free to give me a call tomorrow & I will explain how the system works as it does not use a series type resistor pack as we are used to. 07803719911. Cheers.
 
Dave thanks for the reply mate (wasn't sure you guys would entertain me) [:D]. OK I think I am starting to get this rear engined stuff lol. I take it the fan is not like mine or most others which has distinct speeds controlled by the fan switch position. Rather it blows hot air to the front all the time, which is diverted either to the wheel arches or the heater unit as required. The fan speed for the cockpit is controlled some other way?? I believe there is a fan on the back of the climate control unit. If IRRC the cockpit air will cut in intermittently on the car whist varying the temperature knob. Unfortunately the owners manual has been little help to me.
 
Rob, there are cabin fans at the front that are controlled by the CCU knob. These fans are either side of the main HVAC unit that sits under the panel near the windscreen in the front boot. The rear blower gets the air to that point. As well as the cabin fans there are a series of servo motors that direct and mix the air to the various vents. The small fan on the back of the CCU is there to sample the air so that it can adjust the temperature. It's a very complicated and sometimes temperamental system!
 
Yes mate I was wondering why the fan switch felt more like a pot than a switch if you know what I mean?? Is there a simple way of determining how the HVAC unit should work under normal conditions? The car doesn't have air conditioning. The rear fan is disconnected at the moment but has no intake duct from the engine coooling fan. I have a feeling if reconnected the CCU will do it's stuff and we have just been ignorant as to how it operates. Appreciate all your help so far guys.
 
ORIGINAL: robwright

BTW you are from my glorious home county. Whereabouts are you?
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I'm in the sleepy village of Lymm. Dave knows far more than I do about how it all works and I'm sure will come back with a few pointers. However, it does seem strange that it was disconnected. Almost suggests the previous owner did that on purpose. But also, they might simply have forgot to plug it back in as the blower oftent gets taken off for things like servicing.

In the mean time click on the 'The 964 web site" link in his sig for lots of information [:)]
 


[/quote]
Almost suggests the previous owner did that on purpose. But also, they might simply have forgot to plug it back in as the blower oftent gets taken off for things like servicing.
Click to expand...

The fan was disconnected internally. There are two spade connectors just inside the small cover on the back of the fan. The contacts were quite grubby suggesting they hadn't been connected for some time.
 
Rob,

As Steve mentioned, the HVAC system on the 964 is what some might call "over engineered"!! The rear blower is there just to aid air flow through the heat exchangers, via the sill tubes, up to the mixing chamber ahead of the dashboard. Everything is controlled by the Climate Control Unit (CCU) including the rear blower, the 2 cabin blowers, the sampling fan and the 5 servos controlling the air temperature and distribution. Additionally the CCU controls the blower fan on the oil cooler and the a/c compressor and condenser fans if you have a/c fitted.

The fan speed selector on the CCU is, indeed, a fully variable potentiometer and not a multi-position switch. The speed of the cabin blowers is continuously variable by the CCU via a final stage amplifier mounted under the fuse box in the luggage compartment.

Only when the temperature selector is on the blue dot and the fan speed set to 0 will the rear blower not run. As soon as heat is required in the cabin or air volume needed via the heat exchangers, the rear blower will start up. There is also a supplemental function of the rear blower to aid engine cooling after shutdown although that is quite rare in UK-type temperatures. When cabin heat is not required the CCU closes the entry to the mixing chamber via the 2 mixing chamber servos and the difference in air pressure either side of the flapper valves causes the excess heat to be dumped through the flapper valves to the rear wheel wells.

Cool air enters the mixing chamber from the front air vent via the fresh air servo controlled flap. The remaining 2 servos control the air distribution within the cabin - either up to the screen vents or down to the footwell vents (or any mixture thereof decided via the sliders on the CCU). If a/c is fitted the fresh air flow passes over the evaporator on it's way to the mixing chamber.

As I mentioned before, the 2 speed rear blower is achieved via the coiled resistor mounted in the air flow in the blower plenum. It gets very (i.e. VERY) hot when in circuit and instances of fires have been seen when the resistor has not been mounted in the correct place. Slow speed is the most common mode - high speed is only really used when "Defrost" is selected or when the engine cooling function is being used (by the CCU).

Hope that helps??

Regards

Dave
 
Great write up Dave. You should put that in the HVAC technical section of the 964 website [:)]

And just to add for Rob, check that the resistor Dave mentions is the new type with part number 964 616 550 02 stamped on it and not the old type which has an 01 at the end and has been the culprit for engine fires [:eek:]
 

ORIGINAL: robwright

Dave thanks for the reply mate (wasn't sure you guys would entertain me) [:D].
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Many of us started in 944s and made the leap direct to the 964, a lot of the signatures reflect this. I have very fond memories of my 944S2, which I sold last October and owned for nearly 12 years covering 130K miles.

We're waiting for you, it already looks like your getting the bug[:D][:D]

I know this is off topic but you lost me ages ago [&:]

However...the demister fan on the left side of my windscreen blows at a much reduced flow rate, meaning that only the drivers side defrosts properly. My specialist had a root around and reckoned that they might have to attack it from the front necessitating removal of the fuel tank (and 8 hours labour) so I declined.

Any ideas for a reasonably cheap fix?
 
ORIGINAL: colin129
Any ideas for a reasonably cheap fix?
Click to expand...

Colin,

I'm not sure there is one. The cabin blowers often seem to suffer from bad squeaks before they finally give up the ghost. The only real solution is to replace them but, as you say, it's not an easy job. I'm not completely sure if the whole HVAC unit needs to be removed in order to do the job but it will certainly make it a lot easier to get to the blowers. The left hand one is particularly difficult if you have a/c as the pipes are in the way. The blowers are held in place in what seem to be called "squirrel cages" which are, apparently quite difficult to get removed. Removing the whole HVAC unit does require removing the fuel tank and, I believe, the firewall too and is no mean feat. I had to suffer the cost of all that when JZ replaced the evaporator in my a/c system some years ago. The labour charges were more than the parts cost!!!

Regards

Dave
 
Hi there
I'm having similar problems.
When I turn my ignition on the front and rear blowers blow at high speed and stay on even when I turn the ignition off. I have to remove the fuse to stop it. I have replaced the relay - no luck.
After doing some research it has been suggested that the NTC sensor whic fits into one of the pipes may need replacing, this should then sort the rear blower out. As for the front blowers - well maybe that the servos are stuck.

Before I spend money on a new sensor and start removing the servos - AM I ON THE RIGHT LINES OR TOTALLY ON ANOTHER PLANET - lol.

regards lee
 
Lee, I can at least confirm that replacing the rear temperature sensor stopped my blower from running all the time and restored normal function.
 
Lee,

There is no physical connection betweeen the cabin fans and rear blower. Certainly the NTC sensor (on top of the blower plenum) can cause the rear blower to run. It's there to sense high engine temperature and to engage the rear blower to dissipate the heat. If the sensor goes pear shaped the blower will likely run all the time. However, the relay can weld itself closed but it sounds like you've ruled that one out.

The cabin fans are dsriven through a transistorised amplifier which seems to fail with great regularity. Replacement is the only solution for that one, but they do come up on eBay pretty often.

Finally there is also a possibility that the CCU itself is at fault but it's best to try the cheaper solutions before you contemplate spending big money on a replacement.

Regards

Dave
 
Cheers for the help. will replace the NTC sensor.
Where do I find the transistorised amplifier?
regards lee

P.s I don't get any change from the altering the ccu at all - but like you say hope that this hasn't gone .
 
Thanks for all your help so far guys. It would appear that the rear blower is functioning correctly now its all wired up properley. I think (that is I am certain) that the cabin blowers are not working as there is no airflow when the temp knob is set to cold and the blower speed control is set to full. I presume it should move cold air into the cabin to cool you when it is hot. looks likely to be the infamous final stage amp then. Best start trawling fleebay then lol.
 
Lee,

The blower final stage amplifier is mounted underneath the fuse/relay panel at the rear of the luggage compartment. They come up on eBay pretty regularly (here's one) and there are a couple on there at the moment. The amp is mounted on a large metal heatsink ('coz it gets hot!!) and that is usually included (as with the example I linked to).

Regards

Dave
 
OK chaps hey presto i now have heating but how do i get the thing to blow cold air its a non air con cheers
 
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