Stu, the part number you quote is for a RHD car up to 1990 with no factory air con....so I presume that sounds right for your car?

New units are £1,000+. The first part of the part number is replaced by 993 and there is a minor modification of the wiring connector that needs to be done. A brand new 964 unit came up on ebay a couple of weeks ago and was sold for £350 but it's rare to see them for sale. The old units can be refurbed and I believe there is a place in Italy that does this but it's still £300+. In an ideal world you would want to try swapping the unit with an identical one first because you don't want to spend all that money to find out that it's not the CCU that's at fault.

However, since you now have signs of the rear blower working I would check out the much cheaper final stage relay first which is often responsible for the front fans stopping working. It's under the front scuttle panel next to the windscreen. Used ones come up on ebay (there's one on there for £49 at the moment). I have a spare one which I believe is good (I changed it when investigating my own HVAC problem). PM me your address and I'll send it to you to try.

The plug in the middle of this pic is the location of the final stage relay:



While under the scuttle panel, check out the operation of the servos which will give you further information about the state of the CCU. All the information you need about those is here:

http://www.porsche964.co.uk/technical/servos.htm

I would also remove the rear blower while you're at it and open it where the wires go in. It's common for the electrical contacts inside to corrode and cause intermittent problems with the rear blower. Cleaning them up can solve this.

 

Completed more testing today!

Swapped the suspect heater control unit from the 964 c4 and tried it in my 964t and it operated fine...then tried my 964t ccu in the 964 c4 and nothing..
so that concludes the suspect ccu from the c4 is actually ok it appears!!

Next tried the final stage relay under scuttle as per picture above from my 964t to the c4 and still no blowers at all in the c4

So basically tested all fuses and ok
tested all relays and appear ok
ccu unit ok
final stage relay swapped and still no fans!

So it appears it may be wiring issue or something else.
I put a volt meter on the plug that powers the rear blower and i cant get any voltage to this with the fan switch on and temp **** at hot!

I have tested the majority of components now "apart" from the temp switch and resistor(both situated in the hose that leads to blower) but i am unsure if these would stop fans working??

Also note i have heard the rear fan come on once with the ignition off but never front fans,,,the front oil cooler fan is on all the time with the igntion on now again(this appears to change,sometimes on or off).

Auto electrician may be required now i think

 

Good to hear that the CCU checks out.

Here's some othe things I did at various times when sorting HVAC problems:

1. 30 amp rear fuse was visually OK but rear blower had stopped working. New fuse fixed it.

2. I found that I could remove the rear relay and have the front fans working but only with cold air. This might give you a clue if the front fans are working or if they are both kaput.

3. The plug (shown below) under the scuttle panel was causing a huge current drain and problems with the HVAC operation. Unplugging it, cleaning it up solved the problems;



As mentioned above I've heard of one or more occasions when the electrical contacts inside the blower stopped it working. Opening the blower up and cleaning the contacts solved this (one chap found they weren't even connected!).

Adrian's book says that if your rear blower stops when heat is selected, the ballast resistor at the rear is open circuit. However this shouldn't stop it when it's on the blue dot.

 

Yet again,,much appreciated Steve and this gives me a few more things to try before i think of throwing in the towel!!...As with most things it may be something minor that is causing the blowers from operating,its just finding it!
I will let you know how it goes,thanks again

 

You may find this thread of some help:

https://rennlist.com/forums/964-foru...ear+fan+bypass

Essentially it describes from page 2 that you can jumper the blower relay and with the blower then unplugged, still get heat into the cabin albeit at a lower flow rate (because you have no blower assistance). This would help you confirm whether the cabin fans are working. However, be aware that the jumpers could drain the battery unless you add in a four way relay as shown at the end of the thread. If you find it works this way, you then need to set about finding out why the blower doesn't work normally.

 

Thanks for like Steve...So tried the jumper test in where relay should be and with rear blower disconected no front fans operated at any setting on ccu unit....Then i plugged in the rear fan whilst maintaining the jumpers in where rear relay should be and on comes the rear fan only and no front fans at any dial setting!

Also tried cleaning the plug up under the scuttle panel!

Seems like it still must be a wiring or bad connection issue maybe?
Still not heard the front fans operate but i would be surprised if both were blown and because the system is playing all sorts of tricks it may still be some other issue??

 

From what you've found it sounds to me like the non-working cabin fans are at the root of your problems. The blower works but not in normal mode because there's no demand on it from the cabin fans.

Quoting Adrian's book again, non working fans are caused by:

Fuse #1 blown
Final stage amplifier failed
CCU failed

However, you've eliminated all those possibilities which leaves us with both fans themselves having failed or a wiring issue to the fans. While I agree that it is unlikely that both fans failed at the same time, it could be quite possible that they failed at different times during the ownership of the PO who simply did not get them replaced (maybe they didn't use the car as a daily driver all year round, so didn't get round to having them sorted). I guess the logical step would be to uplug each fan and apply 12 volts to them to see if they come on? Unfortunately, I can't tell you where to unplug them but I guess the cables are hiding behind each fan housing under the scuttle panel.

I would also suggest lifting up the fuse board (remove an allen bolt and undo the thumb screw) and inspecting the connections under fuse #1 to see if there is anything untoward (burnt wires, loose connections, etc).

It might also be worth checking the earth point on the inside of the wing near the fuse box for tightness, corrosion or broken earth wires (there is the main earth for the cabin HVAC assembly plus 3 others attached there).

 

Hi Steve,,,tried 12v to each of the front fans and both operated fine!

The earths looked ok also however under the fuse box i do see some black insulating tape around wires that go to the no.1/no.2 fuse so looks like maybe someone had there hands in there at one time..

Think visit to auto elec on the cards and i will tell him what i have tried so far thanks to your advise and also print off some diagrams for him..

 

Spoke to auto electrician today briefly it seems he also is struggling a bit with the issue..He says though when he was messing about with the resistor inside the plastic tube that gets from the rear blower,the rear blower started to operate although still has never heard the front blowers working yet unless he connects 12v directly to them!

Is there anyway you could some how modify the system to have these front blowers operating when needed!.