kos11-12

Hi Guys,

it seams that I have gremlins in my CCU, it was a hot week end here in France,
I use the air con ,
I delete the rear fan and have the air by pass and the wire from the + from of the coil, everything worked well so far , but this week end, when trying to increase the temperature the fans gradually stop sniping,
I then on purpose increased the temperature to max and they completely stoped working,
when AC is off, the fans are working well at any temperature,
any ideas what could cause the fans to stop ?
thanks

ToreB

There is most probably nothing wrong with your CCU. A missing feedback signal from the rear fan will make the CCU to stop the two front fans.
There are numerous threads about a rear fan feedback bridge circuit on this forum, here's one of these.
Cheers,
Tore

alexjc4

It sounds like you have the wiring in place that Tore mentions above. That would stop the fans dead at anything other than full cold.

If the fans are gradually slowing as the set temperature increases I suspect it is the temperature sensors in the mixing chambers. I'd imagine the CCU is seeing cold air coming from the engine rather than warm and you are asking for more heat so it it slows the fans so as not to blow what it thinks is cold air at you. The faulty sensors would show up as a fault code I think if you have a PTD999 or a Hammer etc. Tore may be able to advise on how to test them.

ToreB

I believe kos11-12's text is subject to interpretation. Faulty mixer chamber sensor would never cause the CCU to shut down or slow down the front fans.
In the case of a missing rear fan feedback signal, some CCU versions slow down the front fans, others stop them completely.
Cheers,

alexjc4

You are the expert in these thing Tore so I do defer to your knowledge, although I'm sure on mine the speed of the fans is determined by the temperature set vs temperature measured, not just the fan speed switch, if that behaviour is a feature of the design then a fault in the temperature sensing might have the effect that I'm understanding from what Kos has written.

ToreB

alexjc4, you are probably correct in your observation, but not in your conclusion.
No 964 or 993 CCU regulate the main blower fan speed depending on temperature readings.
If your fan speed varies with the temperature setting, I would highly suspect you have a problem with the rear fan too. This is a very common fault. If so, check your rear fan operation, low and high speed.
Modern variable and automatic blower fan speed features were introduced in the Boxster and 996.
Cheers,
Tore

alexjc4

I don't have a rear blower, deleted it some time ago, worked the same as i remember before that though. I've bridged the terminals on the relay to provide the feedback to the ccu so the fans run. My hvac works pretty well, no obvious faults, i've replaced servos, main fans and the ccu fan to get everything working correctly. But the fans speeds definately change as the car warms up. Perhaps that's some other feature or bug.

kos11-12

I have the wire like Anto suggested long time , from positive coil to a terminal where the relays was, with a 5 amp fuse ,
I suppose there is nothing I can do,
So I will just have to keep playing between temp and fan speed .

Alex Sol

alex,

you mentioned that you bridged the rear blower resistor? how is this done?

i have same intermittent front blower. no rear blower...

alexjc4

The jumper i have is across the relay. The resistor I left connected and tucked safely out of the way at first, but iirc after studying the wiring diagrams decided to just remove it.