911Thrasher

I have indeed a standard 964RS Lightweight, with the rear heating fan - and was looking to purchase the rear carbon bypass pipe...however after discovering i also need to trick the CCU, i might not be willing to do so, as i havent found a clear answer to:
- will i still be having heat in the car (front fan to operate correctly)
- how to actually trick the CCU

It's a £300 mod, don't want to waste my money if it's to get faulty signals

boxsey911

My opinions are; that the bypass will make no noticeable performance difference, the aftermarket carbon ones don't appear to fit very well (get the plastic Porsche 993 part number one if you must do it) and the heating system will not be as efficient. These are the reasons I've kept the standard system on my C2.

Of course, some will have different opinions on the above. The only reason I can see for fitting the bypass is for those that are on an extreme weight loss program for a dedicated track car where every Kilo counts. Or possibly if the rear blower has gone kaput (expensive to replace).

oldtimer

Well put Steve

In reply to 911Thrasher's request for clarity on what to do

Remove heater blower relay.
Put jumper lead between pins 30 and 87
Put jumper lead between pins 30C and 87C
Run a lead with inline 7.5A fuse from positive terminal of nearest ignition coil to pin 87.

Maplins can supply all the bits ( High St Ken )

You will now get a gentle waft of warm air with flow control from front fans plus you can set the temp control up to what number you desire but dont be in a hurry to get warm

911Thrasher

Thanks Oldtimer - i just spent my money elsewhere (a couple moment ago)...ordered a Dansk Cup exhaust bypass (more noise, and i keep the heater)

At least i have a clear instruction, if i want to get it done subsequently.

Cheers, greatly appreciated

Babalouie

Other salient reasons for doing the bypass include:

- if your engine bay heater blower has packed up; and/or
- you live somewhere where it isn't really that cold

David A

Good info:

https://rennlist.com/forums/993-foru...pass-tube.html

jorijori

Thanks! I'm doing the heater byspass tube & will update with the result.

neffets

This is a prototype i'm testing with internal fan

jorijori

Here is my update, car is 1993 C2 triptronic. I used Porche OEM part# 993-211-941-00 and since I live in hot weather I don't need the heater hence there is no need for me to jump the heater fan relay. I just removed the green fuse 30A.

Highly recommended mod for those who don't need heater, you save a few kgs & better access to engine bay.

crg53

You still have lots of heat with the bypass in place, so no problem there. The big question is, do you have A/C, if you do, the fans for the A/C will not work properly without the jump wire in place. Also I would route the vent tube to the distributor differently than yours, why have it sitting up so high.

jorijori

Hi Steen,

Yes I have AC which wasn't that cold before & is still not that cold after. I did not notice anything different after the mod.

During the day, it can go from 36-40 degree C (100 - 110 F) but AC feels cold at night (30). The main reason AC is not that cold because most of the air blows toward my feet, I think my CCU is bad. So which AC fan may not work correctly?

Okay thanks for your observation, I would reroute my vent tube, I never notice that before.

crg53

There are 2 a/c fans in the lower dash/frunk, they are regulated by the dial on the CCU. Those two fans will only run intermittently, if at all, if you don't have some sort of jumper in place. Start up the car and turn the fan speed dial to full on and let it run for a few minutes, see if it still runs at full speed.

neffets

There is also the engine cooling fan feature you loose with this.
At igntion OFF, the fan run in low speed if the air duct temperature is above 75degC, and shall stop when below 70degC.
That's why i made an internal fan in the duct, so all functions for heating and cooling stays intact

Need sound on

tdiquattro

I have been looking into fitting the bypass pipe, and have a quick question if anyone can help?...

- I have a car with no Aircon (from the factory) but it has the aircon wiring.
- the aircon wiring includes the relay base in the rear engine bay fuse box (unused).

On the aircon relay base is a switched 12v (Pin 86 Violet wire), can I use this as the live feed to fool the heater control unit instead of the coil pack? I think I am right, but would like someone to confirm my thinking if possible. (the violet wire is the output of relay R01 which is on all the time the "X" line is live and is fused at 15A (fuse 14).

If this works it is a perfect method involving a single link wire from one relay base to the other (they are next to each other) if no a/c is fitted. And an easier method if a/c is fitted than going back to the coil you can simply tap into the violet wire instead.

So this will be the coil pack method, but using 12v from the a/c relay base.

Thanks in advance for your help

crg53

There was a post here a while back, that did indeed show how to use that purple wire instead of the coil. I would go ahead and use it, but keep an eye on possible battery drain just in case.