View Full Version : size of ports on coolant flange..

01-12-2010, 08:02 AM
in the process of developing a "very easy do it yourself" electric fan kit for the b5 a4 utilizing the secondary port on the coolant flange on the back of the motor for a thermostatic switch. I am very much still in the beginning stages of development so I figured I would try to figure out as much as I could from forums before tearing into my engine bay. My question is what size are the threads on the coolant flange? Are they both still the same or different?

01-12-2010, 08:22 AM
i dont think there are any threads on the port, i think there is just a lip inside - similiar to the lip that the CTS sits on in the other port.
I would think both ports are the same size, i got an old one in the garage i can check after work..

01-12-2010, 09:47 AM
Yeah, they aren't threaded, they use an o-ring around the sensor and a clip to hold them down.

If you want a fan switch that will fit in that port, this is probably what you want:



It is a "dual thermal fan switch", so it has 4 pins and 2 temp thresholds. You'd have to check on the temp specs though. It is used on 2000 and 2001 model year B5 1.8T's. Perhaps 99.5 as well. It is, however, almost never listed for these cars for whatever reason. It is used on the B5 S4 and C5 A6 2.7T as well, and many others I'm sure.

I don't think the coolant flange is really where you want a fan switch, though, is it? Usually they are put in the lower rad hose or bottom of the radiator so it reflects that actual exiting water temp of the radiator. In the coolant flange I think the water temp in the radiator will be somewhat abstracted by the thermostat, no? I suppose as long as the temp switch was set higher than the thermostat temp it would be alright. Maybe I'm wrong here.