How the coolant tank sensor works
 

For anyone that cares, here's how the coolant equalization tank level sensor works. (Or at least a partial explanation.)

I recently replaced my coolant tank, and since it's such a PITA to get at I replaced the level sensor at the same time. Unfortunately, as soon as I started the car up, the alarm light beside the temp gauge started flashing slowly. Grr.

I checked the connections to the sensor etc. No go.

I still had the old tank and sensor, so I cut the sensor area out of the tank:

http://986forum.com/forums/uploads01...1393110772.jpg

The sensor sits in a tunnel in the tank that is sealed from the coolant itself. The float has a metal/magnetic ring that passes over the sensor. When the float is in it's lowest position (i.e. low coolant), the ring gets to this area on the sensor and makes a circuit:

http://986forum.com/forums/uploads01...1393110813.jpg

The exact mechanism of how the sensor is activated I can't explain. Looking at the "blades" on it, intuitively it would make more sense that the ring pulled the them apart rather than together...

To test this I put the old sensor into its position in the old tank mount. I put a meter on the two little prongs coming out of the sensor to measure resistance. With the float in the "full" and "low" positions, I got 0 and 0.7 ohms respectively. Made sense; no circuit with tank full, and a warning circuit with the tank low.

When I put the meter on the new sensor in the car, I got around 0.7 ohms resistance, no matter where the float was. That meant there was a closed circuit all the time, and therefore the warning light.

Out comes that tank (again). Oh joy.

Clickman. thanks for going through all of this trouble and posting up this info! I'm sure that it will help someone in the future. I love learning about these cars and can't get enough.

Y'r welcome. I've gotten a lot out of this forum and feel it's my duty to contribute wherever possible.

I picked up my second new sensor yesterday, this time from the local Porsche dealer (the first new one was from Vertex). There were some raised eyebrows when I pulled out my meter and the old tank section to test that the new sensor was working before I paid for it...

Very interesting, thanks so much for explaining these.
I guess I love learning about these cars as much as thstone. :D

That looks like a magnetic reed switch. The internal "reeds" are magnetic, and are pulled closed when the magnetic ring in the float comes near.