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Old 02-22-2014, 02:15 PM   #1
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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:

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:

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.
2001 Boxster, 5 spd, Seal Grey
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Old 02-28-2014, 02:15 PM   #2
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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.
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Old 03-01-2014, 06:46 AM   #3
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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...
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Old 03-06-2014, 10:30 PM   #4
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Very interesting, thanks so much for explaining these.
I guess I love learning about these cars as much as thstone.
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Old 03-07-2014, 02:53 AM   #5
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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.
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