Coolant Level Switch Explained

Bali · Jan 25, 2023

This post shows how the coolant level sensor works and how to test it.

It resides inside an air cavity on the bottom of the coolant expansion tank sealed off from the coolant. You can remove the sensor without worry of a coolant shower. Oddly, you remove it by rotating it clockwise 1/4 turn (as opposed to counter clockwise), similar to a left hand threaded bolt.

The sensor works using magnetism. A washer shaped magnet on a coolant float stick activates the sensor when position is nearby. The most common reason I've seen for the coolant level sensor to not work is a poor quality (cheap ebay) expansion tank with a non functioning float/magnet.

The sensor is a normally open magnetically activated reed switch that gets closed when the magnet washer on the float is in the normal range of coolant level, near the reed switch. A closed reed switch keeps the coolant level light off. A very low coolant level opens the reed switch turning on the coolant level light. The photos below show the coolant level sensor open with no magnet and then closed with a magnet.

For some reason, you cannot push the red float stick down far enough to cause the coolant level light to come on. I have tried!
See @Sapote’s explanation in this post.

The windshield washer fluid level sensor is identical and can easily be swapped in as a test. You can also just put a magnet near the reed switch and watch it close up.

Wiring diagram for coolant level switch.
Coolant Level Switch Wiring Diagram

pic below shows open reed switch

pic below shows closed reed switch caused by a magnet

cut open expansion tanks

TobyB · Jan 25, 2023

Nice!

I felt really silly the first (and last) time I drained the tank to change the sensor.

For some reason, you cannot push the red float stick down far enough to cause the coolant level light to come on. I have tried!

Since I am paranoid about coolant, and the sensor is an almost foolproof check, I always check a new one to make sure the low coolant light works BEFORE I fill the system.

t

Sapote · Jan 25, 2023

I'm curious of why the reed switch doesn't open (light turned ON) when the stick is at its highest position, where the magnet is so far away from the reed switch.

Sapote · Jan 27, 2023

I checked a spare tank this morning, and finally learned how it works.
1. The reed switch opens when the magnet is at its lowest position to trigger the light ON.
2. The part that holds the magnet is not rigidly attached to the red rod, but sliding within a fixed section on the rod. This means the rod total travel distance can be longer than the magnet travel distance. This is the reason why with coolant properly filled in the tank and pushing down the rod to its bottom doesn't trigger the light ON, because the magnet is still higher than the reed switch.
3. Only when the magnet is very close to the reed switch to cause the switch to open and trigger the light ON.

bonestockE36 · Jan 26, 2023

Having the reed switch closed to keep the light off makes sense in terms of reliability. If the wiring gets damaged you'll get a light. Which is better than damaged wiring and not getting a light when low on coolant because the closed switch can't complete the circuit with bad wiring.

Bali · Jan 27, 2023

If someone has a spare ET and sensor, can you do some multimeter tests with the red float stick in various positions and report back?
Photos would be a bonus!

Coolant Level Switch Explained

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