Fluid Level sensor

Thread Starter


Joined Jun 10, 2022
I'm trying to install a low fluid sensor in my dump truck to activate a low coolant warning light in the dashboard, I have 12v on 1 side of the light in the dashboard (So i guess i'm only missing the ground to activate the light)... I have tried a lot of different things and schematics but I just can't make it work, it's driving me nuts.

The old sensor is this one (https://www.fozmula.com/product/s285-fozmula-capacitance-type-coolant-level-switch/) Tried the wiring diagram that the and troubleshooting guide manufacturer provided but didn't work (maybe I'm missing something but redid and 100 checked the connections and still no work) So I assumed the sensor is shot and bought this one (https://www.amazon.com/gp/product/B0994QK9TV/ref=ppx_yo_dt_b_asin_title_o00_s00?ie=UTF8&psc=1) because silly me thought it was easier to wire (just 2 pins so I assumed I just needed 12v+ and ground) but nope. at this point I'm lost...

Thanks in advance, and I really appreciate your help.


Joined Feb 19, 2018
Stop connecting things to the wiring harness until you know what each wire is supposed to do and what the replacement sensors do. You run the risk of ruining the sensor and maybe more by guessing.

Is the bulb good?

Does the wiring harness utilize all 4 pins on the original sensor? If not, which pins have wires going to them? Datasheet.

Thread Starter


Joined Jun 10, 2022
The harness was cut off when I rebuild the electrical system, so I don't where each cable goes.... the bulb is brand new (also using led t5 bulb or this doesn't matter), already did the guessing part in the truck and in my work bench (Following the datasheet diagram), and with the new 2 pin sensor only connected +12v on one pin and ground


Joined Feb 19, 2018
The datasheet diagram is only part of the information you need as the initial state of the device, whether powering up in water or not (its capacitive, it requires a reference state), is set at the factory per the customer requirement. Also programmed at the factory is whether the output states are active in water or out of water (whether the outputs are normally active or not essentially). This means the schematic diagram, at best, gives a vague generalization of what it might be doing. But being the harness clip was cut off, lets skip that part.

I understand the new sensor is 2-wire, but is that the same number of wires the harness has for the sensor? How many went to the original sensor? What are the voltages on those wires with the engine running (not across each other, but each referenced to chassis ground)?


Joined Sep 17, 2013
Going by this schematic, (for an unspecified vehicle), a 2-wire sensor doesn't connect directly to the bulb. Rather, it connects to the ECM and the ECM drives the bulb.
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Joined Nov 6, 2012
The Sensor You are working with is NOT a "Switch".
Unless You can obtain a proper Factory-Wiring-Schematic, and duplicate it exactly,
You are not likely to get this type of Sensor to work.

Question #1
Do You have a wire that causes the Warning-Light to illuminate when You touch the wire to Ground ?

2 )
Do You have a "Coolant-Recovery-Tank", or just a Radiator with a Pressure-Cap and Vent-Hose ?
If You don't have a Coolant-Recovery-Tank, I would highly recommend that You get one.
A visit to a Junk-Yard will supply a huge array of options, or,
You can purchase a high-end, Welded-Aluminum-Tank from suppliers such as Summit-Racing.
It should hold at least 2-Gallons for a big Diesel-Truck.

3 )
If the Truck did not come with a Coolant-Recovery-Tank,
the Radiator-Cap will have to be exchanged for one that is
specifically designed to work with a Coolant-Recovery-Tank, ( 2-separate Rubber-Seals ).

4 )
Get a custom made Fluid-Level-SWITCH from this company ...........
Tell them exactly what You have,
( a Light-Bulb that needs a Ground, and,
the measurement down from the top of the Tank where You expect the Warning-Light to illuminate ),
and they will supply You with exactly what You need to make
your Warning-Light work reliably, and as expected.

5 )
The best option ..........
Don't depend on an "Idiot-Light" to keep your Engine running properly.
Quite often it's far too late when the Light comes on.

Forget about the Warning-Light and do a proper inspection of your Truck
before You start it, and at the end of the day.
Check the Gauges every 2 to 3 minutes of run-time during the day.