Coolant Level Sensor Controller

Does anyone know what is inside the “controller” for the coolant level sensing system. Photo attached. Mine may have gone bad…the line from the sensing pin in the coolant tank is intact to this unit but the warning light now stays on…
Can it be fixed…has anyone looked inside it…??

Rumor has it it’s a standard GM item. Might be easier to replace than repair?

Where would I find this say on eBay???

Well, I admit it doesn’t seem to pop right up. Here’s a couple of promising links:

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And a discussion for you:

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Great! Thanks Kirby.

Not that mine is the final word but all the GM coolant modules I ever sold were always square-ish/box-shaped


Anyhow, back to Jags, I’ve always wondered what really goes on inside that little cylinder. Can anyone explain briefly how it works?


It’s normally the single IC that fails, an electronic supplier will have

Jim Brighton UK XJSC 3.6 Manual

the module in my 93.5 coupe has a part number DAC7808 instead of RKC5259, but is looking exactly the same as described in the thread mentioned above
the fix was easy following John_John1 's suggestions

A quick Google search reveals there are a great many different ideas here. It might be possible to sum them up as the 1-wire (ours), 2-wire, and 3-wire – but I’m not sure it’s that simple. Many of them involve a little float on a pivot, usually designed so the whole thing can be inserted through a threaded hole. There’s one popular design that screws onto a radiator a couple inches down from the top, but nothing protrudes into the header tank; rather, the coolant flows into the external sensor. No idea what’s inside the sensor. And a whole bunch of the “controllers” have 5 connections.

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The low coolant warning module is a nand gate oscillator Several nand
gates coupled together, so it oscillates. When the water level touches
the probe it stops the oscillation & that is detected to light the
warning light. Changing the single IC (very cheap less than pound)
normally gets it going again.

Jim Brighton UK XJSC 3.6 Manual

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Wow. Rimmer Bros wants a hundred bucks for that thing! Yeah, time to install a new IC and swap out a couple of resistors.

Oooh! Check this out!

It’s always worth having a look, and a sniff, inside these kinds of modules. If they haven’t been potted in resin then they are ripe for being repaired with simple component replacement. Mid 1990s and before, you should be dealing with basic through hole components. Often the failed component can be spotted easily - leaky/bulging capacitor, burnt resistor or transistor. I fixed my headlamp dim/dip control unit, which had a failed transistor, for 50p + P&P - and that gave me 5 replacements.

Interesting!!! Does this mean that the little pin contains a piezoelectric vibration thingy to shake in the water…rather than just a simple stainless pin…how does the circuit detect the state of the oscillations…Intriguing little system we have here…and I was thinking it was detecting micro currents flowing in the coolant to earth…
Maybe my replacement of the pin with a bolt is a flawed strategy and this is what killed the unit…???

the pin is just some stainless bar, the electronic part is in the small cylinder box,
the principles are above, see post from Jimandhelen

coolant makes contact between the said said pin and ground (from the header tank)
if coolant goes down, the pin is no more at ground , the circuit activates the transistor to switch the dashboard signal on
2 delays are added from the secondary circuits using the capacitors :

  • one to prevent false alarms on the dashboard (ie : coolant has to be low for a few seconds before the light goes on)
  • on to show the light at startup, for 10s or so

in most cases, the capacitors are on their way out (ie delay is much shorter) or dead
the transistor and IC are also potential suspects : check the post from John, in the linkl I posted above

Advanced Dynamo Regulators contends that one of the reasons these things fail is because the coolant level warning light pulls more current than the IC was designed to handle. Perhaps one solution would be to change that warning light bulb out for an LED. ADR offers their solution, a PCB with a control circuit for the warning light that can handle the current – but it’s listed as out of stock.

I’d like to suggest a third idea: A separate little bit of circuitry that can be spliced into the wires between the controller and its plug that does the same thing that ADR’s revised circuit board does: takes the load off the IC. So, can one of you electronic weenies draw us up a little schematic? I suspect it’s just a transistor of some type, spliced between the 12V, ground, and warning light leads on the controller. If it’s small enough, perhaps it could just be heat-shrunk onto the leads. If it’s bigger, perhaps a black box of some sort. Or maybe it could be jammed inside the controller container.

you’ll still have the capacitors (condensers) slowly failing, and in need for a replacement after 20 or 30 years, and replacing those needs to open the controller

if you have the controller open, the fix is easy, just follow John_John1 suggestions in the same thread you posted above

the change of value for the resistor prevents the IC to blow again in the future

So how does a circuit that provides oscillating voltage detect when liquid is present.
Is it possibly to do with capacitance…and the high frequency circuit makes the difference in capacitance between water and no water a measurable thing…???

that’s possible, or just using the coolant as a short circuit between the pin and the ground, , creating an open circuit when the coolant is below the pin