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What resistor should I use to disable the defective coolant sensor light?
My 2009 RX-8 has a defective coolant level sensor; it's a factory defect that didn't crop up until after the warranty expired. (figures.) Replacing the sensor apparently involves replacing the overflow tank, which may also involve replacing the radiator too, because the plastic hose nipple on the radiator tends to get really brittle over the years. I don't want to go through all that trouble, I just want to shut off the damn light on my dashboard. I can pop the hood and check the coolant level for free, after all, and I have to do that regularly anyway since the engine burns oil.
If the coolant level sensor works the way I think it does, then it's a variable resistor, and the light only turns off when the resistance is below a certain value. Short-circuiting the wiring plug would hypothetically turn off the light, but it would put unnecessary strain on the ECU and possibly burn-out something, because the delicate sensor-monitoring circuitry isn't designed to handle the amount of amperage that would flow through that circuit with no resistance.
So it would be helpful if someone with a functioning coolant level sensor could grab a multimeter and test what the resistance of the sensor is when the coolant level is full. Then I can connect a resistor to the wiring plug and make that damn light shut off without risking damage to the ECU. Has anyone done this yet? If so, what is the correct resistor to use? If not, can someone check it now? Thanks.
On the series 1, it isnt nearly that smart. I just unplugged mine and the light went away. Now I check the coolant when I check the dipstick (once a month). Im not certain about series 2, but it might be worth a shot.
Mine's a series 2. I don't think I ever tried unplugging the sensor. I assumed the sensor was configured to generate increasing resistance as the fluid level drops, and so the ECU would sense infinite resistance from having the sensor unplugged and respond the same as if the sensor were reading "empty".
EDIT: You're right, unplugging the sensor turned off the light. I can't believe they would design the sensor circuit that way. Still, I would prefer to fool the ECU "correctly", in case it generates a non-critical error code. I have to be able to pass inspection every couple years, and I'd like to not fail on a technicality.
Last edited by fyrstormer; 06-30-2018 at 05:46 PM.
Just unplugged mine(S2) to find out what would happen. The light stayed off.
So that means the light would trigger from a lower resistance.
Personally, I like having that light, so if you have a sudden leak then you can pull over before it's way too late. Just buy a new one from Mazmart and they give you a new hose to replace the one you need to cut to avoid damaging the radiator nipple.
Last edited by UnknownJinX; 06-30-2018 at 06:01 PM.
I replaced my coolant reservoir yesterday. I saved the rad hose for last. I left it connected and just very carefully rotated it until I could get to the other end of the hose on the reservoir.
My old reservoir had turned a dark yellow. It was impossible to tell the fluid level unless you had a flashlight and it wasn't full daylight. I decided since it was hard to read I would just replace it then I don't have to worry about it. Plus that warning light is full brightness at night which I hate.
OR you could fix properly with a new overlfow tank..
Of course I could fix it properly with a new overflow tank, but I don't feel like spending hundreds of dollars (I live in an apartment, so draining my own coolant in the parking lot is not an option) just to turn off an idiot-light. But I'd like the hack to be not-immediately-obvious so it won't get noticed during an inspection. If it were a safety-critical system, I would spend the money to fix it properly, but it's just an idiot-light on a 9 year old car. It's not worth the cost to fix it.
Engine overheating is really deadly for this car, though. Coolant seals are a lot harder to replace compared to head gaskets. A $100 tank beats a multi-thousand-dollar rebuild.
I personally peek at the coolant level every time I fill up, but the light is even better because it immediately notifies you about the problem.
I didn't ask if it was a good idea, I asked for the measured resistance of the coolant level sensor when the float is in the "full" position. I would not be contemplating this if I thought it were a serious risk.
Last edited by fyrstormer; 06-30-2018 at 11:38 PM.
There is no resistance. It's just a simple continuity switch. This is from the S2 service manual.
Basically:
Continuity = light ON
No continuity = light OFF
And I stuck my smart multimeter into the two sockets and the multimeter just didn't respond, so there is no continuity. Makes sense.
So yeah, just put an electrical tape over the plug. If you put a resistor in place, the light will still be on because there will be continuity.
I actually wish it's the opposite. I'd prefer the light to bug me to the risk of losing too much coolant. The two AutoX events I have been to so far both had a car spilling coolant. Don't want to be that guy.
Considering a new tank with sensor will run about $150, and a reman is over $3k just for the engine, just ignore it, or put a little piece of electrical tape over it so you don't have to look at it.
I'd urge you to replace the tank, too. Not expensive in the grand scheme of things. My old Saab suddenly lost its coolant on the road - and it was the idiot light that alerted me. Had I waited to routinely eyeball coolant level next time I had the hood up, it would have been too late.
06-30-2018, 04:20 PM
