low coolant light
Thread Starter
Burning Brakes
Joined: Jun 2009
Posts: 1,135
Likes: 226
From: Roanoke Indiana
low coolant light
My 89 L98 has an intermittent low coolant light. It doesn't over heat. The coolant isn't low when I check it cold and there's coolant in the overflow tank.
Also I just replaced the sending unit, which made no difference. Anyone else have this problem?
Also I just replaced the sending unit, which made no difference. Anyone else have this problem?
Safety Car
Joined: Mar 2017
Posts: 4,202
Likes: 1,323
From: LeClaire Iowa
I am not an expert by any means but if you are sure it is full and you just replaced the sensor only two things can be wrong. The new sensor is bad or the wiring has some kind of short. There may be other things that are way over my head. Dan
Zen Vet Master Level VII
Joined: Mar 2016
Posts: 5,218
Likes: 1,174
From: Southlake, TX
My gut teels me you have air in the system as the light is intermittent. The sensor is binary in operation and will read off/coolant, on/no-coolant. When the water drops below the sensor, it tells the sensor to turn on the light.
In order to get that light to stay out, you have to make sure that ALL of the air is out of the system. I'd wait till cool and then open the radiator cap and start the engine. When you rev the engine, the water level drops and then you want to add fluid. When you let up on the idle the water comes to the top. Put the cap back on as the excess will flow into the over flow tank.
In order to get that light to stay out, you have to make sure that ALL of the air is out of the system. I'd wait till cool and then open the radiator cap and start the engine. When you rev the engine, the water level drops and then you want to add fluid. When you let up on the idle the water comes to the top. Put the cap back on as the excess will flow into the over flow tank.
Racer
Joined: Nov 2015
Posts: 294
Likes: 40
From: Massachusetts
If you had that much air in the system, I would think after running the car hot for a while the car would boil the coolant and puke it back out into the lower clear reservoir (guess how I know). Try bleeding the system first.
If not the sensor is broken. GM likes to use a simple float sensor. They do get gummed up sometimes and/or mysteriously break. I've had intermittent coolant lights on other GM vehicles until the sensor goes completely. The sensor is part of the black assembly at the top with the metal cap. Not sure if you can just replace the sensor or if you have to replace the whole assembly.
If not the sensor is broken. GM likes to use a simple float sensor. They do get gummed up sometimes and/or mysteriously break. I've had intermittent coolant lights on other GM vehicles until the sensor goes completely. The sensor is part of the black assembly at the top with the metal cap. Not sure if you can just replace the sensor or if you have to replace the whole assembly.
Safety Car
Joined: Mar 2017
Posts: 4,202
Likes: 1,323
From: LeClaire Iowa
86-90 Corvette does not "use a float". The sensor is a simple probe into the radiator tank. There is a module that reads resistance on this probe to ground through the coolant. Something like less than 10,000 or over 25,000 ohms is Not OK, but within that range IS OK.. It is a range, not an absolute value, so it is not "binary". (I'm sure these numbers are not accurate, I don't recall the values offhand, this is for illustration). Many things can affect the resistance measured at the probe. Glycol concentration, mineral content of make-up water, air (as mentioned). There is also a time-delay between the sensor detecting an out of range condition and turning on the light. So "air bubbles" shouldn't affect the reading, unless there is a large void in coolant. Keep in mind that at cruise conditions, the thermostat is the biggest restriction to flow. The radiator will "empty" to keep the engine full, pressure drops after the thermostat, so the radiator level will drop even though the engine temperature remains within operating range..
I have seen radiator probes that have corroded away so that only the diameter of the probe is exposed to coolant, thus sending high resistance and a false-low indication. The probe used to be available separately from GM. I have no idea if it is any longer. The probe is retained in the radiator tank with two plastic clip/latches, and sealed with an o-ring. It just pulls out.
If you are certain that coolant level is adequate, you probably have a corroded probe. I'd attempt to source a new one before removing the old one. You can't just disconnect the probe to turn out the light because of the "range" of resistance expected.. You have to insert the expected resistance in the probe circuit, or remove the dash pad and disconnect the module. It's on the RH side behind the breadloaf.
I have seen radiator probes that have corroded away so that only the diameter of the probe is exposed to coolant, thus sending high resistance and a false-low indication. The probe used to be available separately from GM. I have no idea if it is any longer. The probe is retained in the radiator tank with two plastic clip/latches, and sealed with an o-ring. It just pulls out.
If you are certain that coolant level is adequate, you probably have a corroded probe. I'd attempt to source a new one before removing the old one. You can't just disconnect the probe to turn out the light because of the "range" of resistance expected.. You have to insert the expected resistance in the probe circuit, or remove the dash pad and disconnect the module. It's on the RH side behind the breadloaf.
Team Owner
Joined: Sep 2003
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From: Freedom is never more than one generation away from extinction.
St. Jude '03 thru '24
Unplug the sensor and connect the plug wire to ground. The low coolant lite should go out.
My 86 would get fiesty after a coolant change. So knowing that the system was packed and full I'd do this for a day or two then connect it back to the sensor.
Always worked for me.. YMMV
My 86 would get fiesty after a coolant change. So knowing that the system was packed and full I'd do this for a day or two then connect it back to the sensor.
Always worked for me.. YMMV
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Thread Starter
Burning Brakes
Joined: Jun 2009
Posts: 1,135
Likes: 226
From: Roanoke Indiana
Unplug the sensor and connect the plug wire to ground. The low coolant lite should go out.
My 86 would get fiesty after a coolant change. So knowing that the system was packed and full I'd do this for a day or two then connect it back to the sensor.
Always worked for me.. YMMV
My 86 would get fiesty after a coolant change. So knowing that the system was packed and full I'd do this for a day or two then connect it back to the sensor.
Always worked for me.. YMMV
Can't remember if I ever changed it. My LS1 Firebird did a lot of gurgling after I shut it off, a new cap fixed that.
2nd Gear
Joined: Sep 2018
Posts: 2
Likes: 0
86-89 Corvette does not "use a float". The sensor is a simple probe into the radiator tank. There is a module that reads resistance on this probe to ground through the coolant. Less than 10,000 is OK, over 30,000 ohms is not OK and will trigger Low Coolant.. It is a range, not an absolute value, so it is not "binary". Many things can affect the resistance measured at the probe. Glycol concentration, mineral content of make-up water, air (as mentioned). There is also a time-delay between the sensor detecting an out of range condition and turning on the light. So "air bubbles" shouldn't affect the reading, unless there is a large void in coolant. Keep in mind that at cruise conditions, the thermostat is the biggest restriction to flow. The radiator will "empty" to keep the engine full, pressure drops after the thermostat, so the radiator level will drop even though the engine temperature remains within operating range..
I have seen radiator probes that have corroded away so that only the diameter of the probe is exposed to coolant, thus sending high resistance and a false-low indication. The probe used to be available separately from GM. I have no idea if it is any longer. The probe is retained in the radiator tank with two plastic clip/latches, and sealed with an o-ring. It just pulls out.
If you are certain that coolant level is adequate, you probably have a corroded probe. I'd attempt to source a new one before removing the old one. You can't just disconnect the probe to turn out the light because of the "range" of resistance expected.. You have to insert the expected resistance in the probe circuit, or remove the dash pad and disconnect the module. It's on the RH side behind the breadloaf.
I have seen radiator probes that have corroded away so that only the diameter of the probe is exposed to coolant, thus sending high resistance and a false-low indication. The probe used to be available separately from GM. I have no idea if it is any longer. The probe is retained in the radiator tank with two plastic clip/latches, and sealed with an o-ring. It just pulls out.
If you are certain that coolant level is adequate, you probably have a corroded probe. I'd attempt to source a new one before removing the old one. You can't just disconnect the probe to turn out the light because of the "range" of resistance expected.. You have to insert the expected resistance in the probe circuit, or remove the dash pad and disconnect the module. It's on the RH side behind the breadloaf.
I tried and it won't go. I white lubed it a little, as well. I don't want to brek it. By the way, the sensors are available through NAPA.
Team Owner
Joined: Sep 2003
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From: Freedom is never more than one generation away from extinction.
St. Jude '03 thru '24