LLY low coolant sensor

[Dan Herr]

My low coolant sensor has failed.

The dealer reports that the entire surge tank has to be replaced in order to replace the sensor. Is that correct?

[DmaxMaverick]

It's true. The sensor is molded into the tank. But......

It may not be bad. If you've added water to the tank recently, it may not sense the level correctly, and give the false indication of a failure. The density of water is considerably different than Dexcool. I ran across this while doing head gaskets on a 2001 Malibu about a month ago. An independent shop and the Chevy dealer both told the owner the sensor was bad, and the tank needed replacement. He did, but the low coolant indicator didn't stop. Turns out, the system had no Dexcool in it (water only, and I suspect the head gasket failure was related), and the sensor never saw the level. Anyway, after the head gasket replacement and a fill with 50/50 Dexcool the sensor and engine was happy. I did some testing with the sensor using a scan tool to read the PCM coolant level indicator (on/off). Less than about 25% Dexcool didn't show a level. That doesn't mean the system had less than 25%, but if you add water to the tank, you are diluting the mixture, and may cross that line. I don't know, but depleted Dexcool may also cause issues with the sensor (specific gravity changes), so maybe it's time for a coolant system service, if the mix is otherwise correct.

[Dan Herr]

Thanks DMax.

Dexcool ratio is correct - so I guess I need sensor/tank.

[DmaxMaverick]

Check the connector and harness as well. The wire is very thin, and can break inside the insulation.

[Dan Herr]

DMax,

If I unplug the harness from the sensor, I still get the low coolant message.

And if I jump across the terminals on the disconnected wiring harness plug, I still get the low coolant message.

If the sensor is a simple open/closed relay, this would lead me to believe that I have a harness problem.

Do I need to troubleshoot the harness? Should there be a particular resistance value across the terminals?

[DmaxMaverick]

I ran across the same thing on the Malibu. The old tank was tossed before I saw it, so I couldn't compare old with new. I did check it in comparison to my '01 HD and 2000 Impala, and they appeared the same. One thing is didn't do is a resistance check through the sensor, which is likely a value seen by the PCM (not just a open or closed circuit). I'm out of town now, but I can check it once I get back home tomorrow. The on/off I was referring to was the value seen on the scanner, not the actual sensor operation.

[WhiteTruck]

The low coolant switch is closed when the level is correct. Jumping the terminals should turn off the message. The light green wire has a 5 volt reference voltage from the ECM. This gets grounded to the black wire by the switch when the coolant is at the correct level. In your case, there is likely an open in one of the circuits. Check for the 5 volt reference voltage at the connector with it unplugged and the ignition in the run position. If this is OK, then check for continuity to ground on the black wire.

[Dan Herr]

White Truck,

I have voltage on the green wire, but no ground on the black wire.

Can I just establish a new ground to the frame?

Or does the ground need to connect to wherever it is supposed to run (some computer?) in order to properly complete the circuit?

[WhiteTruck]

The ground wire goes directly to the block. If you don't want to trace it, I don't see any problem grounding it with another wire. My illustration is inaccurate, as it shows the location at the top of the block in the rear on the passenger side. Problem is, the picture is of a gasser, so not positive if it really goes there. I would have thought it would run to the lower right front of the block where several ground connections exist. I don't want to mislead you, so unless you want to trace it to this point, go ahead and run another ground to the block.