what is the procedure for testing the gauge coolant sensor on a 91 240? The 700/900 FAQ mentions a link for measuring resistance, but I don't see any link, and regardless, the FAQ describes two terminals on the gauge sensor whereas my 240 has only one.

my temp gauge started to become erratic after changing coolant (9 times out of 10 needle shows no movement, but then suddenly it works fine). there appears to be steady voltage coming from the gauge.

if you mean the water temp gauge sensor which send a signal to the cluster gauge remove the yellow wire and ground it to the block with the key in second position.

if the gauge moves to the max the gauge is good and the sensor is suspect

You may have some air trapped in the system at the highest point which is the thermostat. Try squeezing the upper radiator hose while the engine is idling to fill the area with coolant.

Did you remove the thermostat?

Dan

thanks for both of these responses. The gauge is working when I ground it. It defintely seems to be on the sensor side. But i'm interested in the suggestion about air bubble and thermostat--I did replace the thermostat. If I squeeze the hose while it is running, will this remove the bubble, or do I need to actually open the system?

Hello,

Since you did change thermostat did you happen to notice the little metal toggle pin in the perimeter plate. It is suppose to be at the top of rotation or in relation to gravity on the topward side.

This little jiggle port has two functions. One is to weep by so there is some flow by the thermostat so it does sense the coolant.
The other purpose, is to vent air bubbles that can slip by when the engine is off. When the coolant is stagnant this happens.
Actually it should rise more quickily in the radiator because it's less turbulent there and the coolant is cooling and settling.

Air is lighter than most liquids used in cooling systems. It will rise to the highest point in any fluid system every chance it gets.
Again more chance in the radiator. If you happen to notice there is a tiny tube at the top end of the radiator that goes to the recovery bottle.

When the engine is cool, you can try loosening the cap and give the top hose a few fast squeezes.

If the recovery bottle's coolant jiggles, that should mean that the top hose is full or nearly full and the radiator has burped it's air. If if it makes lots of sloppy sound then maybe not.

In this case, make sure the little hose mentioned, is clear and consider checking the rotation of the jiggle pin on the thermostat.

Repetitive cooling cycles will clear a air pocket.
It helps to run the heater core wide open.
If it's air bound that might stop some circulation force since its connected on the return side or inlet of the pump. Maybe you did that already?

Check the bottom hose on the radiator for softness as it might be collapsing if you had any overheating indications in the past or future?

If you happen to have an Volt / Ohm meter you can check the sensor resistance for an ohm reading.
It should have a higher resistance number when cold than hot.
The lower the resistance the closer it is to being like the time you held the wire to the engine block and it shot the gauge to the top.

That by the way, that can be very hard on the inner workings of a gauge as it might over heat it internally, just saying!
A 10 ohm resistor or a home made test light with clips and a tail light element, will put it up there somewhere high, safely! As it will limit the current draw a bit.

I dont know how accurate this information is to the gauges with the compensation boards of nineties cars but the engine sensor on late seventies, to early eighties, used these resistances.
I think or imagine the sensors should still be both the same used.

Temperature Resistances
50C = 122F 282 Ohms
100C = 212F 60 Ohms Around center or above is my guess as most thermostats are 87C +
The rate is not linear but maybe 110 ohms = 190F, if 20C more gets you 20 less ohms.

A normal reading on the gauge should be center or less on most cars.
That's why there are no numbers. A guess -O- reference meter is easier to make and cheaper. All in all, it's better than the Ole' calloused hand, on the top radiator hose trick! (:-(

120C = 248F 40 Ohms Beginning of the Red Zone

Go between the terminal and a good bare ground on the engine block and then check it to the battery negative side post for comparison.
Just in case the engine block has a bad ground intermittently. This should be very rare without other issues going on with the car.

I'm guessing but did the gauge made you change the thermostat? This might just be a weak wire or connector.

Hope this helps trace down the intermittent changes. Sensor, connectors or wires.

Phil

great info, Phil.

The gauge didn't make change the thermostat, I just did that as routine maintenance when I put on new radiator hoses. I just checked that the bleeder pin was installed at the top and went through your suggestions for looking for air bubbles. It doesn't seem like there are any. The gauge works fine when I ground the wire to the engine block. It appears that the sensor is the culprit. I still find it odd that the sensor would suddenly fail immediately after I changed the hoses and fluid...seems fishy to me.

I forgot to mention that I took the Ohm reading and it was spot on--exactly what you said to expect. It was higher when the engine was cold, and when the engine got up to temperature it stabilized at exactly 110 Ohms. So it seems that the sensor is fine.

I grounded the gauge wire again just double check, and it immediately sprung to life and went up towards the top of the range.

So I'm baffled: the gauge appears to work, the sensor appears to work. I've double checked the thermostat is installed correctly and re-bled air bubbles.

Any thoughts?

If the engine service harness (coolant temp, oil pres., starter, etc) is separate from the FI harness (not like a '93 with an all in one harness) that goes to a big gray multi-wire connector on the firewall I'd take it apart and clean all the blades/receptacles. -- Dave