Comparing Engine Coolant Temperature sensors

[wwii]

1992 300SD OM603.971

I'm trying to figure out if my instrument cluster engine temperature gauge is correct.

The ECT gauge sensor (B13) is installed in a bunghole in the head and the EDS (control) sensor (B11/4) is installed just downstream of the thermostat in its head.

I have the specs for the ECT gauge sensor and the EDS (control) sensor. Can you compare the temperature from those two sensors - given their different locations? Can I expect those two temperatures to be the same?

I got the engine up to ~90c (instrument cluster gauge), turned off the car, unplugged the EDS sensor and measured its resistance - its temperature was considerably less (15-20 degrees) than the instrument cluster gauge indication.

I may have the opportunity to make some better measurements using an IR pyrometer, thermocouple in the thermostat discharge hose, and/or a contact thermocouple. If I can find the connector and rustle up some resistors, I should be able to check the instrument cluster gauge.

thnx

[BillGrissom]

I did similar in my 1984 & 85 300D's, but don't know if they translate to your car. I posted w/ the resistances vs temperature I found in a search, plus my measurements w/ IR gun and substitution resistors. Yes, the sensors for the dash gage and EGR have much different responses.

Recently I pulled up my post because dealing with an apparent overheating in my 1985. It started running ~100 C on the dash and the IR Gun confirms "too hot". Changing the T-stat didn't help, nor looking for debris in all hoses, and I did purge out all air. Now suspecting either the water pump impeller separated or a crack in the head. I did get flow from the top hose if I rev'ed it, but not while idling, which didn't seem quite right. I'll sure do many more tests before I finger the head. Finally, the oil cooler top tube doesn't get near as hot as the bottom, so it doesn't seem to be flowing much. Perhaps something happened when I last changed the oil filter.

[Mxfrank]

Why would you expect the temperatures to be the same? One sensor measures water as it's exiting the head, the other measures at the point it's introduced to the engine. I would expect a 10-20 degree differential.

[wwii]

Originally Posted by Mxfrank

Why would you expect the temperatures to be the same?

That was my question - I didn't know what to expect. But I did think that they were both measuring coolant temperature after it had been heated up.

Which one measures at the engine inlet? - seems I have a conceptual problem, back to the drawing board.

[Mxfrank]

The thermostat is the inlet to the motor. The back of the thermostat sits in the bypass passage, which loops hot water back from the head. So it's reacting to head temperature. But the front of the thermostat faces the radiator outlet, which is the coolest water in the system. When the thermostat is open, the sensor just downstream is seeing cooled water.

The hottest water temps are in the head, which is where the gauge sensor is located. The design delta T of most cooling systems is 20 degrees, but could be less depending on load, ambient conditions and wear.

[wwii]

Yep, I had the flow direction wrong. #$% @!#$.

How about comparing a head contact temperature to the gauge indication - should they be pretty close?.

[Mxfrank]

That should work, as long as you are close to the sensor.

(FYI, Mercedes mounts the thermostat on the "bottom" side of the cooling system..you're probably used to cars where the thermostat is mounted at the water exit.)

[wwii]

Thank you.

[wwii]

I'll correct myself yet again.

After looking at a better diagram of the coolant circuit I realize that I had the flow direction correct but mistook the head outlet as the thermostat because of its shape. I should have posted a picture - I'm sure you all would have seen that immediately.

Now things make more sense, I wasn't going crazy, and I can see that the EDS coolant sensor indication should correspond to the gauge indication.