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Temp reading too low - sensor values


Cromagnon

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Hello everybody. I bought a 2002 a month ago and noticed that the temp reading is too low when the engine is fully warm. The needle barely leaves the blue zone, rising one or two millimeters at all above the blue. I checked the thermistor resistance and showed values from 560 Ohm when cold to 200 Ohm or a bit less when hot (80°C or a little more). I runned the test again but with the sensor out of the car and in hot water on the stove, and the readings were the same.

 

It seems to be OK but I am not sure. I searched the net to no avail. But if it is OK, what is the cause of the missreading?

 

Do someone have some helpful information about this topic?

 

Thanks in advance.

 

Santiago

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It's hard to see with the temp gauge in place in its housing, but there are temperature numbers on the very edge of the gauge face (in centigrade).

 

The bottom of the blue zone is 50 degrees; top of the blue zone, 67.  About 4pm is 83 degrees, about where an 80 degree thermostat should open.  The bottom of the red zone is 110 degrees and the top of the red zone is 117 degrees.  

 

You can always check your car's gauge with a mechanical pocket thermometer, what HVAC folks use--looks like a pen with a dial gauge on one end.  

 

To insure your car's gauge is reading correctly, make sure 

1.  the gauge itself is properly grounded (if it flickers when you turn on the headlights, turn signals or light the brake lights, that's a sure sign of a bad ground on the gauge)

2.  the threads on the sender unit are nice and clean, and aren't wrapped with teflon tape.  The sender grounds through those threads

3.  the ground strap from the - terminal on the battery to the car's body is nice and clean.

 

Let us know whatcha find so we can add to our corporate knowledge.

 

cheers

mike

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What thermostat do you have installed?

 

When I got my car it had an 80 degree thermostat (Part # : 11 531253249/80, I think), and the temp gauge was almost exactly horizontal when warm. Then I replaced it with the 75 degree version (Part # : 11 531253249/75) and now the temp gauge goes about a third of the way up (much closer to the blue zone). There's apparently an even lower temp thermostat (71 degree). I imagine that's basically down in the blue zone.

 

Not sure what's best. I like that now if something goes wrong I would have a lot more warning time before the engine way overheats, but I don't know if running at lower temps like that is bad for an M10. Probably depends on the oil you use. All that is kind of black magic to me.

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Not sure what's best. I like that now if something goes wrong I would have a lot more warning time before the engine way overheats,
and there will be a lot more water in the oil.  Oil running cooler than 180F will have a lot of condensation in it from the blowby gasses.  Lab check the oil and see if it happen.
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Hi everybody and thanks a lot for your help. In fact the I have the instrument cluster on my bench and the temp/tank module is separated from the rest. I connected it to a battery and tried with external resistance. I found that with some 75 Ohm the needle is in the middle, and the sensor is 180 Ohm when warm.

 

What I didn't do and will do this very morning is to run the same test with the sensor in a can of water and see what happens. This will be a more accurate test and will let me see if there is some teflon as Mike suggested. I also noticed the markings in the edge as you said and really are barely noticeable.

 

I will also try to find what p/n is the thermostat installed in the car. I took duly note of the p/n that other folk mentioned. Will check it.

 

The oil is Castrol GTX 20W50 changed a week ago along with all the coolant and coolant hoses. The operating temp is around 80°C metered with a digital thermometer.

 

Again thanks to you all.

 

Cheers 

 

Santiago

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I took off the sensor and it turned out to be a VDO 120°C p/n is 801/1/6.

 

Just for clarification, I was talking about the thermostat valve that decides when to start sending coolant through the radiator, not the electronic temperature sensor. It's the T shaped valve connected to three coolant hoses on the front passenger side of the engine.

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Sorry. I was that zeroed on the temp sender that took your post wrongly. The thermostat seems to be the original one. I cleaned and checked it couple of weeks ago while changing the hoses. I put it in a water can and see it open and close with the temperature increase and decrease. As long as I can know it worked fine. The engine temp measured in several points with the digital instrument never went above 80 - 85°C.

 

But after my last post I runned the test explained before with the VDO sender and the instrument. When the temperature reached 80+ degrees the needle began to jump erratically. It jumped from just above the blue zone to the middle of the scale and stood there with some jitter. Then when the water began to cool down the jitter and the hics started again. It seems that I am before a faulty sender.

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Look carefully at , and clean associated grounds (um, that means all grounds) before blaming the sender. Also the voltage regulator is grounded through the securing screw. There is also a pigtail harness which can be attached to the back of the gauge cluster which will help calm jittery gas and temp gauges.

 

I've never seen a defective stock temperature gauge.

 

Good luck,

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Is there any hot air coming out of the heater when you turn it to full hot on the controls for the water valve? Is it really hot or just warm? if the gauge isn't getting out of the blue, it won't feel very hot. If it feels red hot and your gauge is jumpy it's probably a bad ground on your gauge cluster. See aforementioned pigtail harness.

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Hi everybody. I took off the earthing straps (two), cleaned the terminals with sand rag and with a bronze brush with the drill, cleaned the surfaces in the engine and body, changed some screws and washers and started the engine and let it warm up but in the driveway.

 

The reading turned out to be almost right. I say almost because it is a bit lower than what it should be, but nothing like before. I think that when I drive the car in the street the reading will be accurate. 

 

I frankly don't know if the problem was with the earthing or the thermal shock of the last Saturday. Anyway it seems that the problem is solved.

 

I want to thank to all of you that posted his help. 

 

Cheers 

 

 

Santiago

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