Coolant Sensor Vs Gauge Anomaly?

[Dongskie2]

Good day guys!

Here I have a 1991 4D56TDic.

I recently replaced the coolant sensor because I believe it was sending the incorrect signal to the temp gauge on the dash.

Previously I installed a separate and independent coolant temp gauge so I have a 2nd opinion.

Attached are photos and the round gauge says coolant temp is around 95deg C

But on the dash gauge its almost overheating!!

Is the new coolant sensor wrong or defective?

The old coolant sensor never goes never kicked-up 3/4 of the gauge even when the round gauge says 95deg C.

What should i look into?

[Dongskie2]

[old Jack]

You have now had 2 different sensors connected to the dash gauge with similar results, this points to a wiring or gauge fault.

OJ.

[erad]

A temperature gauge is merely a micro ampmeter. The circuit is provided with a stabilised power supply and the sensor is a variable resistor, the resistance varying according to the temperature - higher temperature equals lower resistance, and it is not linear. So as the temperature increases, the resistance lowers and the gauge reads higher.

The higher reading on your OEM gauge is due to a lower resistance in the circuit. This could be due to a short circuit in the wiring or, more likely in this case, a dud sensor. My Max Elery W/S manual states that the resistance for the sensor should be 104 Ohms (+/- 13.5 Ohms) @ 70 Deg C. Check the resistance between the terminal and the metal surface of the head. Obviously you won't get exactly 70 Deg C when you measure the resistance but it will give you some idea. If it is much lower, suspect eh sensor.

[Kaldek]

Mitsubishi on their older cars run two separate coolant sensors. There is:

- A three-wire 5-volt referenced "Engine Coolant Temperature" (ECT) sensor which sends signals to the ECU
- A one-wire sensor which connects the dash gauge to ground, and via changes in its resistance alters the current passing through the gauge, which in turn changes the needle position.

The one-wire sensors fail *all the time*. Just replace the sensor and your problem will go away.

Having said all that, 95C seems high to me but if this is a diesel I've got zero experience on diesel engine temperatures.

[Kaldek]

I just checked and 95C is apparently still high for a diesel.

The Mitsubishi dash gauge will usually read about 3/4 when the engine is 95C. An old/failing temperature sensor reads higher than usual, so I suspect that your engine is running too hot AND your sensor has failed.

At 82-85C the dash gauge will read just above the cut out, or a little above 1/3rd. As the engine climbs over 90C, the Mitsubish gauges rise *rapidly*.

Can I just ask if your aftermarket gauge has its own sensor or if you're tapping one the existing sensors?

[erad]

Cannot say for sure with a Pajero, but on the old Magnas, they used to have 2 coolant temperature sensors - one fro the gauge and the other for (I think) the emissions control or the auto transmission. The latter one was just a thermal switch which went from OFF to ON. Maybe you have one of these?

[TomW]

erad, that’s correct for the Pajero as well. The auto Pajeros have both a sensor for the temp gauge and also a switch which inhibits overdrive until the engine is warm. If you inadvertently installed a switch instead of a sensor (they look the same) you would see this behaviour as the gauge is effectively grounded by the switch and showing max hot.

You can validate this by measuring resistance between the new sensor output terminal and ground, assuming it is a switch or is faulty it will show basically 0 ohms when the engine is hot.

In relation to the factory gauge it’s not the best even when working correctly and has quite a large dead zone, I simply left mine disconnected and rely on the aftermarket gauge.

[stumagoo]

you have an auto - I have to ask what sensor are you replacing and where is it - the auto actually has up to 3 temp sensors 2 are single wire units one is a 2wire unit (sorry Kaldek its not a bigger one) the 2 wire as above is for the ecu only and on the v6 ( i dont know the configuration on a diesel auto unfortunately) the single wire one closest to the radiator is the temp gauge on the dash the single wire furtherest from the radiator is not a sensor but a switch - when you get to the preset temp it earths allowing the OD to kick in I wonder if you have either

A switched the wires for the temp sensor and the temp switch over
B got a temp switch and not a sensor for your gauge

[Dongskie2]

Hi guys, thanks for replies.

I replaced the sensor on top of the 4D56T cyl-head itself, its the big one with 2 pin combo perpendicular to each other. One pin is for the coolant gauge and one pin is for the heater timer control.

The OD sensor switch is mounted on the base of the water port where the radiator hose is attached and another sensor switch on the other side of the base for the FAN assist when temps over 115deg.

Speaking of OD sensor switch, the one on mine is busted and totally gone with half of the sensor body still inside the thread but no more top half, hence I earthed the socket and my OD worked.

Yes I thought too that 95deg C is too hot, I was told that it should stay 85deg-90deg only.

Thanks for the ideas and comments mates this sure helped me a lot and i'd know where to go from here.

Maybe the viscous coupler is bust?

[Dongskie2]

Can I just ask if your aftermarket gauge has its own sensor or if you're tapping one the existing sensors?

It's got its own sensor.

[TomW]

Easiest answer is possibly someone installed a higher temp thermostat inadvertently, or its not functioning correctly?

According to the workshop manual the diesel should have a 77 degree thermostat, with full open at 90.

If the viscous fan had a problem it would tend to show up when airflow is low (i.e. round town the temperature would creep up) or under heavy load.

A good test is on the highway at speed, if the viscous fan is dodgy or the radiator not performing to spec then the temperature should drop down to around 80 degrees with the increased airflow. If the temp stays at 95 then more likely you have a thermostat problem (i.e even with increased airflow the thermostat is maintaining its set temperature.

[erad]

You say that the extra temperature gauge has separate sensor. Where is that sensor mounted?

[Dongskie2]

You say that the extra temperature gauge has separate sensor. Where is that sensor mounted?

On the hose port of the cylinder head where the upper radiator hose is mounted. There is a vacant threaded hole plugged with a 12mm head bolt.

[Dongskie2]

Easiest answer is possibly someone installed a higher temp thermostat inadvertently, or its not functioning correctly?

According to the workshop manual the diesel should have a 77 degree thermostat, with full open at 90.

If the viscous fan had a problem it would tend to show up when airflow is low (i.e. round town the temperature would creep up) or under heavy load.

A good test is on the highway at speed, if the viscous fan is dodgy or the radiator not performing to spec then the temperature should drop down to around 80 degrees with the increased airflow. If the temp stays at 95 then more likely you have a thermostat problem (i.e even with increased airflow the thermostat is maintaining its set temperature.

Thanks for this.

I do have the 76.9deg TStat

But I haven't checked my viscous fan..

Thanks mate!