Oil Temp sensor wiring harness issues

OilLeaks

Need electrical help!!!

Problem: The wiring harness on the right side of the engine w/ 4 plugs (oil temp, oil pressure, power steering pressure & alternator) melted on the turbo manifold. The previous owner repaired the wires, but now it always reads high oil temp even when it's cold. I unplugged all the end plugs from the sensors on the part of the harness that had been repaired & oil temp still reads high. That leads me to believe there may be something upstream that is causing the issue. None of the wires appear to be grounding out though.

Anyone know what else could cause the false high oil temp reading & how to fix it?

dts3

What exactly do you mean when you say they don't appear to be grounding? You need to rule this out with a multimeter as your very first step, if you haven't already done so.

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SonicVQ

If the oil temp is showing a high temperature with the sensor unplugged, I suspect the oil temp signal wire is shorted to ground.

To verify the wiring:
With the sensor unplugged and ignition on (and engine not running) check for voltages at the harness:
• One wire should have about 5 volts from ground.
• The other wire should have 0 volts (or close it) from ground.


To verify the sensor:
• measure the resistance across it:
• at 10C / 50F it should be around 3600 - 3900 ohms
• at 20C / 68F it should be around 2100- 2900 ohms


Often, when troubleshooting it is good to know what the correct/working voltages should be.

Using a multimeter, measure the voltage across the oil temp sensor, with the ignition on but without the engine running. Use the table below to verify correct operation - or just look at your gauge

Temperature & voltage table for our oil temp sensor (and the coolant sensor, they are the same part)

Temp C/F ---- Voltage
11C / 52F ---- 3.84 volts
16C / 61F ---- 3.68 volts
24C / 75F ---- 3.36 volts


Good luck!

OilLeaks

The repaired wires are okay, I just can't tell what happened further back into the harness. I presume the wires weren't damaged further away. I'll investigate it further.

OilLeaks

So just to clarify... I check the wires for voltage one at a time by putting the red multimete probe on one wire & the black probe on a common grounded area? One should read 5v & the other 0v? The wires are green / black.

Under normal circumstances, if the oil temp sensor is unplugged, does the car default to a high reading or what should happen?

The sensor was replaced & is new.

SonicVQ

Yup, you got!
A good ground is any unpainted aluminium part of the engine. Make sure to press the ground probe firmly to ensure a good connection.

If the oil temp sensor is unplugged, the indicated temp should be around -40C / -40F.

OilLeaks

I did this & got 0v on the black wire & 4.79v on the green wire. I presume it's okay then?

OilLeaks

The ecu may have problems then. 😕
I'll check that next. Ughhhhhhh

SonicVQ

I would agree.

SonicVQ

That's possible... when wires melt and short out bad things can happen

I will have a look at the oil temp gauge wiring diagram and see if anythings stands out.

If you have an OBD scanner and smart phone/tablet there are ways to check the oil temp using apps like Torque Pro or OBD Fusion (with the $15 enhanced diagnostic add on)

This way we can verify the exact oil temp the ECU is seeing.

If you have an Android device, here is how you can configure the Torque Pro (~$5) app to display engine oil temp for free:

SonicVQ

I checked the triple meter wiring diagram and it is isolated from the engine oil temperature circuit. The ECM reads the oil temp and sends it to the triple meter via the CAN bus.

I am starting to suspect the ECM

The engine oil temperature circuit (and every other ECM sensor input) uses 5 volts.

Is it possible when the wires melted the bare 12v alternator wire touched the bare oil temp wires? If that happened, 12 volts would have been sent to the 5 volt sensor input.

We know the 5 volt supply is good, as you measured almost 5 volts with the sensor unplugged. AND the ground is good, since you had 0 volts.

The engine oil temperature circuit inside the ECM connects to a "multiplexer" (multiple inputs with a single programmable output) before it goes to the analog to digital converter in the main CPU (actually a SH7059 32 bit RISC microcomputer) I suspect one of those two items may be damaged.

OilLeaks

I have an EcuTek tune, so I can see all the parameters. It always reads 275 degrees regardless if its cold or hot. I have an ECU on the way so I can test it & see. I'd almost rather it be the ECU because that's easier than tracking down unknown electrical gremlins. 😐

Rusty

It's reading 275F. It should be close to limp mode. It's pulling a whole bunch of timing. The ECU starts to pull timing around 240F. The higher it goes, the more it pulls until limp mode.

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OilLeaks

The false high oil temp is causing the throttle bodies begin to close under boost in higher rpm & drop to 50% in between shifts. Hopefully I can get it resolved soon. Feels like a stutter on accelerating

SonicVQ

Can you modify the Oil Temp map?
If you can, you could temporarily re-map the high temp to a lower temp and see if the engine runs better.

I know on ECUTek you can:
(Stock map from the top is: 205, 191, 124, 110, 99)

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