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Old 01-14-2020, 12:45 PM   #11
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Default Re: Temp Gauge reading high with cold engine

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Originally Posted by koenb View Post
In theory the ECM could have logic to compare values of oil temp, ambient air temp, and coolant temp and decipher that oil temp is near ambient (cold engine) and coolant is not, and throw a fault, but I doubt that exists.
The NCV3 does this. Using an oil pan heater prior to a cold start will cause an oil sensor plausibility code. Similar effects will happen if the air or coolant sensors have ridiculous values. The T1N does not have this advanced logic, for better or for worse.

I have not checked, but if you unplug the coolant temp sensor I think it defaults to a sane, but very hot value.
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Old 01-14-2020, 08:25 PM   #12
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Default Re: Temp Gauge reading high with cold engine

Hmm that's neat, but seems almost uncessary given the human logic involved when contradicting information shows up. But, I suppose, some vehicle owners might need a light to tell them when something isn't working as it should.

Anyhow, the resistive values chart from the training manual in post 6 would make me think if it were unplugged it would register max cold and a short would be max hot.
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Old 01-14-2020, 10:00 PM   #13
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Default Re: Temp Gauge reading high with cold engine

Quote:
Originally Posted by koenb View Post
Anyhow, the resistive values chart from the training manual in post 6 would make me think if it were unplugged it would register max cold and a short would be max hot.
The words on the earlier page (also posted) state that:
If the leads are shorted to ground (i.e. zero resistance) it thinks 140 C ( 284 F)
If the leads are open or shorted to positive (i.e. infinite resistance): - 40 C (-40 F)

The manual doesn't say it, but i would expect that having the sensor leads shorted together (again, zero ohms) would also read +140 C (284 F)

--dick
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Old 01-15-2020, 01:37 PM   #14
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Default Re: Temp Gauge reading high with cold engine

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Originally Posted by syncro_G View Post
I used my Autel - as far as I can tell, there is only one coolant temp sender, the oil temp is the only other temperature sensor I can cross reference with. I don't think the computer has any way of knowing if the temperature is wrong. (Even on a cold start, the booster fan is blowing on the radiator as if it were hot.)
So the autel agrees with the dash gage?
If so, and if the wires are in good order why not just get a new thermostat with sensor?
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Old 01-18-2020, 08:19 PM   #15
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Default Re: Temp Gauge reading high with cold engine

I've been really busy with other things - sorry for the slow reply.

It looks like the T1N doesn't have a plausibility check for the water temperature. I've seen those for other sensors and this should have triggered one if it existed.

I ordered a new sensor, plugged it into the harness (without installing it into the housing) and the reading remains high (~180F with a cold engine)

And same reading with the harness disconnected from the sensor. So I assume I have a short somewhere in the harness as @Midwestdrifter suggested.
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Old 01-18-2020, 08:29 PM   #16
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Default Re: Temp Gauge reading high with cold engine

Next step is to unplug the ECM's connector and test directly from the correct pins in the connector. If that shows a ground, short, or open at least you know it's in the harness.
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Old 01-19-2020, 02:45 PM   #17
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Default Re: Temp Gauge reading high with cold engine

If the harness looks good and you cant spot the obvious damage, I would be tempted to run a new wire pair from the ECM to the sensor. There is an unused rubber blanking plug above where the ECM harness passes through the firewall, so you can run the wires there. Obviously use waterproof splicing techniques in the engine compartment.
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Old 01-26-2020, 02:03 AM   #18
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Default Re: Temp Gauge reading high with cold engine

I finally got to spend about an hour poking around before being called away.
The wiring diagram in the manual indicates there are no breaks in the wiring between the sensor and the ECM but wire colors at the sensor socket don't match. This usually indicates there's a connector in line somewhere.

With Both ends disconnected (ECM and sensor) I measured resistances.
From the sensor socket, I wasn't able to measure any shorts or leaks to power or ground.
From the ECM, I measured 3K ohms to ground on both wires.

That was curious but I was called away. Hoping to get back to it Monday.

This is starting to get annoying because the temperature sometimes reads over 270F which makes the engine cut back. I can feel the engine cut back and surge as the needs wanders into and out of the red.
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Old 01-26-2020, 02:56 AM   #19
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Default Re: Temp Gauge reading high with cold engine

So the colors don't match? I would be inclined to think the schematic is wrong?

3 ohm is about the lowest most meters will read.
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Old 01-26-2020, 09:46 AM   #20
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Default Re: Temp Gauge reading high with cold engine

Are you reading the ECM pin numbers for 'OBD' or 'EXCEPT OBD' ?

And what year is your T1N?
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