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Chasing Engine Temp Sensor Problems, Lesson Learned 200

posted by  J.R. on  Sun Apr 12 05:48 CST 2009
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last visit: Tue Feb 1 20:22 CST 2011
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While this has been discussed periodically

http://www.brickboard.com/RWD/index.htm?id=535708&show_all=2

I thought I would share my little adventure and save someone some pain. My current adventure's complete story is the subject of another thread

http://www.brickboard.com/RWD/volvo/1340712/220/240/260/280/hard_starting_rough_ridle.html

When I declare victory I will close that out. On with the Murder Mystery

Using Bentley's I was checking all of the likely suspects, to include the Coolant Temp Sensor that feeds information to the EZ-116K (ICU) and LH 2.4 computers. When I checked it that day, the car had been driven and I read the resistance off #13 in the LH harness. It was 230ish ohms. Close enough.

Moving on I checked the rest of the suspects/sensors. Nothing out of line.

Soooooo, frustrated like you read about, I start over. Maybe I missed something on that dark and stormy night.

This time, I check the Temp Sensor first. It just kept nagging me. Why was it running so rich at start, why the rough idle, why when it was warm did it behave, why why why?

Cold engine. Temp 55F
At #13 probe point in the LH, resistance was 112,000 ohms. WHAT!?
At #2 probe point in the ICU, resistance was 112,000 ohms. WHAT AGAIN!?

GO to the engine, pull the connector off, check each side of the sensor against a ground. Same readings. Okay everything is apart, good time to check the resistance across the harness and make sure there are no shorts to ground. .2 ohms per wire, no shorts.

RATS! Bad temp sensor. Explains a lot. Computers think the car is at the coldest place in the universe so it gives it more fuel and retards the timing.

Lesson learned. I should have made at least two measurements. One hot and one cold. With time, I should have warmed it up in the morning and taken measurements every hour to see how its doing against the curve and taken temp readings using a infrared temp sensor. One part of Bentley's sez take it out and test it at three temps. That sensor is a pain to R&R.

Hope this helps someone. Ordering in a new temp sensor. I swear I replaced that thing about 5-6 years ago, but the notebook doesn't record it.

Cheers.

R,
J.R.







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    Chasing Engine Temp Sensor Problems, Lesson Learned 200

    uid 27's pic
    posted by  Art Benstein on  Sun Apr 12 10:19 CST 2009
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    last visit: Fri Oct 17 20:10 CST 2014
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    112K each side to ground is a strange reading. It could be explained by a broken, crystallized joint where the two thermistors are grounded at the housing tube, and that would also explain the temperature dependent behavior NOT designed in. I would read across the two terminals as a double check of meter and probe, to see you get 2X the expected resistance of one element. I suppose there's time enough to do that when you've changed the part.

    That would be very tough one to find based on your symptoms, unless, as you say, you take readings cold and hot, AND get lucky to find it in its non-working state. Another means of catching this, is to backprobe the pin on either the ICU or ECU for voltage. You could leave the voltmeter connected while you drive. The ECU measures the thermistor's resistance by applying +5 through 2.7K and deriving the temperature from the divided voltage - a little less than 2.3V at a cold start where the coolant temp is about 70F. It drops from there. So if your probe sees nearly +5, you've caught it open circuited (or nearly so) and you'd be able to watch when the thermal expansion "fixes" the poor connection and the thermistor pair finds its ground reference once again.

    But knowing to look for it? That's how we should benefit from your experience. If that reading was no mistake, I think you've solved your rough idle/ intermittent hard start.
    --
    Art Benstein near Baltimore

    Some days you're the bug; some days you're the windshield.







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      Chasing Engine Temp Sensor Problems, Lesson Learned 200

      posted by  J.R. on  Sun Apr 12 11:08 CST 2009
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      last visit: Tue Feb 1 20:22 CST 2011
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      Art,

      I would loved to have done the voltage measurement test. When I get the new part, I'll take the measurements again (old part then new part) (resistance and voltage) cold. I have a nifty tool to probe the wire (alligator clip with a probe in the clip so I'll connect up and see what happens driving.

      I too can't figure out the high resistance.

      I just hope in the end, it is the holy grail... until my next adventure

      R,
      J.R.








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    Chasing Engine Temp Sensor Problems, Lesson Learned 200

    posted by  walrus3 on  Sun Apr 12 09:22 CST 2009
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    last visit: Mon Dec 2 14:20 CST 2013
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    They don't go bad very often but they sure do sometimes!
    Good info!
    --
    George Downs, Bartlesville, Heart of the USA!







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