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B23F bad AMM or coolant temp sensor 200 1983

posted by  jimo14 on  Fri Apr 27 08:16 CST 2012
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last visit: Mon Oct 6 18:50 CST 2014
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Can a bad coolant temp sensor mimic the same bad running problems as a bad air mass meter? The only way my wagon will run and stay started is with the air mass meter unplugged ("limp home mode"). Before I spring for a new AMM ($), I'd like to be as sure as I can as to what's causing my wagon to run like crap.

I tried running tests yesterday probing the sides of of the ECU connector (plugged in) as per the Bentley manual, but don't trust my results. In most cases, I got no readings (ohms, continuity, etc.). I chalk this up in large part to my inexperience and lack of know how.

My engine wiring harness is new, so I'm confident it's not an issue with that. I have 2 ECUs, have tried both, same results. Could the one wire O2 sensor be a culprit in this bad dream? I just solved a very bad running condition in my '91 240 by replacing the O2 sensor (3 wire). Thanks for any insight here - Jim








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    B23F bad AMM or coolant temp sensor 200 1983

    posted by  someone claiming to be Chris Huntsville AL on  Tue Mar 25 07:35 CST 2014 [RELATED]

    hi my name is Chris I live in HUNTSVILLE AL Look carefully at the grounding points / strap to the frame!








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    ANSWER B23F bad AMM or coolant temp sensor 200 1983

    posted by  LoveMy240Wagon on  Fri Apr 27 09:58 CST 2012
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    last visit: Tue Dec 31 09:52 CST 2013
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    A bad coolant sensor COULD mimic the issues a bad AMM could cause, mostly because a bad AMM can cause a variety of running conditions.

    If your coolant temp. sensor is bad then the engine thinks it is cold when it is not. Therefore it will add fuel and run rich. The Oxygen Sensor SHOULD (in theory) notice the extra fuel in the exhaust and correct the A/F mixture however that has not been my observation. I've had a rich running condition where the O2 sensor was operating correctly (I checked the output voltage) but still did not correctly adjust the air/fuel mixture.

    The good news is that it's very easy to test your coolant temp. sensor. Pull it out, dunk it into water of a known temperature, measure the resistance across the terminals and match it up to the chart in the Bentley Manual.

    Here it is for the B230F LH2.2: http://repairguide.autozone.com/znetrgs/repair_guide_content/en_us/images/0900c152/80/06/41/99/small/0900c15280064199.jpg

    "Connect the ohmmeter between ground and terminal 2. Resistance depends on coolant temperature (see chart). As a reference, if the coolant is at 68°F (20°C), look for readings of 2300-2700 ohms. If the coolant is at 176°F (80°C), expect readings of 300-360 ohms. Hint: if the ohm reading is incorrect, perform the test again at the pins of the sensor. If the readings are now correct, you have a wiring problem, not a sensor problem."

    This post has been marked as an answer to the original question.



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      B23F bad AMM or coolant temp sensor 200 1983

      posted by  jimo14 on  Fri Apr 27 10:25 CST 2012
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      last visit: Mon Oct 6 18:50 CST 2014
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      "If your coolant temp. sensor is bad then the engine thinks it is cold when it is not."

      Can this also be the case even when it actually is cold? As in initial start up? With the AMM plugged in, it will start easily enough, then immediately die.

      And yes, it definitely runs WAY rich. Btw, I'm thinking if I pull the intake manifold to get at the ECT, I might as well go ahead and replace it with a new Bosch.








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        B23F bad AMM or coolant temp sensor 200 1983

        posted by  LoveMy240Wagon on  Fri Apr 27 11:00 CST 2012
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        last visit: Tue Dec 31 09:52 CST 2013
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        Yes, that can be the case. I believe there is only one way that the temp sensor fails so when it does fail, it thinks it is -40 degrees which causes it to dump fuel in.

        I should have mentioned that you don't really have to remove it, just check the resistance at the ECU. I check mine when it is cold so I know it's around 60F. This is not a test where you are splitting hairs; if it failed then it will give you a resistance that is way off. At least that has been my experience.

        I doubt the Oxygen Sensor would cause it to run extremely rich.

        Another thing you could do is go to Radio Shack and pickup a resistor that corresponds to the ohms at say 110F. Pull the wires off of the temp sensor and attach them to the resistor. The resistor will mimic the temp sensor output for 110F and the computer will adjust accordingly. If the sensor is the culprit you will see a difference in how it starts and runs. But this really isn't neccessary if you have tested the sensor and it is good.







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