Your IAT sensor, 5V reference, and you
03-15-2015, 05:26 PM
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Staging Lane
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Your IAT sensor, 5V reference, and you So I just finished swapping an LS1 into my S10 and I have this nagging issue where the IAT is reading abnormally. I first noticed it when I started the truck for the first time and my IAT read 114* when it was cold and ambient was around 55*. I ohm'd the two wires and they came back OK showing 0.1-0.2Ohms. I read the voltage from the sensor and it was about 3V. Also when unplugged, my scanner would read a temp in the 60's instead of -40 like it should.
So now I'm thinking the PCM is bad and it needs to be replaced, when a friend suggested stealing a 5V ref from an unused pin (EGR in this case). I wire up an extra IAT pigtail I had and now I read ambient +/- on a cold engine, good right?? But now when I unplug the sensor, I see the temperature I was just reading.
Am I wrong in thinking that I cannot use a different 5V reference than what is intended? I measure 5V from the new reference, so I assume it is ok to use?
Why don't I ever read the default -40* when I unplug the sensor?
So now I'm thinking the PCM is bad and it needs to be replaced, when a friend suggested stealing a 5V ref from an unused pin (EGR in this case). I wire up an extra IAT pigtail I had and now I read ambient +/- on a cold engine, good right?? But now when I unplug the sensor, I see the temperature I was just reading.
Am I wrong in thinking that I cannot use a different 5V reference than what is intended? I measure 5V from the new reference, so I assume it is ok to use?
Why don't I ever read the default -40* when I unplug the sensor?
03-16-2015, 07:39 PM
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I have used both the wires in the harness and a pigtail from another harness and they both Ohm'd out ok and had the same results. I wish it would have been that easy haha.
06-17-2016, 12:48 PM
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Hate to dig up an old thread but did you ever resolve this issue OP? I have the same problem; IAT is 155C at ambient and 50C when unplugged (according to Torque App). I used a meter and both wires traced 0.3 Ohm of resistance. Measured 5VDC at reference and no continuity between chassis ground and reference, when IAT unplugged. No resistance between signal and chassis ground should mean PCM ground is good. Checked the Thyristor and it measured 32.54 kohm at ambient, which should be normal.
If I had a short somewhere on the power side I would have continuity between the reference and ground, no? I do not currently see this. The Signal and chassis ground have continuity but that's expected as it returns to chassis ground through the PCM.
Also it seems like its somewhat intermittent as I have seen logs of my IAT where it reports ambient on cold start. I am considering a bad PCM but I have a gut feeling its something else, like a bad ground somewhere else or an iffy 5V supply. In general it probably does read high against ambient (located right behind the rad) whne hot which results from heat soak but I think its made worse from this issue.
If I had a short somewhere on the power side I would have continuity between the reference and ground, no? I do not currently see this. The Signal and chassis ground have continuity but that's expected as it returns to chassis ground through the PCM.
Also it seems like its somewhat intermittent as I have seen logs of my IAT where it reports ambient on cold start. I am considering a bad PCM but I have a gut feeling its something else, like a bad ground somewhere else or an iffy 5V supply. In general it probably does read high against ambient (located right behind the rad) whne hot which results from heat soak but I think its made worse from this issue.