3.5 volts across anunplugged engine coolant sensor?
03-19-2014, 10:04 PM
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3.5 volts across anunplugged engine coolant sensor? I'm trouble shooting an ECT reading on a 4.8/5.3 with no DTC's. PCM readout is 50-60 degrees cooler than the actual temp so fans wont come on and engine is hot. With the engine off and the ECT sensor unhooked the voltage across the pins on the sensor read 3.6V. Is this normal? It's the only thing I have found that has me scratching my head but maybe resistor type sensors hold voltage?
03-20-2014, 01:02 AM
#2
The ECT sensor circuit is not +5V referenced, if you meassure across the connector you won't see 5V even with sensor unplugged.
You need to test the sensor using a ohmeter, check this chart
https://ls1tech.com/forums/15884132-post2.html
You need to test the sensor using a ohmeter, check this chart
https://ls1tech.com/forums/15884132-post2.html
03-20-2014, 09:55 AM
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I have a mechanical gauge hooked to the passenger side head that reads 50 to 60 degrees cooler than the readout I'm getting on my monitor hooked to my OBD2. If I turn the key off the reading on the monitor instantly switches to read the same as the mechanical.. turn key on and it switches back to read 50 degrees cooler. This also happened to the HP tuner when I was having a tune built. this rules out the monitor.
I unhooked the sensor connector and jumped the +5v reference wire to the signal wire and the monitor reads 284 degrees as it should as a diagnostic check. I went one step further and started the truck like this to make sure the PCM would kick the fan on and it did.
I then unhooked the pcm connector and tested continuity of these two wires from the the sensor plug to the PCM plug and they were good.
I plugged the sensor connector back in and checked voltage across the circuit from the pcm blue plug (wire 63 and 74). This is where I saw the +3.5 volts. I unhooked the sensor plug and checked voltage across the sensor and got a reading of +3.6 volts.
If this resistor type sensor does not store a charge then I must have a +12 battery voltage short because the key was off.
I unhooked the sensor connector and jumped the +5v reference wire to the signal wire and the monitor reads 284 degrees as it should as a diagnostic check. I went one step further and started the truck like this to make sure the PCM would kick the fan on and it did.
I then unhooked the pcm connector and tested continuity of these two wires from the the sensor plug to the PCM plug and they were good.
I plugged the sensor connector back in and checked voltage across the circuit from the pcm blue plug (wire 63 and 74). This is where I saw the +3.5 volts. I unhooked the sensor plug and checked voltage across the sensor and got a reading of +3.6 volts.
If this resistor type sensor does not store a charge then I must have a +12 battery voltage short because the key was off.
03-20-2014, 10:57 AM
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The PCM pushes current down the sensor to read its
voltage (then figure out resistance and calculate to a
temperature number). So there should be a voltage
developed (unless it decides to fault and give up).
High voltage, high resistor value in the sensor mean
low temp to the PCM. Open circuit ought to read -40
or so.
voltage (then figure out resistance and calculate to a
temperature number). So there should be a voltage
developed (unless it decides to fault and give up).
High voltage, high resistor value in the sensor mean
low temp to the PCM. Open circuit ought to read -40
or so.
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03-20-2014, 12:24 PM
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Open circuit reads -39. jumping the connection with a wire reads 284. So that is correct!
Why is there voltage (+3.6V) across an unhooked ECT sensor?
Where is the voltage coming from?
key is off, PCM connectors are off, ECT sensor plug is off and the reading acroos the sensor is +3.6V!
Why is there voltage (+3.6V) across an unhooked ECT sensor?
Where is the voltage coming from?
key is off, PCM connectors are off, ECT sensor plug is off and the reading acroos the sensor is +3.6V!
03-20-2014, 06:41 PM
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You mean the sensor itself is generating voltage, with
the harness off?
Only thing that comes to mind is some kooky corrosion
cell type action. Wonder what the sensor would read if
you pulled it out and baked it dry. Or, for that matter,
just what's left of the business end.
the harness off?
Only thing that comes to mind is some kooky corrosion
cell type action. Wonder what the sensor would read if
you pulled it out and baked it dry. Or, for that matter,
just what's left of the business end.
03-20-2014, 08:27 PM
#11
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Unscrew the sensor, remove it from the head, and repeat the measurement.
How are you measuring the voltage, do you have your DMM set to 20VDC (just sanity checking your technique)...?
Measure VDC between coolant (at radiator neck) and engine block... if it is appreciably non-zero then the coolant may be acting like an electrolyte and/or you're missing a ground strap.
03-20-2014, 08:38 PM
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It's a DMM set to 20VDC. I'll try what you said and see what happens. I was just reading up on phantom, ghost and stray voltage across resistors. I don't quite understand the physics of it but understand that is exist ( but not really...like a ghost).
03-20-2014, 09:13 PM
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Joecar thanks for your sanity check because it made me double check again before I made a mess on my floor. My DMM did not register voltage on 20VDC but did on 20K Ohms. It must have been on the wrong setting...stupid! stupid! stupid!
This is good and bad because I'm out of ideas on the temperature issue. I'm going to clean everything up, put it together and try again unless anyone has a another idea. If it still don't work I'm going to abandon those wires and run new ones to the pcm!
This is good and bad because I'm out of ideas on the temperature issue. I'm going to clean everything up, put it together and try again unless anyone has a another idea. If it still don't work I'm going to abandon those wires and run new ones to the pcm!
03-21-2014, 10:10 AM
#14
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Is the sensor grounding properly in the head? If you put to much teflon tape on sensor maybe its not making a good ground. Are you saying the sensor has 3.6 volts or the wires going to the sensor?
03-21-2014, 04:11 PM
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Is it possible for a +12v short at any wire that uses information from the ECT reading to be back feeding thru the entire system and cause the ECT signal to read incorrect voltage. For example if the fan signal wire was shorting to positive voltage then backfeed that voltage thru the pcm and interfere with my ECT signal. I'm not sure if that's possible but it would help me understand the incorrect temp reading when the key is on and correct reading when the key is off.
one thing I noticed today was that I I let the mechanical gauge get to 200 degrees and the monitor was reading around 120. I turned the key off then on and the reading jumped to 140. I did it again and it jumped to 160 and that was as high as I could get it. The mechanical was still reading 200. The sensor is new and I did not use any tape.
I should also note that this is the second engine in the truck. the first one was junk and I did not have a water temp gauge at the time but the fan did kick on and off at least telling me it was reading temperature somewhat correct. The new engine is using the same PCM and wiring harness I built myself. The only thing modified before the second engine was installed was taping and looming the harness and I wonder if something happened during that process to cause this issue!
one thing I noticed today was that I I let the mechanical gauge get to 200 degrees and the monitor was reading around 120. I turned the key off then on and the reading jumped to 140. I did it again and it jumped to 160 and that was as high as I could get it. The mechanical was still reading 200. The sensor is new and I did not use any tape.
I should also note that this is the second engine in the truck. the first one was junk and I did not have a water temp gauge at the time but the fan did kick on and off at least telling me it was reading temperature somewhat correct. The new engine is using the same PCM and wiring harness I built myself. The only thing modified before the second engine was installed was taping and looming the harness and I wonder if something happened during that process to cause this issue!
03-23-2014, 08:54 PM
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I abandoned the old ECT wires and ran new ones to the PCM. it did not help. Then I warmed the truck up to 195 degrees and shut it off then installed a second PCM in to just check the temp reading. it showed 185. Guess I'll be replacing the PCM.
