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Old 08-13-2019, 09:27 PM
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how to test for reference signal ?

If you have a circuit with a 5v reference signal, from the computer, how
do you test for that signal?

Do you test across the send and return of a sensor?

Can you test by red lead on the signal, and ground lead grounded?

I have done plenty of wiring and diagnosing on the 12v -ground system, but I have never really needed to test sensors/computer. I have always just replaced sensors, and I guess I have been lucky enough to never have had a computer problem, or a short in the system.

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Old 08-14-2019, 07:56 AM
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Pretty sure it's sense wire and ground that you want to check for proper output. Check across power and ground for 5v. Be sure the ignition is on so the power and ground are active.

Russ
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Old 08-14-2019, 10:07 AM
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You can test between the 5v reference and any ground, if you test across the sensor ground in the same connector that also will show the ground to the sensor is good basically doubling up the test steps. If you don't get a 5v signal then you could check the ground at the engine or battery to see if it's a bad ground or 5v signal. If you get 5v then you know the 5v reference and the sensor ground which goes through the computer are both good.

Also try not to use test lights on computer circuits unless specifically instructed to in a test proceedure. Try to use a digital meter with high impedance. That will help prevent letting the magic smoke out of the wiring.
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Old 08-14-2019, 10:55 AM
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Test it for power to ground at the ecm plug/pin if you're testing the ecm for output. Some vehicles have more then one sensor referenced off the same 5v circuit. Testing at the sensor plug at the end of the line doesn't eliminate the wiring as the issue or another sensor.
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Old 08-14-2019, 12:00 PM
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You have 2 wire sensors, and 3 wire sensors. 3 wire sensors have the 5v reference, a ground, and the actual signal returning to the ECU. TPS's are 3 wire, you can check for 5v and ground with the sensor disconnected, but you need the sensor connected to check the output back to the ECU. You can use a small paperclip to slip into the connector from the wire side to get a reading with it connected.

2 wire sensors, like temp sensors have a ground and a return signal which is going through a pull up resistor to the 5v ref. in the ECU. If you disconnect the harness, one will read ground, and the other will read close to 5v. Connect the harness and the signal drops to somewhere between .5 - 4.5 depending on the temperature.

Computers are programmed as to what is normal for each sensor. As long as the signal is in its range even if it's wrong, it believes it. The temp sensor should never go all the way to ground, or all the way to the 5v reference, or the ECU will determine that there is a short to ground, or to power, turn on the check engine light, ignore the sensor, and insert a value of it's own to allow it to keep running.

I check the voltage at the sensor, if there is a problem, then, I will check it at the ECU.

You can't always rely on the numbers a scan tool is showing you, if the number is not changing, it might be generated by the ECU, check it at the sensor.

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