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Hi, when I test my ect signal wire there's 4.87 v ignition on. If I put my multi meter on battery positive and the other lead on the signal wire, ignition off I get battery voltage. If this wire was shorted there would not be 4.87v ignition on. Is this a valid test to establish wiring integrity? When I disconnect the ecu battery voltage is gone. Can anyone advise, thanks.
The reason is your meter has an impedance (fancy name for resistance) of somewhere between 300,000 and 10,000,000 ohms. The circuit you are measuring will have a ressistance of at most 4,000 ohms so there will only be a very tiny volts drop from battery voltage.
You can verify the sensor by measuring the sensor resistance disconnected, these sensors vary from manufacturer to manufacturer but are typically what is called a negative temperature coefficient thermistor, which is just a resistor the resistance of which drops as the engine warms up.
I don't have the numbers for your engine but typically I would expect about 3,000 to 4,000 ohms on a cold morning and a few hundred ohms when fully hot.
A better test is to back pin the signal wire and measure the voltage between the signal wire and battery ground as the engine warms up as this checks both the sensor and wiring integrity.
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Last edit: 8 years 10 months ago by Andy.MacFadyen.
