Been a while since I've had electricity class. I'm tring to check to see if a water temp sensor is working correctly. The sensor has 2 wires which connects to the ECU. If I disconnect the wire at the sensor, I get 1 wire with 4.99 volts and the other has like .03. Once I attach the wire back on the 4.99 volts drops to .98. Should the supply voltage drop like this or should it stay at 4.99 volts.

 

Where are you putting the negative of the meter? On the engine block? No the supply should not drop.

 

That sounds about right. One of the wires is the SIG RTN (ground) and the other (ECT) is floating high when you take it off. This wire goes to Pin #7 of the ECU and then to a chip inside and does not have a pull up source, only a cap to ground.

 

Yes placing the negative from the meter to a metal ground to take readings.

With the wire connection unplugged I get 4.99 volts as a supply voltage to the water temp sensor. When I plug it back on the supply voltage goes to .98. I thought the supply voltage should stay the same and the output back to the computer would vary. Does this sound right. This is not a ford I'm working on but I tought that all vehicle where real similiar.

I have a car that is running really rich (EFI motor) and seems like it has something to do with the computer or wiring.

 

I do not know what you are working on and the information I gave above was for the Ford Ranger in your signature. Ford does not use a supply voltage to the ETC sensor.

 

No, sorry it is a nissan stanza. I tried useing a nissan forum but didn't get much help.

 

Variable resistance sensors are likely to use an internal pull-up resistor to 5V on the signal wire of the sensor, which will give that voltage on the signal wire when no sensor is connected. As soon as the sensor is connected, the voltage will drop, even on the wire that previously showed 5V. The voltage is reduced based on current - when there's no current through a resistor, there is no voltage drop, but as soon as the circuit is completed, there will be a potential difference across the resistor, both the internal resistor to 5V and across the variable resistance sensor itself. Ford and Nissan probably use the same method, even if the pull-up resistors are different.

 

Thanks,

I know that the water temp sensor itself works. checked it with the meter and the resistance changes as the water heats up and cools down.