Electronics/wiring help

[corsair231]

Hey guys, not a Buick specific problem but a general electronics question. Can a magnetic field travel via a wire? I know, it's an off the wall question but let me explain why I'm asking.

The particular vehicle I'm dealing with is a 1st gen Dakota v-8 pick up. Situation is I had a leaky intake gasket. Pretty straight forward r&r job. I finished the install, hooked my harness back up and started the truck to check everything out. My factory temp gauge started off like it should on cold and as the truck warmed up it went up the scale as expected. The problem is it never stopped. The truck was not running hot but the needle slowly climbed all the way past the hot mark and proceeded to bury itself at the bottom of the dial and would not come back. I though I may have just stuck my needle so I took the cluster apart and the needle moves freely but went back to hot as soon as I turned the ignition on. I happened to have another cluster so I changed it out and the temp gauge did the same thing.

I then figured I had either pinched a wire or left a ground off. I double checked and have checked several times now and cannot find either. The switch is a simple open switch that grounds as the temp rises. It appears to be operating properly. I unhooked the harness from the switch and started the truck. The needle swung all the way to the cold side of the gauge and buried itself at the bottom. The factory check is to unplug the harness and the gauge should read cold. Ground the plug and the gauge should read hot. It will not move no matter what I try. The rest of the gauges seem to work properly.

The only thing I found out of the way with the wiring is the wire from the switch was running right behind the alternator and touching the housing at the center where the magnetic field is created. I know an electrical charge passed through a wire winding around a iron nail will create a magnetic field but is it possible the magnetic field from the alternator has traveled through the harness and somehow polarized my temperature gauge? I had touched nothing in the dash before this problem manifested and I can't find any wiring problems so I'm at a loss as to why my temp gauge won't work. Ideas?

TIA

 

[mikestertwo]

The temperature sending unit is not a switch. It is a variable resistor. The fact that the gauge reads cold when you disconnect the wire and reads hot when you connect the wire back up tells me your sending unit is bad. It's shorted to ground.

 

[corsair231]

The temperature sending unit is not a switch. It is a variable resistor. The fact that the gauge reads cold when you disconnect the wire and reads hot when you connect the wire back up tells me your sending unit is bad. It's shorted to ground.

Sorry, poor choice of words. I know the gauge works on a resistor circuit and a light is basically an on/off switch but my gauge does not move at all. It is all the way off the scale. It went all the way past hot when the truck was first started and stayed. I changed gauges and it did the same thing with the truck cold. Put the original one back in and straight to off the scale hot again. I unhooked the sender and started the truck and needle swung to off the scale cold. Now it is stuck there and will not move even when I do the factory test procedure and ground the wire lead. It still reads cold with the harness shorted to ground. The sending unit seems to be working since I have an open circuit when the truck is cold and a path to ground when the truck is at temp.

 

[mikestertwo]

The sending unit seems to be working since I have an open circuit when the truck is cold and a path to ground when the truck is at temp.

That doesn't make any sense. The circuit should never be open or shorted to ground. It's a variable resistor. The gauge could be mechanically stuck at the cold position now when you opened the circuit. What resistance does the sending unit read when the engine is cold? What is it when the engine is up to operating temperature?

 

[corsair231]

Thanks Mike but the problem has somehow been solved. Still don't know what the cause was but I unhooked everything looking for a pinched wire or a missed ground. Unhooked the entire harness from the sensors and unhooked the harness from the junction where it goes into the firewall. Found nothing but put it back together and the gauge started responding. Although I couldn't find it I guess it either had to be a bad connection or possible bad wire in the harness. Could have been either shorting to ground or possible power bleed from another circuit. Either way it seems to work now. Just chalk it up to a gremlin.