electrical help - temp gauge

[todd reid]

I could use some help from our resident electrical gurus:

My temp gauge moves swiftly up to appox 210 degrees whenever the key is rotated to the run position - even when the engine is off and stone cold.

fuel gauge does not follow suit - goes to approx. 2 gal and stops - which is probably correct (haven't driven lately so not sure).

Installed one of Ed's solid state voltage regulators approx. 18 months ago. I'm thinking a voltage regulator problem would effect both gauges.

The temp sender seems relatively bulletproof (unlikely to fail, but not impossible), and I would expect a burned out gauge to read zero - not high, so what am I looking for??

Thanks in advance to all for your time & consideration!

 

[Tom H]

Fuel gauge seems normal, so your regulator is almost surely OK. Immediate run up to near, but not quite, full scale- sounds like a defective temp sender, or more likely, a shorted wire. Pull the green wire connector off the temp sender. Temp gauge should drop to zero. If not , somewhere the insualtion is worn off and the wire is shorted to chassis. Or maybe as you pull the wire off you'll see something wrong right there. Reconnect and see if it goes back to max. If so, looks like a failed sender. Not very common for them to fail completely or even this badly. I have never seen one totaly shorted. Nor totally open, for that matter. But they do go off cal. These are not "bulletproof" senders. The sender is essentialy a thermistor, a temperature variable resistor , and one of the least reliable components ever invented.

Tom

 

[Paul A]

I agree with Tom's analysis. You may want to temporarily replace the wire from the sender to the gauge with a "test wire". If you get a more appropriate reading it will tell you the fault is in the wiring. If you still have an unusually high reading it is likely a faulty sender. Pete

 

[Tom H]

One more thing to check. Assuming it goes to zero when you pull the connector off the sender and goes back to near max when reconnected, you might look to see if there is some build up of rust or gunk around the terminal on the sender. Clean that off and recheck. If still the same, then the sender is kaput.

Update: I have 2 failed temp senders in my collection. Both have failed by having too high a resistance, rather than too low, like Todd's seems to be. Too high a resistance would cause a too low reading. I'm bettin' on a partial short somewhere, maybe just a wire fragment dangling across the connector terminal on the sender.

Tom

 

[todd reid]

Tom & Paul,

Thanks for the replies - I will report back after examination. In the mean time, could I ask a follow up question? How does this circuit work? There seems to be a single wire running to the temp sender, does that mean the sender is the ground?

 

[Tom H]

Yes, the sender housing is grounded. The single connector terminal on the sender is insulated and connects to one end of the thermistor inside the sender housing. the other end of the thermistor is connected to the sender housing and thus connected to ground when the sender is screwed into the thermostat housing.

The complete circuit is : from + 12V battery term to the regulator, then 10 V out of the regulator to one terminal on the gauge, then form te other terminal on the gauge to the sender, thru the thermistor, and then to ground. Very simple circuit. And inside the gauge the ciurrent flows thru a small resistance heater. That heater heats up a bi-metal strip, which is connected to the needle. The bi-metal strip bends when heated. Depending on the resistance of the sender, more or less current flows thru the heater and thus the needle moves more or less. The sender has least resistance at highest temp.

Tom

 

[todd reid]

electrical help - temp guage

Just reporting back. Pulling wire from temp sender did nothing - gauge still read 210. Running new wire from gauge to sender returned things to normal, but raises questions about the integrity of my wiring harness???

Again, thanks for the help Tom & Paul!