Temperature gauge

[Silent Thunder]

Can anyone figure this out? My gauges work except the temperature gauge. Here is what I've done: I grounded the wire at the sending unit and the needled will move all the way up to hot. I have purchase two new sending units and installed them (without Teflon tape) and no difference. I then changed the gauge and no difference. What else could it be? I'm assuming the voltage reduction is working at the dash board since oil pressure is fine and alt meter are working. Gas gauge only goes to read half (or slightly above) even when full but that's next either a ground issue or tank sending unit. Any ideas???

[jopizz]

Make sure you have the ground strap that goes from the engine to the firewall. If it works when you short the wire then it can only be a sender or ground problem.

John

[Silent Thunder]

I installed a brand new ground strap, no difference. I know there are different senders that break in the middle of production of the 65. But I assume that is a diameter issue and since this is the correct size I believe I have the correct unit and resistance??

[jopizz]

To test it remove the wire and put an ohmmeter from the intake to the top of the sender. With the engine cold you should see little or no resistance. Start the car and let it warm up. You should see resistance as the engine gets hotter. If you don't then it's either a bad sender or it's not grounded correctly to the intake.

John

[Silent Thunder]

Thanks, I will try that this weekend.

[jopizz]

Dave pointed out that I explained it backwards. With the engine cold you should have HIGH resistance and it should go down as the engine gets hotter. Without a meter in front of me I'm lost.

John

[Silent Thunder]

Either way when I set my meter to continuity it should change from hot to cold. When its cold the needle should peg and then as it warms the meter should drop? Or do I too have that backwards?

[jopizz]

Another way to look at it is when the engine is cold the sender is open to ground and as it heats up it shorts to ground. So whatever way your needle points when it's cold it should move the opposite way as it heats up. If it doesn't move at all then you have a bad sender or it's not grounding to the intake. I assume you are seeing coolant when you remove the sender. Other tests you can try are running various ground wires from different points on the car to the sender body and see if your gauge moves. It's unlikely you have three bad senders so I suspect a ground problem.

John

[simplyconnected]

Bear with me for a minute...
Your gauges react to current flow.
When the key is off there is no current flow and the gauge needle drops. The same happens when you disconnect the wire (anywhere in the circuit).

We regard this as HIGH resistance because current cannot flow through high resistance.

Notice that I didn't specify which gauge, because they all work the same way.

Yes, a solid ground is absolutely necessary for the Constant Voltage Regulator and all sending units (gauges don't need a ground and polarity doesn't matter). Also know, when the C.V.R. gets old, it fails in one of three ways:

1. Commonly, it gets erratic. One moment it passes too much current then it passes too little current. The way mechanical CVRs work is just like your turn signal flasher unit, pulsing 12-volts on and off at 50% duty cycle which effectively cuts current in half, simulating six volts from a 12-volt power source.
   
2. The CVR can short internally, meaning it passes a steady 12-volts all the time. <--this can burn your gauge out.
   
3. The CVR can burn open inside which outputs nothing.

So, shorting the wire at the sending unit only proves one thing, the gauge is working. It tells very little about the CVR. For that, you need to see the frequency and how steady it is. My Fluke meter has a frequency setting. If the numbers are steady, that is good. If they are all over the map or not pulsing, change the CVR.

[Silent Thunder]

thank you. I will check it out.