I just converted a friend's 49 F1 to 12v negative ground. It has a 53 Mercury v8 flathead. Everything seems to be working correct except the temp gauge. It's reading "H" no matter if the key is on or off. Here's what I have and found while trouble shooting...


I'm supplying the 3 gauges with 6v with this converter...

With ignition on, I get 6.1v on the temp sender wire(not hooked to anything).


If I ground the sensor wire, the gauge doesn't move, stays on "H". It should move to "C", right?


The gauge worked before the 12v conversion. Is there something I'm missing? The three gauges are fed +6v to the metal straps on back of them and are grounded by the panel case.


Am I missing something?

 

I'd try loosening and re-tightening the screws that hold the actual gauge into the back of the panel (grounding). Also, there is a remote possibility that one of the gauge posts got turned/twisted too much when you installed new wiring and broke one of the internal gauge wires. Other than those I would suspect a bad gauge.

 

Are you still using both senders on the engine? When you say you are grounding the sender wire, is it the wire leaving the dash unit, or the one at the sender with one terminal that you are grounding?

The sender with 2 terminals is really a normally closed switch. Power from the dash gauge runs thru that before going to the 1-terminal sender. If the 2-terminal switch has failed Open, ground the wire at the 1-terminal sender won't do anything.

 

I should have mentioned that only the 1-terminal switch is being used on the passenger side. A previous owner eliminated the 2 terminal switch(probably didn't want to spend the money to replace it). The 6 volts is making it to the 1-terminal sender, which is where I removed the wire and grounded to engine to test.


The sensor itself appears to work. After running the truck for about 20min, the resistance at the sender pulled the voltage down to 4v with wire attached. If the gauge was working, what would 4v put it at? Little past halfway? Does 0v = "C" and 6v = "H" ?


Thanks for the help!

 

Guess that is a possibility. If I remember correctly, the temp gauge is where the +6v wire tied into. I'll check it out on Friday when I get back to the truck.

 

Look at the schematic above. The sender doesn't work on varying resistance. It heats up and makes/breaks a full-voltage contact.

You can test the dash gauge by connecting a 1.5v C-cell battery across it. It should read 1/2 scale.

With everything hooked up, a voltmeter connected to the wire on the sender should show alternating low voltage and full voltage.

 

So my buddy calls me and says he switched the wires around on the temp gauge and now it's working. This had me baffled so I ran over there to see what exactly he changed. I'm still baffled why it's working how he changed them...

Here's how I had it hooked up....




And here's what he did(swapped the sender and power wire)....





This does not seem right? This way the other 2 gauges are being powered by the temp sender output.

Should the power and sender terminals be switch on the oil and fuel gauge as well?

 

I suspect with the way he has it wired, the temp gauge is grounding thru one of the other senders or gauges? How does power get to the other gauges?

I don't see anything wrong with the way you had it, with one caveat; the gauge pod isn't a particularly good place to ground the power supply. It may not be making good contact thru the dash to "real" ground.

 

I don't understand how the gauges are working. Something isn't right.

 

You changed the system from positive ground to negative ground when you went to 12 volts. That's why he swapped the wires.

 

Yes, but originally the positive was supplied through the metal housing. I switched the polarity by supplying the +6v to the 3 terminals strapped together, and negative ground through the metal housing.

 

The gauges aren't polarity sensitive, except for the ammeter