Temperature gauge question

[Tony Smith]

Polarity makes no difference.

The gauge works by current flowing though a wire winding around a bi-metal strip. As the winding heats the strip, it bends, deflecting the needle on the gauge. The current flow, and the frequency of the circuit opening is dependant on the temperature of the sender on the Cyl Head.

For the gauge to correctly indicate, it needs the correct voltage and amperage flowing through the wire to cause the strip to heat at a certain rate, but polarity doesn't change the heating of the bi-metal strip.

The gauge operates with MORE current showing a COLDER indication, and LESS current showing a HOTTER indication. A full 6v across the gauge should move the needle fully to cold.

Tom, I'd be putting a Multimeter on the back of the gauge with the Ignition on + engine cold, and then again as the engine is running at operating temp (and showing off the scale).

To me, it seems like you are putting less than 6v through the gauge. This could be caused by the 12v reducer not giving you a full 6v at the gauge, or defective wiring with too high a resistance, or a poor ground (either at the Instruments, or engine to hull).

There is also the chance that the gauge or sender are defective, but as they seem to be showing some of the right functions, I would say they are working OK if given the correct diet of electrons.

[Lynn Eades]

Tom, on 6 volts it would work correctly. You need to buy a step down resistor from the likes of Gotham parts, Macs autos or Dennis Carpenter or lots of other vintage parts suppliers, to cut your supply (only to that gauge) to 6 volt The gauge is a 6 volt unit and will not function correctly on 12 volt.

[Tom Millward]

Hello Gents,

I have a voltage step down resistor, and recently changed it to see if that was the issue, but it made no difference. I will check the voltage at the gauge next and try to rule out the senders or gauge. Thanks for the advice.

Tom

[Grant Bowker]

Please understand that I'm not familiar enough with Ford and or Carrier electrics to be considered a "reliable expert". I have the impression that Ford 6V systems actually ran at 4.8 V in part. If I'm right on this and if the converter in use is designed to give 4.8V and if the statement that the gauge system is performing as if it is getting too little voltage are all correct - could this all fit together to make sense?

[Tony Smith]

Originally Posted by Grant Bowker

I have the impression that Ford 6V systems actually ran at 4.8 V in part.

Ford Ignition Systems do run at a reduced voltage through a resistor to give approx 4.8v to prolong the life of the points in the distributor, but the rest of the electrical system (Generator, battery, lights gauges, horn, etc) are not part of the reduced circuit, but use the full 6v (which is itself a nominal figure, as the Gen and the battery, when the engine is running and the Gen is charging, actually put out between 7.0v and 7.3v).

[Bruce Parker (RIP)]

Originally Posted by Tony Smith

Ford Ignition Systems do run at a reduced voltage through a resistor to give approx 4.8v to prolong the life of the points in the distributor, but the rest of the electrical system (Generator, battery, lights gauges, horn, etc) are not part of the reduced circuit, but use the full 6v (which is itself a nominal figure, as the Gen and the battery, when the engine is running and the Gen is charging, actually put out between 7.0v and 7.3v).

Agreed. The resister stepping down the voltage is to the ignition coil only.

[Lynn Eades]

To add to Bruce's input. The voltage going to the coil is the full 6 volts (battery voltage) This is to aid in supplying good voltage to the coil at start up. As the resistor heats up, so does the voltage (to the coil) drop to the 4.8? (4.5?) volts. The ignition resistor has a tin cover with holes in it (the cover is to stop you burning yourself + other things. Make sure your gauge resistor connects in from before that.
The problem with diagnosis via internet is that those attempting to help usually don't get the full details, and so the chances of getting it right are a bit like trying to win lotto.
Having read a bit about faults in Jeep fuel gauges, the gauge body has to have a good earth in the dash panel. In turn, a part of the gauge circuit needs a good earth inside the gauge body. These are common faults in old Jeeps. There are many things to consider.
Tom, the resistor you are using, Is it matched to the gauge? What I mean by that is that the gauge needs to receive a 6 volt input while the load (the gauge) is live (operating)
Does your carrier have 1 sender or 2? (one in each head?) In your left bank you should have one with a single terminal, and in the right bank you should have a sender with 2 terminals.(wired in series) The single terminal one does a gradual signal while the 2 terminal one is just a "hot on" "cold off" switch. I dont think Ford made a separate single terminal one for a 12 volt gauge (matched pair) but your original gauge is 6 volt so you have to use it that way (with correct resistor)
The system (less the resistor) is covered in the info that Tony directed you to. If you cant sort it out your self, then point the info to your 50+ y.o. auto sparky.
The Ford v8 is like 2 separate engines and the water only meets in the radiator. If you took the two wires off the R.H. sender and connected them together and insulated the join, then you could pull the sender out of the R.H. head and fit a separate bowden tube (capilliary) type gauge to that bank. It would then give you an idea of what's happening.
Lastly, you want the engine to run as hot as possible, but at the same time, to never have it boil. Good luck!