flathead original gauge 12 volt conversion
 

I switched my truck over to 12 volt a while back and I am still having trouble getting the gas gauge and the temperature gauge to work. the oil pressure gauge works great. but i dont think its a problem with the gauges, i think its the way they are wired. does anyone know how to wire the temperature switch and the fuel sender for 12 volt negative ground?

I wish I could help here, but I bought an original wiring harness for my truck when I converted it to 12v. All the wires were already in looms, it was just plug and play. As far as the fuel gauge, the only thing I could think of is maybe the sending unit's bad?:-huh

Did you use new 12 volt aftermarket gauges? or are you using the orig ones? if so the 12 volt upgrade may have fried them.

If you have the stock gauges you need to provide them a constant 6V for them to work. If you put 12V to them they are probably cooked.

Well, that's where I'm stumped. By lookin at the truck it looked 6 volt but it had some 12 volt parts like headlight bulbs. And it has a half way wired up new wiring harness. But it seems as tho someone converted it from original 6 volt to 12 and someone else half ass wired it back to 6 volt. So I'm not sure if those temp switches are 6 or 12 volt

Get to testing with a simple volt/ohm meter. Than we will all know what volts ya got where Bud.

This link may help you. Van Pelt has quite a collection of wiring diagrams. Let me know if those don't work for you as I have collected some others.

http://www.vanpeltsales.com/FH_web/flathead_drawings_electrical.htm

Here is a picture that I took of the back of the gauge cluster. Provide 6v from a voltage regulator to the strapped side of the temp gauge (actually any of the strapped terminals will do) then the other terminal goes to the respective sensor for each gauge.

The gages require 6 volts, but the lightbulbs you just change over to 12 volt. I like to use the LM7806 integrated circuit voltage regulator. It takes up to 36 volt input and regulates to 6 volt output. Each regulator is good for 1 amp, so just use 1 per gage. Will not work for heater fan or elec wipers, those need higher amps. The LM7806 are small and you can hide them behind the gage panel. They should be mounted to a heat sink, a simple strip of metal or similar works great. Three wires: input, output and ground.

Just generically speaking, on any old car or truck with 6 volt, the ammeter works on either 6 or 12 volt, no need for any conversion except change the lightbulb to 12 volt. Mechanical temp or oil pressure gages do not need converting beyond the lightbulb.

Use #53 or 57 bulbs (12v) to replace the 6v #51 OE bulbs. I prefer #57 as they are twice as bright as #53. There are some LEDs that appear a bit brighter but their light is blue white (3000K+), they are fragile and many break on installation, some don't fit the hole, some have a light that is too focused, most can't be dimmed (not a problem if you keep the standard light switch) and they are expensive.

well, from what ive been readin, it seems as tho that the temp gauge only reads hot or cold, no in between. apparently, when the coolant temp hits 205 degrees somethin clicks in the gauge and turns to hot? i think is what i understand...

The stock temp gauge reads temperature just like you are used to on every other car / truck. The six cylinders had a regular (for Ford) sending unit that varied resistance with temperature. Voltage is supplied by the gauge and the sending unit provides the ground. The hotter the sending unit its, the the lower the resistance and the hotter the gauge reads. The confusion is in the sending unit set up on the V8s since there is both a sending unit and a thermal switch. The driver's side head had the same sending unit the six had. The passenger side head had a thermal switch as a kind of safety back up. If the passenger side head got to boiling temperature the thermal switch closes and throws the gauge to reading full hot.

wow, thanks so much 38 coupe, that makes perfect sense now. What an awful design lol. any ideas on how to fix that and get a normal reading?

I'm not sure that it makes much difference other than to save on extra wire length but I believe that the single pole sending unit is on the passenger's side and the double pole switch is on the driver's side. The wire from the gauge runs to lower pole on the switch then a wire goes from the upper terminal on the switch through the center holes in the plug wire holders thereby keeping it off the manifold over to the sending unit. (I wonder if that is different for the Aussie units?) ;)

petemcl is correct, the double pole is in the driver's side head and the single is in the passenger head.

I also was wrong when I said the hotter the temperature, the lower the resistance. It is exactly opposite, the hotter the higher the resistance. The double pole thermal switch opens when it gets too hot, cutting off the ground entirely, which sends the gauge to full hot.

I really should post less when I am tired...


Try the following steps:
1 - First make sure you are getting 6V to the gauge.
2 - Second make sure the gauge still works by grounding the sending unit wire momentarily. The gauge should dive for cold when the wire is grounded. Don't do this for too long as you can send too much current through the gauge and damage it.
3 - Third, if the gauge works hook it directly to the sending unit (the single pole unit) and see if the gauge now works. If not you likely need a new sending unit.
4 - Fourth, if the gauge now works put the thermal switch (double pole unit) back in the middle of the circuit. If the gauge stops working you need a new thermal switch.

I hope this helps.