The Ford Barn

J Witt · 05-07-2016, 01:16 PM

I have been experimenting with the fuel and oil pressure gauge pair, mainly trying to get the fuel gauge to read a bit closer to the truth.

My car has an NOS fuel gauge that has been mostly pegged at the top of the scale. It has a Drake aftermarket sender in the tank.

I noticed that the gauge would drop down from Full some with the motor off, but would move back to the upper limit when the engine was running, so I made some measurements of the voltage and current for the gauge circuit under the two different conditions and found the following:

Engine running, system charging -- 7.4 volts at 215 mA, apparent ohms = 34.3
Engine stopped, battery voltage -- 6.2 volts at 176 mA, apparent ohms = 35.3

What I thought I needed was the needle to read even lower by about the difference between the two voltages, which was about 1 volt. This would require about 1-2 ohms added resistance in the gauge circuit, shooting for a voltage of about 5 volts across the gauge instead of 7.4 with the car running.

After some head scratching and looking at the circuit diagram, I decided to measure the voltage drop across the ignition dropping resistor and lo and behold, there was about 5 volts at the connection to the red wire.

I cut the black/green wire loose from the 6.4 volt side of the resistor and moved it over to the other side (5 volts side) and now the gauge reads a little below a half tank and it took just about 7 gallons to fill it up. I think I can get it a little closer now by bending the float arm, but the gauge is now reading (I think) within the scale range, so that should be possible.

This makes me wonder if there is a mistake on the wiring diagram.

The oil pressure gauge was down a little but wasn't much affected by the voltage change.

Hope this is of interest,

John

JSeery · 05-07-2016, 01:20 PM

Be interesting to see how the ignition resistor holds up to the increased load.

J Witt · 05-07-2016, 01:49 PM

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True, time will tell in all of this stuff. I didn't think an added 200 mA would have much impact though, I believe that's a just a fraction of the ignition current.

05-07-2016, 01:16 PM	#1
J Witt Senior Member Join Date: Mar 2015 Location: Acworth GA Posts: 534	'36 Fuel and OP gauge information I have been experimenting with the fuel and oil pressure gauge pair, mainly trying to get the fuel gauge to read a bit closer to the truth. My car has an NOS fuel gauge that has been mostly pegged at the top of the scale. It has a Drake aftermarket sender in the tank. I noticed that the gauge would drop down from Full some with the motor off, but would move back to the upper limit when the engine was running, so I made some measurements of the voltage and current for the gauge circuit under the two different conditions and found the following: Engine running, system charging -- 7.4 volts at 215 mA, apparent ohms = 34.3 Engine stopped, battery voltage -- 6.2 volts at 176 mA, apparent ohms = 35.3 What I thought I needed was the needle to read even lower by about the difference between the two voltages, which was about 1 volt. This would require about 1-2 ohms added resistance in the gauge circuit, shooting for a voltage of about 5 volts across the gauge instead of 7.4 with the car running. After some head scratching and looking at the circuit diagram, I decided to measure the voltage drop across the ignition dropping resistor and lo and behold, there was about 5 volts at the connection to the red wire. I cut the black/green wire loose from the 6.4 volt side of the resistor and moved it over to the other side (5 volts side) and now the gauge reads a little below a half tank and it took just about 7 gallons to fill it up. I think I can get it a little closer now by bending the float arm, but the gauge is now reading (I think) within the scale range, so that should be possible. This makes me wonder if there is a mistake on the wiring diagram. The oil pressure gauge was down a little but wasn't much affected by the voltage change. Hope this is of interest, John __________________ Houston, Tranquility base here. The Eagle has landed. Last edited by J Witt; 05-07-2016 at 01:52 PM.

05-07-2016, 01:20 PM	#2
JSeery Member Emeritus Join Date: Nov 2012 Location: Wichita KS Posts: 16,132	Re: '36 Furel and OP gauge information Be interesting to see how the ignition resistor holds up to the increased load.

05-07-2016, 01:49 PM	#3
J Witt Senior Member Join Date: Mar 2015 Location: Acworth GA Posts: 534	Re: '36 Furel and OP gauge information Sponsored Links (Register now to hide all advertisements) True, time will tell in all of this stuff. I didn't think an added 200 mA would have much impact though, I believe that's a just a fraction of the ignition current. __________________ Houston, Tranquility base here. The Eagle has landed.