47 merc fuel gauge quits when engine starts

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1947 Mercury 12 volt positive ground. Original gauges. Getting back to this project and ignored non working fuel gauge for a long time. I thought the gauge wasn't working but noticed that when I flip the ignition switch on the gauge works until I start the engine and then it stops working.

I didn't do the 12 volt conversion. Looks like they installed a large resistor to the oil pressure gauge and maybe not to the fuel gauge but I'll check that later to be sure.

I'm assuming that once the engine is started the extra voltage from the generator is causing the fuel gauge to stop? Maybe gauge needs a voltage regulator?

Thanks for any suggestions,
 
The reduced voltage to the fuel gauge should be the same regardless of whether the source is the battery or generator. However, since the system is DC, I wonder if accidental reversal of the two wires on the fuel gauge would cause this. When you flip the key on (without starting) does the fuel gauge show the actual fuel level ? If so the tank unit must be working. As you remarked a voltage reducer is installed at the oil gauge but recommend you check the wiring to see if the fuel gauge is included on the wiring involving the oil pressure gauge. On the other hand, without checking my own car that was converted, I seem to remember that one of the gauges does not require a reducer - others here may comment.
 
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I checked voltage at the fuel gauge. There is a 50 Ohm sand cast resistor going from the harness to the oil gauge with a brass buss linking to the fuel gauge. Voltage with switch on but engine not started is 5.9 volts and then when the engine starts voltage drops to around 3 volts. Oil pressure gauge works although I don't know how accurate.

12 volt positive ground system.

Any idea on why the voltage drops when the engine starts? I assume the generator is still set for positive ground. I installed a new voltage regulator and followed the polarity directions asking for terminal F to be momentarily touched to the battery line on the VR. Maybe those directions are for negative ground and not positive ground?

Thanks,
 
i'll just say that ( in my '41 deluxe) when i removed the 8v 'farmer's hack' that prevented the car from starting and the gauges from working properly. the car was transformed. the 6v system now works as well as any 12v system i've ever owned.
 
Check your gas tank sending unit to make sure it's in good working condition.
Also, do a Fordbarn search on the topic converting to 12V or 12V conversion. Tons of info available. You may find that installing a Constant Voltage Reducer will solve your problem. That's what I did. However, I had to isolate the stock BATT gauge from the rest of the gauges and install a 60 ohm resister in the line that feeds only that gauge in order to make the gauge read properly. I posted the info in the SEARCH.
 
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19Fordy said:
Check your gas tank sending unit to make sure it's in good working condition.
Also, do a Fordbarn search on the topic converting to 12V or 12V conversion. Tons of info available. You may find that installing a Constant Voltage Reducer will solve your problem. That's what I did. However, I had to isolate the stock BATT gauge from the rest of the gauges and install a 60 ohm resister in the line that feeds only that gauge in order to make the gauge read properly. I posted the info in the SEARCH.
Click to expand...

the reason may be, according to the wiring diagram on my '41, that the ammeter and charging system is on a completely separate circuit than that involving the temp, fuel and oil pressure gauges and probably the lighting too.
 
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