Fuel gauge voltage issues

G

Goldfinch28

Member
I have a ‘36 ford with an original fuel gauge with a recently purchased fuel sender. Bench testing with 6 volt battery results are a perfectly working gauge, but installed and engine is running the gauge spikes to over full and stays there. The generator is putting out 7.5 volts. Any thoughts on why the gauge would spike while running and not while bench testing?
Thanks, Louis
 
On the later vintage vehicles (mid to late 1940s) the OE gauges are not compatible with the repop senders. I am less familiar with the 1930s equipment but the problem sounds identical.

We'll await the smarter folks.
 
Try installing a resistor in the line to the sender to reduce the voltage to 6 or 6.2 volts.
Can someone with a stock set up measure the voltage to the sender when the engine is running and let us know what it is?
 
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Guys, read posts #2 and #3. Gage works fine with ignition on / engine off. Then with engine running, gauge pegs full scale.

I don't have an answer to this. This is a mystery to me. 7.5 volt generator output shouldn't cause this, I don't think.
 
King-Seeley thermal gauges are very reactive to voltage fluctuations and a 6v vehicle does not operate at 6v but rather more like 7.3ish and fluctuates which will noticeably register in the gauge readout when running non points style sending units

TEST to confirm that your gauge is ******* past Full due to the increased chassis voltage experienced during generator output (after confirming the wiring connections are all going where they should):
Remove the fuel sender from the tank and then ground the sender frame with a jumper wire and leave the sending unit signal wire connected to the sender and to the gauge. Get a friend to help with the next bits
1) Turn the ignition on and actuate the fuel sender to Full and confirm what the gauge reads. Then actuate the sender to Empty and see what the gauge shows (thermal gauges are slow to respond so give it a good 30 seconds or more in either direction).
2) Start the vehicle and hold the gas pedal down just enough to increase the rpm slightly (we want to be certain the generator is outputting and that isn't always the case at idle) and repeat the above process. You will almost certainly have two very different gauge responses between 1 & 2 because the voltage has increased significantly in 2.

NOTE: You do not want to leave your gauge needle pegged past Full for any extended amount of time because you are very likely destroying the gauge internals with a signal it was never designed to handle. OEM King-Seeley points sending units act as voltage regulators of sorts because the signal is never constant, the points are always making or breaking so you never get constant chassis voltage flowing through the gauge. Modern constant wound sending units provide a constant signal at all times and thus full chassis voltage is always being applied.

EDIT: This basic scenario is also true for temp and oil pressure gauges with their sending units (OEM are points making and breaking, modern are constant).
 
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What AbbottInstrument said!

IMHO it's best to use original King-Seeley senders with the original gauges...
 
Is the 7.5V at idle or higher RPM? If at idle when the RPM increased, while you drive, the voltage will become higher. 7.5 is high at idle. You can adjust the generator third brush for lower volts.
 
Mate, your wire from the gauge to the tank is earthed somewhere.
Disconnect it from the sender unit at the tank end.
If it's still pegged past full, your wire to the sender is earthed somewhere. If it goes back to empty, the sender is crook. But you say you've bench tested it (I assume you used YOUR sensor in that test. If so, then like I say... your wire from the sensor to the gauge is shorted. I wouldn't be too excited anout the high voltage.
 
measure voltage when ignition on, then measure with engine running. it should give you a good idea of where the problem lies.
 
Thanks for the tips, I’ve done all the tests and I believe I’ve narrowed it down to the aftermarket sending unit is not compatible with the original gauge as was suggested. So, I’m at the point where I’m looking for an original NOS King Seeley sending unit, would anyone have one or suggestions on where I can source one?
Thanks,
Louis
 
Goldfinch28 said:
Thanks for the tips, I’ve done all the tests and I believe I’ve narrowed it down to the aftermarket sending unit is not compatible with the original gauge as was suggested. So, I’m at the point where I’m looking for an original NOS King Seeley sending unit, would anyone have one or suggestions on where I can source one?
Thanks,
Louis
Click to expand...


Much as I hate to say it, Evilbay. That's where I found one a few years ago.

Or you could post a wanted ad here on the Barn.
 
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