70s dodge gauge help / pulsing 5 volts on fuel sender line

Redrig

Master Chief Petty Officer
Joined
Oct 13, 2009
Messages
860
Hello all,

I am at my wits end on this one and could some help.

quick back story - 78 Plymouth , motorhome , the stock fuel gauge " sorta works " on a completely full tank it reads about 3/4 full .... therefore I dont trust the bottom of that gague. it does drop fairly accurately though , I just never let it get too low.

Last year dropped the tank to install new hoses , checked the sending unit and everything looked great 73-10 ohms and that checked.

This weekend I was trying to install an aftermarket gauge just so I have a better idea of what my fuel level situation is.

When I hook up the aftermarket gauge it basically sweeps all the way , every few seconds when the ignition is on. Almost like a flasher bulb , it will go from no volts to almost 5 volts every few seconds . When the key is off it reads normally ( right now im about 22 ohms from the sender) but as soon as I turn the key on or run the 5 volt thing starts to happen.

My plan was to "T" the fuel sender line and maintain the original gauge , but add the aftermarket one too

Oddly enough - this does NOT have an impact to the OEM gauge and I conformed the voltage is present there too.

anyone have any ideas on how a pulsing 5 volts would end up at the fuel sender line ? its very odd to me
 

matt167

Rear Admiral
Joined
Sep 27, 2012
Messages
4,168
Mopar gauges of that era, are 5v reference. They drop voltage with small regulators..

This will clue you in to how it works
 

Redrig

Master Chief Petty Officer
Joined
Oct 13, 2009
Messages
860
Mopar gauges of that era, are 5v reference. They drop voltage with small regulators..

This will clue you in to how it works
I'll be danged ....... I was pulling my hair out trying to find how voltage is ending up there. Blinkers , flashers , etc.

Hmmm , back to the drawing board I guess. My plan won't work knowing that.

Thanks for the info !
 
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