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Instrument Panel Voltage Regulator


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I am attempting to get a somewhat functional fuel gauge in my 73 Mach 1. 

Current problem ( no pun intended): When I turn on ignition , gauge goes from E to about 1/2 tank with tank full as fuel is consumed it moves down some but never to E ( even when I ran out of gas recently. ) except when ignition is off battery disconnected etc never reads E

 

if I short yellow wire to ground needle sweeps to a bit above 1/2 ( should go to full as I understand it )

 

ground is good - ground wire to tank 0 ohms ground wire to rear bumper or leaf spring ( rust scrapped off ) or neg battery all 0ohms - Ground is always my first suspect but here I think it is ok.

 

sending unit tank full reads 14ohms ( I also have the common full is not full premature fuel shut off so a bit Hi is expected, full is not full)

resistance empty 73 Ohms

 

When I check the potential at the yellow wire pulled off the sending unit my ( very high quality ) analog potentiometer sweeps from zero to 5 volts at about two sweeps per second. It is my understanding that the OEM Instrument Panel Voltage Regulator puts out 5 volts and works by rapidly opening a closing a circuit in response to thermal heating on the 12v side of the regulator, but this should happen too fast for meter to see it happening. 

It seems that I should see a constant 5v or if I can see the opening and closing of the internal circuit of the Instrument Panel Voltage Regulator

shouldn't I see a sweep from 0-10 basically a slow always positive a/c source ( RMS voltage=5  much like -120 to + 120 has RMS of 120 )

        

Should I suspect the Instrument Panel Voltage Regulator  or the gauge itself ? 

if it helps Oil pressure nor temp ever hit  H either, but I do not think these parameters have ever been abnormal and have not tested further

 

should the wiring harness be considered suspect ( I know it always is to some extent in these cars ) or is the fact that the yellow wire is hot (even of at wrong potential ) probably enough to looks elsewhere first.

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I'd replace the CVR first, as it is easy and cheap to do. If that doesn't fix the problem, I'd check the contact for that line at the dash connector, to make sure the contact is good and clean. If that doesn't solve the problem, then the issue is likely in the underdash harness, and you'd be best sending it off to some jerk named Midlife to take care of things.

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Thank you for the quick reply. I’ll replace the CVR and see how it goes?

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