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1941 - All Electric Gauges Read Low

Started by Paul Phillips, April 30, 2017, 06:09:19 PM

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Paul Phillips

Trying to sort this last item from the restoration, and it has me stumped.  Car has a new wiring harness, and gauges are a mix of the originals (that worked) and from a panel bought from another 41 owner & CLC member reported to work fine - I chose the ones for best cosmetics.  All electrical gauges - Fuel, Temperature and Amps want to read very low.  I have triple-checked that the gauge wiring matches the diagrams.   All electrics on the car seem to work fine other than the gauges, generator charges the battery, lights & turn signals work, etc.

Diagnostics - I can measure battery voltage on the small terminals for the Fuel & Temp, and on the ammeter terminals.  I can measure voltage from the temp sensor to ground and watch it go down as the engine warms (and sender impedance drops), so that basic circuit seems to work.  I did not try the same on the fuel sender, since it is difficult to change its setting w/o removing the sender from the tank.  Fuel sender has an aux. ground from one of the sender mounting screws to a chassis ground screw in back.  I tried connecting an auxiliary ground from the instrument housing to frame, with no change. Turning on headlights causes a small drop on the ammeter, but it is already well to the discharge side.  With battery disconnected, the ammeter still reads low, so it may be its own problem and not related the what Temp & Fuel are doing. 

What am I missing?

Thanks!
Paul
Paul Phillips CLC#27214
1941 60 Special (6019S)
1949 60 Special (6069X)
1937 Packard Super 8 Convertible Victoria
1910 Oakland Model 24 Runabout

Jay Friedman

Paul,

I don't know '41s well and I'm certainly not an electrical expert, but on a '49 the fuel and temp gauges share the same power source circuitry, with grounding through their respective sending units.  They are voltmeters, so any resistance in the circuits will make them read low.  Since they are both reading low, I suggest to check the wire providing their power source according to the '41 wiring diagram.  Also, check the temp gauge's place where it screws into the motor since paint in that place will create resistance.  Same with the fuel gauge, though you have an auxiliary ground wire there.
1949 Cadillac 6107 Club Coupe
1932 Ford V8 Phaeton (restored, not a rod).  Sold
Decatur, Georgia
CLC # 3210, since 1984
"If it won't work, get a bigger hammer."

Paul Phillips

Thanks Jay

I think I have been over these areas (actually multiple times!).  I have the same voltage reading at the gauge terminals as the battery, and I have also tried a temporary aux ground for the temp sender to rule out a weak ground on the head.  No luck with that.  My understanding is there is a balance circuit in the temp & fuel  gauges that requires Batt & Gnd locally at the gauge - I was wondering if there were nuances of that current flow that could be affecting how they were reading.  Even starting to wonder if the weight of a lite coat of fresh paint on the needles for cosmetics could have added too much mass for the gauge to overcome.  I can't imagine that every instrument out of my mix-n-match is bad, but it almost seems like that happened.

Any other ideas out there???
Paul Phillips CLC#27214
1941 60 Special (6019S)
1949 60 Special (6069X)
1937 Packard Super 8 Convertible Victoria
1910 Oakland Model 24 Runabout

bcroe

I don't know anything about 41 gauges.  But electronic meters have the needle balanced
in 2 axies, and it doesn't take much to throw them off.  If you have an extra with the
needle painted, you might try rotating it and seeing if the needle moves around. 

Bruce Roe