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1929 Fuel gauge wiring and troubleshooting

Started by m-mman, December 08, 2017, 03:44:13 PM

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m-mman


I had the fuel tank sending unit for my 1929 rebuilt but now the set up still isnt working.
The wiring diagrams in the shop manual are weak. (too small to really see the details) and it is NOTHING like a 1950s circuit.

The system seems to use two resistors in the gauge unit with two wires going to the sender which then acts as a variable resistor between them. It seems to be like a Wheatstone bridge type circuit(?) but I cant understand it enough to test and troubleshoot.

Does anybody have experience with this circuit and how to test it?
1929 341B Town Sedan
1971 Miller-Meteor Lifeliner ambulance
Other non-Cadillac cars
Near Los Angeles, California

CLC #29634

bcroe

Not having seen this, I am going to guess there are a couple wires
from the gauge going to ends of a resistor in the tank.  A slider in
the tank is grounded, and slides up and down the resistor to ground
it at one point, shifting the current flow from one wire to the other. 

If this is correct, you should be able to unhook the 2 gauge wires
from the tank: ground one and see the gauge go to E; ground the
other and see it go to F.  That would demonstrate a good gauge.  If
correct, resistance from either tank connection (dozens of ohms) to
ground should vary with fuel level. 

All wrong, could you show a picture?  good luck, Bruce Roe

m-mman

#2
This is the wiring diagram.
Two wires going to the tank and they each vary resistance to ground in inverse proportion to each other. (never seen that before) I had mine rebuilt and it functions as designed.

On the gauge unit there are three terminals.
One for 6 volt power. Terminal 3
A resistor between it and terminal 1
Then between terminal 3 and 2 there is WHAT??

I can measure a resistance between 3 and 1 but nothing between 3 and 2.
This wiring is all within the very delicate gauge unit. The needle flips back and forth when the gauge is tilted.
I suspect I will have to send the gauge out to be rebuilt somewhere.

1929 341B Town Sedan
1971 Miller-Meteor Lifeliner ambulance
Other non-Cadillac cars
Near Los Angeles, California

CLC #29634

bcroe

Thanks for the picture; looks like the guess was spot on.  So disconnect
the 2 wires from gauge to tank.  Grounding one gauge wire should give
E; the other F, to prove the gauge. 

Use an ohmmeter on the tank to verify resistance between wires
as fixed; it should vary from a wire to ground as fill level changes. 
Note the total resistance, a test circuit with the same resistance
can be used to exercise the gauge.  Bruce Roe

J. Gomez

Jim,

It would be similar to what newer senders/gauges function with a slight different since the gauge is not grounded (per the illustration), so both grounds are needed from the sender to move the needle to either side.

The older gauges have three connection although only two are wired one is +voltage, one is negative (resistance ground from the sender), and one internal which is ground (this is the one you do not see).

You setup provides a resistance ground from the sender on either side “E” or “F” with the sender acting as a potentiometer with ground on the center to less resistance ground on one end and more on the other and vice versa, versus newer one as variable resistance with more or less to ground.  Attach drawing is for reference to my comments above.

So an open coil on the gauge would only give a slight function of the gauge (depending on where the open coil is) and will stop at that point while the sender is moving.

I would agree with your theory the gauge would need to be refurbished.

Good luck..!
J. Gomez
CLC #23082

bcroe

Quote from: J. Gomez on December 10, 2017, 03:12:49 PM
Jim,

It would be similar to what newer senders/gauges function with a slight different since the gauge is not grounded (per the illustration), so both grounds are needed from the sender to move the needle to either side.

The older gauges have three connection although only two are wired one is +voltage, one is negative (resistance ground from the sender), and one internal which is ground (this is the one you do not see).

You setup provides a resistance ground from the sender on either side “E” or “F” with the sender acting as a potentiometer with ground on the center to less resistance ground on one end and more on the other and vice versa, versus newer one as variable resistance with more or less to ground.  Attach drawing is for reference to my comments above.

So an open coil on the gauge would only give a slight function of the gauge (depending on where the open coil is) and will stop at that point while the sender is moving.

I would agree with your theory the gauge would need to be refurbished.

What does that picture have to do with the 29 gauge?  Test the gauge before rebuilding it.  Bruce Roe

J. Gomez

#6
Quote from: bcroe on December 10, 2017, 08:18:18 PM
What does that picture have to do with the 29 gauge?  Test the gauge before rebuilding it.  Bruce Roe

To show the difference function between the 1929 and later ones, as I pointed out just for “reference to my comments above”.  ;)
J. Gomez
CLC #23082

bcroe

Quote from: J. Gomez on December 10, 2017, 08:47:23 PM
To show the difference function between the 1929 and later ones, as I pointed out just for “reference to my comments above”.  ;)

OK, thanks now I understand.  Bruce