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  Topic Review (Newest First)
12-24-2016 01:51 AM
Darth Quote

At the full nominal voltage of 12 Volts to ground (few windings in the circuit it reads full. If you have a bad ground (with a lot more resistance than 60 Ohms) then it will always just sit there at empty and laugh at you!

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For those who read this, I think Dave meant to say " If you have a bad ground (with a lot more resistance than 60 Ohms) then it will always just sit there at full and laugh at you!

A 60's GM with a bad ground on the fuel gauge will show way past full.
12-23-2016 08:10 AM
rbwinston
Quote:
Originally Posted by Big Dave View Post
A DC motor (all DC motors from fans to toy trains) rotate faster as the voltage increases, or slower as it decreases. They also rotate backwards if you reverse the polarity (+ with the -, and the - for the +).

Now you can reduce line voltage by adding resistance to the circuit. In the case of a toy train or your gas gauge sending unit it utilizes a rheostat (some ni-chrome wire wrapped around an insulating form that has a contact point that sweeps across the windings to vary the voltage). In the case of the toy train it controls the speed of the locomotive by varying the voltage to the DC motor. In your gas gauge the float moves the rheostat arm depending upon the gas level that reduces the amount of voltage that goes to ground. As the resistance increases to 60 Ohms (more windings) the voltage to ground drops and the gauge reads empty. At the full nominal voltage of 12 Volts to ground (few windings in the circuit it reads full. If you have a bad ground (with a lot more resistance than 60 Ohms) then it will always just sit ther at empty and laugh at you!

DC circuits are extremely easy to understand if you understand the function of resistors, batteries and the concept of a switch or load. In fact it is as easy as PIE (the letters used to express Ohm's Law) that is used to calculate values to predict what the reading should be ahead of time. Or allow you to correct or tune a circuit to make it behave as expected (say you are trying to use a 12 volt battery in an older than a 1955 Chevy that used a six volt battery for everything). Plug in a 12 volt battery in a six volt car and your volt gauge and your gas gauge go up in smoke though your head lights are much brighter till they also burn up.

From your description of the problem you have either a bad switch or a blown (burned open) resister. My money is on the switch being bad as it isn't feeding any power on high speed. My guess is the contact points in the switch have burned away and are not making contact to complete the circuit.

Big Dave

Sounds more than reasonable. I'll start there. Thanks!
12-19-2016 12:30 AM
Big Dave A DC motor (all DC motors from fans to toy trains) rotate faster as the voltage increases, or slower as it decreases. They also rotate backwards if you reverse the polarity (+ with the -, and the - for the +).

Now you can reduce line voltage by adding resistance to the circuit. In the case of a toy train or your gas gauge sending unit it utilizes a rheostat (some ni-chrome wire wrapped around an insulating form that has a contact point that sweeps across the windings to vary the voltage). In the case of the toy train it controls the speed of the locomotive by varying the voltage to the DC motor. In your gas gauge the float moves the rheostat arm depending upon the gas level that reduces the amount of voltage that goes to ground. As the resistance increases to 60 Ohms (more windings) the voltage to ground drops and the gauge reads empty. At the full nominal voltage of 12 Volts to ground (few windings in the circuit it reads full. If you have a bad ground (with a lot more resistance than 60 Ohms) then it will always just sit ther at empty and laugh at you!

DC circuits are extremely easy to understand if you understand the function of resistors, batteries and the concept of a switch or load. In fact it is as easy as PIE (the letters used to express Ohm's Law) that is used to calculate values to predict what the reading should be ahead of time. Or allow you to correct or tune a circuit to make it behave as expected (say you are trying to use a 12 volt battery in an older than a 1955 Chevy that used a six volt battery for everything). Plug in a 12 volt battery in a six volt car and your volt gauge and your gas gauge go up in smoke though your head lights are much brighter till they also burn up.

From your description of the problem you have either a bad switch or a blown (burned open) resister. My money is on the switch being bad as it isn't feeding any power on high speed. My guess is the contact points in the switch have burned away and are not making contact to complete the circuit.

Big Dave
12-18-2016 10:13 PM
rbwinston
Heater Motor Question

I don't quite understand the relationship between the resistor and the fan motor. It appears the motor is on high when the resistor is bypassed, medium and low through the resistor. Assuming that's true, how can I determine if the motor is at fault or the resistor is at fault (seems to run on low speed only). Possibly wire the motor directly to the battery and see what happens?? Or could the switch be at fault?? I do see a faint spark when moving the switch between positions but that may be normal. This is a '62 without A/C (blower motor has a single wire to it).

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