SV: Converting (+) to (-) ground


Bo Strander

Hello Ed,
Thanks for a very instructive and useful lesson!
This kind of tech-instructions are really useful.
Best regards from a chilly Stockholm.
Bo the swede
----- Original Message -----
From: WB6WSN
Sent: Friday, January 16, 2004 12:28 PM
Subject: Re: [amphicar-lovers] Converting (+) to (-) ground

----- Original Message -----
Sent: Friday, January 16, 2004 2:15 AM
Subject: Re: [amphicar-lovers] Converting (+) to (-) ground w/alternator


Thanks for the alternator input. I recently changed a Ford from 6 volt
positive to 12 volt negative. Had advice from several about how to handle
Instrumentation. Most said I should put some resistance into the dash
circuit among other things. One of my knowledeable mechanic friends said
both the change from 6 to 12 volt and the polarity change didn't require
special. Other than reversing the ammeter leads, he was right. The fuel
and temperature gauge and oil gauge all work fine and required no resistance
stuck in. I don't know why not, but he had done the same to his car and told
me it caused no problem. The radio and/or clock would not work on 12 if
designed for 6 volts, but the dash instruments are apparently unaffected. I
dealt with the clock issue yet, so it is unconnected. Even the starter is
original 6 volt. I know of many who have made the 6 to 12 positive to
switch and made no change to the starter. With the 12 volts negative where,
Fords, they had been 6 volt positive, they start like bandits after the
change and the reversed polarity has no effect on the starter working. It
very well break the Bendix spring at some point, but that is simple enough
replace. Of course changing from 6 to 12 absolutely requires changing to a
volt alternator or generator and the advantages of the alternator over the
generator make the choice a no brainer unless one is trying to retain the
"original" appearance in which case the generator needs to be rewound or
replaced with
a 12 volt in the same case.. Vic "Splash" Nelson

Electric auto instruments, like the gas gauge, water temp gauge & oil pressure
gauge, are not simple series circuits; rather, they use the "bridge" concept.
(This can be accomplished electrically or mechanically within the gauge
housing.) Without getting into a deep description of bridge design, here's what

Electrical power is applied to the gauge, and is split into two paths. One
path goes through a coil to ground, and pulls the pointer in one direction. The
other path goes through another coil, to the sensor, and then to ground, and
pulls the pointer in the other direction. As the sensor changes resistance (due
to temperature, gas level or pressure), more or less current flows in this path.
The ratio of current in the "element" path, versus the "fixed" path, determines
the position of the gauge pointer.

The slick thing about this method is that it's independent of applied voltage.
If the applied voltage varies, the current changes in both paths at once,
canceling out the effect on pointer position. The pointer is affected only by a
change in the path ratio, and the only variable element is the sensor element.

The really fast minds will likely have guessed something by now. Notice I said
that the pointer position is solely dependent on the RATIO of the paths? If you
have a positive ground system, you are applying negative voltage to the meter
terminal. If you change ground systems, to a negative ground system, you will
apply positive voltage to the meter terminal. What will happen is that the gauge
will still work, but it will work backwards! (This is the reason behind opening
the gauge and reversing the coil connections to convert the gauge to opposite

Now you know what moves the pointer.

But there may be a problem in simply switching from 6 VDC power to 12 VDC
power. The power drawn through each "path" will increase. (A little trip through
Ohm's Law. Amps = Volts / Ohms & Watts = Volts x Volts / Ohms . The current in
a circuit varies linearly with the voltage, but the power varies as the square
of the voltage. In other words, when you double the voltage, you double the
current, and you quadruple the power.) The coils in the meter are wound with
very tiny wire, and the 4x power increase may overheat the little coils,
resulting in shorter gauge life or even rapid burn-out of the coil. It's a
gamble with unknown odds.

Clocks are a different problem. Most of the old electric clocks use a small
spring powered movement. When the spring is "unwound", a little switch closes
inside the clock, allowing current to flow through a coil, creating a strong
magnetic field which pulls a lever, which winds the clock spring. (The spring
isn't very big; it usually only runs the clock for a minute or two, and then
needs to be rewound. This is the reason that, if you listen very close and
patiently, you will hear the clock spring being "rewound.") Changing polarity
will create a magnetic field that tries to "push" the clocking winding lever,
instead of "pulling" it. It probably will not work unless you open the clock and
reverse the rewind coil leads.

The design of the starter motor coils and brushes allows it to turn in the
correct direction for cranking the engine regardless of applied voltage
polarity. But there are two common starter designs.

One system allows the starter motor to start turning (with no load). As the
starter motor shaft begins to rotate, a spiral grooved gear at the end of the
shaft lags the shaft rotation a bit, which moves the gear away from the starter
motor and into engagement with the flywheel gear. This is called a Bendix gear
system, and allows the starter to begin turning before it's heavily loaded. And
once the engine starts, the gear tends to automatically disengage the starter
motor (with a lot of nasty grinding noise!). This system will work with twice
the applied voltage, but again, you will apply 4 times the power to the starter
motor, and that will mean much more stress on the starter relay, the brushes,
the battery and the Bendix gear. And again, since you are pushing the design
limits of these parts, you are gambling in the dark.

The other system is often used on heavier engines, or when a starter reduction
gear drive is used. In this system, when the starter relay closes, it applies
power to a heavy solenoid, which pulls a lever, forcing the Bendix gear to
rotate and move into engagement with the flywheel gear. As the lever moves, it
also closes the circuit for the starter motor, and the motor begins to spin the
Bendix gear. This system gives a stronger "push" to the engaging gears, coping
with the higher torque starter motor. Notice where I said the solenoid "pulls
the lever"? If you put a reversed polarity onto this system, the solenoid will
try to "push" instead of "pull" and, just like the clock, it won't work.
(Fortunately, I haven't heard of this kind of system used on an Amphi.)

El Cajon
67 Rust Guppy

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