The Stovebolt Site Forums The Shop Area The Electrical Bay Ammeter question

I’ve performed a 6v to 12v conversion on the major components of my 1955 first series truck. For the purposes of seeing if I can get the original 235 engine to crank I was going to attempt starting it without a 12v to 6v reducer on the ammeter. I’m thinking that I shouldn’t damage the existing ammeter and the existing ammeter (if it even works) should only show 1/2 as much charge (amperage is now cut in half) with the new 12 volt system, right?

The ammeter doesn’t care what the voltage is and will function perfectly with a 12 volt supply. No worries!

It only samples milliamps to display by the meter movement. The rest goes thru a big buss bar called a shunt. As Phil says, no problem.

A dash ammeter is a good example of the "measure with a micrometer- - - -mark with chalk- - - -cut with an axe" principle. All ammeters have a "shunt" that bypasses the meter movement and only uses a tiny trickle of juice to move the needle. Doubling the voltage doesn't change the deflection of the needle much, if any. PM me for a work-around modification to prevent the needle from pegging out during heavy current operation right after startup.
Jerry

As Jerry says, you can see by looking at a 55.1 "ammeter", it is kind of mis-named. It's a Discharge/Charge indicator. Like an idiot light. No numbers, three hash marks. Not an electrical lab ammeter. The meter is set up with a resistance ratio/balance between the shunt and meter movement so as to have an "equalibrium" value. This will show middle needle (or slightly right) when "equalibrium" is met. Needle indicates tpward D or C or light on/off when skewed. This should basically apply to 6V or 12V.

A good reliable indicator is when your lights get dim something is going wrong somewhere. When truck dies and /or won't start, something is already wrong somewhere. I never seem to spot a problem ahead of time.

An idiot light is the best idea for "GEN" indicating. Attracts more attention than a meter.

I've had pretty good results by adding a little more shunt when converting to a higher output alternator than the usual 30-35 amp generator. Putting a loop of stainless steel cut from a radiator hose clamp across the ammeter terminals prevents the needle from pegging out after startup, and there's just enough resistance in the stainless to bleed a little current through the meter movement and make the needle deflect. Accurate- - - -nope- - - -but it prevents damage to the meter movement.
Jerry

I added a shunt as Jerry recommended. It reads a bit lower then before, but does work. In retrospect, I would make the shunt a bit thinner in the mid section so it reads truer. Here is what it looks like now.

If you are still using a generator you do not need to modify the amp. meter. If you are going to use an alternator which has much more output then you should add more shunt to the meter. With the added shunt it will naturally show a lower reading. that is to compensate for the higher output range of an alternator.

George