Ammeter Fire

R

rfitzpatrick

Well-known member
After installing alternator, my 20 volt ammeter will peg out for just a short drive, settles down to just a couple amps after I'm underway. Is this a fire hazard -- should it take longer to get back to just a couple amps?
What are the danger of one staying pegged.
Tks

6 Volt Pos. Ground -- NU-REX Single-Wire --- 30 Amp
 
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I've never heard of an ammeter fire, but then I've never heard of a lot of things. Maybe the new generator has to charge hard at first because the battery rapidly losses its charge when sitting for awhile. Can you swap batteries and see if you have the same results? You didn't say if the system is 12v or 6v, but that shouldn't make a difference. I brought that up because the 6v single wire alternators can do some peculiar things.
 
After cranking a bit to get it started, its normal for the battery voltage to drop a little. The alternator's regulator senses that and increases the charge to bring the battery back up. Then it settles back to little or no charging. This is normal.
 
The amp meter is probably okay. Did you install a 30 amp or a 60 amp alternator?
Your amp gauge has to match the out put of the alternator. If it’s a 30 amp get a 30 amp gauge.
 
This isn't a fire but scorch marks are a start. This is just from the ammeter going bad though.
 

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They will get hot and smoke (ammeter) if the two knurled plastic brass based nuts on the back are loose.

Good ideas to snug them down now and then.
 
ndnchf said:
After cranking a bit to get it started, its normal for the battery voltage to drop a little. The alternator's regulator senses that and increases the charge to bring the battery back up. Then it settles back to little or no charging. This is normal.
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Agreed.

JOHN CT said:
Your amp gauge has to match the out put of the alternator. If it’s a 30 amp get a 30 amp gauge.
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Not necessary to change the ammeter. It measures only battery in/out. Mine doesn't go above 20A after start. Your battery may be old/weak or got more discharged from hard starting. Excess current won't hurt the ammeter except if the needle slams the peg and gets bent.
 
I've seen several dash panel lights short out in there over the years. A person has to make sure that little wire is well insulated.

The condition of the battery and the wiring has a lot to do with wire or component problems. The wires have to be secure and in good condition or resistance builds at the weak spots. Resistance equals heat and a person wants to keep heat to a minimum. The modern reproductions of the ampere meter or ammeter use plastic for the insulators. Bakelite could take a lot of heat but some plastics just melt & catch fire if they get too hot.

One wire 6-volt alternator conversions all seem to have internal voltage regulators that keep the voltage up around 7.5-volts or more. This is really too much voltage for a 6-volt system. They should be closer to 7.1-volts. Just enough to overpower battery voltage by a fair amount. I'd feel safer with a 12-volt conversion set up myself.
 
Most alternators really should have a 30/30 meter instead of the 20/20 stock one. The ampere meter in my Model T when I looked at it, though that doesn't have a bulb in it, wasn't a bulb but is sure was glowing!
 
This is why ammeters aren’t used anymore, they have to ‘see’ full system load, subjecting them to it…voltmeters are common today,they don’t have to see the load, just the voltage present in the circuit.
 
FWIW - With more modern voltage regulators, a voltmeter is a better choice and really all you need. But with a generator using a cutout, there is no automated regulation. The battery regulates the generator voltage. The generator's 3rd brush position regulates the output amperage. I prefer an ammeter for a stock generator/cutout system so I can see the amperage output and adjust the 3rd brush as needed. If I were running an automatic regulator with a generator or alternator, then a voltmeter would be my choice.
 
rfitzpatrick said:
After installing alternator, my 20 volt ammeter will peg out for just a short drive, settles down to just a couple amps after I'm underway. Is this a fire hazard -- should it take longer to get back to just a couple amps?
What are the danger of one staying pegged.
Tks

6 Volt Pos. Ground -- NU-REX Single-Wire --- 30 Amp
Click to expand...

A couple of suggestions:
- tighten all the connections on the back of the ammeter, including the nuts next to the case that hold the ammeter posts tight. Also, be sure to tighten the outer nuts. On original ammeters, these are Bakelite insulated….some repros use plastic insulated nuts, and the lower-quality repros use up insulated nuts. In any event, all connections have to be very tight.

- replace the 20 amp ammeter with a 30 amp one. This may make you feel more comfortable as you’ll be able to see charging status rather than a pegged ammeter.

Good luck !
Jim
 
I've heard of putting a piece of #14 (AWG) wire for a jumper across the back of the ammeter, for a shunt, so the meter only reads 1/2 of the actual current.
 
OK, so I installed a 30 ammeter. On first start-up, needle goes to three notches plus
side of meter after 15 amps. It'll quickly decay to about two notches of center.
Tks

New Battery 6 months installed, tho, I don't know its age
 
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Have you checked the specific gravity of the battery? I ask because when I first got my A it had an alternator and it would peg the ammeter at max charge. After chasing a few things, I checked the battery's specific gravity. It was way low - under 1200. The battery was just a year old, but it must have been badly sulfated. No matter how hard the alternator charged, it couldn't overcome the degraded battery. I put a new battery in and it immediately went to a very low charge rate.

It worth checking the specific gravity.
 
Ammeters are still commonly used in aviation but all the systems I've worked on over the years use either a purpose made shunt or a custom made shunt wire set up to give an amp meter reading without worry of overloading the indicator. The shunts have a relatively heavy bypass copper or brass strip and the custom shunt wire set ups have a shunt wire that is calibrated for resistance by the length and gauge of wire used for the shunt bypass.

It would be rare to use all of the capable output from an alternator on a model A Ford. The only way the meter would ever get pegged out would be if the battery is extremely weak or shorted or if there was a major short malfunction of a main conductor wire in the system. A very high resistance due to a loose wire can draw a lot of current and create excessive heat as well. Make sure your wires to the ammeter and terminal box all remain clean and tight. I would chuck the plastic nuts on the ammeter and put some hex nuts on there. I've found the plastic nuts melted before. Bakelite works but the cheap plastic may not.

I'm not sure what the one wire 6-volt conversions are capable of for output but it's likely around 30 to 35-amps. Most conversions don't even list the maximum output. The original 10SI GM alternators were around 42-amps but that is on 12-volt negative ground systems. Some 10 SI units and the 12 SI units were available with a lot more output capability.

A person can use a 20 or a 30-amp indicator if they want. Neither one is all that good of quality and I've had both types.
 
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A ground return path with high resistance (couple of ohms) from paint or rust can cause the regulator to produce higher voltage. Check the charging voltage at the battery. If more than 7.4 volts, this may be the problem. Run a dedicated 12 awg ground wire between alternator and battery ground posts if necessary.
 
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