Posted by docmirror on June 22, 2021 at 21:23:29 from (75.3.255.251):
In Reply to: Ammeter??? posted by db0941 on June 21, 2021 at 16:59:05:
First, a caution. If the OPs ammeter is completely disconnected the ALTERNATOR may not be bad after all. It just may not be providing current to the loads of the tractor. The coil wire comes through the key switch, and is often jumpered around the ammeter if that goes bad.
Now on to the 'why' of each meter. An ammeter became popular on cars and trucks as different features were added that used electricity. The generator or alternator puts out a rated current at a defined RPM. When loads increased in cars and trucks, there came a time when the standing load of the vehicle exceeded the power output of the generator at idle, and the battery would be providing current for the car elec loads. It was an indication that the loads should be reduced, or the engine run higher to provide more current. It would also indicate a failing charging system as the ammeter would be swung to the negative '-' when running at higher speed.
The voltmeter has taken over in most cars and trucks, for two reasons. First, it's cheaper to run a voltmeter in the dash than to run a large copper wire from the batt '+', to the relay/fuse board where all the loads are tapped. Next the voltmeter gives an indication of charge of the battery when the key is on, and will show a sag in voltage in a failing charging system. Along with the voltmeter, there is often an idiot light indicating discharge during normal operation, if the operator doesn't check the voltmeter often.
In a tractor, my vote is for the voltmeter, unless the tractor has a lot of external elec loads. The voltmeter goes from the '+' side of the ign switch where the coil wire attaches, to any negative chassis ground '-'. This provides no voltage flow when the key is off. When the key is on, it will show the resting voltage of the batt, and will be around 12V. As the engine is cranked, that voltage will sag due to the large amperage used by the starting circuit. Once the engine is running, now the voltmeter will show charging voltage about 14.3 to 14.7V, or - it will show a low voltage below 12V indicating a failing charging circuit.
Even the smallest and oldest generators produced far more current supply than the coil required. That was to charge the battery quickly after starting, so that the tractor could be switched off and on during the day. Most old tractor gens are 25 to 35 amps at 2100-2600RPM at the gen pulley.
If it were me, and I had to choose, I would prefer the voltmeter. The old tractor coil is only going to draw maybe 2-3 amps at 12V, and the difference in the meter deflection is pretty small. A lite could also be included with a diode to show discharge if the charging system fails.
OP, have the alternator tested off the tractor before you start messing with wiring. If it is working, then you need to diagnose the wiring, and find out how to reconnect the ammeter. Test the ammeter with your multimeter set on 200 ohms. The resistance across the two big terminals will be very small, about 0.1 to 0.3 ohms or so. If it is an open circuit, the ammeter is bad, and will need to be replaced, or changed over to the voltmeter.
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