Ammeter???

Hello, As I am new here and am somewhat new to old tractors, I do not quite know how to ask this, so here goes nothing. I have a Farmall M. Not a Super M just plain M. The tractor had been converted to 12 volt by a previous owner. I was having trouble keeping the battery charged and after some checking found that the alternator was bad. I also discovered that the ammeter was completely disconnected. So my question is before I install the new alternator how do I (re)install a new ammeter? To what do I connect the positive and negative to? Thank you for any help you can give me.

I just had the same issue on a 60 year old tractor. This is the decision I made on my tractor, which may not be the right decision. I will
garantee you that you will get very hostile conflicting opinions on this. Apparently, for an ammeter to work properly, all the current of the
system must flow thru the Ammeter. I think that is a lot to ask of 60 year old wiring insulation, frayed and bare in spots. I chose to
replace the ammeter with a voltmeter which just measures if the system is charging or discharging. Just my take on the issue. Only a trickle
of the system flows thru the gauge.

Be it is + ground due to having an alternator the charge wire from the alternator goes to the + side if I remember right and all other wires like ignition go to the other side. Also I remember right the side the alternator wire hooks to is the right side as your sitting on the seat

(quoted from post at 19:59:05 06/21/21) Hello, As I am new here and am somewhat new to old tractors, I do not quite know how to ask this, so here goes nothing. I have a Farmall M. Not a Super M just plain M. The tractor had been converted to 12 volt by a previous owner. I was having trouble keeping the battery charged and after some checking found that the alternator was bad. I also discovered that the ammeter was completely disconnected. So my question is before I install the new alternator how do I (re)install a new ammeter? To what do I connect the positive and negative to? Thank you for any help you can give me.

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The old ammeter maybe fine and the previous tinkerer had no clue and disconnected it .
When equipped with an alternator the tractor will be negative chassis 99.9999% of the time .
Ammeter - will go to the battery + . Ammeter + will have the alternator connected to it . Along with wiring that will supply the lights .
The coil will have to be a 12V unit with approx 3.0 ohms across the primary. The coil - should go to the points .

An amp meter measures current flow, be it coming from the alternator going into the battery, which
is good, or flowing from the battery to run the ignition and accessories, which is not good.

So, there is no ground connection to the ampmeter.

You will need a #10 wire from the battery, usually connected to the starter solenoid post, the same side the battery cable is connected to (this keeps the wire away from the corrosive battery). This will connect to one side of the amp meter. Nothing else goes on that post but the wire from the battery. If there is any other wire on the battery cable, it needs to come off and be rerouted to the other post of the ampmeter, along with all other ignition and accessory loads. Typicaly there will be 3 wires on the other ampmeter post, one going to the ignition switch, and one for the lights or any other unswitched accessories, and a #10 wire that will connect to the output (BAT) post of the alternator.<p The connections need to be well made, preferably soldered, not just crimped. This is a high load junction, any loose or poor connections will soon overheat and fail, probably why the original ampmeter was disconnected.

The ampmeter will probably not be marked - or + on the terminals. It's just trial and error which post to use for the battery. If it reads backward, simply reverse the wires on the ampmeter.
Amp Meter Wiring

If it was me, I would just put in a Volt meter. Easier wiring, same results. Just my 2 cents...

A bit of a correction needed. ALL current does not flow through the ammeter. Starter current is larger than what an ammeter's range is, therefore starter current does not flow through the ammeter. Ignition, lights, and any accessory's current does flow through the ammeter. Sorry if I'm too wordy in explaining this.

Voltmeters and ammeters are both easy to wire, but results are not quite the same. It isn't wrong to have both in a system. The ammeter measures the water flow and the voltmeter measures the water pressure. That is a rough analogy and hopefully helps with the understanding.

i wrote water flow and water pressure with quotation marks but they didn't appear on the final.

Hello db welcome to YT! Steve@advance and
Buickandeere have good descriptions of what the
connections should be. The link Steve gave to the diagram
takes you to a multiple return search result so that is a little
confusing. I will direct link his top diagram. Your M will differ
if it in stock configuration because instead of a solenoid it
will just have a heavy contact switch that energizes the
starter, most are operated by a foot lever just to the left of
the brake pedals. So in the diagram place that switch in
place of the solenoid and you will not have the starter
button that would energize the solenoid. Main thing is as
was described, one side of the ammeter will go directly to
the post on the foot switch where the battery cable that
come from the positive cable connects and that will be the
only wire connecting on that side of the meter. The wording
for the ..solenoid wiring starter button.. is placed in that
diagram so it is a little confusing just disregard it and follow
the wire connections as shown. And also as pointed out the
starter motor amperage does not go through the ammeter.
If you make the connections and the gauge reacts
backwards as the alternator shows a discharge when
running and the light draw shows a charge you just need to
swap your wires side to side on the ammeter. Hope this
helps to clarify the answers to your question.
Generic 12 volt conversion diagram

(quoted from post at 19:59:05 06/21/21) Hello, As I am new here and am somewhat new to old tractors, I do not quite know how to ask this, so here goes nothing. I have a Farmall M. Not a Super M just plain M. The tractor had been converted to 12 volt by a previous owner. I was having trouble keeping the battery charged and after some checking found that the alternator was bad. I also discovered that the ammeter was completely disconnected. So my question is before I install the new alternator how do I (re)install a new ammeter? To what do I connect the positive and negative to? Thank you for any help you can give me.

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oor feller...... got some good advice and some bad advice. Shame! Now to determine the difference. When people don't "know",they should not post! :evil:

(quoted from post at 18:14:24 06/21/21) If it was me, I would just put in a Volt meter. Easier wiring, same results. Just my 2 cents...

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I disagree on both accounts.

To wire up an ammeter, you install it in the wire running from the battery (or starter lug) to the box on the steering post.

A voltmeter is a whole separate additional circuit. It needs to be "switched" so you don't just run the battery dead when the tractor isn't running. Way more complicated. Not complex, but more complicated.

Ammeters and voltmeters tell you two entirely different things. A voltmeter won't tell you whether the angry pixies are coming into or going out of the battery while the tractor is running or how quickly that is happening. An ammeter does, but it does not tell you the state of charge of the battery like a voltmeter. So there are advantages and disadvantages to each.

Ron .... for some strange reason, the double quotation marks will not show on this site ..... but the 'single ones' will for some reason.

Good question So my question is before I install the new alternator how do I (re)install a new ammeter? To what do I connect the positive
and negative to?

NOTE 1 Since it has an Alternator Im assuming its now NEG Ground NOT POS Ground as some old tractors were !!!!!!!!!!!
NOTE 2 Since many (cant say all) Ammeters are labeled for a NEG Ground, their + is the SUPPLY INPUT (to UNgrounded + battery post) while
the - is the LOAD OUTPUT (to Alternator and loads like lights and ignition)

HOWEVER I CANT GUARANTEE THAT so if you wire it up and it reads bass ackwards simply reverse its terminals

IFFFFFFFF its wired correct if you turn on lights or ignition etc not running it should swing over to - discharge but + charge when
running if alls correct.......If it does the opposite simply reverse the leads


1) To measure the current (other then starter motor) flowing INTO (Charging) the battery and that flowing OUT (Discharging) it needs to be
wired BETWEEN THE BATERY AND ALTERNATOR with Loads (Lights, Ignition etc) also on the Alternator side.

2) To do that the SUPPLY INPUT ammeter terminal connects to the UNgrounded battery post often (but not necessarily) via where the big
battery cable wires to the INPUT BATTERY side of a starter switch or solenoid,,,,,,,,,,,,The ammeters other LOAD OUTPUT terminal wires to
the Alternators big main output stud/post

EASY PEASEY SUPPLY side of ammeter to UNgrounded battery post (or a terminal connected thereto like at starter switch input),,,,,,LOAD
side of ammeter to Alternators big output stud/post along with other loads like lights and igniton etc

3) Does it need a new ammeter??? maybe the old one is okay as it measures current flow NOT if the tractor is 6 or 12 volts. An old ammeter
may only be a 20 or 3 amp while an alternator may pump 40 to 60 or more amps so an old may not be best way to go !!!!!!!

4) Most alternators require NEG Ground, typically an ammeter is labeled for NEG ground vehicle....IF THATS THE CASE AND BOTH ARE TRUE
??????????? But I cant know that for sure Im ONLY assuming that but if not its easy to swap wires

THE AMMETERS + IS THE SUPPLY INPUT SIDE (wires to battery or to a terminal connected thereto) if my assumption is correct ??
THE AMMETERS - IS THE LOAD OUTPUT SIDE (wires to alternators big output stuc/post)

You DID NOT ask about a Voltmeter but it wires across the battery + to + and - to - EASY PEASEY two wires PLUS switched so its ONLY
connected when tractor running.. Switched so its not working when tractor isnt running

You DID NOT ask, but an Ammeter measures current flow into (charging) or out of (discharging) the battery
a Voltmeter measure battery voltage

Not being there I cant say for sure this is correct BUT REMEMBER the above is for a NEG ground tractor and assumes (maybe yes maybe not)
the ammeter is labeled for typical NEG ground vehicles such that + connects to battery and - to alternator IF NOT REVERSE LEADS

John T

For a modern NEG ground vehicle many ammeters Ive seen are labeled + for the battery side and - for the alternator side.....If he has an
Alternator Id bet its now NEG Ground........If the ammeter is like Ive seen the + wires to battery and - to alternator BUT I CANT SAY OR KNOW
FOR SURE HOW HIS AMMETER IS LABELED

You sticking with Ammeter - to battery ??? It may be but NOT knowing his ammeter labelling system I sure cant say

Take care Glen hope to see you again sometime

John T

(quoted from post at 21:14:24 06/21/21) If it was me, I would just put in a Volt meter. Easier wiring, same results. Just my 2 cents...

Click to expand...

Not the same

(quoted from post at 21:14:24 06/21/21) If it was me, I would just put in a Volt meter. Easier wiring, same results. Just my 2 cents...

Click to expand...

Not the same

Yo Rich, hope is all your way, you say Be it is + ground due to having an alternator

But aren't most Alternators NEG Ground ??

Best wishes now take care

John T

How about a new wiring harness that is not all chewed up . And has labels for where everything is connected . https://www.wiringharnesses.com/About-Us.html

I agree Ron, ONLY the charging current INTO the battery as well as the discharging current OUT of the battery such as lights, ignition and other small loads etc flow through the Ammeter NOTTTTTTTTTTTT the starter current which may be hundreds of amps UNLIKE maybe 0 to 20 or 30 or so amps typical (other then starter) in an old tractor.....

BOTH an Ammeter (measures current flow maybe 0 to 20 or 30 or so amps in an old tractor) PLUS a Voltmeter (measures battery voltage) are handy to have even though most old tractors only use an ammeter

To each their own is fine by me use whatever you like one none or both lol

John T

Yes got me on one of those moments

Hey Rich I have them daily and more as I grow olddddddddddd grrrrrrrrrrrrr

Best wishes

John T

I was probably thinking + for the non ground but put it backwards but here lately I have had a lot to think about and try to get done

I try to follow that 'rule' but we aren't perfect and mistakes do creep in.

Ammeter or voltmeter? Both serve a purpose as others have posted. And some say one is better than the other, personal preference. When there is a starting or charging problem, are people asked what does your ammeter say? Or are they asked, what does a voltmeter read at the battery, starter, etc.? Yes, I am leaning towards voltmeters as I change some, especially those with low current requirements as the ammeters more often than not, just appear to sit on 0. Voltage changing after starting, is easier seen to me and if it runs in the 13 to 15 volt range, it is charging even if the ammeter shows 0. If there is no switch to shut a voltmeter off, then an ammeter is the better option.

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.

Wow, thank you to everyone that responded. She's up and running again. What a wealth of knowledge here.

Sounds similar to Mr Thinker a couple of months ago .

So this all brings up a question. If mine has an ammeter and I wanted to add a voltage gauge, can I hook it up to read the battery voltage 24/7 even with the ignition off? Or would the voltage gauge have some current draw with it? Then I could always know what the state of the battery is when I look. Usually during the winter I keep a HF battery maintainer on any batteries sitting in the cold.

Suppose I could always tie wrap one of my many freebie HF multimeters near the battery as well.

Voltmeters do draw a minute amount of current that will eventually drain a battery. Just wire the voltmeter to a switched circuit.