School me on the reasons why this is done

Because it is a weak link and an under designed system for today's higher output alternators. So the ammeter and associated wiring and connectors are the weak links. End result is melted wiring and fire potential.

That nailed it.

Start adding amperage that was not needed when these cars were new and things can go wrong.

Some people try to overthink the original engineering and things can go badly.

All the early Mopars with amp gauges routed all the current flow through the amp meters, those meters weren't design to handle much more than 80 amps so when we started putting in 100 amp alternators it caused issue with the wiring and amp meters failing, melting, and catching on fire

Not so much the ammeter, per se. But poor connections on it & the bulkhead connectors.

I've seen a number of melted ammeters over the years. It just wasn't a very robust design during the 60's and 70's. I assume they fixed it at some point but I had a '74 Dodge truck with a melted ammeter. When they started to use the big 100 amp alternators the engineers got serious and upgraded the whole system. I think they eventually got really smart and eliminated the ammeter. Not sure when that was for sure but you don't see ammeters in cars anymore. Someone eventually figured out that shoving all the charging amps thru the firewall and back out again wasn't the brightest idea.

You dont have to have higher output alts to have problems.My 65 was always a standard alt and very little accessorys.Same with the 4 Ram Chargers I had.All had the amp gauge starting to melt .Ive had a couple that were almosy ready to start a fire.Just take the wires off screw them together and tape up and install a volt meter.problem solved.A few of the Rams would actually cut out the amp meters were so bad.Rocky

One of the issues, even on a standard low AMP alternator is the 50 year old wiring. Corrosion in the wire, poor connection and the Bulk Head connection are all issues that add to the problem.

Here is a long answer if you really want to know what is going on. Trace out the wiring in the service manual and you will see how it works.

Some correct answer some incorrect. All current is not routed through the amp meter. Basics: Battery can source more amps then alternator. 700 Cold Cranking Amps etc... The alternator is limited to a design current based on mechanical conversion of rotating magnetic field to create current. Rectified through diodes. So early amps from alternator limited to about 37, 50 amps. If you look at the wiring diagram, the alternator does not have fusible links, like the battery does in later cars. A short in the wiring before the car would start, while just ignition switch on, could source that 700amps from the battery which would quickly melt your 12, 14, 16 gauge wire. The alternator could not sustain that high a current.

The output of the battery goes directly to one end of the amp meter (and starter). The output of the alternator goes to a splice joint to feed ignition, fuse panel, etc, while one of the splices feeds the other end of the amp meter. The only thing going through the amp meter normally when you start the car, is the alternator feeding amperage to recharge the battery. Based on the voltage difference between alternator and the battery. Current flow from high voltage to low voltage. The alternator when good, will have a higher voltage then a new battery. The alternator runs everything via the splices before it ever gets to the amp meter. A normally running car, would have a trickle charge of the battery after it levels out from starting the car. (Yes battery can be sourcing some current since it is in parallel with alternator and two uneven power sources will load share to a degree, but the one with a higher output voltage will source the majority of the load).

Weak links are the terminal connections at the bulkhead. Poor connections as mentioned on the amp meter. Bad voltage regulator, or bad diodes on the alternator which would reduce alternator voltage. NOW the battery is feeding current via amp meter, back to the splice and even to the alternator. The other weak link is that splice. They route the alternator output to the center of the dash on a 70 B body, then back to the column for ignition switch. All that excess wire bundled together creates heat, raised resistance, more heat.

So the only thing the amp meter monitors is how much the alternator is charging the battery. Or with car not running, how much current the battery is sourcing for anything on.

A simple 10 gauge wire from alternator output to battery simplifies the battery charging and prevents sending the current into the dash, and then back out to engine compartment, when both the battery and alt are in the engine compartment. A volt meter placed on the alternator output to ground, tells you what you need to know. Alternator voltage output is higher then normal battery voltage.

So even with a 100 amp alternator your not at risk other than your amp meter does not have a fusible link to limit the now much higher amperage available for your original wiring. Yes can overload the amp meter and wiring now, if the battery is dead. Will act like a short to the alternator. If your interior wiring remains the same, and all your new loads are in the engine compartment with separate wiring, and fuses and relay (electric fans, water pump, separate wire to fuel pump). That amp meter is not at risk. Current is not going through it as long as the battery is good.

Current kills, so running wire from alternator through bulk head, back to battery, just so you can have a gauge tell you if your battery is charging or discharging is a little archaic with more modern alternators and regulators. A Voltmeter can do it and does not jeopardize the wire.

But the alternator is still beneficial where the voltmeter will not protect you:, Let say you developed a short on the ignition wiring laying across the engine. You turn on ignition but not start. You see amp meter peg battery discharging because of the direct short. You immediately turn ignition off and investigate. Early cars without any fusible link your ignition is not really protected from the battery. A voltmeter would not really tell what is going on. Voltage would droop, but not enough to warn you. 100 amps to fry a 16 gauge wire not a problem for the battery.

I left my amp meter on, but what I did do was double up my bulk had connectors, and I also sent the alternator output to the ignition switch directly, not to the splice and then back. I have fusible links on the battery output. When I start the car, I watch amp meter. I know where it indicates and how fast it comes down, even when sitting for a month.

My car prior to purchase had the engine harness burn up. I found that that overheated the ignition wire under dash at the splice and had to repair all that wiring. Had nothing to do with amp meter, but a shorted engine harness allowed all that current to go into dash harness to get to the ignition switch and back out. Bulk head connector melted too. Repaired it all and modified it for a more efficient electrical wiring. Without an amp meter you rely on blowing a fuse, breaker, or fusible link to know you have a problem. An Amp meter can show you a developing high current situation that a voltmeter can not. Just like watching oil pressure.

If an ammeter grounds into the instrument cluster, have fun. Those little cork donuts aren't much of an insulator IIRC.

I'm not a fan of ammeters in cars. If someone wants to run them, great... it's their car. There are plenty of people that run down folks that dislike ammeters and normally have comments similar to "Have you ever seen one cause an issue like this...." why yes I have .

All good, pick your part, pay your money!

For those who have done that MAD Electrical alternative, did you drill the hole in your bulkhead connector or did you choose to put a new hole in your firewall?

I don't like the mad approach.

I prefer to run a wire with fusible link from alternator to starter relay, bypass ammeter and leave the rest of the OEM wiring in place if in good shape. Check the bulkhead for clean connections and run it.

You can usually find a location in the firewall that has a hole already to pass through.

I drilled out the 2 holes in the bulkhead with some extra length of those 2 wires behind the firewall in case I need to seperate the bulkhead halves again. I kept the ammeter & no issue so far but I do have a stock alt & no added devices (and a good battery) & I cleaned ALL terminals/connections in the dart including grounds

I'm still learning electrical... when you say bypass the ammeter, do you mean you are still leaving the ammeter stock and functioning?

I just read your first post. I guess you probably disconnected it.

Bypass as in remove completely from the system.

I build the wire around wires in 6 & 8 ga with fusible links. Will custom build to length with buyers measurements. I had an ad in each area on this site, probably gone with the 1/4ly purges.

Rob

You will want to connect the 2 sides together if you "bypass" the ampmeter.

the main reasons:

-Underrated alt outputs at iddle from factory
-Incorrect sourcing of added accesories from batt side ( this is incorrect when you have ammeters )
-Upgrade batteries when the real need is the good alt. A bigger capacity alt with stock alt is worst for the system
-Lack of understanding of what the ammeter reading means &/or how it works.
-lack of mantenience ( yes, electrical require some checkout, just like your tires, brakes, oil... )

every thing of this makes to causes more stress on the charging system than the needed one, including the amm

and no, the bulkhead meltdown is not amm's fault

for more an old thread I wrotte up and some recent discussions on FBBO

http://www.dodgecharger.com/forum/index.php/topic,33574.0/all.html

https://www.forbbodiesonly.com/mopa...icking-time-bomb-factory-ammeter.177831/

https://www.forbbodiesonly.com/moparforum/threads/dissection-and-study-of-a-mopar-ammeter.199179/

https://www.forbbodiesonly.com/moparforum/threads/poll-about-ammeter-reading.198930/

and more info from a member here:

http://www.heritech.org/cuda/Charge.html

we have got LOT of long discussions about this here, but hey are old and I won't search them

Yes, I've seen burned bulkhead connectors as well as burned ammeters in the wrecking yard on 60's and 70's era Mopar cars. Both of those areas can be problems.

People adding accessories on the battery side is a problem since that forces additional current thru the ammeter. That is a good point that no everybody thinks of. I had that discussion with a guy not too long ago since he was adding EFI and electronic ignition. The instructions from Holley say that both the ignition system and the EFI system need to be connected directly to the battery. They also want the fuel pump connected to the battery. So that is an additional 30+ amps of stuff. If it is all connected to the battery on a classic Mopar with an ammeter then the alternator is going to need to send all the power to operate those items thru the ammeter. The instructions don't say anything about how to wire the system if you have an ammeter.

In my car I shunted the ammeter and installed a voltmeter just to avoid the issue of sending so much current thru the ammeter. I could've installed the EFI, ignition and fuel system on the alternator side of the ammeter, but that would've required more work and it isn't how Holley wants things installed. They want to use the battery as a large capacitor to filter out noise for the EFI computer.