Technical Alternator Wiring Fire, What Now?

Shown above is the wiring in one of my projects that is not HAMB eligible. However I was driving it home from school today when I had a minor electrical fire. My question is what did I do wrong in the wiring that would cause the wires to heat up and catch fire? The switches in the diagram are toggle switches because of the nature of the project with bypassed many electrical systems. Just curious what I did wrong and any help is appreciated.

Thanks!
Trent Rogers

 

I should add that the battery did charge with this configuration of wiring and the amp meter did work. None of the wiring got hot until I started to use accessories such as the heater and headlights at the same time. Thanks in advance!

 

What exactly melted? Wiring fires are always caused by too much amperage for the wire size somewhere...

 

Any fuses??

 

Overload or dead short.
Overload = cable size not sufficient for load
dead short = +ve to -ve short circuit

 

Should have specified, the wiring running from the battery to the alternator switch and the wire from the alternator switch to the amp meter were the only affected wires. The rest were unharmed. Thanks!

 

More info needed. As above, fuses & locations? What's the switch rating? Wire gauge(s) used? Ammeter rated for alternator output and using a shunt?

 

Too small wire for the additional load...

 

1. I don't understand why a switch is needed in the main output circuit. Cars with an amp gauge don't need one. Engine off means no flow from alt. Diodes won't let power flow back.
2. A 30 amp and up switch is not easy to find. I'd guess the wiring and the switch were undersized for output and failed.

Fix? Remove switch and replace wire from amp gauge to battery with proper size for alt output. Research amp gauge bypassing for more safety.
EDIT, you want to switch the trigger wire, not the output. Probably a good idea to have the alt checked before running again, too.

 

What is the amp rating of the alternator? And the amp gauge? A lot of factory amp gauges were rated for lower amperage as they were used with generators. It seems the wire was not able to handle the load. Going to a larger wire may transfer the problem to the gauge. X2 for amp gauge bypassing or shunt.

 

need fusible link in wire to battery. diodes do fail, and cause short with ignition off or on. why using amp gauge and not voltmeter? if using heater, etc need a proper fuse panel.

 

I agree that the most likely problem is circled in red. While the amp meter may not have been capable of handling the full output of the alternator, It is a safe bet that the toggle switch was way under rated. As the full amperage in the system passed through the wires, amp gauge, and that toggle switch, the switch probably melted/shorted internally. This created a dead short and rapidly went down hill from there. Wires melted and the caught on fire. You are lucky you didn't lose the whole car.

 

What gauge wires did you use?

 

Most cheapo toggle switches are not made for a lot of amps, and usually that would fail first. Not so lucky in your case

 

Usually 15 amps or less.

 

Believe what Steve stated. He is our guru of Auto Electrics.

Even though your wiring hookup was not what any of the rest of us would do or suggest it worked until you added a load to it. That means that the switch or wiring wasn't rated for the amp draw or the blower motor was drawing way too many amps.

It doesn't make a damned bit of difference what the alternator is rated at, What counts is the load draw on the system. The alternator is only going to put out what the system asks for.
Headlights and tail lights don't draw a lot unless you get into high wattage halogens.
a regular AM-FM radio cd player might draw 5 amps unless you add an amp
Power windows/power seat only draw when they are being used. They can have a pretty good draw.
A heater blower motor on high can draw up to 30 amps. Run with the headlights on, the radio on and turn the heater on and you might be drawing 40-50 amps through a wire or switch designed for 20.
I save every usable alternator wiring diagram that people post on the net here https://photobucket.com/bucket/c5e8...74/album/694af3ea-994a-4ac7-af37-2e4f6ad7da37

These three are actually all the same. I've wired a dozen alternators with this basic diagram over the past 40 years.

 

It still comes down to this Volts, amps and ohms. In water hose terms Volts are the pressure, amps is the flow and ohms is the kink in the hose or resistance. Resistance in any electrical system creates heat. A wire that is too small or a switch that can't handle the amp load and it gets hot and you have an electrical fire.

 

I agree with Steve. Most of the time.. LARGER WIRE. I use , on the 1950 Buick, 8ga wire. And that is not large enough when the battery is way down and the alternator is putting out near max. My daily uses a 2 ga.

Ben

 

Here is a better version [more HAMB suitable ]

I haven't a clue why there is a switch on the alternator charge wire [unless some rocket scientist is using this to stop a faulty Alternator draining the battery overnight]

Your schematic ^^^^ is wrong!
The battery connects to the + side of the ammeter. And the ignition AND alternator are both connected to the - side of the ammeter.
The ammeter should only show charge / discharge to the battery.

Most of the time when there is heavy loads [headlights on etc] the Alternator is maintaining charge instead of going back to the battery via the ammeter.
The only time an alternator risks frying an ammeter is when a high output alternator is trying to recover a flat battery [or low battery]
You can make a "diode shunt" that acts like an electrical "relief valve" to take the excess current once the ammeter is maxxed out.
Ammeters have progressive voltage drop [increased resistance] whereas diodes require a forward drop of approx 1.15v to open.
So usually the ammeter has less voltage drop than the diode so it flows current. As the ammeter gets maxxed out the ammeter has a greater voltage drop than the diode
There is a point of equilibrium where both have the same drop and the ammeter stays at that level and the diode flows the excess [below this the ammeter is business as usual]

here is a rough schematic of how to make a "diode shunt" using 2 x 100a-150a Vishay diodes parallel and facing different directions.


I made one for my friends old truck so he didn't have to butcher the dash [and everything worked properly]

 

^^^^ that shunt set-up is good. It does matter about the alternator output, which is why wiring harness manufacturers include specific instructions and a bypass wire diagram for alternators of 80 amps or more.

 

Interesting; I did something similar but just used normal wire for the shunt circuit. The problem was an ammeter designed for a 15 amp generator used on a vehicle with a 60 amp alternator. I didn't want to see it maxed out when starting with a low battery. It works as designed, but doesn't move the needle a lot at low current flow. The diode shunt might give more needle movement at low current flow. Nice design.

 

Could be too small a wire size, but also the switch rated too small for loads the charge wire puts out. Once the switch begins to fail it builds resistance which could cause the heating throughout the charge wire also. Not sure any toggle switch is rated high enough amperage to work in the charge circuit.

 

At one time, reman alternators, for Fords, at least, came with a new wiring plug and partial harness along with a cardboard wire tag attached to the rebuilt alternator instructing the installer to always cut off the original plug and wire in the new plug provided.

I haven’t seen that for a long time, but I haven’t replaced too many alternators lately either.

Apparently the original plugs were prone to developing high resistance over time. That leads to heat…..and electrical fires.

 

Fixed! Larger gauge wire did the trick. It’s wired the same way as in the original diagram shown above just with larger wire and a better or higher amp rated switch. I did double check and the amp gauge is rated for 60 amps which is far more than this alternator will ever put out. Looks like I’ll have a heater and lights again this winter. Thanks for all the input!

 

You still need to connect it properly.
The way you have it, ALL the alt charge goes through the Ammeter.
And it will only read ammeter charge and not discharge unless the Alt regulator is faulty [which I suspect is the reason for the switch]
This is the correct way to connect an Ammeter to a charging system.


Any high load accessories like heater, A/C, headlights, sound system etc are connected on the "A" side of the ammeter [usually after the ignition switch]
If all these accessories are operating at once and the Alt is keeping up it doesn't go through the ammeter. [ but any surplus charge to the battery does] The ammeter reads charge.
If the Alt cannot keep up with the accessories the battery slowly discharges and the ammeter reads discharge.

Your Alternator switch is not needed ,but can be optional ,if you insist on using it [it goes in the dedicated Alt wire]
All you need to do is swap a few wires around.

 

Add a high current relay, which the present switch operates. Then wire both alternator output and ignition switch to the ammeter per the previous post.

 

Or a decent diode, like we do on race cars with an isolator switch. [master cut-off]
This allows the Alt to charge but not discharge. And it comes in real handy when working in the engine bay.[there is no live wire from the battery]

That switch in the post #1 should not be needed unless the Alt has a faulty regulator.[and discharges when not running]

 

The schematic that @Kerrynzl provided is a good set-up. That is what I plan to use in my builds.

 

A switch that is turned "on" is the same as the dead short you described. In essence, when you flip a switch to the "on" position, you create a dead short in the switch.