Underhood Ammeter Bypass

[1970FuryConv]

Any guess as to what size inline fuse I would use for this instead of a fusible link?

I have always read that a fusible link is better than a fuse in an ammeter bypass.
I've also read that a fuse or circuit breaker should normally be handling 80% or less of its capacity.
If you had a 35 amp alternator, by that rule, you'd need a 44 amp fuse approximately.
Of course, if you had a 60 amp alternator, mathematically you'd need a 75 amp fuse.
In either case, I would think you'd rather use a circuit breaker than a fuse. Or you could just use a fusible link.

 

[Big_John]

I have always read that a fusible link is better than a fuse in an ammeter bypass.
I've also read that a fuse or circuit breaker should normally be handling 80% or less of its capacity.
If you had a 35 amp alternator, by that rule, you'd need a 44 amp fuse approximately.
Of course, if you had a 60 amp alternator, mathematically you'd need a 75 amp fuse.
In either case, I would think you'd rather use a circuit breaker than a fuse. Or you could just use a fusible link.

The wire size is really the determining factor.

Let's say you have a 75 amp fuse for that 60 amp alternator, but were only using a #14 wire. Yea the wire is undersized, but for this example we'll skip that.

The wire will burn before the fuse pops.

So you want to look at wire size in the circuit first and size the fuse so it goes before the wire.

I posted this earlier in this thread.

 

[Big_John]

I should add that the fusible link is the best option.

 

[USSMOPAR]

Substituting a fuse/breaker etc for the fusible link that is poor engineering and not a good practice as the system is designed for a fusible link.
These are the simplest electrical systems and as usual it get the over anal-ization so prevalent

The bulkhead should be bypassed completely with the red and black wires going thru the firewall properly installed and connected

The alternator bypass requires a fusible link, all cars need the bypass and a few more grounds btw. The gage is not "inaccurate" with the bypass. It indicates charge and discharge as it should.

The 1972 Chrysler engine wiring diagram shows the bypass - look it up
The 1972 E body does not

 

[Ross Wooldridge]

Agree 100% - and once completed, those mods should render the car's electrical system far safer. The only concern some have is if the amp gauge fails, which on some it's prone to do... I think this occurs when there is corrsion at the terminals, creating heat cycling which loosens the connections and then the cycle repeats - sooner or later the gauge can't take the spikes etc., and things fail.

My thoughts are on that are simply to ensure that the terminal connections to the studs on the back of the amp gauge are clean and tight. Checking them every few seasons is a good idea. This is something I feel should be written into maintenance schedules for cars after they've been driven over a hundred thousand miles. However, back then it was assumed by the factory that cars would likely be at the end of their life cycles and people would be buying new cars. Now we know differently!

Always disconnect the battery when doing this of course.

 

[Ghostultramarine]

Thought I’d give you a thank you for this.

 

[Ross Wooldridge]

The collective "you" is warranted. Lots of people have chimed in here, and in other similar threads.

I trust you have worked out the issues..

 

[Ghostultramarine]

I hadn’t done this yet, until today. Feel a bit better having done it.

That said, I still haven’t tried starting it yet - I have the battery disconnected still as I found 4 new bulbs under the dash day before yesterday (the 2 for the ashtray and the 2 between them for the fan buttons(?)), have to change them and wire in a fuel gauge, temp, volt and oil pressure gauges.

 

[watchfatha]

Thank you for this knowledge. As a big believe in the "Carpenter's Rule" (measure twice, cut once) I have a question of clarification: Is that a TYPO where you say "But it still works-until is shorts out"- Do you mean after the mod the gauge still works and the car still works regardless of whether there comes a time when the gauge burns out, OR do you mean that doing this mod will eventually LEAD TO the gauge burning out. Thanks! (FWIW-intended applications are 64 and 66 Imperials, which are notorious for melting Ammeter gauges). Much appreciation.

It took longer to do the thread compared to the wiring,LOL!!
But I cannot stress enough how important this 15 minute job is .
What it does is piggy back on the ammeter circuit. This wire takes 50% of the load away from the ammeter itself.
We all know the ammeter gauge is the achilles heel in our beloved old Mopars.
The fusible link is a safeguard should the alternator short out and not burn the dash or car to the ground.

You wil notice that the ammeter gauge will not swing as crazy as before and will not read 100% accurately.
But still works--until is does short out but the car will still function normally on a day to day basis.
An option is install an afermarket volt gauge.
Simpy hook it to a switched 12 volt source and ground it to body. done.

In laymans terms the Red wire at starter relay goes through the bulkhead and eventually to the ammeter gauge--which is hot all the time.
Hot I mean live as in anything that is run by battery no ignition key needed (headlights,hazards,dome light lighter,etc)
The black wire on the ammeter goes to the ignition switch which as you know powers anything that needs the key on.
So when the ammeter shorts out you have NOTHING!
This simple little addition is worth its time,trust me.

I cannot take credit for this mod.I first read about it in MOPAR ACTION
20 years ago writen by Richard Ehrenberg.
Hope this helps.

 

[Ross Wooldridge]

It's a typo, and to clarify, the mod will NOT result in a short out situation, unless the mod is done incorrectly. What he meant originally is that although the car still works prior to the mod, it may short out at any time due to the poor original design of the system, and its age related degradation in quality. The mod is a huge improvement in overall safety of the charging system.

 

[watchfatha]

It's a typo, and to clarify, the mod will NOT result in a short out situation, unless the mod is done incorrectly. What he meant originally is that although the car still works prior to the mod, it may short out at any time due to the poor original design of the system, and its age related degradation in quality. The mod is a huge improvement in overall safety of the charging system.

Noted with appreciation.

 

[cbarge]

Thank you for this knowledge. As a big believe in the "Carpenter's Rule" (measure twice, cut once) I have a question of clarification: Is that a TYPO where you say "But it still works-until is shorts out"- Do you mean after the mod the gauge still works and the car still works regardless of whether there comes a time when the gauge burns out, OR do you mean that doing this mod will eventually LEAD TO the gauge burning out. Thanks! (FWIW-intended applications are 64 and 66 Imperials, which are notorious for melting Ammeter gauges). Much appreciation.

To clarify, if the ammeter was working before the bypass and it shorts out after the bypass, the car still functions normally.
The only difference is the ammeter will sit in the middle and not move..
The bypass eliminates the chance of the ammeter causing any dash fires.
Hope this helps!

Cheers

 

[watchfatha]

To clarify, if the ammeter was working before the bypass and it shorts out after the bypass, the car still functions normally.
The only difference is the ammeter will sit in the middle and not move..
The bypass eliminates the chance of the ammeter causing any dash fires.
Hope this helps!

Cheers

Actually, yes. And you solved two mysteries: 1) the answer to my current question, and 2) the reason my former 70 Polara ran fine but oddly there was never any indication from the ammeter gauge......and an extra wire on the alternator. Who knew????