Circuit Breaker on Ammeter Bypass

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dartman59

dartman59

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I'm doing the typical ammeter bypass using a fat 8 gauge wire from alternator to starter relay (overkill?). I want to use a circuit breaker as opposed to a fusible link, so I don't have to worry about having a spare. Is this a good idea? What is safe amperage for 8 ga? I'm thinking of something like this:

http://www.summitracing.com/parts/bss-cb185-50/overview/
 

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My plan is to continue to use my red and black wires with fusible link at the bulkhead connector and splice red and black at the ammeter, leaving it dead. My BC is in great shape. I have a '71 non-rally dash (boring) and thinking of converting the ammeter to a voltmeter must may just hang the voltmeter down below the dash with my oil gauge and temp gauge.
 
I think if you went to the electrical forum and searched it, you'd find a lot of info in that subject. I'm curious to see what you find out. Good luck
 
basically a fusible link has no "moving parts" so to speak. no places for tarnish to take hold and increase the resistance over time. when the resistance gets to high things melt.

so as long as you maintain the connectors / breaker it will be fine.

if you leave it ignore it you will have a meltdown.

if it's a street driven car with moisture under the hood (rain at times etc) i'd stick with the fusible link. or use a sealed box for the breaker
if it's trailer queen/garage kept never sees rain it wont matter

for what it's worth i've only seen fusible links fail at night when the headlights were in use, usually because the headlight switch is worn out / corroded / needs replacement. switches dont last forever. headlight/brakelight/tail light sockets get cruddy too
 
This unit has a good rating for water/dust; it is IP67. So that part is good.

I would be rating the breaker for the expected load. If you are placing it so as to replace the fusible link, then it will not be in line with the new bypass direct from alternator to starter relay; this will be in the line from the starter relay to the inside of the car. So it will not be protecting the 8 guage wire; it will be protecting the wiring inside the car.

The biggest loads on that are inside the car in stock form are typically the headlights and blower motor, then blinkers and brake lights. That load ought to be in the 30-40 amp range, just computing off the top of my head. So, the breaker ought to be rated at no more than 40 A, IMO, and 30A if you want slightly better protection with some risk of false trips when you have the headlights and blower on for a long time. (I am open to other thoughts on this BTW.)

The reasons for this small rating are:
1) This is a thermal breaker; that means that it has to pass enough current for long anough time to get itself hot enough inside to trip. Flowing 200% of rated current (i.e., 60A through a 30A breaker), it will on average take 20 seconds for this to trip. You can burn up a lot of inside wires in that time.
2) The wires inside fed by the fusible link are smaller and can be melted/burned by the time this thing will trip if you have a partial short. In reality, the wires in the car are not large enough to withstand a partial short and survive anyway, so all you can protect against with anything in the fusible link location is hard shorts, that will make the breaker trip, or the fuse blow, very fast.

So, I would keep it on the smallish amperage side, for the sake of having some protection and risk some false trips... just IMO. You need to give an idea what the inside wiring loads you have are to decide this; for example, are you doign the headlight relay setup? Or is the headlight set-up stock?

And just don't think you are getting superior protection by replacing the fusible link with this; you aren't. It is all the same for a link or thermal breaker or fuse; there is an overload vs time that has to occur before it will break the circuit. Only a magnetic trip type of breaker will give you better protection. (And I can't tell you where to get one of those for auto use...)
 
That breaker may or may not be a "good idea." Electrically, theoretically, good

Over complicated..........The thing is, you don't EXPECT to "ever" blow the link in the alternator output. There exists many old Mopars, some now close to 50 years old WHICH HAVE NEVER had the original fuse link blow. Think about that.

In the case of the charge wire, the only "thing" which should blow it is a diode short in the alternator which "does not happen" to blow open before the link, or if the wire from the alternator should short.

On the breaker side, there weather exposure and HEAT. To keep the wire short, if you install this in the engine bay, it sees a LOT of HEAT. This de-rates the ampere rating of the device, might be substantially.
 
^^^ Good commnet IMO on the alternator link.

Correct-o-mundo on the breaker derating with heat. Was not trying to get that complicated.....! Here is the mfr's data sheet; it shows a nominal rating decrease of 10% at 150 F, and a trip level INCREASE of about 30% at 0 F. But, I expect that all thermal break devices are going to do this to some degree; I just can't say if a fusible link's rating woud decrease by 10% at 150 F, or by some other amount.

[ame]http://www.cooperindustries.com/content/dam/public/bussmann/Transportation/Circuit%20Protection/resources/datasheets/BUS_Tns_DS_18X_CIRCUITBREAKER.pdf[/ame]
 
dartman59 said:
I'm doing the typical ammeter bypass using a fat 8 gauge wire from alternator to starter relay (overkill?).
Click to expand...
This sentence is confusing. The ammeter bypass really has nothing to do with the way the alternator feeds back to the battery. The ONLY thing the ammeter bypass does is eliminates the ammeter gauge which is the weak link in the OE electrical system.

The ammeter in our cars was a left over item from the generator days when there was less overall current passing through the system. For whatever reason (probably $$) Chrysler never updated to a voltmeter so we are stuck with the antiquated ammeter. The reason the gauge becomes the weak link is because ALL the current that goes through the car passes through the ammeter terminals. So, if there is a short or high resistance somewhere in the system, the excess current created from these situations will pass through the ammeter. The gauge was in fact designed to monitor the system based on overall current but in reality, the gauge itself and wiring is not robust enough to handle too much extra juice going through it and can end up melting and causing a lot of collateral damage.

The weak link idea becomes especially true over time as wires get old and are more susceptible to high resistance and shorts. The bulkhead connector is a major culprit of this particular situation.
dartman59 said:
I want to use a circuit breaker as opposed to a fusible link, so I don't have to worry about having a spare. Is this a good idea? What is safe amperage for 8 ga? I'm thinking of something like this: http://www.summitracing.com/parts/bss-cb185-50/overview/
Click to expand...

I don't think a circuit breaker is a good idea because for one, it can reset. A circuit breaker also has the potential of not working until it's too late, depending on what its rated for. If you melt a fusible link, the system is disconnected from power. In any event whichever circuit protection device you use really only comes into play if you have a big enough problem that is causing excess current to go unchecked through the system.

What is the rest of your electrical system like? Is it in good shape or is it trashed? Is your junk blowing fuses or are you experiencing any bad wiring problems? If you have accessories that draw a lot of amps, think about using relays to shift the load. Your wiring and associated components are old. This stuff was not really designed to handle high output electrical loads unless modified accordingly.

I basically rewired my entire car last year but left some of the OE stuff if it was serviceable. I did the ammeter bypass. I also removed the bulkhead connector, cleaned it out and put everything back together using a lot of new terminals and packed it with dielectric grease. I have a circuit breaker on my car but it's for an auxiliary fuse box that protects stuff like a digital shift light and a tach sender. Not a lot of juice goes through it continuously. The rest of my stuff uses fusible links, solenoids, relays, switches etc. I don't really have any accessories so I don't use a lot of amperage, your situation may be different. I have a Denso 60 amp alternator and my battery is in the trunk with a cut off switch. Everything works good.

Sorry for the long reply, just needed a break from doing something tedious. Hope it helped.
 
I don't understand from your description exactly where you will put this breaker. It should be OK on the feed to the cabin, i.e. where the fusible link normally resides. I would not put one in series with the alternator output, though a large fuse there might be smart (I used 160 A). If that fuse blew, you would see your ammeter come alive again.
 
Thanks for all the thoughts. My wiring is in excellent shape. I, too, disassembled my bulkhead connector and cleaned all the terminals by carefully sanding with 320 grit and even dipped the ends in muriatic acid for a few seconds--really takes it down to bare brass. And I put it all back together with dielectric grease, so that part of things is in good shape. The dash harness is totally uncut and I've had it comepletely apart and all wires are flexible and good. Oh yeah--I also have put my headlights on relays, so I took that load out of the picture.

And ok, you're right RMCHRGR, I am talking about more than removing the ammeter from the circuit--I want to avoid running my charging circuit through the bulkhead, too. :)

I've also always wondered if an alternator is rated at, say, 60 amps, and the battery is dead and you just got a jump start, what prevents the alternator from pumping all 60 of those amps through the charge circuit and into the battery?? I hope someone can school me on that one.
 
Yes, Mr. Grissom, I Was thinking about putting it in series with the wire from the alternator to the starter relay. I've never seen a 160A fuse!
 
A good battery can take pretty much all of the 60A if discharged and if it can be pushed through the wiring resistance.....
 
If you don't know about 'ol Mark and his automotive electrical web page, you should check it out. It's got all kinds of lovely diagrams and everything. Read through it, it's good stuff. You will see what to do with your alternator wiring and the ammeter bypass.

http://www.madelectrical.com/electricaltech/amp-gauges.shtml

BTW, the Denso alt I have works great. Plenty of power for any accessory you can throw at it, lightweight and easy to wire. Its' internally regulated so you get to ditch the OE voltage regulator and associated wiring.

Case in point; a buddy of mine had his rusty old Dodge here at my house recently to take care of a few things. While working on it, I was looking in the trunk for something and noticed there were like 3 or 4 voltage regulators floating around in there. Wonder why.

Also, I worked at a dealership, we jump started cars almost every day, never a problem with feeding too much voltage back into a battery. All of 'em had 120A alternators or better. The jump packs we used can pack a punch too. Alternators usually have a diode or series of diodes that act like a check valve to prevent unwanted voltage from being pushed through the circuit. The regulator senses the magnetic field caused by the extra current and shunts the extra voltage or something like that. They fail when you try to do stupid crap like reverse the polarity of the jumper cables.
 
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