fuse on starter switch, battery, alternator

[TomInCologne]

Hi guys,


situation: i have an alternator (powermaster, up to 60A) and a fuse on the top of my starter switch, Optima battery


I had the situation lately that one of my high-beam lights had a short and as soon as i put the high beam on, the fuse blew. I can't remember wether the engine was running or not. With the fuse blown, i had a look at my ampmeter and it was 'at the stops' so either charging or discharging the battery with mind-blowing current.


So, i had a look at the well known wiring diagram, see below. The fuse is between the starter switch and the junction box, if i understand everything correctly.


So, once the fuse blows and the engine is on, the system gets powered with up to 60A and n fuse will secure the wiring? That sounds like something to improve...

 

[Bruce of MN]

The Bratton's fuse holder comes with a 30A fuse. The starter is fed by the battery, so the fuse doesn't see that high current. It only sees high current if there is a short in the heavy cable to the battery or in the starter.

 

[TomInCologne]

The Bratton's fuse holder comes with a 30A fuse. The starter is fed by the battery, so the fuse doesn't see that high current. It only sees high current if there is a short in the heavy cable to the battery or in the starter.

yesm, but if the fuse is blown and the engine is running, the system is still 'fed' by the alternator, so the current is still high.


this is a problem with cars with alternators, if you set your generator to 10A, then this is not a problem...

 

[Jim/GA]

You are better off adding a fuse to the power supply wire to the headlight switch. Then, when you have a short in the wiring system of the lights, only that fuse will blow. My fuse in this wire is smaller than the fuse at the starter switch, so the fuse at the starter should stay good.

I also have a dedicated fuse in the power wire to the horn, for the same reason. If the horn fuse blows (due to a short in the "hot all the time" horn wire) nothing else goes dead. Only the horn fuse blows. The amp rating on this horn fuse is also less than the fuse at the starter switch.

As you say, the issue is exacerbated by you having a 60A alternator.

 

[TomInCologne]

one possible solution is shown in the attached modified wiring diagram.


positive: te complete car is protected


negative: if the alternator/generator fails you do not see that on the ampmeter



Edit: the attached image is pretty small, sorry. I routed the wire from the battery to the post on the starter, therefore all current from alternator AND battery has to go through the fuse...

 

[TomInCologne]

one possible solution is shown in the attached modified wiring diagram.


positive: te complete car is protected


negative: if the alternator/generator fails you do not see that on the ampmeter



Edit: the attached image is pretty small, sorry. I routed the wire from the battery to the post on the starter, therefore all current from alternator AND battery has to go through the fuse...

no, sorry, that won't work either... let me have another think about it...

 

[nkaminar]

Tom,

The danger with one fuse for the whole wiring system is that if you have a short the fuse will blow and shut down the engine. If you are in an intersection at the time that can be a problem. In my car I have dedicated fuses for different parts that are mounted in a fuse block. See link below.

With the one fuse the alternator should not be able to power the engine if the fuse blows. And even if it did the amperage would not be more than about 5 amps. There is something else going on in your car. The high amperage is an indication that the battery was supplying current to the short. The fuse was not protecting the wiring. What amperage rating is your fuse? Is should be 30 amps.

Here is one fuse block, there are others: https://defender.com/en_us/sierra-4...-source-fuse-block-with-bus-bar-cover-fs40910

 

[Badpuppy]

The original purpose of the 30A fuse was to protect the battery from discharging through the generator when the cutout contacts shorted across. With an alternator, this purpose is moot. It now only protects the battery from a dead short such as your headlight. Several guys have fused horn and lights separately, as Jim has. The Model A was not designed with fuses at all.

With a blown fuse the ammeter should read 0. One side is connected only to that fuse.

 

[CT Jack]

I agree with Neil's approach. I wire my ignition to the "in" side of the fuse. Doing this allows my engine to continue running after the fuse has blown. This method is especially important if you have electronic ignition. Isolating the higher power cosumming devices with fuses is a good approach for minimizing catastrophic failures. Also, using LED lamps greatly minimizes potential causes for blowing fuses.