How many amps to relay? Anyone Know?

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TrailBeast

TrailBeast

AKA Mopars4us on Youtube
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Yesterday I was driving down the road and the car quit.
I carry a test light in the car so I find out there is no main power to the relay from the battery because of a cheesy fuse holder in the line between the two.
I'm sure it used to be a fusable link, but it was replaced by one of those twist together white fuse holders.
I'm going down to the local auto store for a new section of wire and a blade type fuse holder that is a sealable unit.
Question is, what amp fuse should be in that line?

Thanks guys

If I don't get the answer soon, I'll put a 20 in it and get a 30 also and start with the 20.
 
The fusible link doesn't supply only the starter relay. Everything inside and coming back out of the cabin is fed through that link from headlights to tag light.
A maxi fuse may handle the load.
 
I needed to get this taken care of before it got hot, so I just got back with a new piece of red 10 ga wire, heavy duty ends (soldering them) and a 30 amp breaker.
That outta do it.
I have blade fuses inline in most everything, so an upgrade seemed to be in order.:D
Thanks again.



RedFish said:
The fusible link doesn't supply only the starter relay. Everything inside and coming back out of the cabin is fed through that link from headlights to tag light.
A maxi fuse may handle the load.
Click to expand...
 
No, a 30A breaker probably will NOT do it. Redfish said it. The fuse link carries ALL current (except the starter) to and from the battery, including CHARGING current unless you have performed the ammeter bypass.

Look at the simplified MAD diagram:

Follow from the battery, to the start relay to the fuse link, where it branches off "to starter"

NOTE that there is NO OTHER PATH except through the fuse link.

This means that if you have a 50, 60, 80?? A. alternator, ALL of that goes through the link

Follow on down, through the ammeter to the "welded splice" This is where ALL LOADS in the car split off.

The fuse box "hot" buss

The headlights

The ignition feed

ETC

So if you were in a "dark and stormy night" and you just happened to have "everything", lights, heater, wipers, your great big stereo, ?? turned on, and the engine was idling with no charging current, ALL of "that stuff" would be going through the link from the battery to all those loads.

BUT when you rev it back up and your ?? 80A ?? alternator surged to life to bring the battery back up, NOW you have all that charging current going through the link to the battery, so to speak.

IF YOU HAVE bypassed the ammeter, you need to figure what the total amp draw of all accessories are that goes through the link. You "might" get by with 30, but I think that's iffy.

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

amp-ga18.jpg
 
Makes sence.
The amp guage is bypassed.
I have a 65 amp alt.
Just a in dash stereo, (No amp) and I don't drive it at night.
I do however have electric cooling.
Still I will get a 60 breaker just in case and throw it in the trunk in my box O stuff.

Sorry about calling you Redfisk, Redfish.
Typo


67Dart273 said:
No, a 30A breaker probably will NOT do it. Redfish said it. The fuse link carries ALL current (except the starter) to and from the battery, including CHARGING current unless you have performed the ammeter bypass.

Look at the simplified MAD diagram:

Follow from the battery, to the start relay to the fuse link, where it branches off "to starter"

NOTE that there is NO OTHER PATH except through the fuse link.

This means that if you have a 50, 60, 80?? A. alternator, ALL of that goes through the link

Follow on down, through the ammeter to the "welded splice" This is where ALL LOADS in the car split off.

The fuse box "hot" buss

The headlights

The ignition feed

ETC

So if you were in a "dark and stormy night" and you just happened to have "everything", lights, heater, wipers, your great big stereo, ?? turned on, and the engine was idling with no charging current, ALL of "that stuff" would be going through the link from the battery to all those loads.

BUT when you rev it back up and your ?? 80A ?? alternator surged to life to bring the battery back up, NOW you have all that charging current going through the link to the battery, so to speak.

IF YOU HAVE bypassed the ammeter, you need to figure what the total amp draw of all accessories are that goes through the link. You "might" get by with 30, but I think that's iffy.

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

amp-ga18.jpg
Click to expand...
 
You probably have a bad connection at the bulkhead witch is causing high amps, if it were me I would do the bulkhead bypass like the mad dia, go on the website and read it. I did the upgrade on my 66 coronet, and it works flawless and still looks stock, you wouldn't believe how fast the blinkers work now.
 
crlush said:
You probably have a bad connection at the bulkhead witch is causing high amps, if it were me I would do the bulkhead bypass like the mad dia, go on the website and read it. I did the upgrade on my 66 coronet, and it works flawless and still looks stock, you wouldn't believe how fast the blinkers work now.
Click to expand...
He did the bypass....says so right in his post above yours....
 
Right, but depending on how the bypass was done, let's say all he did was jumper the two ammeter connectors together "or not" and ran a big charging wire from the battery to the alternator

This means (in the diagram) BOTH the black (former charging wire) AND the original RED battery lead now act in parallel as "battery feeds into the car.

BUT if either/ both of the bulkhead connectors for those two wires are "iffy" the red one (fuse link) especially, could make enough head to blow the link

ALSO if you do this bypass in this manner, if the black charging wire is left in place, and IF the bulkhead feedthrough is repaird, YOU NO LONGER have the protection of a fuse link, because current can flow-----

from battery -- through bypass wire-- (battery to alternator) -- back through charging wire through bulkhead -- to feed system.
 
It is not as easy as matching amp output of alternator. The current interrupter has to be able handle a short duration over load without blowing. If you put install a 60 amp fuse it will interrupt the current flow when it reaches 60 amps, bang is opens.

But what if a series of loads total 70 amps for whatever reason for a second or so you ask. Don’t forget the battery is capable of supplying 600 to 900 DC amps or so to a dead short, so alternator output is not a reliable target for fusing DC.

This is why Chrysler, and other manufactures back in the day designed circuit protection with fusible links, a short length of smaller gage conductor covered in special nonflammable insulation placed somewhere between battery, and bulkhead connector to protect under wire harness on dash side of fire wall. A fusible link has the ability to handle a short duration current overload by dissipating excess heat before self-destructs.

One should use a rather large slow blow fuse of an undetermined rating probably in the 100 to 150 Dc amp range. This will protect harness from a dead short, but carry enough juice to sub circuits, allowing the fused sub circuits to do their job. The problem with our cars is there are several devices and circuits that are not over current protected, and are directly hot-wired to the battery with only the main fusible link for protection. Sometimes, one of these circuits will short to ground, and because they are of smaller gage wire than the fusible link, those circuits burn up before the fusible link can melt.
 
Frankly, WJ, the fuse link was never "much." A TRUE story

Not long after I bought my 70 sixpack RR, some girl a-holed it, I ended up with a junker /6 Valiant "loaner."

One morning (it DOES get cold in San Diego once in awhile, ice!!! on the mud puddles!!!) the Leading Chief came into the shop and asked (imagine Detective Columbo, eyebrows knitted, terrible look on his face)

"Say, are those loaner cars supposed to squeal and smoke like that?"

So I go out an yup, the alternator is stopped, the oily, loose, worn-out belt smoking and squealing around and around. I shut it down, took a rag and removed the belt, and tried to turn the alternator

IT TURNS OUT that a diode had fallen out down inside, with the wire!! still!! attached!!

When I moved the pulley, the diode shorted (main charging wire) and I watched as the entire underhood harness turned itself into a smouldering, melted, hot gooey mass of plastic and copper, and AFTER it had done that, "phfssszzssstttt" the fuse link blew!!!!
 
When I moved the pulley, the diode shorted (main charging wire) and I watched as the entire underhood harness turned itself into a smouldering, melted, hot gooey mass of plastic and copper, and AFTER it had done that, "phfssszzssstttt" the fuse link blew!!!!
Click to expand...


I did say thouse things disapate heat befor melting... LOL
 
wjajr said:
Sometimes, one of these circuits will short to ground, and because they are of smaller gage wire than the fusible link, those circuits burn up before the fusible link can melt.
Click to expand...

Exactly.

Anyway, I drove it all over the place yesterday with wipers, lights and stereo all on at the same time with the 30amp breaker without issue so I'm going to leave it for now. (Will still get a 60 for the "Box O stuff" in the trunk just in case.)

I feel better about the harness with as low amp of a main breaker as I can get away with.
That way if a lighter guage wire shorts the breaker may save it from flaming up.

I can fix most anything anyway if need be, but a fire is pretty hard to fix.:D

Thanks guys.
 
If you can get away with it , great.

Bear in mind NONE of the wiring going to or coming off the ignition switch is fused, except for the accessory feed, which goes TO the fuse panel.

So if something shorts in the ignition harness..........................
 
The 30 amp main should assist with that though, right?
Since it is the main cabin power feed.

67Dart273 said:
If you can get away with it , great.

Bear in mind NONE of the wiring going to or coming off the ignition switch is fused, except for the accessory feed, which goes TO the fuse panel.

So if something shorts in the ignition harness..........................
Click to expand...
 
Exactly. My point was that the only protection for those circuits was the fuse link, which was way oversize for the wiring in them.
 
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