Question about a old mad bypass repair

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mopar56

mopar56

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So several years ago my son and I did the " mad bypass" on his 70 Duster, at the time instead of using fuseable links we used two ATO style fuse holders with 40 amp fuses, sometimes one of the 40 amp fuses blows, you can always tell because the volt gauge only reads 12v when it happens. My question is...is a 40 ATO fuse not enough? Not equal to the fuseable link, OR could we have another issue? BTW once we replace the fuse, it'll be fine for several days usually.
 
40 Amps is enough in the sense that your car hasn't burnt to the ground. :lol:

I suspect it should be more like 60 Amps, but that depends upon your wiring condition and of course your alternator output, and the loading you place upon it.

A few variables to weigh up .......and it's difficult for anyone to make an accurate assessment without the complete picture.
 
Fair enough, this I a fairly stock duster as far as wiring goes with no excessive electrical load, no AC, or power options , I believe a 55amp alternator, I guess what I was getting at is considering that the car came stock with fusible links is there anyway of knowing what a compatible ATO fuse would be? I guessed a 40 amp fuse was close to a 14 gauge fuesable link?
 
The thing is, the OEM was AFU, both for wire gauge and for fusing, / fuse links. PERIOD, (except maybe for the earliest ones with a 35--37A alternator!!!!

I hope you are using much larger wire than original, and you need to at least size that by the largest alternator you ever expect to use, as well as fuse sizes.

The essence of the MAD swap, however you do your version, is this:

When you get through all the loops and twists in the harness, AND the bulkhead terminals are properly fixed, or bypassed, the old ammeter wires end up in PARALLEL to form one large wire which is now supplying power to the interior, AND there should be no charge current through the bulkhead anymore, which then, RELIEVES those wires of that part of the load which they formerly carried.

The main charge line should now be a nice big fused/ fuse link protected jumper, direct from the alternator output to the battery positive post.

If you have a 45-50 ish alternator, I'd use about 60A fuse on the charge wire
If you have a 65 ish I'd use 70-80

If you have an alternator larger, I'd make DARN sure the wire sizes are up to snuff, and on the charge wire, use 100Aw

If you have the through-the bulkhead circuit properly built with however you modded it, there should be no charge current. That fuse depends on accessories. EFI/ fans/ pumps, huge stereo, whatever. A bone stock car loads wise, you should be able to get by with 40

MAKE SURE the fuse is AWAY from engine heat which degrades amperage

Make SURE the fuse holder is QUALITY, as loose connections heat up the fuse and degrade rating.

Probably the best fuses are the so called "maxi" type or other fuses with screw/ stud connections.
 
Ok, thanks, I will need to review the work done years ago, but I believe other than the ATO fuse holders we used that we followed the " mad bypass" as it was posted on this site, we used 10ga wire so maybe we should increase to 8 gauge and go with maxi fuse and 50 amp, this is what I was thinking is maybe there is a small spike now and then causing our 40amp ATO to blow intermittently, I'll need to re-trace what we did, thanks for the advice as always.
 
The fuse link is a form of "slow-blow" fuse.
It'll take a surge and not blow, the ATO is "instant blow" .
I believe you can get a "slow blow/delay breaker" for the ATO holder, that may work .
 
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The fuse blowing is probably directly related to your alternator output. After the battery runs down following a start or a long crank, the alternator goes full field for a little bit trying to get the battery back up. (This is why alternator belts squeal when people fire up a car with a low battery) and can easily supply more than 40 amps. Usually around the alternator rating and sometimes more. @67Dart273 is right on again. You need a bigger fuse.
 
inertia said:
The fuse link is a form of " slow-blow" fuse.
It'll take a surge and not blow, the ATO is "instant blow" .
I believe you can get a "slow blow breaker" for the ATO holder, that may work .
Click to expand...
Yep, Many years back when Bosch began to design and build antilock brake processors, My brother and his QC crew ran quality tests on multiple brands of 30 amp blade type fuses. Most can pass as much as 40+ amps for some milliseconds before opening. Of course I can't recall any exact figures shown to me. Ambient temperature is a factor too. Placing these fuses in a cool location is the mfgrs engineers responsibility.
A fusible link multistrand wire of proper type and gauge of wire in a silicone casing is more dependable than a slither of metal incased in plastic or glass tube.
And what vehicle feature produces the most varying amp draw in our old beaters, The wiper motor. Pushing the wipers up uses more power than bringing the wipers down again. Because its a gear motor drive, a lot of the workload is absorbed by the gearing but design and subsequent wear in linkage/moving parts does increase the load on the motor. The fusible link will slowly get hotter and hotter and hotter without sufficient time to cool. While a blade type fuse would have blown, a fusible link could smoke and even catch fire.
The engineers found that the more fuses and relays, the better.
Do this, take a nice morning stroll through a large parts yard like LKQ. Take notice of fire damage. It's 99% under the hood. The fuses and relays do their job. Those newer vehicles running 100-amp alternators to power so many heat generating features don't burn the human beans.
 
RedFish said:
Yep, Many years back when Bosch began to design and build antilock brake processors, My brother and his QC crew ran quality tests on multiple brands of 30 amp blade type fuses. Most can pass as much as 40+ amps for some milliseconds before opening. Of course I can't recall any exact figures shown to me. Ambient temperature is a factor too. Placing these fuses in a cool location is the mfgrs engineers respo
Click to expand...

Great info, thank-you .
 
Yes thanks for all the info , o managed to locate 40 amp re-setable fuses that go into the ATO holder, supposedly they trigger slower than a conventional fuse I ordered a couple to try failing that I'm going to go with larger gauge wire and a maxi 50amp I'll post how it goes.
 
mopar56 said:
Yes thanks for all the info , o managed to locate 40 amp re-setable fuses that go into the ATO holder, supposedly they trigger slower than a conventional fuse I ordered a couple to try failing that I'm going to go with larger gauge wire and a maxi 50amp I'll post how it goes.
Click to expand...

A length of suitable "fuse wire", a coupla spade connectors properly attached, viola, "fuseable link" ATO ,
Make a coupla spares for the glove box . lol
Good luck, but I don't think you need it.
 
If this periodically happens every couple days you could be having something like voltage spikes, which could be from your voltage regulator. Make sure you are running a quality voltage regulator. Another thing you may want to do, as was suggested to me by slantsixdan, is to run a ground wire from your alternator case, to your voltage regulator to the negative post of your battery. This now puts your charging circuit all on one plane and should help with voltage spikes.
 
mopar56 said:
Yes thanks for all the info , o managed to locate 40 amp re-setable fuses that go into the ATO holder, supposedly they trigger slower than a conventional fuse I ordered a couple to try failing that I'm going to go with larger gauge wire and a maxi 50amp I'll post how it goes.
Click to expand...
I don't know what is available today but I do know what was in my day and probably still available. There was a component for a fuse box that looked much like the lid from a Zippo lighter. 2 blades to plug it in. Cycling circuit breaker, it was called. They were used to disconnect a circuit/power supply when overheated and close again when cooled. We have a similar device built inside our headlight switch and our wiper switches too in some models. Those at wiper switch were moved to outside the switch in other models just like the one at/on a power top switch.
Enough history chit chat so on to my point... These heat sensitive devices have a limited life span. They will cycle properly for some number of times (God only knows) but eventually they will fail totally. Then open is open. Time to replace it.
So, let's compare it to a band aide in more ways than one. Buy them by the box full. LOL
 
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