Advice to bypass later 'Y style' fusible link for testing/diagnostic purposes?

[swamp-thing]

Hey fellers,

I have a 1980 FJ40 & recently in the process of rewiring my alternator main wire into fusible links etc. because the old wires were cooked and the PO installed a non-oem fusible hack job solution.

Now, wiring close, I'm looking for advice from anyone who has bypassed the fusible link system (safely, temporarily) with replaceable, wired fuses for diagnostic, test purposes?
I know Coolerman sells a testing link used for trucks up through 78 ( single in line wire ) but my rig has both alternator main & Ammeter fuses running into the fusible link (it's basically a Y system rather than the inline found in earlier rigs) I've had a hard time reaching Mark for help.

My Ammeter is the 50 amp one vs 30 amp found on earlier rigs.
However the Amp wiring features two inline 5 amp fuses off the battery positive via fusible link.

I'm fixing someone else's old problem ( now my problem ) and before I throw a new fusible link into my repairs ( expensive to replace if it pops and downtime to get another in ) I'd love to test my work with a failsafe that would trip if there is an issue in my replacement wiring and I can make changes and test again until the result is satisfactory. One that is safe for my system overall that won't cause even more problems.

Eg. would I be good to just wire a 20 or 30 amp fuse into the alternator main to battery ( where the fusible link would normally live ) and then maybe a 5 or 10amp ( the ammeter fuses are 5 amp each ) off the amp meter connector to the battery ( also, where the fusible link would live?) If those hold after my fixes I'd feel good about putting the fusible link back in permanently. Understanding that the fusible link is a "slow" burn to fail and the fuses "POP" when amperage passes threshold therefore not a substitute for the fusible link system.

Sorry if this is confusing but maybe one of you legends has tried something similar.
And before you point it out, YES I know next to nothing about electrical stuff and my only goal is to have the truck back to oem the way it was....

Thanks!!

 

[CruiserInProgress]

I did some rewiring/correcting in that area on my 79, adding back in one of Coolerman's fusible links while verifying all related wiring against the schematic. If you've done all that, you should be good to go.

However, if you want to save your fusible link until you've tested that related, previously cooked wiring, Here's what I learned from Coolerman, who is the guru on the subject.

The fusible link serves two purposes as I understand it. Yes, it's sort of a slow-blow fuse between your alternator and battery, in series with the heavier gauge white w/blue wire. It melts within a protective covering when seeing too much current, and should save the heavier gauge wire it's in series with.

But it's also a shunt, with a small resistance per foot. When current passes through it (to or from the battery) a small voltage drop is created. The more the current, the more the drop. This voltage is what's measured via the 50a meter and moves the needle, ever so slightly.

Fyi, the 30 amp alternator and ammeter are not wired this way, forcing that full current to go through your cluster, rather than a small voltage with the 50a. My ammeter came melted, presumably because the PO didn't realize the 50a meter wasn't actually an ammeter.

So, yes, you could probably bypass (or eliminate) the ammeter function and temporarily substitute a fuse inline where the fusible link should be, for a quick test. You won't get any movement on the ammeter, but if you limit the bypass to just that, you could verify that alternator/battery wire won't fry.

Keep in mind, that heavy white w/blue wire goes to your ignition switch and fuse block as well, so you may want to add to your circuit in steps if practical, just in case the cooked wiring wasn't due to the incorrect wiring around the fusible link.

 

[swamp-thing]

Thank you @CruiserInProgress that is quite helpful info.
For what its worth the truck was running ok prior to me replacing crispy, hacky wires, and has not been started since — however one of my 5amp fuses on the ammeter leg off the fusible link was shorted out. I must admit, I never looked at the ammeter on the cluster to see if it did anything notable and with a blown fuse I doubt it even moved…… i looked carefully at the connections from engine wire harness to dash harness at the passenger duct location and saw no evidence of burn/overload/melt, however after hearing your input I will check the full length to cluster & fuse block. Thank you for that suggestion.

the simple goal is to not destroy a brand new fusible link only to find i missed something clear and obvious, but i also don’t want to ressurect old problems by swapping new wire and feeding a problem further down the chain — I will dig a little deeper to try to eliminate other variables.

 

[CruiserInProgress]

Thank you @CruiserInProgress that is quite helpful info.
For what its worth the truck was running ok prior to me replacing crispy, hacky wires, and has not been started since — however one of my 5amp fuses on the ammeter leg off the fusible link was shorted out. I must admit, I never looked at the ammeter on the cluster to see if it did anything notable and with a blown fuse I doubt it even moved…… i looked carefully at the connections from engine wire harness to dash harness at the passenger duct location and saw no evidence of burn/overload/melt, however after hearing your input I will check the full length to cluster & fuse block. Thank you for that suggestion.

the simple goal is to not destroy a brand new fusible link only to find i missed something clear and obvious, but i also don’t want to ressurect old problems by swapping new wire and feeding a problem further down the chain — I will dig a little deeper to try to eliminate other variables.

Good plan. Look for burned wires, and check for continuity, especially any possible shorts to ground on your related DC circuits.

If you have a blown ammeter wire fuse, that would suggest a lot of current went through it, either due to a miswire or a short to ground somewhere.

Something to look at, that cluster is easy to short to ground (to the dash opening) if ever pulled while power is connected.

Good luck!

 

[swamp-thing]

That blown fuse was on the white in-line and it’s rated at 5amps — I’m curious if that indicates excessive current BEFORE hitting the meter or does it indicate excessive current from the meter area — or is there no way to determine without out checking both directions?

 

[CruiserInProgress]

That blown fuse was on the white in-line and it’s rated at 5amps — I’m curious if that indicates excessive current BEFORE hitting the meter or does it indicate excessive current from the meter area — or is there no way to determine without out checking both directions?

That meter circuit definitely needs a close look, as well as the ammeter itself. Seems like something shorted on one leg or the other.

You might want to disconnect the leads from the ammeter to better isolate the problem. Curious to know if your ammeter fried in the process.

Another thought, if power was allowed to pass between the battery and alternator, and the fusible link was missing, but the shunt wiring was still intact, it would seem that current would flow through the meter, possibly frying it and the wiring to/from it.

Hard to diagnose from a distance, but you've got some things to check out. I can see why you might want to do a test run first.

 

[swamp-thing]

Sage advice thank you!