Needs the fusible link replaced after upgrading my alternator from 30A tot 60A?

[Troopie59]

I’ve got a 1998 HZJ75 Troopy (24V spec). I recently found out the original 24V 30A Denso alternator was replaced with a brand-new 24V 60A Denso alternator. They did mention the alternator was replaced, but didn’t say it was a 60A unit. I don’t mind the extra output, so that part’s fine.
I haven’t had any issues so far, but I’m a bit concerned that if the battery gets really low, it might draw a lot of current and blow the fusible link. Do I need to upgrade the fusible link, or would switching to something like an 80A MIDI fuse do the job? Or is it fine as-is?
The shop (a Land Cruiser specialist) said it’s no problem and that they’ve done this plenty of times, but I’m still not totally convinced. What do you think? Thx!

 

[Troopie59]

Well, I think I found the solution myself with some help from ChatGPT . The charging circuit doesn't stress the fusible link so it is safe. Schematic are a bit confusing because the high power cicruit is not on the schematic. Seems to be typical Toyota.

 

[Eurasiaoverland]

It should be wired through a fusible link. On my 60 Series the only direct connection to the battery is the starter.

Fusible links are matched to wire size, not alternator output. If the wire is rated to 40A with a fusible link and you put 60A through it and it blows, it has done its job. So the real question you want to ask yourself is whether you want to upgrade the wiring.

 

[Troopie59]

Thanks. It makes sense that the B+ to the battery from the alternator to the battery has a fused link, just for safety. AI wanted to please me I guess because in this case I rather liked it did not , just for convinience
In fact there are only two fusible links: one for the Glow circuit (a 2mm2 wire) and one called AM1 with a 1.25 mm2 wire (this is printed on a small black box near the battery).

It looks critical as I bought a new 50A DC-DC charger (Orion XS 1400) to charge a secondary battery. To save the alternator I will limit the current to 30A or so. This was the maximum output of my old alternator. Maybe total current will be 40-50 A (30 A charging and some other users which are not much on a old Troopy. Everything 24V. It seems just to the limit and may be over the limit when my starter battery is not fully charged. I tried to keep them fully charged though.

Everything combined I have to replace the fusible link with a 80A midi fuse I guess. Or is this not safe?

 

[Eurasiaoverland]

Everything combined I have to replace the fusible link with a 80A midi fuse I guess. Or is this not safe?

A fuse is designed to protect a wiring circuit by being the weakest link. If you put an 80 A fuse in a circuit rated for 40 A, what do you think will be the weakest link in the circuit (i.e. the thing that catches fire when excessive current flows through it)? Probably not the fuse. That's why all safety literature tells you never to replace a blown fuse with one of a higher rating.

Not all the fusible links necessarily go through the small junction box. Some will be via a simple plastic connector. If they are missing, I would check on a circuit diagram.

 

[Troopie59]

I took out the fusible link. I’m gonna swap the 1.25 AM1 for a 40A slow-blow MIDI fuse and the 2.00 GLOW for a 60A one. Hopefully I don’t set anything on fire. .

 

[HZJ75 FARMER]

In fact there are only two fusible links: one for the Glow circuit (a 2mm2 wire) and one called AM1 with a 1.25 mm2 wire (this is printed on a small black box near the battery).

If it is anything like the 12 volt models, there should be a third fusible link wire for the headlight circuit.

 

[Troopie59]

If it is anything like the 12 volt models, there should be a third fusible link wire for the headlight circuit.

Yes, you are right, but it is another connector, not the small black connector box with "Fusible link" on it and two wires connected with cable lugs. The previous owner of my vehicle did replaced that link with a blade fuse in a separate (not original) fuse box for some other accessoires. I didn't know it was for the headlights though. Nice to know.

 

[TreadingLight]

As @Eurasiaoverland pointed out, you're only looking at the fusible link, the failsafe, at the end of the chain. You should look at the entire circuit.

Your fuse might handle 60a better than the original fusible link would, but that just means the weak link is now your wiring harness.

In short, you probably need to upgrade the charge wire going to the battery. That is can be done by running a second cable, same gauge, from the alternator to the battery with its own fusible link (or fuse if you want to fit another fuse box).

 

[Troopie59]

I agree, an extra 10-16 mm2 (fuse protected) cable from the alternator to the starter battery is by far the best and safest option. Especially when using the full potential of the 60A alternator. So I consider that. But for now I will limit the current draw to 30-40A, mostly for a DC-DC charger. This is to prevent overheating of the alternator. But a 40A fuse for AM1 seems oke for the moment because the old alternator did already 30A and the cable could handle that. The glow plug circuit exceeds 60A easily, but only for a few seconds so a 60A slow blow (Littlefuse) Midi fuse doesn't seem a bad choice here.

 

[TreadingLight]

I think you may be playing with fire.

how are you going to limit the current to 40a? Is that a continuous load? And for how long? What happens if you have a 30 year old splice in the harness or a ground failing? Will a 40a fuse hum along just fine as your harness is failing?

For ideas about sizing midi fuses as "upgrades" to fusible links, look at what cruisertrash is building for FJ60s.

But personally, I'd keep the fusible link (or replace with a fresh one) and add a second cable. Meets all your needs and does so safely.

 

[Troopie59]

I just measured the current through the AM1 fusible link and it doesn't exceed 40A (mostly between 32A-35A, lights are LED-only) with the DC-DC charger setr at 30A input. I keep my starter batteries (2 x 12V in series) full. I am pretty sure this is safe for the wiring. What is not so safe is trying to max out the DC-DC charger to 50A without upgrading the wiring but 30-40A should be oke. The original alternator was also 30A. I don't want to go to 50A DC-DC anyway to keep the 60A alternator healthy during long charges.

Choosing the right fuses (type and value) is important and I learned more about that after starting this thread. I think cruisertrash you referred to shows a neat solution. I can replicate that by DIY.

As I said already, the suggested extra cable from the alternator is the best solution. It seems simple to do, but I have a very small B+ connector on that Denso alternator. There is no room fo a quality cable lug for a 10 or 16mm2 cable. Any suggestions are welcome.

 

[AirheadNut]

I'm in the process of replacing my original fusible links and the strip fuses from the VW engine harness with MIDI fuses in a BlueSea SafetyHub 150. I've found this document to be extremely helpful: https://www.littelfuse.com/assetdoc...ssetguid=c81c1a21-b903-44b6-b46d-180c48f7c3c5
The primary purpose of fusible links, as I understand it, is to protect the vehicle wiring from a catastrophic meltdown or even fire in the event of a full short to ground somewhere between the battery and fuse panel. They need to be slow blow to absorb the high inrush current from things like window motors without popping.
I came up with the following fuse sizes:

fusible link wire (mm)	fusible link wire (awg)	wire protected (awg)	fuse amperage
0.5	20	16	23
0.85	18	14	30
1.0	16	12	40
1.25	16	12	50
2.0	14	10	60

In my case I only have three of the original four fusible links to replace since I deleted the factory glow system, but also have an 80A fuse for the VW glow system and a 150A fuse for the 120A VW alternator. I'm currently researching options for slow-blow ATO style fuses since I only have four spots for MIDI fuses, I can put the smallest 23A fuse in an ATO socket.

 

[AirheadNut]

I just measured the current through the AM1 fusible link and it doesn't exceed 40A (mostly between 32A-35A, lights are LED-only) with the DC-DC charger setr at 30A input. I keep my starter batteries (2 x 12V in series) full. I am pretty sure this is safe for the wiring. What is not so safe is trying to max out the DC-DC charger to 50A without upgrading the wiring but 30-40A should be oke. The original alternator was also 30A. I don't want to go to 50A DC-DC anyway to keep the 60A alternator healthy during long charges.

Choosing the right fuses (type and value) is important and I learned more about that after starting this thread. I think cruisertrash you referred to shows a neat solution. I can replicate that by DIY.

As I said already, the suggested extra cable from the alternator is the best solution. It seems simple to do, but I have a very small B+ connector on that Denso alternator. There is no room fo a quality cable lug for a 10 or 16mm2 cable. Any suggestions are welcome.

What size is the B+ stud on your alternator? Is there an OEM application for that alternator that you could get the appropriate connector from?

 

[Troopie59]

@AirAheadNut: Thanx for the useful Littlefuse document. Littlefuse publishes also comprehensive datasheets about their MIDI fuses (for each value a separate datasheet with "blow times" versus current data). I think it is important to use fuses from a good reputable brand like Littlefuse, especially when replacing a fusible link with an equivalent fuse value. It sure must be of slow blow type like a fusible link. Not sure all those cheap Chinese fuses have the right specs if you can find them at the first place.

Those Blue Sea safety hubs you mentioned look interesting and better quality than I mostly come across. As is often the case for marine designs. The table you provided is very usefull for all people think about replacing fusible links by fuses.

About the B+ terminal. I will look at it later. What I remember is that the B+ cable connector is recessed a bit with a ring of isolation material around it. You have to be careful not shorting out something to the chassis if you don't use the right one.