Fuse Block instead of Fusible Links

[Torq_Shep]

I am currently re-doing the engine side harness on the comanche as mine got munched on by critters. Repaired a few wires (properly with correct colors, solder, heat shrink, etc). But I have a feeling that my "G" fusible link on the header panel harness may be blown.

 

I am highly considering pulling that harness out and refreshing it as well. However, is there any reason not to just use a fuse block instead of fusible links? Especially if I am hunting electrical problems due to rodents and expect to blow at least another link if I am not careful? 

 

I know the "slow burn" vs immediate blow is a thing but if the purpose is to protect the harness, and the harness is clean and likely isnt going to be a source of significant resistance (anymore) could I just run a small fuse block safely?

[eaglescout526]

Hmmmm you got me thinking. I wouldn't know if their would be cons on this idea but I'm gonna definitely keep my eye on this one. I don't see why a small block wouldn't be a better viable replacement for the links but that's just me.

[Minuit]

This is one of those subjects where no matter what kind of answer I give, someone will have a contrary, "better" answer. I tend not to touch these subjects as a result. It's about like asking what's the best oil to use or asking about putting platinum spark plugs in a 4.0. But I have plenty of time now, so here goes.

 

The replacement fuses will be sized according to the wire gauges present in the individual circuit and the expected current draw in each one. There are a variety of standards for this, and I'll let you choose which one you decide to conform to. That being said, I believe it's very important that electrical work above all else conforms to a recognized standard and not just what Uncle Bubba Joe thinks looks like about the right size.

 

So - I can't tell you what fuse to put in. You need to find that out yourself through study and measurement. But it's not something to be taken lightly. For the fuse to do its job and protect the truck in the event of a short they must be appropriately sized based primarily on the smallest wire gauge in the circuit and expected current draw. Additionally, these fuses must be located as close to the voltage source (the battery/alternator) as possible.

[Torq_Shep]

7 minutes ago, Minuit said:

This is one of those subjects where no matter what kind of answer I give, someone will have a contrary, "better" answer. I tend not to touch these subjects as a result. It's about like asking what's the best oil to use or asking about putting platinum spark plugs in a 4.0. But I have plenty of time now, so here goes.

 

The replacement fuses will be sized according to the wire gauges present in the individual circuit and the expected current draw in each one. There are a variety of standards for this, and I'll let you choose which one you decide to conform to. That being said, I believe it's very important that electrical work above all else conforms to a recognized standard and not just what Uncle Bubba Joe thinks looks like about the right size.

 

So - I can't tell you what fuse to put in. You need to find that out yourself through study and measurement. But it's not something to be taken lightly. For the fuse to do its job and protect the truck in the event of a short they must be appropriately sized based primarily on the smallest wire gauge in the circuit and expected current draw. Additionally, these fuses must be located as close to the voltage source (the battery/alternator) as possible.

Initially what I was planning was to find the standard for each "gauge" fusible link amp draw rating at 12-14v and try those sizes. If I blew them at that point either go back to a fusible link or try one size higher fuse. If the size higher still blew I need to check long and hard for a short and if I couldnt find one go back to fusible link. 

 

But again it will all be subjective. I just don't understand why they didnt just use a fuse block in the first place. 

[coheed]

I replaced my fusible link with just replacement fusible links. Seems like a lot of extra steps when you can get that same AWG link and replace it with the same length and be fine.

[Torq_Shep]

I am 50/50 split on doing it. If I pull that harness that would be the time to do it for sure. 

[coheed]

Another thought is that links will take up less real estate in the bay than a box, even a small one.

[Minuit]

40 minutes ago, Torq_Shep said:

Initially what I was planning was to find the standard for each "gauge" fusible link amp draw rating at 12-14v and try those sizes. If I blew them at that point either go back to a fusible link or try one size higher fuse. If the size higher still blew I need to check long and hard for a short and if I couldnt find one go back to fusible link.

That's how a desirable, rapidly appreciating classic Jeep gets burned down. I hate to be abrasive, but this $#!& is important. The fact that you think that's an acceptable course of action tells me you need to slow your roll and do some more research. The way your post is worded tells me you don't know for sure if the fusible link is intact or not. Grab a meter and check across it for continuity. If the fusible link has continuity, you're looking in the wrong place anyway. Fusible links go open circuit, usually with some level of fanfare, when they blow. If it failed, it's more than likely because of a short to ground somewhere, but sometimes it's just good old fashioned corrosion. In any case, the correct thing to do is never "put in a bigger fuse and see what happens"

 

Electrical work isn't done on feel. You need a wiring diagram and some basic electrical troubleshooting tools. Test light, multimeter, fused jumpers. Start checking every circuit that doesn't work like it should with a multimeter and/or test light. Check for power and ground at each accessory, find out what's missing. Go down the line until you arrive at your problem. If multiple things don't work, see if they share anything.

 

The reason a Renix Jeep doesn't have an engine bay fuse box is this: our Jeeps were designed back in the early to mid 80s. Back then, a lot of what we consider standard practice for electrical system design wasn't a thing. Sealed connectors. Switching things with relays instead of 20 ft of wire carrying full current to a switch and back. Bus bars that aren't just "connect everything to the starter relay." Engine bay fuse boxes and power distribution centers. None of this stuff was standard fare back in the day, and the improvements made in this area in the 90s by Chrysler are one of the most important reasons why the 91 and newer Jeeps are just straight up better than the older ones.

 

Look at the wiring design of 60s and 70s American cars and look back at your '89 Jeep. You'll see a lot of similarities.

[Torq_Shep]

28 minutes ago, Minuit said:

That's how a desirable, rapidly appreciating classic Jeep gets burned down. I hate to be abrasive, but this $#!& is important. The fact that you think that's an acceptable course of action tells me you need to slow your roll and do some more research. The way your post is worded tells me you don't know for sure if the fusible link is intact or not. Grab a meter and check across it for continuity. If the fusible link has continuity, you're looking in the wrong place anyway. Fusible links go open circuit, usually with some level of fanfare, when they blow. If it failed, it's more than likely because of a short to ground somewhere, but sometimes it's just good old fashioned corrosion. In any case, the correct thing to do is never "put in a bigger fuse and see what happens"

 

Electrical work isn't done on feel. You need a wiring diagram and some basic electrical troubleshooting tools. Test light, multimeter, fused jumpers. Start checking every circuit that doesn't work like it should with a multimeter and/or test light. Check for power and ground at each accessory, find out what's missing. Go down the line until you arrive at your problem. If multiple things don't work, see if they share anything.

 

The reason a Renix Jeep doesn't have an engine bay fuse box is this: our Jeeps were designed back in the early to mid 80s. Back then, a lot of what we consider standard practice for electrical system design wasn't a thing. Sealed connectors. Switching things with relays instead of 20 ft of wire carrying full current to a switch and back. Bus bars that aren't just "connect everything to the starter relay." Engine bay fuse boxes and power distribution centers. None of this stuff was standard fare back in the day, and the improvements made in this area in the 90s by Chrysler are one of the most important reasons why the 91 and newer Jeeps are just straight up better than the older ones.

 

Look at the wiring design of 60s and 70s American cars and look back at your '89 Jeep. You'll see a lot of similarities.

I completely agree with you. Which is why I broke down and ripped the engine harness out after finding chewed wiring just to make sure I didnt have any more critical issues. I didnt even consider a fuse block until I was at the parts store and I couldnt find readily available links and got to thinking about just switching to a fuse block. 

 

The wiring sizes and fusible links were engineered that way for a reason. I understand and respect that. However, fusible links are old school and more difficult to diagnose (marginally) then just throwing the meter on the posts of the fuse. I have not done enough research on the subject of swapping them although if I pull the front harness also "ALL" of the links are being replaced with new links (or possibly fuses). 

 

[Minuit]

3 minutes ago, Torq_Shep said:

The wiring sizes and fusible links were engineered that way for a reason. I understand and respect that. However, fusible links are old school and more difficult to diagnose (marginally) then just throwing the meter on the posts of the fuse. I have not done enough research on the subject of swapping them although if I pull the front harness also "ALL" of the links are being replaced with new links (or possibly fuses).

I think fusible links suck and should be done away with. That's not what my previous post was aimed at. My previous post was aimed at this:

 

Quote

Initially what I was planning was to find the standard for each "gauge" fusible link amp draw rating at 12-14v and try those sizes. If I blew them at that point either go back to a fusible link or try one size higher fuse. If the size higher still blew I need to check long and hard for a short and if I couldnt find one go back to fusible link.

 

Fusible links are outdated, but replacing them with increasingly larger fuses until they don't blow anymore when you have not verified the circuit's integrity is not an acceptable way of doing it. You need to put together, at a bare minimum, a list of each fusible link protected circuit and the gauge of every single wire in that circuit. There are recognized standards for fuse sizing based on wire gauge, expected operating amps, and other factors.

 

I looked at your build thread to get more details of what your problem was that led to the belief of a failed fusible link.

 

Quote

Turns out ai probably blew a fusible link and I can power the fuel pump by jumping either pin 30 or pin 87 to 12v so I have a short. Damn mice probably did a lot more damage than I thought. 

This means that the relay is energized (if jumping either pin to 12V allows the fuel pump to run) but has an open circuit between the battery and terminal #1 which corresponds to pin 30 of the relay. The most likely suspect for this is fusible link 10-2 under normal circumstances, but with rodent damage anything is possible.

 

You want to avoid messing with fusible links if you possibly can for now. That fusible link leads to a 10-pin connector on the right hand shock tower through a red 10-gauge wire at pin #5.

According to the FSM, the part of the connector with rounded edges leads back to the battery via the starter relay and fusible link. The squared off side of the connector leads to a splice which branches to the fuel pump relay. I don't know off the top of my head what color this connector is or exactly what it looks like, but it should resemble the diagram.

 

Unhook the battery. Multimeter on low range ohms (or continuity beeper if your meter has it), test between the 10-pin connector pin 5 "rounded" side and the positive battery terminal, and then test between the other side of that connector and terminal #1 of the fuel pump relay socket.

 

Both sides should show near zero resistance, but one of those tests will probably reveal an open circuit. Report back what you find. If there is an open or high resistance between the + battery terminal and pin #5 of the "rounded" looking part of the connector as the diagram shows, suspect the fusible link but look for chewed wires in that area. If there is an open between the fuel pump relay socket and the "squared" half of that connector, suspect a chewed wire at some point between the connector and splice or between the splice and fuel pump relay socket pin #1.

 

Clear as mud?

[Eagle]

2 hours ago, Torq_Shep said:

I completely agree with you. Which is why I broke down and ripped the engine harness out after finding chewed wiring just to make sure I didnt have any more critical issues. I didnt even consider a fuse block until I was at the parts store and I couldnt find readily available links and got to thinking about just switching to a fuse block. 

 

The wiring sizes and fusible links were engineered that way for a reason. I understand and respect that. However, fusible links are old school and more difficult to diagnose (marginally) then just throwing the meter on the posts of the fuse. I have not done enough research on the subject of swapping them although if I pull the front harness also "ALL" of the links are being replaced with new links (or possibly fuses). 

 

 

There's nothing wrong with using fuses. A fusible link is a fuse. That's not the problem with your idea.

 

Where you went off the rails was when you described your plan. You said you would research what size fusible link each circuit needs and replace the fusible links with the same size fuses. So far , so goo. BUT ... then you said if the fuse blows, you would increase the fuse size until it doesn't blow. THAT's the problem.

 

In any circuit, the wire size is selected based on the current to be carried. The fuse size is then selected to protect the wire. If you have a circuit that carries 17 amps, it's probably wired with the appropriate gauge of wire for 20 amps and protect by a 20-amp fuse (or fusible link). If you keep blowing 20-amp fuses and then replace the fuse with a 30-amp fuse -- now the circuit isn't proteted, and the entire length of the wire becomes a de facto fusible link. It's like in the old days when houses still had screw-in fuses. A fuse would blow, they didn't have a replacement fuse, so they put a penny in the socket. The lights went on ... and then they wondered why the house burned down.

[Torq_Shep]

56 minutes ago, Eagle said:

 

There's nothing wrong with using fuses. A fusible link is a fuse. That's not the problem with your idea.

 

Where you went off the rails was when you described your plan. You said you would research what size fusible link each circuit needs and replace the fusible links with the same size fuses. So far , so goo. BUT ... then you said if the fuse blows, you would increase the fuse size until it doesn't blow. THAT's the problem.

 

In any circuit, the wire size is selected based on the current to be carried. The fuse size is then selected to protect the wire. If you have a circuit that carries 17 amps, it's probably wired with the appropriate gauge of wire for 20 amps and protect by a 20-amp fuse (or fusible link). If you keep blowing 20-amp fuses and then replace the fuse with a 30-amp fuse -- now the circuit isn't proteted, and the entire length of the wire becomes a de facto fusible link. It's like in the old days when houses still had screw-in fuses. A fuse would blow, they didn't have a replacement fuse, so they put a penny in the socket. The lights went on ... and then they wondered why the house burned down.

Well that is a little more extreme than I had in mind. Only upsizing if it blew due to any of the "slow blow" handling spikes in amperage is not the same as just keep putting in bigger fuses, but I digress. 

 

Managed to wrap my engine side harness today (side without links) after repair of the rodent damage. 3 of the wires going to the fuel pump relay had significant damage so I am hoping that was my issue. If not I will be using Minuit's direction to track down my problem and then likely pull the front harness (headlights and other crap) and assess that one as well. I am suspicious of the circuit going to the fuel pump resistor and need to check that with the meter as well. 

[Eagle]

2 hours ago, Torq_Shep said:

Well that is a little more extreme than I had in mind. Only upsizing if it blew due to any of the "slow blow" handling spikes in amperage is not the same as just keep putting in bigger fuses, but I digress. 

 

 

Doesn't matter why you replace it. The wire is sized for the load, and the fuse is sized for the wire. If the fuse blows, find out why and fix it. Using a higher capacity fuse is not a fix.

[Ωhm]

Fusible links can survive better in a under hood environment (weather wise). If you choose fuses (extra connections), make sure they're well protected. My 0.02¢

[derf]

One other option is to replace the fusible links with automotive circuit breakers.  Some of them are "slow burn" style and will handle the startup surge without popping.

 

Honestly though, I like fusible links.  They don't take up a lot of space, they're easy to connect, and they do their job without any fanfare.

 

You can find the gauge of the fusible link in the factory service manual wiring diagrams easily, and order the fusible link wire online for next to nothing.

[derf]

One other thought.  I seem to remember that automotive "slow burn" fuses exist.  I don't know how easy they are to find but they should work to replace a fusible link.

[derf]

Something like this would be a good option:

 

https://www.mcmaster.com/slow-blow-fuses

[BeatCJ]

Yes, slow blow blade fuses are available. But some quick searching shows that Maxi  Fuses were developed to replace fusible links, and are available where the higher draws are required. I am going to replace the fusible link in my charging circuit with a fuse, but undersize for my circuit.

[AZJeff]

On 4/27/2020 at 2:23 PM, Minuit said:

 

The reason a Renix Jeep doesn't have an engine bay fuse box is this: our Jeeps were designed back in the early to mid 80s. Back then, a lot of what we consider standard practice for electrical system design wasn't a thing. Sealed connectors. Switching things with relays instead of 20 ft of wire carrying full current to a switch and back. Bus bars that aren't just "connect everything to the starter relay." Engine bay fuse boxes and power distribution centers. None of this stuff was standard fare back in the day, and the improvements made in this area in the 90s by Chrysler are one of the most important reasons why the 91 and newer Jeeps are just straight up better than the older ones.

 

Look at the wiring design of 60s and 70s American cars and look back at your '89 Jeep. You'll see a lot of similarities.

I am an old guy, and the first cars I drove were from the 1960's and tended to be hand-me-down Ford products from the family.   Fords in those days did use the "starter relay bus-bar" approach, but on the other hand, they used mostly water-resistant connectors.  Fords were FULL of 'bullet' connectors back from the late-50's until the late 80's.

Why AMC didn't use this approach with their connectors back then is a mystery, unless it was just pure economics.  It's not like bullet connectors were new or proprietary in 1984 when the XJ was introduced.

[Torq_Shep]

So I definitely do not have an 8 pin connector like that. I have a 10 pin one that is similar but no red wires coming from the fusible link to that connector. 

 

From what I can see so far is the fusible link to pin 1 appears intact. <1ohm. But not sure why I no longer have a fuel pump unless I jump pin 87 direct to 12v.

 

I am going to start it by jumping the relay again to make sure I got the harness installed correctly and then pull the front harness because I found what looks like yet another chewed wire on that side...

23 hours ago, Minuit said:

I think fusible links suck and should be done away with. That's not what my previous post was aimed at. My previous post was aimed at this:

 

 

Fusible links are outdated, but replacing them with increasingly larger fuses until they don't blow anymore when you have not verified the circuit's integrity is not an acceptable way of doing it. You need to put together, at a bare minimum, a list of each fusible link protected circuit and the gauge of every single wire in that circuit. There are recognized standards for fuse sizing based on wire gauge, expected operating amps, and other factors.

 

I looked at your build thread to get more details of what your problem was that led to the belief of a failed fusible link.

 

This means that the relay is energized (if jumping either pin to 12V allows the fuel pump to run) but has an open circuit between the battery and terminal #1 which corresponds to pin 30 of the relay. The most likely suspect for this is fusible link 10-2 under normal circumstances, but with rodent damage anything is possible.

 

You want to avoid messing with fusible links if you possibly can for now. That fusible link leads to a 10-pin connector on the right hand shock tower through a red 10-gauge wire at pin #5.

According to the FSM, the part of the connector with rounded edges leads back to the battery via the starter relay and fusible link. The squared off side of the connector leads to a splice which branches to the fuel pump relay. I don't know off the top of my head what color this connector is or exactly what it looks like, but it should resemble the diagram.

 

Unhook the battery. Multimeter on low range ohms (or continuity beeper if your meter has it), test between the 10-pin connector pin 5 "rounded" side and the positive battery terminal, and then test between the other side of that connector and terminal #1 of the fuel pump relay socket.

 

Both sides should show near zero resistance, but one of those tests will probably reveal an open circuit. Report back what you find. If there is an open or high resistance between the + battery terminal and pin #5 of the "rounded" looking part of the connector as the diagram shows, suspect the fusible link but look for chewed wires in that area. If there is an open between the fuel pump relay socket and the "squared" half of that connector, suspect a chewed wire at some point between the connector and splice or between the splice and fuel pump relay socket pin #1.

 

Clear as mud?

 

[BeatCJ]

15 hours ago, Torq_Shep said:

From what I can see so far is the fusible link to pin 1 appears intact. <1ohm. But not sure why I no longer have a fuel pump unless I jump pin 87 direct to 12v.

Wait? Isn't 87 supposed to be 12 volts out? If you have to jump 12 volts to get the fuel pump to work, doesn't that mean you have a problem with the relay?

Do you have power at the appropriate times at the other terminals?

[Torq_Shep]

That is correct. However, if I jump pin 30 it also powers the fuel pump. So I think I have a short somewhere. 

 

 

[BeatCJ]

I don't have a wiring diagram, but in my beady little mind, 30 should always be hot. So 30 to 12 + is open. If you provide 12 volts at 30 is the fuel pump always on? If so, then it sounds as if your relay is stuck closed, too. It sounds to me like the only thing you can say for sure is that the circuit from the relay to the pump is good.

 

I guess I'm a little confused. I would probably test the relay. 30 to 87 should be open without power to 86 and 85 grounded. Power 86 and 30 to 87 should have continuity.

Or maybe I am going too far, your relay is fine and you just don't have power to 30, and you just need to fix that circuit.

[Torq_Shep]

So I did not have power to 30 but if I applied power directly to 30 from the battery it also made the fuel pump run (that is with the relay out). Which to me, seems like something is shorting in 30s circuit back to the fuel pump. 

 

I did find some more screwed up wiring on the other half of the harness and I am going to start tearing into that tonight or tomorrow. Once I repair all of the wires I will reinstall the harness. 

[Ωhm]

With KEY out and everything connected, check for voltage on the following pins:

D1_5 Hot (B+) at all times.

D1_6 Nothing.