buix893

Hey guys, bought a 120 amp alternator at auto zone today for my '97 xj. Quick question...I upgraded the battery wires to #2 gauge. The alternator wire I'm using is #4 gauge..do I need a fuse Link like the stock wire had?..and if so were the hell do I find one that big

jmac108

U really should put one in line that wire is big enough that if it rubs through and grounds to something it could easily start a fire no one wants that. As far as the fuse goes not sure were to buy a fusible link but you could always got to a local auto store and pick up a ANL style fuse and fuse holder they are normally used for aftermarket stereo and u can get a fuse from 50 to 200 amps they work perfectly and there only like 15 bucks

buix893

Thanks...anyone else been in my shoes before?...I would use the stock wire but after putting 2 gauge wire in it seems kinda little compared to the rest

buix893

Anyone?

Radi

Fusible link is just a short section of wire 4 gauges smaller than the wire it protects.
For #2 wire, the fusible link will be #6. For #4 wire, the correct fusible link will be #8.
It should be made of wire with flameproof insulation and enclosed in a flameproof tube or sheath to keep it from contacting the chassis and causing a short if it blows open.

As to upgrading the wire.. the wire capacity should be sized to handle the intended load. A larger alternator alone won't increase the load, just the capacity.
Will you be adding any high-current accessories?

5-90

Not necessarily a fusible link wire (finding it larger than 14AWG is a pain, bigger than 12AWG is damned near impossible!) but some sort of fuse wants to go there.

AGU, ANL, and MEGA fuses are all in common use there. I offer ANL fuses because they're usually easier to find, and I don't like AGU because they're glass tubes.

Your wire size selection will be just fine - but you've bottlenecked your setup if you go much larger with the 4AWG alternator output lead. Be advised (however, since grounds should be the same size as the power feed - or the next size larger - you shouldn't have any trouble.)

Hit me backchannel if you want help with finding a fuse kit.

91 limited owner

Did Chrysler use different wire harness sizes in their production line. Because I have seen alternators (that Chrysler has offered) At different rated amp amounts. From 90 to 136 amp. My own came with a 130 amp alternator. Didn't they use one harness to cover all alternator amp sizes? It would of been more cost effective, to use only one harness in production. And this thread about increasing the wire size, made me wonder. Shouldn't the existing factory alternator wire harness be able to handle a higher amp alternator.

buix893

I just have a aftermarket CD player and stock style speakers...that's it, maybe I should just stick with the stock wire/fuse link for the new alternator

2kcherokee

Check out a car audio store, they have huge in line fuses for their amp power wires all the way up to a double zero gauge.... You will easily find what you're looking for there

5-90

No, they stuck with 6AWG for that lead for as long as I'm able to determine. It seems to be only PD vehicles (and other built emergency vehicles) that get anything larger - and then it's usually 4AWG, maybe 2AWG, depending on anticipated loading (yeah, I've worked on an awful lot...)

As far as the OEM wiring - bear in mind the following:
1) An alternator doesn't always generate max rated output - it will only generate what the regulator tells it to.
2) Due to the ongoing wars between Accounting and Engineering, engineers are forced to be rather optimistic in their design assumptions. The wiring is usually selected using the max available alternator, assuming a 100% output duty cycle of something like 30-40%, and figuring 100% duty cycle at 60-70% output. (The biggest surge in alternator output is usually the first five minutes or so that the vehicle is running, as the alternator has to supply running power and recharge the battery. Typical starter motor loading runs 120-150A for small gasoline engines, 250-280A for large gasoline engines, and can be well over 500A for larger Diesels! Our beloved 6-242 seems to be good for about 160-180A, on average - it's a relatively large engine, but it's also got a relatively low DCR.)

When I design, I usually assume 115% output at 100% duty cycle, except for certain circuits (the starter motor typically has a runtime duty cycle of 7-10% - or less - and a winch may heavily exceed the rating of the wiring, but typically has a duty cycle of 20% - tops - for hobbyists (probably 35-40% for enthusiasts, up to 50% for competitors, and SAR vehicles can see up to 70-80% DC for winch runtime. This is why I ask so many questions when I'm helping with design work...)

Why 115%? Easy - the actual max output of an alternator tends to run in the range of 95-110% of rated output, and the extra 5% gives me a bit of extra margin. I also use a nominal 12VDC when calculating - actual operating voltage runs more usually 13.0-14.2VDC, using 12VDC gives me an automatic safety margin of ~15%, which is standard. (Voltage and current are mathematically related in a closed circuit, cf. Ohm's Law, Watt's Law.)

Anyhow, with the assumptions I typically make, I get an automatic safety margin built into my calculations - and I don't have to think about it.

Radi

You'll be fine with the stock wiring. The alternator isn't going to put out any more current than your electrical loads demand. Use your money to upgrade elsewhere, lol.
Headlight harness is a nice choice for electrical upgrades.

JJCIII

Unless your adding amplifiers or anything like that there is no need to upgrade alternator wiring. In my case i have a custom alternator that can put out over 220 amps, charging a dual top and side post yellow top optima. The side posts i use for stock vehicle wiring. While the top posts i have my big 3 upgrade attatched. Which is a very nice setup.These upgrades let my 98 zj put out the 2400 watts rms that are installed without even a dimming headlight. With much more to be installed.All of these upgrades are very important especially im my case because i have a setup meant for pure SQL without forsaking SPL. Where voltage drop at loads that high can fry your audio components.

5-90

You will be fine with the stock wiring - until you start to use that extra capacity.

I don't say this because I do wiring upgrades, I say this because I've had to put out too many engine bay fires (various causes.) Every vehicle I've ever owned I've had fitted with a fire extinguisher. Every fire extinguisher I've used has been on someone else's vehicle - often, a quick post-mortem reveals that the problem was caused by "cheaping out" on something.

Don't be cheap - you may get away with it for a while, but it will jump up and bite you in the ******* eventually, nine times out of ten. (Some people are just lucky - I've got an uncle like that. He should have burned himself to the ground at least five times a year, but he hasn't. Just goes to show the luck that schmucks can enjoy...)

Radi

I think we're waltzing over the "stock wiring" comment. The stock #6 is NOT sufficient for a 120A alternator. In fact it's borderline deficient for the 90A OEM unit. The battery-alternator wiring in this case is no longer stock.
I was referring to the remaining OEM wiring in the XJ, which IMHO does not need to be upgraded, at least not until future accessories and loads are specified.

Here's my reasoning, for S&G's:
He's upgraded to a 120A alternator and replaced the battery cables with #2, the alternator output wiring with #4.
So far so good. Both should be more than sufficient right up to the alternator's capacity given their relatively short length.
12 volts @ 120A, 36" length of #4 wire: voltage drop 1.7% or 0.2V which is well within anyone's safety margins.
It's actually better than the factory's 90 amp alternator/#6 wire combo, which drops 2.5% at full load.


Since no additional accessories have yet been added, it's difficult to speculate on what else may need to be upgraded. High-current accessories like a winch, lights, amp, etc will certainly require dedicated runs back to the battery, not through the existing PDC or harness. So...as things currently stand, he's fine leaving the remaining OEM wiring, which is what I intended to say, in perhaps a less-than-specific fashion. Additional loads will have to be dealt with as necessary if and when the occasion arises.

5-90

You're on the right path - and, as long as the grounds are at least the same size as the power leads (meaning that the main engine ground should be min. 4AWG if the alternator output cable is 4AWG,) you'll be fine.

I don't think I mentioned - don't forget to fuse! ANL, MEGA, and I think AGU fuses are all available with ratings of 125 or 150A, which will cover what you need (the variation rule is something like actual output being 95-110% of rated output, at maximum, so a 120A can give you 114-132A at peak. This is from one of the long conversations with my own automotive electrical guru - who's been doing rewinds for 20-25 years now.)

I figure Engineering won that battle with Accounting (over putting a fuse in that circuit) for a good reason - and Legal may have weighed in on the side of Engineering! Every vehicle I've worked on (made 1955 or later - haven't worked on anything earlier...) has had a fuse or fusible link in that circuit, so it's a good idea to keep something there...