91 XJ keeps blowing alternator fuse

So for the last few weeks, I've been having to drive my xj everywhere, because my daily driver also broke down, and I haven't had time to fix it. So today on my way to work, the alternator stopped charging the battery, and I only barely made it to work before it quit.
So I replaced the Alternator, since this one was pretty old and crusty. Even with a new one the thing still wasn't charging, so I started checking fuses, and found that my 60A Alt1 fuse under the hood was popped. Replaced this fuse, and checked with the mulimeter, and got 13v! Within a few miles, however, the idiot light came on again, and checking with the voltmeter showed 10v. Dangit.
So I limped it home, and stuck another fuse in, and with the voltmeter on the battery, wached my voltage as I gradually increased the engine RPM. It went up to 13.5v, then I increased the RPM a little more, and POOF! Big orange flash, and the fuse blew, but also made quite a big flame!
This time it melted the crap out of the fuse, and crustied up the box pretty badly.
I've checked wires coming from the battery and alternator, and grounds going from the battery to the body and engine block, and all are nice and clean, as are my battery terminals.
Anybody have any idea what to check next? I would have expected a large voltage spike as the fuse blew, but never got it.
Please help, my only transportation is my motorcycle now...

Did you get an alternator with the same current output? If it’s a 91, I think the ECU regulates the alternator, no? I think it was the RENIX era XJs with the internally regulated alternator. Maybe you can find a diagram of that circuit and check the continuity to the ECU. I’m not a doctor, but that’s where I would start.

Thanks. I will do that and also look into wiring in an external regulator.

What makes you think its overcharging versus something drawing too much current? 13.5 volts isn't overcharging. It's rare that the ECU is bad, and usually the symptom is a blown driver and no charging versus overcharging.

What's the battery voltage look like after a full charge? A shorted cell in the battery could also result in high charging current.

My 94 has two 60A fuses for the Alternator, one at the very front of the PDC and one in the middle.

https://www.cherokeeforum.com/attac...erokee-interior-fuse-box-95-20pdc-20cover.jpg

I ended up cleaning all of my under-hood grounds, as well as every large positive connection, and thoroughly cleaned my battery terminals, even though those looked ok.
Before doing this, I checked resistance between the negative battery terminal and the engine block, and it was like 690 M ohm, so I already knew that was a huge problem. After cleaning the grounds, I was able to get this resistance down to 0.2 Ohm, and the same between engine and body, and body and negative battery terminal. I also cleaned all the oil sludge gunk off of my starter and related cables.
I'm not sure which thing I cleaned actually did the trick, or if it was the combination of all of them, but the maxi fuse is no longer blowing, even when revved to the rev limiter. The wires going to the fuse aren't even getting hot, so that is promising.
I even went wheeling on Saturday and on some muddy hill climbs had to exert the engine a bit, and never had a problem, even on the hour drive to the trails.
Hopefully this means it is fixed!
It is now charging at a stable 13.6V, with very little variation with higher rpm, will go up to 14v or so @ ~2500 rpm.
Thanks for your help, folks! I wouldn't have figured bad connections would cause this.

Jim Malcolm said:

My 94 has two 60A fuses for the Alternator, one at the very front of the PDC and one in the middle.

https://www.cherokeeforum.com/attac...erokee-interior-fuse-box-95-20pdc-20cover.jpg

Click to expand...

My 91 has both fuses, even labled the same, but the 2nd one is just a spare on mine and not connected to anything.

On this vintage of XJ, both of those fuses are used in parallel to give 120 amp capacity to the circuit. 90A alternator + 25% wiggle room (22.5), rounded up (112.5) gives you 120A minimum wire capacity / fuse rating. If you pull both fuses, you should have battery voltage on 1 of the terminals. If you measure the resistance between the two terminals and it's not 0 nominal, you have issues, one of the wires is broken. I am absolutely certain of this.

"if you measure the resistance between the terminals that do not have battery voltage on them and it's not 0 nominally, ..."

Jim Malcolm said:

On this vintage of XJ, both of those fuses are used in parallel to give 120 amp capacity to the circuit. 90A alternator + 25% wiggle room (22.5), rounded up (112.5) gives you 120A minimum wire capacity / fuse rating. If you pull both fuses, you should have battery voltage on 1 of the terminals. If you measure the resistance between the two terminals and it's not 0 nominal, you have issues, one of the wires is broken. I am absolutely certain of this.

Click to expand...

Good point. If it's pulling enough to blow one fuse, it should have blown both.

Something else you may want to try is to ground the alternator body directly to the frame & body. IIRC there is already a tapped metric hole in the alternator body that you'll need to find a metric screw for.. Make-up a heavy gauge wire, with eyelets on each wire end, and attach the wire to the alternator body and grounds to the body and frame. Awhile back someone on this site mentioned the metric screw size, in the alternator body, to do this tip.

Best regards,

CJR

Jim Malcolm said:

On this vintage of XJ, both of those fuses are used in parallel to give 120 amp capacity to the circuit. 90A alternator + 25% wiggle room (22.5), rounded up (112.5) gives you 120A minimum wire capacity / fuse rating. If you pull both fuses, you should have battery voltage on 1 of the terminals. If you measure the resistance between the two terminals and it's not 0 nominal, you have issues, one of the wires is broken. I am absolutely certain of this.

Click to expand...

I need to look closer at this! My alt2 fuse didn't have voltage on it, and didn't have a terminal for both prongs on the fuse.
I've driven the jeep about 200 miles since I got it to stop blowing fuses, with heat, radio, headlights, and wipers on.
I didn't even put a fuse back in the alt2 slot, as I thought it was just a spare on mine.... woops....

Well, that shows how observant I am. The fuse slot that didn't have both terminals in mine was for an ABS system that I don't have. So now I have put another 60A fuse back in the actual alt2 slot. I guess I got lucky to not blow a fuse with only half capacity! I guess with both fuses and clean connections, I should be in good shape.