Blowing fuses in 4 low

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Bjorn86

New member
I've searched and cannot seem to find anything. The past 2 times Ive put it in 4 low I have blown fuse #21 (20 amp) in the PDC. When this happened I was going uphill with moderate throttle applied. I can beat on it in 2 and 4 high no problem, even in 4 low on flat ground it doesn't occur, but once I get on it a little bit the fuse blows. I did a pretty thorough check on the harness underneath and in the engine bay and everything looked good. It killing me as I want to go wheeling but sucks when the XJ is reliably unreliable. Electrical is definitely my weak point. Any ideas?

Oh yeah:
99, aw4, 231, 4x4, lift, 4.56 gears, locked f&r.
 
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Bjorn86

New member
Fuse 21 is for the PCM/ignition coil. I know for sure when it blows the result is no spark. I have searched extensively for loose swinging wires as that was my first thought. I did find one old thread that someone was blowing the same fuse in their 97 when applying full throttle, even on the street in 2wd. They claimed a new alternator fixed it.
 
87manche

87manche

New member
I'd suspect o2 sensor wiring rubbing somewhere.

I guess I could see a bad alternator causing that at full throttle, the PCM controls the field wire, but I don't know why it would only happen in 4 lo.
 
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Bjorn86

New member
I've gone over the harness over and over and haven't found anything. I've let it run and moved the harness all over to see if I might make it blow again. I've even pulled the harness down that runs to the rear o2 sensor and above the trans and it looked fine. As mentioned I don't know why it's only in 4 low, but in trying to wrap my head around this alternator idea. I was thinking, this last time it blew I was on fairly flat ground trying to get my passenger front tire up and over a tree stump. As soon as I put some throttle into it, it blew but I was relatively still as I was trying to climb the face of the stump.
 
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