Alternator wiring question/mysterious "black box" question

[richie]

I have this problem where I start the jeep ('93),the check engine light comes on.I am thinking I need a new alternator.But if I lightly tap on the molded plastic "black box" that is a part of the wiring harness,I can hear the alternator come alive.I was wondering what is molded into the harness inside this black plastic piece.Is it a diode of some sort,fusible link,? I do not want to cut it open but was thinking maybe I have some corrosion in there or something.After tapping it,the charge guage goes up to where it should be,just shy of 14 volts.Thanks,Richie

(yeah I tried search,still a 56k'er here :rattle: )

[outlander]

Yea it sounds like a bad connection somewhere.

[Matthew Currie]

Check all plugs for corrosion, and see if wiggling individual bits works as well as tapping.

Which "black box" are you referring to? It makes a difference in diagnosis. If tapping the alternator itself brings it alive, it might just be a bad connection but it's often a sign of worn brushes or dirty sliprings, a pretty easy fix. If you're tapping up at the relay/fuse box on the passenger side fender well, there are plenty of possible corroded connections, relays, bad hatrness splices etc. to narrow it down to. If you're tapping the main computer on the driver's side fender well, make sure the main plug is clean and properly installed. I've had moisture attack the lower connections on mine.