alternator works intermittently

I have a 96/4.0, I bought a HO alternator off of a friend of mine and took it and had it tested. It tested good about a year ago. I put it on the shelf while I finished my jeep. I have only got to drive it a couple of times after getting it out of the shop and when I start it the alternator shows about 12-12.5 volts, you have to run the rpm's up a little bit and it will go to 14 volts but if you put it in gear and the rpm's fall the alternator goes back to 12-12.5. When I was messing with it I had the headlights on and when I turned them off it went to 14 volts, turned them back on stayed at 14 volts. What sucks is that I had to drop it off at the exhaust shop today to get a muffler put on and I won't get it back until Thursday and I will be wheeling on Saturday. What should I be checking Thursday night and Friday. Battery is good it is a orbital battery.

Check and clean connections first - on the back of the alternator, primary grounds, things like that.

Primary grounds can be found on the fender liner near the battery, rearward end of the engine block (near the distributor, usually on the stud the dipstick tube bracket attaches do,) and the firewall ground strap. The studs and rings on the backside of the alternator should be cleaned just on account of 'cause, since you've replaced the part anyhow. Fine sandpaper, emery cloth, crocus cloth, Scotch-Brite - anything mildly abrasive should strip it clean. Rinse grit away using electrical contact cleaner (you should be able to find that at your local hardware store or parts house. Brake parts cleaner is an acceptable second choice.)

I remember that when I installed the alternator, one of the studs would only let me get the nut so tight it would start to spin. I am referring to the two tiny studs on the back of alternator. I tightened it as much as possible but it may not be tight enough. Is there a way to replace that stud?

before you condemn anything, clean all grounds and try same test. Turn on headlamps and heater blower. See if it still reads low. If it does, verify the voltage at the battery terminals before proceeding. Mine shows closer to 10.5V with all the accessories on, but is still putting 13.9V through the system. Poor circuitry to the instrument cluster can give you false, low readings.

Shorty said:

before you condemn anything, clean all grounds and try same test. Turn on headlamps and heater blower. See if it still reads low. If it does, verify the voltage at the battery terminals before proceeding. Mine shows closer to 10.5V with all the accessories on, but is still putting 13.9V through the system. Poor circuitry to the instrument cluster can give you false, low readings.

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I have checked it with a multimeter the voltmeter is close to the same. I was thinking the same thing.

Use flat washers to take up the slack, if you need to (I think the field coil connections - the pair of smaller studs - are either M4 or M5. The B+ stud - main output - is M6.)

But, it's amazing how many automotive electrical problems have come back to simple bad connections or dirty/failed/failing grounds, so start there.

For that firewall ground strap, you can get a "mid-length" socket (it's between a standard and a deep well, I think you need a 5/8" size) to get to that nut. You rapidly lose clearance with a deep well, and a standard just don't happen.