full battery, everything dead

I have a 1994 XJ with 180K miles 4.0L automatic. I am stumped on this one. Battery has 12V (confirmed from mulimeter) and all the lights are on the jeep when doors are open. 80 percent of the time put the key in the ignition and turn it forward and nothing. No gauges, no radio, no start. The other 20 percent of the time everything kicks on (gauges, radio, blower fan) and when you put the key forward to engage the starter you hear few clicks from engine bay, I am assuming its the starter am hearing, but not the normal starting noise then everything goes dead again, gauges, everything. Almost like there is a short but I don't know where to start

I do have headlight, dome lights, hazards. But no drive cluster, radio, blower fan, no ignition, and even in neutral its still dead.

Check the main harness connection on the firewall under the brake booster. It's a big connector with a bolt in the middle. If it comes loose, it will intermittently kill all the electricity in the interior.

check fuses?

Fuses aren't intermittent.

clean the battery terminals?

Clicking and not starting is usually a low voltage issue from:

• dirty, corroded, or loose wire connections
• internally corroded battery wires
• leaving the lights on
• a failing battery
• a failing alternator

Perform routine maintenance of the start and charge systems. Remove, clean, and firmly reconnect all the wires and cables to the battery, starter, and alternator. Look for corroded or damaged cables or connectors and replace as needed. Copper wires should be copper color, not black or green. Do the same for the grounding wires from the starter to engine block, the ground wires at the coil, and the ground wires from the battery and engine to the Jeep's frame/body. You must remove, wire brush, and clean until shiny the cable/wire ends and whatever they bolt to.

Jeeps do not tolerate low voltage, bad connections, or poor grounds.

Place your DVOM (Digital Volt Ohm Multi-Meter) on the 20 volt scale. First check battery voltage by placing your multi-meter's positive lead on the battery's positive post ( the actual post, not the clamp ) and the negative lead on the negative post. You need a minimum of 12 volts to continue testing. Next, leave your meter connected and take a reading while the engine is cranking. Record this voltage reading. Now connect your positive lead to the battery terminal stud on the starter and the negative lead to the starter housing. Again, crank the engine and record the voltage reading. If the voltage reading at the starter is not within 1 volt of battery voltage then you have excessive voltage drop in the starter circuit.

Typical voltage drop maximums:
• starter circuit (including starter solenoid) = 0.60 volt
• battery post to battery terminal end = zero volts
• battery main cable (measured end to end) 0.20 volt
• starter solenoid = 0.20 volt
• battery negative post to alternator metal frame = 0.20 volt
• negative main cable to engine block = 0.20 volt
• negative battery post to starter metal frame = 0.30
• battery positive post to alternator b+stud = 0.5 volt with maximum charging load applied (all accessories turned on)

Test the output at the alternator with your volts/ohms multi-meter. You should be measuring 13.8-14.4 volts. Load testing the alternator is still recommended.

Have your helper turn the ignition key to START while you tap on the starter with a hammer. If the engine starts, you probably need a new starter.

Have the battery, starter, and the alternator Load Tested for proper function in a test machine that applies a simulated work load. Handheld testers are inaccurate and will often pass faulty parts.

Thanks guys for every ones ideas, So I now had a starting point with every ones suggestions. I believe its the alternator as I took off the alternator plug (4 bolts) and it started right up then. Put the harnesses back on, its dead and wont start.

So I am not double guessing myself, my assumption is correct its the alternator then?

It sounds like something in your alternator harness is shorted. A bad alternator shouldn't prevent the charged battery from turning the engine over.

I drove home about 40 miles one day on a completely dead alternator and didn't know it until the battery died the next day.

It did turn out to be the alternator. It is odd, unless something in the harness got disturbed and its working now (hopefully not the case). But I bought a new alternator and jeep fired right up, guess old one had a short in it? Its holding at 14V on the gauge. Last time a few years ago I got a check engine light before it went. This was no warning, ran fine yesterday and dead today.

But thank you everyone for your help! I had no clue where to start this morning at all until you guys gave ideas of where to look!

Thanks!!!