I see you can use this same meter to test your charging system while running. Have you done that to verify your dash gauge acuracy? One other thing comes to mind is the condition of your starter. It could be a player in this scene.

 

My bad, I watched your video after my last post and see that you have verified you dash gauge.

 

I noticed it had the charging system part on there while testing the battery. To be honest I've only used this thing a few times to check lawn equipment batteries and haven't used that part of it. Should I do the same load/no load test with the engine running?

 

My money is on the alternator not being able to charge all the time. Could be the voltage regulator in the alternator, or the windings with the mod aren't up to the job.

The alternator voltage on mine will drop a couple when idling with a load on it. Sure isn't like my GM stuff, where the needle rarely moves. Maybe it's just a Jeep thing ?

Have you tried a different battery /

 

I use AutoZone top of the line batteries in all of my personal cars, trucks, SUV's, Jeeps, Custom Van and Diesel Pusher Motor Home. Plus 5 of our business diesel trucks 1T Ford Crew Cab's.

There batteries at that price level are among the best out there and warranty the very few times I have ever had to use them has been excellent and even upgraded if needed.

The only thing I don't use them in are my race cars because they are a different voltage battery setup.

 

Another possibility, could be the battery temperature sensor.
I know the later model XJs (I think after '96) have this.
It is linked to the computer, and has some affect on how much output the computer allows from the alternator.
In some cases they have been known to go bad, and affect charging of the battery.
I'm not sure this is the case with yours, as I have heard they may trigger a CEL, but it could be worth a check.


It is underneath the battery, and looks like a small spring-loaded button, that is held against the underside of the battery to measure the battery temperature.
If there was a bunch of crud or dirt in between it, and the battery, it could cause faulty readings?

 

I did think about the battery temp sensor, but it does seem to charge a bit more when the battery is cold, so I thought it might be working correctly. Maybe I could change that out and see if that helps.

The alternator voltage regulator is in the PCM on XJs newer than 1991.

I did some testing today, although I'm not sure it proved anything...

I swapped out my 150A alternator, which seems to make its power at higher RPM with a stock junkyard unit. The junkyard alt appears to be a Napa rebuild from 2007 according to the sticker.

This alternator seemed to help a little bit at idle, but I still have a pretty big voltage drop when I turn on the high beams/KC lights or the electric fan. I'm wondering if my battery just can't handle the load. My previous battery was an Optima yellow top deep cycle, so I don't really have another flooded cell battery to compare to. My friend has an Optima in his XJ, so maybe I will swap batteries with him and see what happens.

 

UPDATE:

So I did some more testing today. My buddy also has a 2000 XJ, so it helps to have a vehicle to swap known good parts with.

We swapped his Optima Yellow top battery over to my Jeep and had the same voltage drop as I did with my Duralast battery. That tells me the problem is not the battery, and I'm thinking it isn't the alternator since I have used 3 different ones. Tomorrow night I will swap alternators with him since his is a known good factory one and test that.

I have a feeling it is the voltage regular in the PCM or the battery cables causing these issues.

When I first installed my beefier battery cables (which go to grounds, starter, and main fuse box), I took the Jeep for a test drive to the hardware store. When I came out from the store, it didn't want to start and acted dead with the first turn of the key. I then turned the key again and it slowly turned over and started. I went home and re-checked all my connections and everything was tight and clean. I haven't had it turn over anywhere near that slow since, but the cables could be the problem. They are significantly larger both in wire gauge and connector size, so I might not be making a perfect connection even though they are tight.

 

How old is your starter? Some starters have more torque than others. Sometimes the brushes or armature gets worn enough that performance is lost. Food for thought.

 

The starter is the original. It only cranks slow on the first start of the day. It cranks right over otherwise. If the starter was the problem wouldn't it crank slow all the time?

 

Not always. You could take the starter off, pull it apart, check the condition of the brushes and armature. A pencil eraser works well for cleaning the armature. Replace the old thickened grease/tar with some fresh stuff. Replace brushes if possible and reassemble, reinstall. (or just get a new starter and keep the original on the shelf for a back-up)

 

Nice thanks for the info, I might try that when I get some time. I did have a nasty oil filter adapter leak for a bit before I fixed it, so I can only imagine what the starter looks like inside.