My alternator puts outs 13.65V at idle, 13.70 when revved, measured at the battery terminals

The battery measures 12.55V and is less than 3yrs old

It seems the starter motor has recently started turning sluggishly

It seems one of these 3 components might be on its way out

Is 13.70V too low ?

I do have a replacement starter motor on hand, so maybe its easier just to swap that in and check

dont know how long the SM and alt have been in there

 

The starter swap would be my first choice. It is easier than removing the alternator and then go somewhere to have it tested. Just my opinion though.

 

If you measure 13.7 volt at alternator, but only 12.6 volt at battery, that indicates that...
1. Battery is drawing down voltage, weak battery. See if battery charges properly or not using a plug in charger. or have battery load tested.
2. Bad connection(s) between battery and alternator. check both positive and negative paths, including the ground points as on engine block. High resistance in wiring is causing a loss in voltage between alternator and battery.

 

Hang on, you get 13.7 with the engine running, MEASURED AT THE BATTERY TERMINALS.

I assume the battery voltage of 12.6 was measured with the engine off, correct?

These numbers seem normal to me.

 

Yes, I think I will change out the starter first, as I have "new" one that I bought years ago, sitting in a box (hope I can find it)

13.7V engine running 12.55 when engine not running

I would expect 14V and 12.65V

 

Is the starter soaked in engine oil? If so, I'd replace it before it leaves you stranded.

 

An update.

Checked battery voltage this morning 12.25V after sitting a day, checked for parasitic drain, was ok

12.25V is to too low, but it started the vehicle ok

after a few short stops, it barely cranked, so I drove direct to my auto electrician mate nearby so he could load test the battery.

It failed the load test.

When we removed it, I could see what appeared to be battery acid on the tray and bottom of battery, not much, but I had this once before, a small crack in a battery caused it to fail

So I have a new battery, the previous one was nearly 3 years old, but you get unlucky sometimes

The starter motor seems to spin good now. It was clean of any obvious oil, but I degreased it anyway, my XJ leaks very little oil

Should know for sure in a week or so

 

Update number 2

Despite the old battery failing the load test, appearing to be leaking and the new battery maintains over 12.6V, the slow cranking issue returned,
so I broke out this little baby, an original AC-Delco that I bought NOS from a Jeep mechanic a few years back just for this moment



turns over like a 24V conversion now

I got a good price on it "not like those Chinese crap" he said.

Says on the box "made in China"

I have 3 spare alternators, all tested in my spare XJ, all show no Voltage and Gen light, so I will have to test them in my DD to see if they do work, or the PCM earth system in my spare XJ is in some way faulty

 

Oil getting into the starters causes issues.

 

I stripped the old SM and found the brushes quite worn, but one was stuck in its slider

I cleaned and lubricated all parts, freeing the stuck brush and assembled

It tested fine on the bench

$A30 for new brushes, I may buy a set, although brand new SM are quite inexpensive, its only so I have a working spare on hand

Not all XJ starters are the same, I also stripped another failed one.
It also had one worn & stuck brush. It had a bearing inside, and a shorter body, the other one had a bush and the body is about 1" longer