Hi, this is from another post, but probably deserves a full thread.
So I move the discussion here .

My 87 XJ Wagoneer has a fluctuating dashboard lightning.
The same for the the headlights, and also for the idling engine speed.
Something is wrong.

I looked at the battery. Here are the data
Car was parked for one full day and a half.
Measurements directly on the battery.
I don't have access to a professional to check alternator/regulator/battery.

Here are my data, with comments in bold from 97GRAND4.0 (thanks!)

- engine off (cold weather) : 12.65v If this was off for 8 hours, it would be good battery full charge.
- engine off + lights on : 11.88v looks like a weak battery
- engine on (cold, idling probably a bit low) : 15.18 to 15.21 (fluctuating a bit) Overcharging. Bad regulator. will kill the battery
- engine on (same conditions) + lights on : about 15.20 Same, overcharging. 14.5 max.

after 5 minutes, I went back to the car to get additional results with the engine revving.

- engine off was now : 12.19v Drops that much in 5 mins, bad battery
- engine on : 15.13v, increasing slowly and not steady (fluctuating) overcharging
- revving the engin : lowering to 15v when revved a bot hard. (The faster, the closer to 15.0v) <---- Why ?

>>> After thought, it could be your only problem is a bad battery. Regulator is charging at max to compensate

Any other way to know ?
- I added grounds (according to Cuiser's guide) is it possible that I did something wrong causing these values (over 15V) ?
- There is a circuit breaker on the ground coming from the battery. It is new and looks goods, can it be a source of issue ?

thanks for your comments and recommandations !

 

Explanation that might be useful

 

Looking at all what talks about over 15v at the battery, I understand I might have an issue with the Voltage Regulator.
Do you confirm ? (it could also be a bad battery by I have no issue to start up (except a slightly long crank))

- Do I have to change the alternator . ( someone has a reasonably priced reference for a 87 Wagoneer XJ Renix 4.0 ?)
- Can I just change the regulator or add an external one ?

I guess the alternator itself is Ok if it delivers over 15V.
I have no "start" problem, except a long crank (3 to 4 seconds)
But the lights + headlights fluctuate
And my Idle is not absolutely steady and too low (but not very bad either)

Thanks for your help.