Roler

XJ always had a slowish crank. Gone through a few batteries over the years.
While working on another unrelated thing (efan) noticed my battery drops voltage from 12.6x (after sitting overnight) to under 12.3V when turning the key to RUN (not START). Battery load tested ok.
Once cranking, the dash meter briefly drops to 10+V and then comes back up. Always starts though. Runs and drives fine with a voltage around 13.8 whilst driving.
Batt terminals and grounds are all cleaned as part of regular maintenance.
Haven't done voltage drop test because the drop in voltage is prior to attempting to start.

The low voltage just prior to start would cause it to crank without much umpf I would say.

I know a small drop is to be expected prior to starting but not down to below 12.3V - ideas about the big drop?

awg

That must mean there is some strong current draw when the key is switched to "On"

I cant really think of anything that should cause that ?

a test light will not allow that much current, what happens then ? It will be glowing like mad..I like the old-fashioned incandescent ones)

(Multimeter may blow fuse if parasitic draw is over 10A)

If everything checks ok, that may mean you have duff main earth or battery cables/ends

Alternatively, how old is your battery. As little as 3 years can be their lifespan, although 5 is more like average

Finally, if your battery holds at around 12.55 V with no electrical load (disconnected) for a few days, it usually means it is ok (but not always)

I have had one duff cell, passed a draw test, failed hygrometer, another had a slight crack in the case, both failed within 1 year

Roler

Yeah I can't think of anything drawing that much with key On either...It's all stock setup

exasemech

Please forgive the stupid question, I know this isn't your first rodeo.

Have you confirmed these voltage changes with a remote voltmeter or is it just what you're seeing on the dash gauge.

It might help figure out what's going to determine if the voltage changes are really happening or if the problem is somehow to the feed to the dash gauge....or what's feeding the voltage regulator.

awg

I assume you are using a digital multimeter and analogue test light and voltage is measured with multimeter at battery terminals, and/or PDC

If the parasitic draw test light glows bright, disconnect the starter motor main cable, and the alternator main cable

These are 2 big suspects as they have a direct high current connection to battery, and it is possible for current to "leak" through them

How old is the starter ?

Roler

Yes this is done with a voltmeter- the dash gauge is just that and not reliable for diagnostic measures


awg- you mean use the test light "in-line"?
I didn't want to risk putting my meter in to measure what could be quite some amps.

Starter connections are clean but starter itself has a loose housing. New starter bought ages ago still needs to be put in...🙄. For some reason I keep postponing because I don't want to risk having to shim it if its out and atm I need it every day as my dd.

awg

Yes, do a parasitic current draw test by disconnecting the battery from + cable and interpose the Test light, which is the correct tool for auto electrical fault finding

Ideally it should be an incandescent, they draw current, the LED ones do not draw enough. A brake light globe is good, draws 2A, if you have a bad current draw it will glow brightly, the globe limits the current, (so as I mentioned in my earlier post, wont blow the 10A multimeter fuse)

The old style will begin to glow just a tiny amount if the current draw is over 50mA, which is about the max acceptable

If it passes this test, (does not glow) it will almost certainly be bad cables, or earths

A 2A draw will flatten a battery overnight though

IJM

Is this a situation where bad battery cables/connectors may be a factor?

97grand4.0

Seems like if he’s getting an immediate drop of .3vdc with key on engine off, that drop isn’t a function of bad connections, but clearly bad connections will hinder proper charging and starting.
To those who say parasitic draw, remember, this is with KOEO so the draw is not parasitic, ..it’s expected. Therefore you are powering on a lot of stuff, including but not limited to the ignition coil so you would expect to be drawing …im guessing maybe 10A there and would see some kind of voltage drop because of that. Parasitic draw refers to something bleeding off smaller amounts of current, like dome lights on, or an anti theft system that hasn’t fully gone to “ sleep” because of bad door lock switches thinking the door is open.
These years of Jeep don’t win awards in my book for the charging system, and the “ normal” 13.8vdc charge rate. I did the hack-in voltage regulator from a dodge truck in my ZJ and am much happier than with the pcm doing it. (14.3v) Another thing to look at is the battery temp sensor located under the battery.

awg

Good answer from Grand, but I would still start with a parasitic draw test...because you have to start somewhere

Yes the fuel pump and other PCM controlled accessories will pull current for a few secs, but then should become quiescent

However, it may still be misleading so you are right

Better would be my 60A analogue Ammeter, but I am thinking the OP does not have one

Perhaps it isnt even a fault, as you suggest

When I have finished fixing my suspension, I will measure mine, but I would be quite surprised if it dropped from 12.5 to 12.3

I agree the XJ charging system becomes very vulnerable as it ages, but so do most. PCM regulation is demanding when you read what is involved

A defective starter motor pulls huge current. It isnt too hard to drop your existing one, dismantle it and check the brushes. If they are very short, they can get stuck in holder etc etc

97grand4.0

Thanks AWG. ( American wire gauge ?)
Yeah where does one get a 60A DC ammeter at a reasonable cost? Clamp on, I assume? Electrician 30 years, still don’t have anything over 10A for DC.
Anywho, yeah, take your ammeter in series with the battery, and turn the key on and see what that pulls.

awg

All it is an analogue Ammeter out of a 60's Jag, reads up to 60A

Ironically. it is not the best way to read the state of a charge system, and most of the current for the vehicle flows through it and a mess of thick wires behind the dash

Some replace with the later voltmeters

I just used a spare out of a wreck, made some cables with clamps, only used it a couple of times

Roler

Yeah Ammeter is not in the arsenal here lol
The charging system is fine, alternator put in less than 3K ago.

I don't have much time the coming days to work with it, but I'd be really curious what others would find if they hooked up a voltmeter and see what happens over the battery terminals on Key On. Like mentioned, I just happened to notice it by accident and there's not a lot of info on this, what the draw on key On usually is and if what I have is abnormal or not. So it would be great to be able to verify!

awg

I am actually on the case here mate,

You would not believe it, but after my suspension work, it had flat battery symptoms, so I stuck it on the charger, and commenced initial investigation as to why

It turns out the "secret switch" the PO had hidden under the console, which I had found and fiddled with did do something, which was activate a relay in the engine bay, which is part of the aftermarket 5 relay setup some PO had installed, presumably for extra lights, and **** knows what else, as nearly all the wires are red or black

turns out this relay was pulling 200mA, (out of spec) once disconnected, it drops to ~15mA (in spec)

I measured the voltage drop from 12.5 to 12.3V when key switched to "on"

When I measure tomorrow morning, I will test that again

Unfortunately, with vehicles of this age, electrical problems are a bane. My XJ has been a bit of a cruel mistress recently, (original) fuel pump failed

Good practice when modifying electrical harness include such things as using appropriate color wiring, tagging any electrical harness you make with tiny yellow tags with permanent marker, and recording the circuit diagram

I am pretty sure this mess of wiring is what caused me to have to take out the headlight switch, searching for the cause of various headlight malfunctions

I say a hearty **** you to whoever done it