Is this an alternator problem?

[fiatslug87]

1990 MJ, 4.0L, 5-speed. Cranked slow but turned over this morning, volt guage stayed down at 9, wipers were very slow, radio barley worked, and signals wouldn't flash. Is this an alternator or battery problem? Had to get to work so haven't checked anything yet. Any advice on how to diagnose is greatly appreciated. Thanks.

[Idiot Wind]

I am not going to be much help here but........

First thing I would do, if you could, is go to an Autozone-type shop, they may diagnose it free. Autozone I know does.

[joshbish]

Once your Jeep is started, disconnect your battery. If your car dies, then you have a bad alt.

[Beached Bones]

A couple simple tests

Put a voltometer on the battery and have someone start it up, if the battery has lost a cell, it will jump down to approx 10 volts while crancking and you'll see all those weird electrical problems. If/when it starts running, the voltage jumps up to 13v-15v the alternator is good.

Or... once Jeep is running, disconnect negative battery terminal. If the jeep dies, your alternator is dead.

[Spanky414]

I had a slow cranker this morning also.

I measure it right at the battery and it was showing 11.5 volts.

I put on the charger for half an hour and it was up to 12.70, it started fine then and the meter in the dash was showing 14 volts with no electrical stuff on.

How accurate is the dash one, what exactly is it measuring?

I'm also wondering if my battery doesn't like the cold weather or if my alt is going south and wasn't keeping it charged.

One other possiblity I guess is I left it in 4wd last night when I parked it but i can't see that drawing on the system unless there is a short somewhere in the switch for the mode light in the dash (242).

[Beached Bones]

I had a slow cranker this morning also.

I measure it right at the battery and it was showing 11.5 volts.

A good battery should hold more than 12v from a charge overnight.


You need to test it while cranking, this simulates the "draw" test that parts stores would put on a battery.

I put on the charger for half an hour and it was up to 12.70, it started fine then and the meter in the dash was showing 14 volts with no electrical stuff on.

How accurate is the dash one, what exactly is it measuring?

Dash one is notorioursly bad at giving a good measurment, it is downstream from a bunch of other stuff in the electrical, and it missed the quick jumps and dips a good voltmeter will give you.


I'm also wondering if my battery doesn't like the cold weather or if my alt is going south and wasn't keeping it charged.

No battery likes the cold, which is why they fail more often when the temps drop.


One other possiblity I guess is I left it in 4wd last night when I parked it but i can't see that drawing on the system unless there is a short somewhere in the switch for the mode light in the dash (242).

this should not draw any power at all.


All in alll I'd say it's the battery. But again, run a voltage under load to be sure.

[CaptTrev]

all good advice above but...

remember to check all connections on alt and especially the battery too!

[Spanky414]

all good advice above but...

remember to check all connections on alt and especially the battery too!

Yeah that was my first thing to check, I tried moving and tightening the connection.

What do you mean by run a voltage under load? Measure the voltage of the battery while its cranking like you stated or take it in to an auto parts place and get it load tested?

What should I get for voltage while its cranking?


What about those battery blanket things that keep your battery warm when its plugged in?

Can I trust the voltmeter on the dash? As long as its at 14 then the alt is good right?

[Matthew Currie]

Yeah that was my first thing to check, I tried moving and tightening the connection.

What do you mean by run a voltage under load? Measure the voltage of the battery while its cranking like you stated or take it in to an auto parts place and get it load tested?

What should I get for voltage while its cranking?


What about those battery blanket things that keep your battery warm when its plugged in?

Can I trust the voltmeter on the dash? As long as its at 14 then the alt is good right?


You cannot trust the voltmeter in the dash for an accurate voltage. However, if you know where it usually resides when running, you can pretty well trust that the alternator is all right if it remains at that level. If it has been running at an indicated 14 with a good alternator it won't get up there with a bad one. In many XJ's certain accessories such as the heater fan will draw the voltmeter down alarmingly when the battery voltage isn't low at all, which is why when you suspect problems you should always check at the battery. You should also be aware that the voltmeter is calibrated using a little magnet that's glued into its casing, and if it jars loose it may give wild readings.

[jeepinxj05]

I just replaced my alt and the gauge sometimes does this. Just rev the engine to about 5-5.5 rpm's and the gauge jumps back up to normal.