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marginal alternator?

METAL MONKEY

METAL MONKEY

NAXJA Forum User
Location
severance, co
I'm trying to decide if I need to replace the alternator or not. It is a '98 w/ the 4.0. Here is the story.

Back in January, wife couldn't get the jeep started. I go out and there is no power what so ever. It had been pretty cold, we've only had the jeep for a few months, didn't know how old the battery was, so I just assumed get a new battery. I go buy a red top and assume all would be well. Fast forward to this week. Wife comes in and tells me the same thing, no power. So, I give the jeep a jump, let it run for awhile and turned it off. It idled just fine, and seemed good to go. We traded cars, she left, and I come back out a few minutes later to leave for work. Jeep would turn over, barely, but wouldn't fire. Tried giving it a jump again, no go. So, I took the battery in to test, and it is still good.

So the question is, do I have a alternator on the way out or a parasitic drain? Here are the measurements.

Test for drain:
1 - 2 mA. Probably not the problem. :dunno:

Voltage measurements at battery:

Jeep off - 12.6
Jeep idling - 13.6
Me throttling up jeep - 13.6
Jeep w/ everything ON - 12.6 (e-fan kicking on) - 13.6.

Never once did I hit 14V. What do you guys think?

Thanks
 
Fuel90 said:
I'm trying to decide if I need to replace the alternator or not. It is a '98 w/ the 4.0. Here is the story.

Back in January, wife couldn't get the jeep started. I go out and there is no power what so ever. It had been pretty cold, we've only had the jeep for a few months, didn't know how old the battery was, so I just assumed get a new battery. I go buy a red top and assume all would be well. Fast forward to this week. Wife comes in and tells me the same thing, no power. So, I give the jeep a jump, let it run for awhile and turned it off. It idled just fine, and seemed good to go. We traded cars, she left, and I come back out a few minutes later to leave for work. Jeep would turn over, barely, but wouldn't fire. Tried giving it a jump again, no go. So, I took the battery in to test, and it is still good.

So the question is, do I have a alternator on the way out or a parasitic drain? Here are the measurements.

Test for drain:
1 - 2 mA. Probably not the problem. :dunno:

Voltage measurements at battery:

Jeep off - 12.6
Jeep idling - 13.6
Me throttling up jeep - 13.6
Jeep w/ everything ON - 12.6 (e-fan kicking on) - 13.6.

Never once did I hit 14V. What do you guys think?

Thanks
Click to expand...

Your alternator is fine (it don't need to hit 14VDC - nominal system charging voltage is 13.6VDC,) so you might want to have a look at your battery again. If it has removable caps (to top off the electrolyte,) check the cells with a hydrometer - I'm almost willing to bet you'll find one of them low.
 
5-90 said:
Your alternator is fine (it don't need to hit 14VDC - nominal system charging voltage is 13.6VDC,) so you might want to have a look at your battery again. If it has removable caps (to top off the electrolyte,) check the cells with a hydrometer - I'm almost willing to bet you'll find one of them low.
Click to expand...

It's a optima red top. All connections looked pretty good.

I did forget to mention that a couple of weeks ago, the negative and ground wire had pulled loosed from the terminal. I only drove it for a few minutes until I got home and fixed it. My harness seems to be pretty tight, so I plan on replacing the battery cables to loosen it up a bit.
 
Fuel90 said:
Never once did I hit 14V. What do you guys think?

Thanks
Click to expand...

due to 97+ having a battery temp sensor, mine rarely gets over 14 volts. mine gets over 14.0 first thing on a cold morning but once the battery warms up the pcm knocks the volts on mine down to 13.54. if you live in a warm climate yours will never get over 14v. mike
 
If you run an optima down then you need to trickle charge it back. A jump or a fast charge doesent seem to hold very long. It's the nature of the beast. They need a trickle charge once in a while.
 
The 98''s symptoms of a bad alternator is usually the guage going to 0 and the "Check Guages" light going on. I had an internittent one, and it would sometimes act up right off, and some days days would pass. A restart would sometimes clear the problem, but it would come back. Your readings appear near normal. Standard vehicle voltage is 13.8V. I suppose that if you had a shorted diode in the alternator, the voltage could read low, but not low enough to set off the PCM's charge error.

If you have those cheap replacement cable clamp ends, I would replace the cables, as those are sooner or later, a problem. Also I would check the engine to chassis cable to make sure it's OK.

Other than that, the PCM is the regulator. If the voltage guage reads normal, all the time, I would say you have battery issues. Even Optimas go bad.

Ron
 
x99j said:
mine gets over 14.0 first thing on a cold morning but once the battery warms up the pcm knocks the volts on mine down to 13.54. if
Click to expand...

Ahhhhhh..... OK then. Mine does exactly that also!!! :clap:
 
If your Jeep sits for weeks at a time without being run; certain circuits create a small amount of draw that will slowly deplete the battery over time.....
The standard lead plate acid bath battery appears to tolerate a slow draw and recharge better than the Absorbent Glass Mat battery.
A couple years ago my Jeep was parked in the garage for an extended period of time; I found the Optima was almost dead and even after a week of driving... the battery still was responding properly.

During troubleshooting this electrical problem I found that when the Jeep sat overnight the voltage would settle at 12.1 with the older battery and when running at engine speeds over 1800 rpms the voltage was 12.9-13.1. With the new Optima battery the voltage overnight would be 12.6 and when running over 1800 rpms; the voltage was 13.6-13.8.

Now... IF my Jeep sits for an extended period of time (> month)... I disconnect the ground cable.
 
Last edited:
ehall said:
Replace the battery cable terminals, that's what caused my last no-start problem
Click to expand...

This is my next step. I'll be sending an order to 5-90. I didn't have any problems yesterday. Took a few trips around the hood, start/stop, etc. Hopefully, it won't occur again 'till I get my new cables.

Thanks for the help.
 
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