[sbc63]

i have a 98 jeep gc 4.0 ltr. stock.

i went to advanced auto to have my alternator tested an they hooked up a portable machine, the thing they told me is that the machine said my alternator was only pushing 22.5 amps and had 13.87 volts.

he said that was very very low of amps and i mite want to think bout getting a new alternator, i know he wasnt tryin to screw me cuz it was one of my friends who hooked it up for me.

so really b4 i rip the alternator out an have it bench tested on the accuall machine i was curious to wat some of your alternators are pushing amp wise, not voltage wise.

also i came home an hooked up a little electronic device that has amperage, voltage, etc and it shot the amperage above 80 off the scale of the machine..

so, i kno the little unit i used was screwed up cuz i thought stock 4.0 ltrs only had 60 or 70 amp alternators....

by the way the reason i had it tested is cuz when i pull up to a red light at nite an have my fog lights and headlights on my voltage gauge goes down to a lil below 12 an normal it runs a hair under 14. in the winter time it wants to stall with the heater on when i pull up a red light at nite.

i think that the low amperage mite be the reason for the jeep to be running like crap cuz everything isnt getting enough power.....

[Redbob]

It sounds like you do in fact have an alternator problem, if you trust the tester at Auto Zone. It is very possible to have a good no-load voltage reading, but poor amperage output. Your other symptoms tend to confirm that there's a problem: low voltage idling with a high load (like having all your lights on) is a bad sign, and low voltage can kill the engine when voltage drops too low to fire the ignition.
I'd ordinarily say, though, that an alternator should last much more than just 8 years - except for Chrysler products.
BTW, you might want to work on your spelling skills...

[sbc63]

sorry bout the spelling i type to quick and just spell things how they sound but anyways..

some other problems i am having is i can hear the ingine an feel the engine stumble at idle and it also misses going down the road.. its not a big problem im just a really picky person, its hardly noticable at all but i thrive on things running smooth.

i've noticed the problem more in overdrive then anything i also can hear it out of the exhaust.

thus, it could be 1 of 300 problems such as fuel filter, o2 sensors, clogged injector, coil, burnt valve, an so on an so forth

i narrowed it down to not being the plugs n wires i just replaced them about a month ago due to this problem, and i just did the cap and rotor today with no luck again..

thats why my friend an i thought it might be a voltage problem cuz one day while leeving work i jumped in after i started it from outside an bumped the gas an it backfired which instantly sparked a voltage problem in my head due to all the symptoms.

[greasefingers]

The 4L engine alternator has a capacity of 90 amps

[sbc63]

so if it has a capacity of running 90 amps why does the tester say 22.5

[greasefingers]

Not sure. I would assume either the test was not performed correctly or the alt is shot.

[sbc63]

i guess im just gonna have to rip the alternator out an have it tested. which is really going to suck

[jeep946]

before you go and do that did you have him test it under a load ( with ac on and fan on high. i ask because the pcm controls alt output. and if no load is on the system you will have 13 to 14 volts but low amps and with a load it will still be 13 to 14 volts but with higher amp readings.

[Redbob]

Originally Posted by sbc63

i guess im just gonna have to rip the alternator out an have it tested. which is really going to suck

Not necessarily.
First, is the battery in good shape, fairly new and of the proper capacity?
If not, it could be sucking all the alternator's output - but *assuming* the alternator test was done correctly, it would seem the alternator is one of your culprits.
Next, or more properly first, how many miles are on the truck? How long have you had it? Is it old enough that tired rings or bad valves could be causing the misfire?
Have you run a compression test?

It's logical that a problem with the ignition would be more noticeable in overdrive because that's when the rpm would be lower while the cylinder pressures would (should!) be high. Assuming that you didn't accidentally damage sparkplugs or wires while installing new ones, I'm stumped as to where to look next.
Rather than simply playing the parts change game, you might consider turning it over to a well-equipped shop and paying them to do the diagnostic work for you.

Tip: Don't tell the mechanic what you have done to the truck; if you've done something incorrectly, and you mislead him, sending him off down a blind alley, he'll be really annoyed - ask me how I know!

Good luck;
- R

[sbc63]

well i had an optima yellow top, cuz i have 3 tv's and a huge sterio system in there, but unknown to me optimas dont have a long life, so it had a dead cell in it, so i just bought a new battery, works good, less then a month old.

the jeep has 134,000 miles on it an i havent done anything to it except recently, which was cap, rotor, and plugs n wires.

the machine that he hooked up to it puts a load on it automatically, it checks battery, alternator and the diodes.

i've yet to do a compression test on the motor but i highly doubt it has badd valves or rings, altho it could be possible,

i also tried using some marvel mystery oil. if u dont kno wat that is, its basiccaly the same thing as sea foam, u put it in your brake booster vaccuum an it causes it to smoke like a blown head gasket, when i did this, it hardly smoked at all, i just figured the cat took care of most of the smoke..

when the car is sitting at idle, the motor stumbles n shakes , its noticiable but nott too badd, i no this has something to do with the problem but i cant figure out wat is causing it.

there are no codes being thrown either.

i have already popped my hood at nite an checked for arking around the plugs wires n cap so i can pretty much narrow that outta the picture

[jeep946]

check or replace the coolant sensor it could be lien to the computer causing a lean or a rich condition. i just had that happen to me, it was telling the computer the motor was running a 33 degs. so the computer was over fueling the motor. check for voltage changes in the map sensor. should be like 1.8 to 2.1 volts on a warm engine at idle. that would be where i would start at.

[SPL Tech]

First of all do not assume that Advanced Auto parts did the test right and don’t assume that they have the slightest idea of how to bench test an alternator. I do not have an Advanced Auto parts where I live but I have herd stores upon stores about how someone took their alternator to Advanced and they did not test it properly. I would not be suppressed if they tested it improperly with you too.

The stock alternator is 90 amps but it can easily put out over 140 for short periods of time.

What does your OEM voltage gauge say when your engine is at idle? That’s the very first thing to check. At idle on a warm day a voltage level of about 13 volts is nominal.

[greasefingers]

Optima’s are known to last longer than standard batteries.

If indeed you do need a new alternator then I would not replace it with a 90 amp one since you have all those Mods that tax the alt. Use a 120 amp or higher one.

[Woody]

If your Optima was bad, you should have had it replaced, they have a great warranty.

I think you are running too much stuff for your stock alternator to handle. Time for a Mean Green!

[Varkyl]

Just because the alt was only putting out 22 amps at idle doesn't mean it is bad. Remember it's not going to output its max at idle. Also remember the alt and its output are computer controlled, so there are alot of things like battery temperature that come into play when looking at the alt's voltage and amperage.

Your other problem is more than likely clogged or dirty injectors.
I just recently put new injectors in my ZJ. Before I had done that I had noticed that the idle was rather rough and thought it needed a tune-up. Well after the injectors were replaced it idles silky smooth.