Jump to content

alternator question

Recommended Posts

I went out to start my 89 mj a couple days ago in the morning to drive to work and no electronics were working. I assume dead battery, jump it and it ran fine a few days until this morning its dead again. I do have a sound system with a 2000 watt amp and run ac and my voltage has been in the red while driving since, so i assume my alternator is tired and can't keep up.

If I'm wrong on assuming that, let me know but I'm gonna get it checked anyways. But assuming it is bad, should i go for a 100a upgrade from advanced auto parts or will that have the same issue possibly?

Link to comment
Share on other sites

1st test the battery to see if it has discharge going on it, could be bad and not holding a charge.

2nd test alternator. many new battery charges can test alternator and the battery.

Most alternators put out 60 to 90 amps at 1500 to 2500 rpms. Of course to recharge what it takes to start the car normally need to run the truck for 10 to 15 minute. can be more or less in speeds. idling does not really put enough charge out to restart unless your idling for a vast amount of time. Take in mind if your using a 2000 watt system and using 1000 watts that is about 83.3 amps. So it is pushing what your alternator can handle. During those times your using half your power of 2000watts your using both battery and alternator. if you don't shut system off and allow to recharge you will end up with dead battery in no time.


If your going to push your truck like that, best bet to get a larger alternator. They have 200 amp and higher alternators that will do for the power you need.

Link to comment
Share on other sites

battery and alternator testing is free at most auto parts stores. :thumbsup:  


if both test ok, I would just drive without banging the sound system super loud for a few days and see what happens.


also, clean all your grounds and battery cable connections.  be sure to upgrade those if you upgrade the alternator.

Link to comment
Share on other sites


You can also do the old school test, start the truck then pull the positive battery cable, if it dies your alt is bad.

I wouldn't recommend this, you can fry electronic components very easy doing this " test "
I have never encountered any problems doing this on any of my three RENIX MJs, which the OP has.
Link to comment
Share on other sites

  • 2 years later...

I know this is a dead thread but just wanted to say it was a short that was draining my battery over night and turns out my gauge is off in the voltage. 


Just posting an update for anyone with a similar issue

Link to comment
Share on other sites

I'm not sure if this is true or urban legend, but this is the concern:

With the battery in the circuit, voltage from the alternator is "stabilized", if there should be a surge in voltage it will be absorbed by the battery. Sort of a built-in "surge protector".

If the battery is disconnected with the engine running, as soon the battery cable is disconnected, the alternator's output may spike upward, before the voltage regulator has a chance to adjust the voltage downward. This voltage spike, at least in theory, could be damaging to electronics.

Personally, I would avoid removing a battery cable with the engine running. I have done so, from time to time, years ago, and always got away with it.  Looking back at it I probably shouldn't have.

Besides, it's really unnecessary. You can get a cheap voltmeter at Harbor freight for about five dollars, and this gives you way more information.

If you want a real quick check to see if the alternator is charging, with the engine running just hold a screwdriver or wrench or some iron or steel object right by the alternator case. If the alternator is charging, there will be a strong magnetic field, which will attract the metal. If the alternator is not charging, there will be no attraction.

Hope this helps!


Link to comment
Share on other sites

I have used the batter cable pull method many times with no problems.....maybe the warning is for more modern hi tech ecu's that we have.....


but to answer the question better....get a DC volt meter.....with the truck off what is the battery voltage.....should be more that 12....I like 12.3-12.5......now start the truck and take another reading.....a healthy alt will read between 12.7 and 14.5....lower is no good .....(probably brushes)....higher is no good....something internal is fubar

Link to comment
Share on other sites

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.

Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.


  • Create New...