Alternator Not Charging/Low Voltage Issues

[Mike L]

Last week I was having an issue with the alternator and battery cables in my 98 Cherokee with the 4.0L. The original 90 amp alternator and cables had over 214k on them.

The alternator was not charging the battery and it was running off the battery alone. Long story short I had it towed and started tearing it apart.

I pulled the alternator and had it bench tested at two different AutoZones and both places showed it was putting out over 14 volts.

The battery is a 10 month old Optima Red Top.

I figured it was time to replace my battery cables as they looked pretty bad and the alternator tested good:
The battery + to starter is now 00AWG
The battery - to the block is now 00AWG
The rest are now 4AWG: battery - to chassis, battery + to PDC, alternator output to PDC, and block to chassis

The upgraded 4AWG charging cable is connected to the PDC with an inline Mega Fuse, similar to CharlesS setup:
http://www.naxja.org/forum/showpost.php?p=245429748&postcount=2

After getting everything back together I was getting 13.5 volts with the engine running and I thought everything was back to normal.

On my way home from work yesterday the voltage gauge on the dash was reading 12.5 and after a few minutes the voltage gauge dropped out to 9V. I understand the gauge dropping the 9V is normal in the 97 and up when it is reading low voltage for a prolonged amount of time.

When I got home I checked the voltage with the engine off and got @ 12.5V, with the engine running I was getting the same 12.5V. I removed the connector at the back of the alternator and cleaned the connectors and reinstalled it. I then rechecked the voltage: engine off was 12.5V and with the engine running I was getting 13.36V. I checked the voltage at the battery, PDC terminal, and the B+ terminal on the alternator and was getting the same 13.36V. I drove it around a couple times last night and it was giving me the same voltage readings so I figured it was okay.

Fast forward to the commute to work this morning and the dash voltage gauge was reading 12.5V again and a few minutes later the gauge would drop out.

I understand the PCM is what regulates the alternator. What are some other possible issues? The battery temp sensor is under the battery and I am not getting any codes.

 

[Tim_MN]

From your descriptions, it seems that your alt has issues when it warmed up. You should be getting 13.5-14.2 volts from the alternator at 2000 rpm's. I would start with a top quality new alternator or a rebuild from your local auto-electrical shop. "Lifetime Warranty" rebuilt alternators from Advance/IdiotZone/PepBoys are very often junk. Even though the battery is newer, I would have it load tested.

Here is the best DIY alternator/battery testing thread - http://www.jeepforum.com/forum/f27/primary-battery-starter-alternator-electrical-testing-1171690/

 

[Mike L]

Even though the alternator tested good on a bench test at AutoZone, I am wondering if their test is accurate. At lunch the voltage gauge on the dash was reading 12.5 with the engine running. I smacked the alternator and the bracket with a hammer and the voltage went over 13V. I did not have a multi meter with me to see what the actual voltage was but this to me indicates a possible issue with the alternator.

 

[5-90]

Even though the alternator tested good on a bench test at AutoZone, I am wondering if their test is accurate. At lunch the voltage gauge on the dash was reading 12.5 with the engine running. I smacked the alternator and the bracket with a hammer and the voltage went over 13V. I did not have a multi meter with me to see what the actual voltage was but this to me indicates a possible issue with the alternator.

No, their test is not accurate. Those benches usually test voltage, not current, and current is the more important value (if the alternator is losing output capability, it can still show as 'good' if tested with minimal current draw. The parts house benches, from what I've seen, still lack the ability to really "wring out" the alternator and put a heavy load on them. Most benches won't draw enough current to drive my flashlight...)

Your "percussive maintenance" points to either a loose internal connexion (possible) or the brushes are sticking in the holder (more likely) and not making good contact with the sliprings. Brushes not making good contact will reduce current output, by reducing contact area through which the current can flow.

As far as the testing thread - I got tired of typing it out, I got tired of doing copypasta, so I posted that thread verbatim on six or seven different boards - including this one (should be in OEM Tech.)

 

[ehall]

I have had sporadic voltage drops twice. First time was a blown ANL fuse (similar to your maxi) where the ends of the fuse were flopping around and making random contact sometimes and flopping apart at others. The other time was a damaged field terminal (the two small wires on the back of the alternator)--one of the threads was rough and the nut had worked its way loose, so the wire wasn't making reliable contact with the terminal.