Alternator Not Charging Battery

First time posting for me hopefully it will be successful. Over the past 2 weeks my 1988 XJ in dash gauge has been jumping between 9 & 14 volts. At first it seemed like it dropped when I was in gear and the ended up doing it in neutral and idling. Very recently I tested the charge and decided that the alternator was done for. I swapped alternators and I still have the same problems. The next thing that came to mind was maybe it’s the regulator; however they are built within the alternator. I can’t imagine the regulator being shot on a brand new alternator so I simply ruled that out. Basically now I’m sitting scratching my head wandering what it can possibly be. Basically no charge going to the battery. Advise please…. Thanks, DK

Perhaps a busted fusable link? Check the main wire that runs from the alternator to the fuse distribution block. ( Measure resistance across the wire with it disconnected from the fuse and distribution block.)

So we don't need to go over all the test you ran, what tests did you make that told you the ALT wa bad and not something else?

Most local auto part stores will test your battery, starter, alternator while still in your vehicle for free. Assuming your alternator is good, I would check your battery cables and connections. I have seen cables that look in very good shape on the outside that when cut open revealed very bad corrosion.
Been there done that

Also, have the battery tested. If the battery is around 5 years old, then it could be going bad. Better to replace it, if its getting old. When it goes, it could take out your alternator. Also, maybe the gage is bad or connectors faulty.
Tom

Is your battery going dead? Thinking maybe it's just the gage bouncing around and you not actually having a problem. Doubted this is the case, but just an option.

Your alternator doesn't have a regulator as such. The charging is regulated by the engine controller which modulates the ground to the field coils to give the desired charge rate. If you suspect it is not charging, first measure the voltage across the battery with the engine running. You should get around 14 volts. If the alternator isn't charging this will quickly fall to 10 volts or less. You should also get a check engine light on and a fault code recorded.
As you have already replaced the alternator I would check the continuity of all the wiring to the alternator. Pay special atention to the small wires that go into the moulded block bolted on the back. These sometimes fracture inside so the connection is broken to the ring terminal on the other side of the block. Also check the ground terminals are secure as well as the main + wire.

aardvark4x4 said:

Your alternator doesn't have a regulator as such. The charging is regulated by the engine controller which modulates the ground to the field coils to give the desired charge rate. If you suspect it is not charging, first measure the voltage across the battery with the engine running. You should get around 14 volts. If the alternator isn't charging this will quickly fall to 10 volts or less. You should also get a check engine light on and a fault code recorded.
As you have already replaced the alternator I would check the continuity of all the wiring to the alternator. Pay special atention to the small wires that go into the moulded block bolted on the back. These sometimes fracture inside so the connection is broken to the ring terminal on the other side of the block. Also check the ground terminals are secure as well as the main + wire.

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The problem is with a 88 it don't have a check engine light.
The regulator is built in to the Alt in that year, but other wise good advice.

D'oh! Read it as '98. Yes, it will have a regulator then.

Thanks to all for replying, here is an update after my hunting trip. My brother went wild and thought he got a faulty alternator so he replaced it again, no such luck. Since then we have check all the wires and connections and everything looks fine, but then again looks can be deceiving. I bought some new cables which I hope to replace this weekend.

I know it's not the guage as we tested the battery with a volt meter as well and it was mirroring what the guage was doing. The battery is good as well as I got it tested. What's weird is that the guages jump rapidly liek as if it charges and then doesn't. We put a drain on the battery and the alternator charged it quite rapidly and seem to run okay and then it starting jumping all over the place again.

This weekend I will try some more things. Thanks again to all for advice, first time posting and quite impressed. Cheers,

DK

You on the right track now, the trouble you describe is often caused by bad cables.