Alternator?...or Battery?

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Sep 29, 2004
Houston, the lower bowel of TX
Ok, so for the past couple days I've noticed my 62 has been cranking a little slower and takes a few extra seconds to fire.
No big deal 'til last night when it was significantly weaker and hesitant to fire. The horn was faint and the headlights dim. The stock voltmeter was reading pretty low and didn't change despite engine rev or accesory usage.
Got home and parked it.

Today it was weak and hesitant to start. When it fired the idle speed went immediately to 1500+ rpm...NEVER done this before.
The battery is a Sears Platinum Group 65 (a stout battery) that is 7-8 months old. Had zero problems and it is a daily driver so no sitting for long periods.

If this battery were dying I would think the alternator would pick it up more esp. at higher revs. I wonder if the alternator has gone south and I have been running on just the battery and now it has finally gotten weak. (??)
The battery is on a charger now to try and refresh it.

The batt terminals read close to 12v (not running) on the handheld voltmeter earlier today.
Have not gotten the dash light christmas tree effect (yet) that can be common with a dying alternator.

Anything is possible but this battery should not be dying.

Well when you get a chance get a reading.

If the voltage is around 14.4V your alternator is fine.

If it is lower, it could be the alternator, or the voltage regulator. Once you get a reading it will be easier to tell.
if the alt isn't charging, you should get at least two lights. do the brake and battery lites work?
i'm guessing the slow starts were the first of the day? how did it do the rest of the day?
Have a quick check of the alternator brushes, should only take 10mins or so to pull cover off alternator, yank them out and check them.
My guess is they are worn down enough so it doesn't charge very well, but still making contact so the lights don't come on.
That's what happened to my 61, it was all fine till I went for a 4hr drive with all the electrics on, the alternator couldn't keep up, and batteries went flat.


I charged the battery on the driveway and re-installed it and it sat overnight. The next morning it spun the motor like a greased top, fired right up, good idle, everything fine, dash gauge reading in the middle (12) at idle as usual.

Today is the second day of starting, and driving around with the AC on, etc...ran the headlights some last night. The dash gauge is slowly creeping below the 12v line. Engine rev doesnt really bring the needle up.

Just went out and checked while typing this....Dash gauge reading consistently just below middle 12v mark now. Current voltmeter (handheld analog meter) reading at the battery terminals:
At idle = 12v
At 1500 rpm = 12v
At 1500 rpm with hi beams on, fan on, rear heater fan on, rear defrost on = 11-11.5v
Back at idle= 12v
(My sequoia just measured 14v+ at idle w/hi beams on)

So it seems this big Platinum battery has taken and held a charge just fine and is keeping up best it can.

I dont know alternators but does this point to a failing alternator to you?
Sounds like the alternator is toast. It should be putting out 14v at anything above 1200 RPM. Make sure that the battery terminals are clean and that the wire from post on the alternator to the battery isn't corrodid. You might even want to try a temporary wire from the lug on the alternator to the battery to make sure you don't have a wiring issue - replacing the white w/ blue trace.

If it is the alternator and you need to replace it do yourself a favor and remove the air pump completely - makes accessing the alternator much easier.

Also try a search for FJ62 alternators - lots of good info on rebuilt vs. remanufacturer vs. Mean Green
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Ok, well now that you got a reading, that eliminates the option that the battery is toast.

Most likely the problem is with the alternator or regulator. like mentioned above there is a way to test if it is just the regulator.

Take a length of wire that will reach from the positive terminal of your battery the field terminal on your alternator, I forget which one is the field.
Attach the one end, and make sure it can't arc to anything.
Then start your truck and touch the wire to the field terminal.
While it is attached have a buddy take a voltage measurement across the battery terminals.
If it is higher than 12V your regulator is damaged.
Turn off the truck and head to a parts store to grab an external voltage regulator.
Wire that up and you should be all set.
You don't have your location posted - if you were local i'd loan you a spare Alt.

it sounds like an alt to me. Won't hurt to check/clean your contacts on battery. check your ground and clean up anything that doesn't look right.

Let us know what fixes it - GL!
Start truck, pull negative cable, if engine dies, alternator is prob.
Start truck, pull negative cable, if engine dies, alternator is prob.

Not entirely true, the alternator might still work, it just might now be receiving voltage to the field terminal causing it to generate electricity.

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