Alternator repair/ rebuild question

I have a 90’ Toyota pickup that is slowly losing its alternator. Over the past month, especially on cold mornings, I will start my drive to work and notice dim headlights and a dash volt meter reading around 14v. After a few minutes, it will jump up to its normal 16v and the lights will brighten and everything will be fine the rest of the day. Lately, it has been taking longer to “kick in” and it has been having trouble getting above even 12v now until I get about 10 miles down the road. The battery light is also beginning to flash now, but goes away when the alternator kicks in. I have been looking at alternators online and found that I can buy parts, ie. bearings, voltage regulators, brushes, etc. I would like to save some money and try to repair my existing alternator rather than buy another one. Based on the symptoms I listed above, does anyone know with any reasonable level of certainty what part of my alternator may be going bad and need replacing? Or, since I’m not a mechanic, maybe there’s another problem I’m not thinking of? I’ve also noticed that in the afternoon when it’s warm outside, the alternator works fine. The battery is only 3 months old and I know that’s not a problem. Thanks to anyone who can offer some advice.

The first thing you should do is inspect and thoroughly clean all battery cable ends and check fusible link end connectors for corrosion since a dirty connection somewhere can cause an in and out symptom.
I think there not one, but two, fusible links between the alternator and battery. Alternator current flow, which is fairly high amperage, goes through both of these and it does not take much for an end to burn or corrode.

If everything is fine in that area and if you need an alternator your best bet is to replace it as a rebuilt unit.
The problem is more than likely in the brushes or the regulator. By the time you factor in the cost of this plus bearings, etc. you’re still sitting there with a dubious alternator.
The DIYer generally does not have the expertise or equipment to check things like field coils, armature windings and laminations, etc. and when done you could still have a faulty alternator.

The normal voltage range of a good charging system should be between 12.8 to 15 volts. You shouldn’t have 16 volts, that is too high. I recommend you change out the alternator as soon as you can.

You can rebuild the alternator yourself but the time it takes and for the money spent I don’t think it is worth it. If something happens to the alternator later on you will usually have a warranty with a rebuilt unit to fall back on. If one of the parts fails in your own rebuilt unit you are back in the same boat again.

If your alternator is charging at 16 volts you are going to ruin the new battery if you haven’t already. Replace the alternator, don’t rebuild it.

16 volts is certainly high, but I’d first question the accuracy of the gauge. But the voltage change is something to be concerned about.

The worn part in your alternator is probably the brush. In a Toyota alternator, this is pretty easy to change and the brush is fairly inexpensive, about $8 I believe, compared to $100+ for a reman alternator or $400+ for a new alternator. Bushings are pretty cheap but more labor intensive to install and sometimes require special tools for pulling off the pulley. I’d do the brush for now and if problems show up later, go for a reman.

Go over all the connections you can get to between the alternator and battery, and clean the grounds where they make contact with the body or frame. Remove any and all corrosion with a stiff wire brush. If nothing changes I think this is a nice project to indoctrinate yourself into the world of “gear-heads” with. Sure, if your time is worth anything the replacement with a “rebuilt” sounds attractive. Only thing is that I’ve had so many rebuilt “whatevers” that went bad within a month or two of installing them that I went back to rebuilding everything myself. By doing it yourself you’ll learn something about how it works and save a few bucks in the process. Instead of forging ahead on your own, the best way to go would be to find an “old timer” who is a “backyard” mechanic and will get you started and help you along. Start by removing the alternator (usually just a couple of bolts and a wire connector). Then scribe a line across both halves of the case, so it goes back together just as it was assembled. You’ll need a $10 volt ohm meter (usually less than that from Harbor Freight Tools). Have the guy who understands how it works explain how to check the diodes and regulator, and run simple short and open tests on the windings. Since you’re in there you might as well install a new pair of brushes, no matter what condition you find them to be in. You’ll also need a toothpick or paper clip to hold the brushes out of the way when you reassemble things. Fix any internal corrosion you find inside as described above. You may find a bad connection, bad diode in the rectifier, bad regulator, or bad diode trio. None of these things cost anywhere near what replacing the alternator with a new one does. Clean the shaft and rear bearing with a rag and put a dab of fresh grease in there, too. Then you can begin to join the ranks of those of us you see on the side of the road with the hoods up on our beaters, trying to get home. You can also recognize us in public areas; we’re the ones with perpetually grimy hands and fingernails.

Thanks for all the advice and encouragement. After being surpised at how quick and easy it was to remove the alternator and get it apart, I think I’ll take a shot at fixing it. I love a challenge too much to take the easy way out. Worse case scenario, my bad alternator will stay bad and I’ll be out just a few extra bucks. But, I usually have a knack with things. So this may be my indoctrination into the so called “gear heads” after all. Wish me luck.

Also, if DIY-ing it doesn’t work out, if you have a good auto-electric shop in your town, they can often fix an alternator for a fraction of the price of buying a rebuilt one.

Good luck to you. I renew my GM and VW alternators with new bearings and slip ring followers (brushes) and leave the old electronics in if they work. Engineers would call old, working electronics burned in and no kidding; more reliable than new! Take a look at the slip rings and if they are deeply grooved, then chuck the rotor up in a collet lathe or a lathe with a three jaw chuck being careful to set the TIR (total indicator reading) of the shaft end near the slip rings to less than .0005" using a dial indicator and then turn the slip rings down to smooth, taking very light cuts and running fairly slowly so as to not shift the rotor in the collet/chuck. I used to work where I could access a lathe for this but have a friend who has one of those $300 Chinese lathes. If you have no lathe, then plan on installing new slip ring followers more often.

I apply a cursory dielectric test to the rotor and stator isolated, of course, from the diodes with a 140 volt ac indicator light with a special isolation transformer that I have.

Some stores sell alternator rebuild kits that have what you need to renew your alternator.

Save a ton of money and go for it!

Thanks for all the responses and advice. In the end, I didn’t have time to really check everything out inside my alternator. I basically just checked the fusable links, all the wiring and connectors, and the brushes. Since everything was fine except the brushes appearing to be heavily worn, I went ahead and replaced them with a brush assembly from a Toyota dealer ($21.00) and crossed my fingers. Well, that was over a week ago and my alternator has been working great! My voltage is back up and stays up. (As far as the “16v”, I was extrapolating between the 12v mark and 18v mark on my display. When things are working good, the needle sits about 2/3 towards the 18v side and alway has since I bought the truck years ago. Whatever the actual voltage is “supposed” to be, I don’t know, but I do know where that needle is supposed to sit on my guage- wrong or not- and that’s at around the 16v area.) Anyway, thanks for all your help.

Glad you got the alternator running again. As far as the voltage goes you should have no more than 15 volts going to the battery. More than that will cause problems like shortened life of the battery and headlights and possible other things. I suggest you have the battery voltage read using a voltmeter across the battery and running the engine to around 2,000 RPM. If the battery voltage exceeds the recommended level there is a regulator problem.