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Alternator started failing on my truck last week. So I pulled a good one off another truck, and popped it on. Was only getting ~10 volts out of it, so I replaced the voltage regulator with another good used one out of the drawer. Still only getting ~10v to the battery. Tonight I threw on another good alternator I had around... You guessed it, it's putting out 10v. (I'm measuring directly at the alternator, by the way.)

Automotive electrical is still somewhat elusive to my comprehension (obviously!), so I could use a little help here. I can keep throwing parts at it until it works, but then I'll be in the same boat next time something happens...

Thanks guys!

OK follow these steps in order and you will fix it. The charging system is dead simple. (this is assuming early alternator).

1. Check battery by measuring voltage across terminals. If it's less than 12.5 volts then charge it. If it won't reach or stay above 12.5 volts get another battery.

2. Check battery cables - check voltage between starter relay and engine block. If it's at battery voltage (around 12v) keep going. If it's more than .5v below battery voltage fix the cables or clean the terminals.

2. Turn on ignition, engine not running. Check voltage at the IGN side of the voltage regulator. Is it close to battery voltage? If yes, keep going. If not, figure out why no voltage is getting to regulator.

2. Check voltage at FLD side of regulator. Is it the same or close to battery voltage? If not, check the regulator ground. If it is grounded well to the firewall, and still no power at FLD with engine off and ignition on, change regulator.

3. Check alternator wiring. Voltage at 1 field terminal should be battery voltage. Voltage at the BATT terminal should also be battery voltage. The second field terminal (if there is one) should be grounded. If there is no power at the BATT terminal the main power wire is damaged somewhere. Check the firewall connector. If there is no voltage at the field terminal, fix the wire from the regulator FLD to the field terminal on the alt.

4. Start the engine, check the voltage at the battery. Is it 13.8v? If it is not, then the alternator is bad. If it is, then you have fixed the problem.

You make it sound easy. haha Thanks, I appreciate the layman's terms! It might be a few days before I get a chance to touch the truck, but I'll report back when I do.

This is great, thank you! Mine truck just stopped charging as well, so I've been slowly tracking down the problem, but it's looking more and more like the alternator. I've never rebuilt one, must be time.

OleRed66 wrote:I've never rebuilt one (alternator), must be time.

Just an FYI, the brushes aren't hard to do as I recall, but if the bearings need to be replaced, they require special mini pullers/press. I have original one off my '68 sitting on the shelf waiting to find a competent shop to rebuild it for me.

Check this out if you want more amps out of your round-back alternator...

http://store.alternatorparts.com/repair ... its-4.aspx

If you have a good casing, get the rebuild/upgrade kit(s). Round back casings are getting hard to find. And my engine won't take the next generation alternator (stock 69 318), hits in a couple spots. If you have a modern stereo in your truck, get the upgrade, unless you like really dim headlights (36ish amp alternator, CD player, 8 speakers).

WD wrote:If you have a good casing, get the rebuild/upgrade kit(s). Round back casings are getting hard to find. And my engine won't take the next generation alternator (stock 69 318), hits in a couple spots. If you have a modern stereo in your truck, get the upgrade, unless you like really dim headlights (36ish amp alternator, CD player, 8 speakers).

I have a square back casing on my '69 318 right now? But only because that's what I had laying around. I don't need the extra juice, since it's a real truck. Radio and other creature comforts need not apply.

Oh, and Rob, I started following your directions yesterday. Turns out all my VRs were bad... Have one on a parts truck I will check out soon, and then can continue the tests you laid out so well. Thanks again!

Used your instructions. No voltage on the ignition terminal of voltage regulator.

There was voltage on the ignition switch, but not at the firewall. I cut the wire, spliced in new, and now it's charging. I had all the parts so all I spent was time!

Thanks. Poor nearly 50 year old wiring is starting to show its age.

Demulsion, mine is strictly a farm truck these days. Radio was only used when I was living in western Washington so I could get the traffic reports. Lived in Buckley, worked in Tukwila and Tacoma. Needed that much volume to cover the wind noise, my truck should really be a parts junker. 9mpg and a dire need for a full rewire and modern brakes (Memphis drivers make the I-5 corridor look really tame, posted speed limits here are the recommended minimums, 30+ over is normal). Truck hasn't moved in 2 or 3 months. Not planning to bother licensing it, never gets more than 5 gallons of fuel at a time poured in.

If it had a 251 L6, 4spd and a t-case I'd be more inclined to fix it up. Or if I get around to swapping in my 413/727 combo.

Square case hits the valve cover and the face of the cylinder head on mine.

OleRed66 wrote:Used your instructions. No voltage on the ignition terminal of voltage regulator.

There was voltage on the ignition switch, but not at the firewall. I cut the wire, spliced in new, and now it's charging. I had all the parts so all I spent was time!

Thanks. Poor nearly 50 year old wiring is starting to show its age.

Glad to help!

Found my last spare VR, and it was good. Everything checks out now, thanks!

Yee haw!

Nothing like a good charging system.