charging system question

[406cid70Chevy]

My truck drains the battery when it's running. So I checked the battery with a multimeter, it's at 12.8 volts with the engine off. With the engine on it's at 12.6 volts. Shouldn't it be at about 14.2 volts when the engine is running? I have the old style externally regulated alternator. Are there any tests I can perform to determine if it's the alternator, voltage regulator, or another source? Could bad grounds cause this? Thanks!

[Jeepster376]

Originally Posted by 406cid70Chevy

With the engine on it's at 12.6 volts. Shouldn't it be at about 14.2 volts when the engine is running?

Yup

Are there any tests I can perform to determine if it's the alternator, voltage regulator, or another source?

If you know how to energize the field on your alternator, you can eliminate the regulator to determine if it's bad. Otherwise you could just bring the alternator into an auto parts store and have it tested.

Could bad grounds cause this? Thanks!

Check that your regulator is well grounded to the core support. The alternator is grounded directly to the block through the mounting bracket, and your battery ground cable should also be direct to the block. The block should also be grounded to the firewall.

[jef5150]

? I have the old style externally regulated alternator. Are there any tests I can perform to determine if it's the alternator,

try this, it only takes 2 minutes,,pull a cable off the battery while its running,if it dies, its a 99.8% chance the alternator is bad. its easier if you loosen the cable clamp before you start it,,,, then you can start checking other things (dont do this on newer vehicles with ob computors) jeff

[Baradium]

with lower voltage with the engine on, you already know the alternator isn't charging at all.

Voltage should be 13-15 volts with the engine running. Careful with bypassing stuff, don't want to burn up anything.

[406cid70Chevy]

I went and got the alternator tested at autozone, it was putting out 250amps...pinned the needle. The guy didn't know what that meant, he just said it's not right. The alternator can't really put out that many amps can it?

[Jeepster376]

Originally Posted by 406cid70Chevy

The alternator can't really put out that many amps can it?

Not for very long.

He probably didn't test it properly as externally regulated. It's your regulators job to control the alt output. Sounds like your alt works OK.

[GSFMECH]

I agree with Jeepster, sounds like the alternator is ok. I would pull the cover on the regulator and check the points and inspect the coils. Sometimes you can spot a problem with the coils such as overheating etc. and see if the points are pitted or carboned up. You can clean them with a points file but they never last long after you knock the factory edge off them. I would just replace the regulator if it were me as it is more than likely almost 35 years old anyway.

Mike

[Cat Fuzz]

Originally Posted by 406cid70Chevy

I went and got the alternator tested at autozone, it was putting out 250amps...pinned the needle. The guy didn't know what that meant, he just said it's not right. The alternator can't really put out that many amps can it?

I think the guy doesn't know what he's doing. All the alternator bench testers I've seen cannot check the current output, just the voltage. We don't have Autozone here, but if its anything like Schucks, the people there don't know squat. They only know what the computer tells them. 250 amps would fry your wires.

[1972C10]

yeah 250 amps out of a delco alt is impossible well maybe not but very hard

Its hard to get 200 amps out of a big leece neville train alternator

And if you need a reg ive got one that was on the truck for a week then i went to an int reg alt ,

[406cid70Chevy]

Well I took the alternator to another parts store and it tested out ok. It's supposed to put out 14.4 volts right?

So I bought a voltage regulator there and when I got home I popped the cap off the old voltage reg. and found a blown/fried wire. Well that was at least part of my problem, because it still doesn't charge at 14.4 volts. I let it warm up and revved it a bit and it got up to 13.8 volts on the revs, but then would drop down to about 13 volts at idle. What does this mean?

To double-check I tested my car at idle, it was at 14.4 volts.

Another question, what is the purpose of this resistor? that plugs into the volt. reg. in the pic?

Thanks for all your help guys I appreciate it.

[1972C10]

I never could get over 13 volts out of mine thus the reason i went to internal reg.

[Jeepster376]

You should get ~14.4v at the stud on the back of the alternator, and 13.8V at the battery. Sounds like you're right on the money.

[406cid70Chevy]

So the voltage is not supposed to stay constant then? It's supposed to fluctuate with engine rpm?

[Jeepster376]

That's normal, as the alternator does not work to it's fullest at idle RPMs.

['68OrangeSunshine]

That's not a resistor, it's a capacitor. It's not normal [stock], there should only be a ground lead off your front engine/headlamp harness with a self-tapping screw to ground the side of the regulator box to the system. Best guess is that capacitor was for Hi-Fi or CB noise reduction. If you didn't install it yourself -- you probably don't need it. A capacitor doesn't flow current constantly, it stores it up then bursts it thru at intervals. That could be your gripe right there.

[kbs71]

These old style voltage regulator hardly worked when they were newly designed. Back when these trucks were first manufacture,d. all they were worried about was ignition, lights, a blower motor and maybe a radio. Since then they have added more accessories along with us owner creating a larger draw on the alternator. These newer external volate regulators are produced overseas and are no longer the quality that they once were. Hence they don't work as well. I installed a new altenator (35 amp) and then I went through 4 of them in 3 months along with a battery from the charging system not operating correctly. I bought a internally regulated altenator kit and installed it along with a 96 amp altenator. I also put in headlight relays. I get 14.4 amps out of the altenator all the way to the battery I no longer have weak lights or a weak ignition system. I love the new system. I did not go to a one wire system because I think that is just basically a battery charger and doesn't actually act like a altenator. It only took me about one day some soldering and the trucks charging system is now fantastic.