Alternator weak?

[Flash]

72 with 318. At idle I am only getting 13.5v at the battery. If I increase rpm is rises some, but only to around 13.7.

When driving with headlights etc they will turn off sometimes and at idle the turn signal blink very slow compared to a higher rpm.

Also any idea how to identify this alternator only has a a number stamped on it. 2005191

 

[Don Frelier]

Those numbers seem fine to me.
Some of the headlight switches have a breaker built in (not sure about a 72).
Sounds like the breaker is tripping.

 

[Flash]

Those numbers seem fine to me.
Some of the headlight switches have a breaker built in (not sure about a 72).
Sounds like the breaker is tripping.

Copy, do the breakers auto reset?

Also anything else power goes dim or slows down. Like the turn signals slow down considerably when at idle versus high rpms while driving.

 

[dadsbee]

How old is your battery?? Load test it. Nothing wrong with the voltage output of your alternator.

 

[PlymCrazy]

13.5-13.7V sound like pretty acceptable numbers to me

 

[Flash]

How old is your battery?? Load test it. Nothing wrong with the voltage output of your alternator.

Just a little over 1 year.

 

[Flash]

13.5-13.7V sound like pretty acceptable numbers to me

Gotcha, could it be something else then that is causing the lights to dim and turn signal to slow down at idle?

The voltage does drop with headlights on and turn signal on.

 

[RJRENTON]

According to the title of your inquiry, the original alternator would be a round back dual field, single groove sheave, and using the electronic voltage regulator......that is IF no one has changed it to a square back design. It would use the electronic voltage regulator. Typically, the standard alternator was rated at 46 amps output, slightly more if the vehicle had A/C. If you have access to a FACTORY SERVICE MANUAL (FSM)....follow the procedure for load testing the alternator. IMO....DO NOT TAKE THE ALTERNATOR' TO THE AUTO PARTS STORE AS THEY CANNOT DUPLICATE THE FSM CONDITIONS. Its possible that the alternator has a bad diode. This would reduce the capacity by half, but under light load, the alternator may produce correct voltage.
BOB RENTON

 

[Don67Satellite]

Another possible option. There are many threads here on running relays for the headlights to lighten the electrical load and significantly brighten up your commute. You can buy plug n' play units here from @Crackedback as well as other options online.

 

[Photon440]

Output could vary with temperature as well. This is from the 1971 FSM:

 

[RJRENTON]

Another possible option. There are many threads here on running relays for the headlights to lighten the electrical load and significantly brighten up your commute. You can buy plug n' play units here from @Crackedback as well as other options online.

You DO NOT NEED TO ADD, REWIRE, OR CHANGE ANY FACTORY WIRING....SIMPLY TROUBLE SHOOT LOW ALTERNATOR OUTPUT PER FSM PROCEDURES......
BOB RENTON

 

[PlymCrazy]

Gotcha, could it be something else then that is causing the lights to dim and turn signal to slow down at idle?

The voltage does drop with headlights on and turn signal on.

At idle it's typical to see the voltage drop with the headlights dimming and a slowed blink with the turn signals. As a kid dad taught me one way to test to see that the alternator is working is to shine the headlights on the garage door at night with the engine idling. A little bump on the gas pedal and there should be an obvious increase in brightness if the alternator is charging. Same with the rate of blinking of the turn signals.
The more sure way of testing what's happening is with some type of volt meter as the guys are advising you here.

Edit: Essentially what I'm saying here is what you're seeing sounds very normal for this older style charging system.

 

[Don Frelier]

I don't think there is anything wrong with his alternator.
Read post #2

 

[Don67Satellite]

You DO NOT NEED TO ADD, REWIRE, OR CHANGE ANY FACTORY WIRING....SIMPLY TROUBLE SHOOT LOW ALTERNATOR OUTPUT PER FSM PROCEDURES......
BOB RENTON

Just your opinion of course. However, you are somewhat correct. No one NEEDS to, but it is an option as stated.

Agree with @PlymCrazy that this dimming behaviour is normal with an aged charging system. This is where using relays can help.

As for the headlights turning off, I would check what @Don Frelier said. Good luck.

 

[Photon440]

 

[Nacho-RT74]

72 was the first year for the SQUAREBACKS ALTS...

roundback ”dual fields” was 70/71 only deal.

just my opinion of course…

 

[dadsbee]

Bob's got a pinky ring, ya gotta listen to him!

 

[Rusty knuckles]

I don't think that's low voltage but would check the ground on the engine if you are having problems then change the voltage regulator. It's also possible to have blown a diode in the alternator (back half of the case) but I don't think that's the case in this situation.

 

[Don Frelier]

Are you sure it's 13.5 volts because everything should work fine at that level?

 

[Flash]

Another possible option. There are many threads here on running relays for the headlights to lighten the electrical load and significantly brighten up your commute. You can buy plug n' play units here from @Crackedback as well as other options online.

I thought about converting to LED ithe future would that help any?