Test an Alternator ?

Tori · 09:28 PM

Is there anyway to test the alternator before removing on these cars ?

I suppose that's a dumb question given what i already know about the situation, but imma P-car newb and maybe there is something to learn here.

Here's my deal......

Battery was dead after a drive today, had to have it jumped. We put a volt meter on the battery once it was running (both at idle and revved) still same 10.5v battery voltage.

My battery light does come on with the key before starting engine, and is NOT on when engine is running. This is the part that baffles me a tad. If the ALT. is ka-put, why isn't the car recognizing it ?

So, before i yank the ALT out, is there anything i can/should do first ?

Cooper911SC · 12-12-2018, 07:30 PM

Using the alt. light as a diagnostic tool:

1. no light with ignition "on" not running
a. bad regulator
b. bad light
c. open slip rings
d. bad brushes

2. light stays "on" with key "off" - shorted diode/diodes

3. light glows brightly while running
a. open diode/diodes
b. shorted rotor/stator winding
c. shorted alt. light wire to ground
d. bad regulator

4. light glows dimly & gets brighter with higher RPMs - voltage drop,
e.g. @ alt. B+ post, @ starter connection, or @ battery connection

5. light flashes at low RPMs - bad slip rings

6. light glows fairly dimly at all RPMs - open diode/diodes

Thanks to Loren at Systems Consulting!

If you’re only seeing 10.5V it’s not charging... time for a rebuild or replacement swap in.

Cooper

Tori · 12-12-2018, 07:35 PM

Thanks Cooper, the 10.5 was merely battery voltage. Could have just as well been 8.2, 6.1 ....etc.

The 6 highlighted ways of using the light to diagnose does not cover my situation.

I guess i just assume it's "tits up" ???

Targalid · 12-12-2018, 07:56 PM

Your alternator light seems to be working. If the alternator is not charging you will see the alternator light on or flashing with the engine running. I am thinking you may have some dirty connections. Clean the battery cables at the battery and also at the grounds. The negative cable goes to the body near the battery. Disconnect it and give it a good cleaning. This needs to be shiny and clean before you reconnect the cable. Also check the ground strap between the tranny and the car body, under the car. These are two obvious places to look. With the engine not running your charged battery should be 12.6 volts or higher. With engine running, the voltage should be between 13 and 14.4 volts. Check your battery cables and the main ground strap for solid connection and report back. Then we will move onto the alternator exciter signal through pin 11 on the 14 pin connector.

Cooper911SC · 12-12-2018, 07:56 PM

Ok, here’s the basic first test.

1.) Use a Volt meter to check current battery charge with car off. (Sounds like 10.5v innyour case)

2.) Start the car. (Jump if necessary)
With the vehicle idling check voltage at the battery terminals. If the alternator is working at all the voltage should be above the 10.5v you are seeing.
A normal functioning alternator should put out ~13.2 to 13.8 max.

I can’t see your profile so not sure what 911 we’re talking about here... if late ‘70’s through the SC era with external voltage regulator then *possibly* just that...

Start using the Pelican search function as all the info. is here in the archives. 😄

timmy2 · timmy2's Garage

Clean up the connectors as mentioned, then Check the resistance between body ground at rear fuse panel and engine case.
Will tell you if you have a bad ground from body to trans/engine/alternator if it reads anything above about 1 ohm. Ideally it should be the same as touching meter probes together. (Close to zero)
Had someone chase alternators and Regulators and batteries due to no charging.
Turnout to be a high resistance connection at the transmission bolt. 3 ohms from engine to body was enough to stop it from charging due to voltage drop.

Tori · 12-12-2018, 09:19 PM

Quote:

Originally Posted by Targalid

Your alternator light seems to be working. If the alternator is not charging you will see the alternator light on or flashing with the engine running. I am thinking you may have some dirty connections. Clean the battery cables at the battery and also at the grounds. The negative cable goes to the body near the battery. Disconnect it and give it a good cleaning. This needs to be shiny and clean before you reconnect the cable. Also check the ground strap between the tranny and the car body, under the car. These are two obvious places to look. With the engine not running your charged battery should be 12.6 volts or higher. With engine running, the voltage should be between 13 and 14.4 volts. Check your battery cables and the main ground strap for solid connection and report back. Then we will move onto the alternator exciter signal through pin 11 on the 14 pin connector.

Thank you for all those suggestions, I just put the engine back in the car 2 weeks ago. Ive put about 125 miles on it since.... i wonder if a fully charged battery would take me that far, and its not been charging all this time?
Anyway, i know for a fact the ground to chasis underneath the car is perfectly, shiny clean, made sure of this during install. The battery terms are perfectly clean as well along with the batt. ground to body.

A few things I suspect are the DME and the connectors in the engine bay - the 4 pin at firewall and the other one under the plastic cover by the ign. coil. Both of those connectors are piss poor IMO. just dont seem to be a tight fit. Same with my DME, the connector is not very tight. Does any of the ALT. function run thru any of those?

I also wonder if i hooked the wires up at the alternator in the wrong places. ?

Tori · 09:30 PM

Quote:

Originally Posted by Cooper911SC

I can’t see your profile so not sure what 911 we’re talking about here... if late ‘70’s through the SC era with external voltage regulator then *possibly* just that...

Start using the Pelican search function as all the info. is here in the archives. 😄

Car is in my sig line. But just realized that doesnt show on a phone.
Anyway, 1987 3.2

I did quite a bit of archive reading but did not see any test procedures, just lots of parts swapping. I even replied to a guy to find out what his final out come was since his thread never listed his conclusion.

Rawknees'Turbo · 12-12-2018, 10:07 PM

Tori, before you spend a bunch of time troubleshooting, I would suggest that you remove the battery and take it to a local auto parts store and have it load tested. I spent a bunch of time chasing my tail (not like THAT!

) years ago when my box truck had similar symptoms and all it turned out to be was one of the batteries had two shorted cells. I felt like a real dumbass then!

T77911S · 12-13-2018, 02:47 AM

charge the battery. if it will not charge replace it. once charged take it/car to advance and have the battery tested.
once you have a good battery in the car have them test the charging system.

ITS FREEEEEEEE!!!

clean the battery to chassis connection . I am assuming you cleaned the battery connections

Tori · 12-13-2018, 07:26 AM

Quote:

Originally Posted by Rawknees'Turbo

Tori, before you spend a bunch of time troubleshooting, I would suggest that you remove the battery and take it to a local auto parts store and have it load tested. I spent a bunch of time chasing my tail (not like THAT!

) years ago when my box truck had similar symptoms and all it turned out to be was one of the batteries had two shorted cells. I felt like a real dumbass then!

The battery will take and hold a charge,... but even if it did have a couple bad cells, why would that keep the alternator from producing voltage ?

Rawknees'Turbo · 12-13-2018, 07:47 AM

Quote:

Originally Posted by Tori

The battery will take and hold a charge,... but even if it did have a couple bad cells, why would that keep the alternator from producing voltage ?

Have you measured the alternators running voltage output with a multimeter (didn't see that in your posts above, butt might have missed it)? If not, you can test it by measuring running current at any hot wire (such as the small fuse block in the engine bay).

Tori · 12-13-2018, 07:52 AM

Yes, measured at the battery. I suppose the next step is to verify output as close to the ALT. as possible.

911obgyn · 911obgyn's Garage

Alternator charge circuit comes through 10g wire at alternator to starter lug at starter to battery through 2/0 (large) cable directly to + battery terminal,4 connections. Ground runs through many more as previously posted.

Tori · 12-13-2018, 09:14 AM

Quote:

Originally Posted by 911obgyn

Alternator charge circuit comes through 10g wire at alternator to starter lug at starter to battery through 2/0 (large) cable directly to + battery terminal,4 connections. Ground runs through many more as previously posted.

Perfecto, this i can work with..... Thank you for that info. I will chase the flow and see where it leads me. I feel since this is a fresh problem since the engine rebuild, it most likely is self inflicted.

T77911S · 12-13-2018, 09:50 AM

do you connect the ground to the block on the back of the alt

Tori · 12-13-2018, 12:24 PM

Quote:

Originally Posted by T77911S

do you connect the ground to the block on the back of the alt

I don't have an exact memory of that, but will check once the ALT gets pulled out.

KNS · 12-13-2018, 04:13 PM

I had the exact same problem once. The connection at the starter (from the alternator) was not tight. Tightened it - problem solved.

Tori · 12-16-2018, 04:28 PM

Quote:

Originally Posted by KNS

I had the exact same problem once. The connection at the starter (from the alternator) was not tight. Tightened it - problem solved.

I was hoping you were onto something,..... as i couldn't exactly remember tightening that connection. Finally got time to put the car on the rack at work and check it. No joy.

All tight, well and good.

Will continue to dig this week.

Tori · 12-20-2018, 12:30 PM

Heading toward pulling the alternator to check connections and verify if i have it wired correctly. Although once i have it out, i don't know how i'm going to do that as i cannot find a wiring diagram for an '87 alternator.

I was able to take this pic, (this is as far as i got while on lunch today) Next will be to try to disconnect the wiring and pull the alt. out. Can it be tested after removed ?

Is there a easier way to get to the nuts holding the wires on ?