Charging system mystery

[hpaulb]

I've had troubles for two years with charging. Alternator has a new regulator and been tested by three shops on the bench. In-car was only reading 12 volts and engine would stall if I took the positive terminal off the battery. Shorting the rotor pin to 12 doesn't make a difference on the car. Should have kept running off the alt. Just cleaned the terminals on the battery, alternator and starter as it goes there first. Now I'm up to about 12.8 volts but still stalls with the terminal off. Not charging. Turning on the lights doesn't make a difference. Should see ~ 13.2 nominal and ~14 loaded. Anyone have a suggestion? All I can think of is grounds but were ?

[mytrplseven]

When you start your car, before you rev the engine, what is the voltage at the battery? If it's 12v or less the alternator field is not excited and therefore not charging. If you rev the engine over 2500 rpm and the battery is showing over 13 volts, then the alternator is charging. In these conditions, the blue wire that goes to the alternator doesn't have 12v on it to excite the field. This is commonly associated with the alarm system.

If your alternator is charging immediately after starting, you'll see 13 or more volts at the battery terminals.

[gtgt__bangbang]

I would be happy to be wrong but isn't pulling the + off while eng running hard on the alt diodes ?

[hpaulb]

Used the method before successfully on removing the terminal. Should be enough load to keep the alt producing power.

Cleaned all the pos and gnd terminals i could find. After startup, i read about 12. 4 on idle and ~12.8 reved up now. Load up the system and it doesn't change at all. 12 volts on the totor all the time. On the alt rebuild guys bench, my alt read 13.2 nom and 14 on loaded charge.
Can't figure out why the difference or if I'm just reading the actual battery. The past two years it runs for some time then the battery starts running down. The radio start cutting out at this point and i bring it home to a charger.

[Jfrahm]

I suppose either the alternator is bad but testing good (with no significant load?) or you have a really big drain. I have little regard for most alternator bench tests. Personally I would score a good used alternator, or if that is difficult rig a bunch of tests. Have you tried an exciter circuit with a test lamp to replace your D+ input? Does your warning lamp work?

You could also see how fast the battery drains with the alternator disabled and the car running. If it's like 5 minutes that'd be a clue. You can also check battery voltage over time with the car on (not running) and off as a sign of excessive load.

In my 968 I just got 12.7 volts with the key off, 12.3 with the key on, engine off (radio was on) and 12.08 with the heater fan on full blast. After turning they key off I got 12.5 volts which climbed back up to 12.7 after a few minutes.

Generally in these situations I find the alternators get weak and can't charge much but can sort-of be lived with. I have not tried rebuilding a weak alternator so I do not know what the issue might be.

-Joel.

[hpaulb]

The last place i went did the bench test with a load. At the moment i get about the same voltages as you have listed. With the lights on, it goes down to 12.0. I have a Fluke meter hooked to the cigarette socket so I can watch from my seat over time. I don't see why it read correctly on the bench but lower in the car. I'v never seen the dash light on?
Question: It the rotor voltage always the same? or is there some variation dependent on how much charge current/voltage is needed? I'm diagnosing a generator regulator on my Generac at the same time. One of the tests it to jumper it directly to the battery. This forces out a nominal voltage. That regulator actually has to put out higher than that for full output and varies dependent on load.

[Jfrahm]

The dash light is critical to the excitation circuit. I'd try disconnecting your D+ and wiring in a switched jumper through a 2w lamp to D+

The stock circuit I think has a resistor as well as the lamp but this should work as a test.

[hpaulb]

My mistake, the red light is on before the ignition then goes out when running.

Can someone spend a moment and read the voltage on the D+ terminal. Easiest place is the wiring. There is a molex connector with a thin blue thick red over the brake servo. Red is the starter solenoid power and the blue is the rotor D+. Ground the meter and push the red probe up the connector. It would be beneficial to this forum if the following three condition were red:
Ignition on acc
Idle voltage and higher RPM
Same with a load, AC, Radio, Lights.

[Jfrahm]

I got 1.5v with the key on, engine not running (discharge warning lamp on) and 14.8v in all running conditions. The voltage dropped briefly with a load added but then bounced back up. The dash gauge, typically lazy, showed more like 13 volts.

I still feel that you have a sick alternator and the bench testing is flawed in some way.

[Rasta Monsta]

wiring fault - replace alt and starter harnesses

[87m491]

Run a ground, Jumper cable or bump start line direct from alt body to negative terminal and check the voltage. 25 cents say you have a bad ground.

Originally Posted by hpaulb

The last place i went did the bench test with a load. At the moment i get about the same voltages as you have listed. With the lights on, it goes down to 12.0. I have a Fluke meter hooked to the cigarette socket so I can watch from my seat over time. I don't see why it read correctly on the bench but lower in the car. I'v never seen the dash light on?
Question: It the rotor voltage always the same? or is there some variation dependent on how much charge current/voltage is needed? I'm diagnosing a generator regulator on my Generac at the same time. One of the tests it to jumper it directly to the battery. This forces out a nominal voltage. That regulator actually has to put out higher than that for full output and varies dependent on load.