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  #1  
Old 03-16-2007, 07:34 PM
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What should the alternater charge be at idle?

After a lot of work I thought my electrical woes were behind me. Not so. After a good run last night to charge the car up she's as dead as a Dodo today. Autozone charged up the battery and claims its good. I put it in the car and drove to my indie. He put a volt meter on it and at idle it's showing 12.3 volts. He says it should be 14 and it's not charging the battery properly. So it looks like I'm in for a new alternator. Just looking for confirmation that it should be showing 14 volts charge with the engine running before I commit to a new alternator.

- Peter.
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  #2  
Old 03-16-2007, 07:43 PM
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You might need a voltage regulator instead of an alternator...$50 and 5 minutes versus lots of dough......its on the rear of the alternator.
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  #3  
Old 03-16-2007, 08:35 PM
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If a battery is fully charged then the alternator does not have to recharge it. So it might read only 12.3 volts. If at idle, all the switches were on, headlights, heater blower rear defog etc. then the alternator would increase the charge rate to around 14 volts. The power to be measured is NOT voltage but amperage. More load added, more amperage needed. I've seen many alternators provide decent voltage but very little amperage. You need to have amperage output tested. It should be approx 90% of the alternator rating. I.E. alternator rated at 55 amps should produce at least 50 amps under load. If you have no idea how old the voltage regulator is, replace it as a maintanence job. Also, an alternator will not quickly recharge a battery unless the battery has enough voltage to EXCITE or TURN ON the field within the alternator. In other words, jump starting a battery that has only 6 volts in it will not be enough to excite the alt. field. The car will start, run fine but the alternator is only running the switches that are on. It will not replenish the battery soon. So, next morning MY BATTERY'S DEAD? occurs!
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Old 03-16-2007, 08:40 PM
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Thanks for the replies. Ohiomike. Already replaced the VR.
Carnut. That's interesting. Alternator is a Bosch unit. Installed six years ago.
I drove the car for almost an our last night on the freeway, with the lights on obviously and I think the thing should have charged the battery under those circumstances if it were working properly shouldnt it?

- Peter.
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2000 GMC Sonoma
Formerly...
2002 Kia Rio. Worst crap on four wheels
1981 240D 4spd stick. 389000 miles. Deceased Jan 08
1984 123 200
1979 116 280S
1972 Cadillac Sedan DeVille
1971 108 280S
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  #5  
Old 03-16-2007, 08:41 PM
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if its below 12.9 volts @ idle, replace the voltage regulator and/or alternator.
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  #6  
Old 03-16-2007, 08:54 PM
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Quote:
Originally Posted by carnut View Post
If a battery is fully charged then the alternator does not have to recharge it. So it might read only 12.3 volts. If at idle, all the switches were on, headlights, heater blower rear defog etc. then the alternator would increase the charge rate to around 14 volts. The power to be measured is NOT voltage but amperage. More load added, more amperage needed. I've seen many alternators provide decent voltage but very little amperage. You need to have amperage output tested. It should be approx 90% of the alternator rating. I.E. alternator rated at 55 amps should produce at least 50 amps under load. If you have no idea how old the voltage regulator is, replace it as a maintanence job. Also, an alternator will not quickly recharge a battery unless the battery has enough voltage to EXCITE or TURN ON the field within the alternator. In other words, jump starting a battery that has only 6 volts in it will not be enough to excite the alt. field. The car will start, run fine but the alternator is only running the switches that are on. It will not replenish the battery soon. So, next morning MY BATTERY'S DEAD? occurs!
I'm not sure where to start with this one.

Power is measured in neither volts nor amps. The regulator works from voltage. If you have 12.3 volts at idle, something has failed. Since the nominal voltage of the battery is above this value, this indicates a slow discharge.

Testing the full-load amperage is a helpful test, but so is checking the voltage with a number of consumers turned on.

Alternators do need excitement, but only the first time they are energized. After that, there is enough residual magnetism to excite the field with no battery present. Generators, on the other hand, use ONLY the residual magnetism to self-excite, and thus must be "polarized" upon installation to prevent the chance of the generator starting the wrong way (and then welding the points in the regulator).
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Old 03-16-2007, 09:03 PM
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Quote:
Originally Posted by carnut View Post
If a battery is fully charged then the alternator does not have to recharge it. So it might read only 12.3 volts. If at idle, all the switches were on, headlights, heater blower rear defog etc. then the alternator would increase the charge rate to around 14 volts. The power to be measured is NOT voltage but amperage. More load added, more amperage needed. I've seen many alternators provide decent voltage but very little amperage. You need to have amperage output tested. It should be approx 90% of the alternator rating. I.E. alternator rated at 55 amps should produce at least 50 amps under load. If you have no idea how old the voltage regulator is, replace it as a maintanence job. Also, an alternator will not quickly recharge a battery unless the battery has enough voltage to EXCITE or TURN ON the field within the alternator. In other words, jump starting a battery that has only 6 volts in it will not be enough to excite the alt. field. The car will start, run fine but the alternator is only running the switches that are on. It will not replenish the battery soon. So, next morning MY BATTERY'S DEAD? occurs!

I need to correct a little misinformation here. A good alternator working properly will output about 14 volts. It's output is unrelated to the quality and level of charge that the battery has, nor is it related to the load on it at the time. Only if the amp load is nearing the capacity of the alternator will the voltage output of the alternator drop.

Your alternator/battery voltage when running should be 14 volts, if it's 12, you're running the car off your battery and not charging it. You're alternator is bad.

An ammeter is nice, you can look at the direction of current flow (to the battery is charging, away from the battery is discharging), but a voltmeter will suffice. Car makers went from ammeters to voltmeters years ago to eliminate the need for heavy gauge wire (that can handle the car's entire current load) running through the engine compartment to the cabin and back. A voltrmeter can use thin gauge wire and provide adequate information.
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Old 03-16-2007, 10:17 PM
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Take a meter and make sure your glow plugs are not staying on. A test light will do as well. After a couple of minutes there should be no voltage on those plugs.
Watching your ceiling light for an increase in brightness as they shut down is okay as well and easier. I think someone like me would be a little upset if I missed it on my car. In your case it does sound like the alternator though.
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  #9  
Old 03-16-2007, 11:33 PM
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The alternator will not charge the battery at idle if there are many loads on, it needs more RPM to output its maximum current. Thus the entry in the owner's manual to turn off rear-window defogger and other high-current loads when idling for extended periods to avoid draining the battery.

Mercedes unfortunately didn't seem to understand the American driving habits: sitting in the car with everything on at idle while waiting outside the store/school/etc., so they didn't install (IMO) adequate alternators through the '80s. In '85 my Audi alternator was a 90a bosch, the '87 M-B a 60a. The Audi could run the bun-warmers while sitting in traffic, the M-B would eventually end up with the ABS light on unless I turned off the rear-defog and bun-warmers.

If you jump start an '80s Mercedes with a dead battery, drive it for a while with moderate 12v use to charge the battery, idling with the heater running will be a very slow charge if at all.
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  #10  
Old 03-17-2007, 02:46 PM
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Originally Posted by barry123400 View Post
Take a meter and make sure your glow plugs are not staying on. A test light will do as well. After a couple of minutes there should be no voltage on those plugs.
Watching your ceiling light for an increase in brightness as they shut down is okay as well and easier. I think someone like me would be a little upset if I missed it on my car. In your case it does sound like the alternator though.
I'm pretty sure they are not staying on. I tested this by letting the glow cycle complet till I heard the relay in the engine compartment click. Simultaneously the brake light brightened noticably with the relay click so I think that indicates the glowplugs are functioning properly.

It's looking like I need a replacement alternator. I'll have to wait a month or two to build up some spare cash before I take the plunge on that one.

- Peter.
__________________
2000 GMC Sonoma
Formerly...
2002 Kia Rio. Worst crap on four wheels
1981 240D 4spd stick. 389000 miles. Deceased Jan 08
1984 123 200
1979 116 280S
1972 Cadillac Sedan DeVille
1971 108 280S
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Old 03-17-2007, 02:46 PM
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