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  #1  
Old 11-06-2003, 08:25 PM
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Join Date: May 1999
Location: Calgary, Alberta, Canada
Posts: 190
Question about battery voltages in 1988 300E. Kinda long.

I replaced my alternator as my old one was causing a flickering lights etc (bad diode). I put in a used one and now that it is cold out all the accessories are running (Heater, Defrost, heated seats etc.) I parked somewhere the other day and the car would not start. I boosted it and all was fine, but I have been eerie about keeping on accessories that are not necessary. Today I checked the voltages with the car at idle and everything off and it was close to 14volts. With everything on it was dipping down to 11.? at idle. I changed the original Bosch regulator with the brushes and put on a Beru that I had recently replaced from my old alternator. It did not make much of a difference. Should the voltage fluctuate slightly while it is running? I understand that as components get older they usually take more juice to run. Am I just going to have to live with this? Any suggestions?

TIA,
Steve
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1997 E320
1990 Nissan 300ZX Twin Turbo (Stage X+)
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  #2  
Old 11-06-2003, 09:08 PM
JetForeman
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I'm not overly familiar with your model car but I do know this past summer I was concerned with running voltages on my E320 and the lowest I could get it was 12.5v at idle, with A/C on, Aux. fans running, blower motor on high, headlights on, well you get the picture. When I brought the revs up from idle it would slowly creep up to around 13.8v. 11v sounds way too low, perhaps a new regulator will cure it.
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  #3  
Old 11-07-2003, 10:59 AM
moedip
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Before you condemn the alternator - it could be your battery. Use this procedure to check it. Charge your battery. Let the car sit for 8 hours. Check the voltage on the battery without trying to start your car - it should be 12.56 volts MINIMUM - a real good battery will be about 12.72 or higher. If your voltage on the battery is 12.1 volts after 8 hours of rest - you have a shorted cell. If your battery reserve is too low because the battery is bad as determined from the above test - your alternator will not be able to keep up with the current draw of your car causing the low 11 volt reading because your battery does not have the reserve to keep it above 12.5 volts. Cold weather will make things worse with the battery. Good luck
Morris
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  #4  
Old 11-10-2003, 06:04 AM
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Join Date: May 1999
Location: Calgary, Alberta, Canada
Posts: 190
More test specs.

Tonight I had my car die again while I was driving with the accessories on. After I got it running again, I checked the voltages and with the fan and lights high, it was 11.8volts. With the fan off and lights on it was 13.2volts.

I don't know if it is not charging the battery completely or what. Is the fan drawing to much current? Could the fan be screwed? I think I will get the charging system load tested tommorrow.

TIA,
Steve
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1997 E320
1990 Nissan 300ZX Twin Turbo (Stage X+)
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  #5  
Old 11-22-2003, 06:32 AM
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Join Date: May 1999
Location: Calgary, Alberta, Canada
Posts: 190
Is the 70AMP alternator too small?

Is it possible that the alternator could ever keep up to having all the accessories on? With the rear window defrost, headlights, heated seats, and climate control, I am drawing about 56 amps. Is that a lower output than it should be? That is when my battery voltage drops to about 11.8volts at idle. Is this normal? When I rev the engine a little it comes back and with everything off, it is around 14volts.

TIA,
Steve
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1997 E320
1990 Nissan 300ZX Twin Turbo (Stage X+)
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  #6  
Old 11-22-2003, 09:28 AM
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Join Date: Mar 2003
Location: Southern California
Posts: 2,036
Alternators only output about half the rated amps at idle speed, so a 70 amp rating will not produce 56 amps at idle, which means the battery has to carry part of the load, which is why the system voltage drops below open circuit battery voltage.
Full rated output is usually achieved by about 2000 engine revs. Also, alternator output is less the colder it is (open circuit battery voltage also drops in cold weather), so a high electrical load may mean the battery is discharging. Alternator output is typcially rated at 80 degrees F. If system voltage is less than about 12.5 volts the alternator is not producing enough current to carry the load and the battery is discharging.

Especially if you drive short trips in cold weather with a high electrical load the battery may end up loosing charge. The cure is to reduce the electrical load or drive longer/farther.

Duke
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  #7  
Old 11-22-2003, 09:31 AM
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Join Date: Jul 2000
Posts: 18
With no load and revved engine, 14V is about right.
Alternator output varies with revs and only reaches the 75A rated output around 3000rpm.
At idle the output is about half the 75A rated output.
With the 56A draw at idle part of the load was being supplied from the battery.
It is likely that your alternator is OK. The battery may be faulty or you may just be drawing too much from the system.
Check that the alternator drive belt is not slipping under load!
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  #8  
Old 11-22-2003, 09:41 AM
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Join Date: Mar 2000
Location: Gainesville FL
Posts: 6,844
The answer is NO!! The alternator in an up to 89 300E is a POS. It has a hard time keeping up without seat heaters.

The problem is both under total capacity and general poor performance. The alternator is probably a 70amp unit. Recent E320s have a 120amp unit. But this isn't the real problem as the 1990 on 80amp 300E alternator does just fine.

If you were watching current flow you would see why you are dropping below battery voltage. Its sort of like a bank account. If the money going in doesn't match or exceed the money going out then the pile (battery voltage) gets smaller. You either need to change your spending habits or get a better paying job (bg).

THe problem with the standard 70amp alternator is that it only produced about 25-30amps at idle speed. At about 2000rpms it will produce the 70a and should handle the load. The 90- 300E 80a alternator produces about 60amps at idle and will power all those debts.

It turns out that the later alternator is no more money than the POS. It installs similarly with the exception that one must change the $50 alternator harness. If you can't find the number in the archives (for the harness), write me on Monday, we stock the harness and never use the POS.
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Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician
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