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  #1  
Old 01-18-2015, 07:07 PM
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Alternator producing 30 amps/idle - load 50 amps...

More amps at idle possible?
-Smaller alternator pulley?
-Larger than 80 amp alternator?

89 420 SEL...

I've noticed my alternator only puts out 30 amps when at idle (around 600 rpm).

Yet if it is cold and nighttime... Headlights on, rear window defrost on, heater on full blast, etc., then the load on the electrical system is 50 amps! (Higher engine rpms will produce 80 amps.)

I've read of higher amperage alternators for other models of Mercedes (100 amp) which will fit a W126 car. Anyone know if those put out more amps at idle than the 80 amp alternator?

Or is a smaller alternator pulley available (which would turn faster at slower rpms)?

Other solutions? (Move to Florida and roll down the windows? )

Or maybe not a problem and that is why there is such a large battery on these cars - runs off that at traffic signals?

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  #2  
Old 01-18-2015, 07:21 PM
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Sounds normal, alternators increase the amperage output as RPM's increase.

If there is more draw/load than the alternator is putting out, then yes it borrows from the reserve capacity of the battery until the output exceeds the load again and then the battery builds the reserve back up. Have seen batteries be totally discharged and cars towed because of idling in severe weather with lights, heater, wipers, electric defrosters and radio turned on. Keep this in mind if sitting in severe weather...do not use what is not needed.

30 amps at idle is excellent output as long as the voltage stays up at the same time.
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  #3  
Old 01-18-2015, 08:26 PM
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To some extent the battery is a buffer to cover low charging at idle. A smaller pulley will give more charging at idle but the alternator can be damaged at high engine RPM. If you never run to red line a smaller alternator pulley can be a fix.

Thirty amps sounds OK but without another car to compare it to I can't say for sure. If you have an open diode in the bridge charging at low RPM will suffer.

Not all high amp alternators put out more amps at low RPM, some have worse low RPM output than a low amp unit. You need to search for RPM / Amp output curves to be sure.
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  #4  
Old 01-18-2015, 08:34 PM
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I go with Sugar Bear!
The size of an alternator was determined by the allover load expected at worst case. The battery size is also included into that equation.
A small battery capacity with a large alternator and a heavy load, would constantly stress the battery. (Fast Battery discharge, fast alternator re-charge and so on) Ultimately, that battery would not live for very long.
The electrical system has to be balanced and designed to the requirements.

If your 80 Amp alternator allows 80 Amps of current to flow, with full load, I'd be happy, but would suspect the battery to be on it's way out. A 80 Amp alternator goes rarely up to that current (or very brief), since the battery would have to be completely discharged.

One can get an idea if the electrical system has somewhere a problem, if the headlights dim, while switching other consumer in and out. But most likely, this may be related to a corroded ground system.

Normally, the alternator supplies only if the battery drops below a preset voltage. Depending on that battery voltage the alternator would try to charge the battery back to about 13.8 Volt. As the battery voltage increases the current will decrease until the battery reaches the voltage regulator cutoff level. That's when the regulator takes the alternator out of the loop. During driving with some consumer on, this probably happens rarely, cause the battery gets discharged and then re-charged by the alternator, constantly. Without any significant load (consumer) the current maybe very low, perhaps in the range of only 10 Amps. With a 72Ah battery (healthy) it would take about 7 hours to discharge to 11.8 Volts. If my memory doesn't play tricks on me, the regulator will switch the alternator in at 12 Volt (nominal battery voltage) perhaps earlier at 12.6 VDC.
The current and voltage from the alternator is dynamic and one can not say it is good or bad unless the load condition is known.
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Last edited by oldtrucker; 01-18-2015 at 08:51 PM.
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  #5  
Old 01-18-2015, 09:08 PM
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Quote:
Originally Posted by Elektri View Post

Or maybe not a problem and that is why there is such a large battery on these cars - runs off that at traffic signals?
Not a problem unless you only use the car for a half mile each way commute. My dad had an issue with that back in the 80's and found that if he took the car for a 20 minute drive once a week then all was well.
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  #6  
Old 01-19-2015, 03:23 AM
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Quote:
Originally Posted by Skippy View Post
Not a problem unless you only use the car for a half mile each way commute. My dad had an issue with that back in the 80's and found that if he took the car for a 20 minute drive once a week then all was well.
Right! This is a second car I mainly use for short trips in the winter - or don't use it for awhile. In the summer I'll take it for long drives - and that charges the battery.

No problem if I don't drive the car - very few parasitic electrical draws like with newer cars. But a bunch of short trips at night and it seems it would be a losing battle with the battery charge.
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  #7  
Old 01-20-2015, 07:13 PM
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Ok folks, now listen to this!!!!

I have been testing/tinkering/reading all about alternators... I turned on everything on the car as a test and heard a v-belt squeal when I turned on the rear window defrost...

So I tightened the alternator belt and now suddenly I am getting 48 amps at idle! That is 20 amps more output at idle from the alternator than I was getting!

So question: Can an alternator belt be "slipping" if it is not tight enough and you would not hear that "squeal"? (In my case perhaps slipping a little with a little load, and an all out squeal with a larger electrical load?)
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  #8  
Old 01-20-2015, 10:18 PM
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Yes, belts can slip without being noisy. A sign of slipping belt other than noise is when the pulley it rides on gets hot due to slipping. Belts that have been slipping often get glazed on the sides and are prone to more slipping and often need to be replaced. Good find!!!
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  #9  
Old 01-21-2015, 12:38 AM
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Thanks. I think I will get a new belt for a little "Murphy's Law" protection.

FYI - For people with alternator problems reading this in the future...

I tested my alternator with a "clamp amp meter" (AC/DC 600 amp capacity) to measure amperage as well as a multimeter set on DC to measure voltage.

The test readings from both of these indicated ENTIRELY DIFFERENT results! The voltage indicated 12 volts DC at idle (indicating not charging). But the amp meter showed that the alternator WAS in fact working - just not enough amperage.

So if just using a multimeter to measure voltage, you might be mislead into thinking the alternator was not working. Best to have both test meters so you can see exactly what is going on.

Note auto parts stores do not realize that cars have circuits greater than 10 amps. That is the maximum capacity of the amp meters they sell. So don't bother going there to buy one (might inform them that cars these days have 15, 20, 30, and larger amperage circuits. Not to mention 80/100/150 amp alternators.)

Anyway you can get a 600 amp AC/DC clamp amp meter on the internet for around $80. Some are ONLY AC, be sure to get AC/DC!

Another fascinating thing I learned about the Mercedes/Bosch W126 car alternators is that the instrument cluster "Batt" light bulb MUST be at least 2 watts. If less, the alternator may not work!

Or if the instrument cluster "Batt" light is burned out, the alternator WILL NOT WORK!

This is because that batt light circuit provides + power [via a blue wire on my alternator] to the alternator "rotor" which is an electro magnet basically. That rotor needs a bit of electricity to magnetize it to get things going with the alternator - sort of jump start it.

This is called "excitation" or an alternator "exciter" circuit.

I'm still trying to find technical information [pdf/white paper type stuff] about the voltage regulators which are used in these alternators. The manufacturers of these regulators seem to want to keep all their technical information to themselves...
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  #10  
Old 01-23-2015, 01:18 PM
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The automotive charging circuit is "dynamic" not static!
Until the regulator takes the alternator out of the circuit, there will be a current flow.
The voltage always goes hand in hand with the current.
For example: If my battery is at 12VDC and the Alternator produces 14VDC after rectification, there is a 2VDC potential difference. Current will stop to flow only, if the circuit is disconnected (Regulator will do that) or the Battery has reached 14VDC and the circuit is still closed. With battery at 14VDC and the Alternator producing 14VDC there is no potential difference anymore and current will stop flowing.

This is what the ohms law is all about, trust me, it doesn't matter if it is applied in vehicle, a TV, a computer or any other electrical consumer, it will be the same law.
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Mostly, I don't know notin, I just know where to look.
I am looking back, to over 30 years in Electronics Design.
Electrons don't care if they move in a car, computer or relay!

95 W124 E320 M104.992 - Because, I love to repair, naaaah!
Over 221,000 Miles
Cheers,
Norbert
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  #11  
Old 01-23-2015, 07:50 PM
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Well before fixing the tightness of the belt, the output of the alternator was 30 amps, the load was 50 amps, and the voltage was 12 volts.

After tightening the belt, the load was smaller than the amperage output of the alternator and the voltage was 13/14 volts.
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  #12  
Old 01-25-2015, 05:14 PM
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Quote:
Originally Posted by Elektri View Post

I've read of higher amperage alternators for other models of Mercedes (100 amp) which will fit a W126 car. Anyone know if those put out more amps at idle than the 80 amp alternator?

Or is a smaller alternator pulley available (which would turn faster at slower rpms)?


The AL129X which works on the 616/617s also fits the M116/117s... I have been using it on my 560 for a few years now. It is rated at 115A, stock belt works.

Yes, you can buy a 2.5" alt pulley to replace the typical stock 3". You will not find anything smaller than a 2.5" for a single v-belt. You will not overspin the alternator as the motor does not turn nearly enough RPM for that to occur.
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  #13  
Old 01-25-2015, 05:18 PM
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A slipping belt can screw up the effeciency of an alternator, that's for sure.
The alternator is also a converter, mechanical energy into electrical. So a t a load of 50 Amp at 13 Volt, I take this number as an example, the electrical power would be 13V x 50A = 650 W
Some of the mechanical power is lost in heat, mechanical friction etc. so assuming the alternator works at 90-95% efficiency about 5 to 10% more of mechanical power is needed. In the example roughly 700 Watt would be needed. One horse power is about 746 Watt.
Thats almost one horsepower that the belt is looking at.
__________________
Mostly, I don't know notin, I just know where to look.
I am looking back, to over 30 years in Electronics Design.
Electrons don't care if they move in a car, computer or relay!

95 W124 E320 M104.992 - Because, I love to repair, naaaah!
Over 221,000 Miles
Cheers,
Norbert
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  #14  
Old 01-26-2015, 01:53 PM
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The v-belts do need occasional adjustment as they wear, I had that happen a few times on my 617, not so far on my M116 though. New belts seem to start slipping after about 5k miles as they wear in and need adjustment again.

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