Author Topic: Charging circuit on 1967 MB 250SL  (Read 7550 times)

jcochran@cox.net

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Charging circuit on 1967 MB 250SL
« on: August 07, 2014, 00:56:56 »
Hi,
 The charging lamp is out on my Pagoda (battery is dead) and I was hoping someone would know of a trouble shooting procedure to find out if I have a bad alternator, regulator, lamp, etc.

Thanks,
Jim

alchemist

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Re: Charging circuit on 1967 MB 250SL
« Reply #1 on: August 07, 2014, 01:33:01 »
It is possible that the charge light bulb is burned out. To test the alternator, turn the engine on and let it it run and turn the headlight on. Measure the voltage at the battery terminals. If you get more that 13.5 volts, your Alt is OK. If you get around 12 volts, your Alt is bad.

jcochran@cox.net

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Re: Charging circuit on 1967 MB 250SL
« Reply #2 on: August 07, 2014, 01:58:22 »
ZALSAIGH,
 The voltage is 12.5 under load but it is my understanding of the schematic that if the charge bulb is burned out I will get no voltage to the excitation circuit and thus no output. If the bulb is burned out does it still deliver a signal to the exciter (shunt circuit?)? I really appreciate your help with this.

Thanks,
Jim

garymand

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Re: Charging circuit on 1967 MB 250SL
« Reply #3 on: August 07, 2014, 21:57:28 »
I think you are correct.  No bulb, no current, no charge.  Try a jumper light from pin D+ on the regulator or D+ on the alternator, they are the the same point, to fuse #2.  That is a parallel light with the dash light.  It will turn on the regulator if the alt is good or stay glowing if the alt is bad. 
Gary
Early 250SL German version owned since 71, C320, R350, 89 Porsche 944 Turbo S

alchemist

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Re: Charging circuit on 1967 MB 250SL
« Reply #4 on: August 08, 2014, 02:29:53 »
Jim: 12.5 V under load indicates that either your ALT or the voltage regulator (on the right hand side fender) is bad. From my experience with Mercedes, the indicator light is a warning light and it does not work most of the time. That is to say, a burned bulb does not interrupt the circuit. I have had several bad ALTS in different models; the light bulb never comes on. The last incident was with 2004 ML350. One way to find out, is to take the ALT to either AutoZone or Advance auto, they will tested for you for free. Advance Auto carries an Alt for 280SL for only $75 with life time warranty. If the Alt tested OK, then you should focus on the voltage regulator. They are available at several websites and e-bay. Good Luck.

Jonny B

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Re: Charging circuit on 1967 MB 250SL
« Reply #5 on: August 08, 2014, 15:22:46 »
I replaced my alternator a year or so ago, when we did it, we also replaced the voltage regulator with the Bosch recommended replacement. The new one is solid state. The noticeable (from the outside difference) is that the protective "can" is shorter. Easy to remedy, just carefully remove the can from the old regulator and replace, yellow tape for the seal is readily available (Home Depot, etc.) and voila, modern reliability with the look of the original.
Jonny B
1967 250 SL Auto, DB 568
1970 280 SL Auto, DB 904
1966 Morris Mini Minor

jcochran@cox.net

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Re: Charging circuit on 1967 MB 250SL
« Reply #6 on: August 08, 2014, 17:51:07 »
Thanks for the help! I'm picking up a new alternator this afternoon and I'll let you know if that solves the problem. It seems somewhat ridiculous that the charging circuit would depend on a dash light - if that is the case. I recently installed a new regulator and I liked the idea of re-using the old cover - I think I'll ytry it.

thanks,
Jim

ctaylor738

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Re: Charging circuit on 1967 MB 250SL
« Reply #7 on: August 08, 2014, 19:31:59 »
My experience and understanding is same as garymand.  You need the warning light in the circuit to "excite" the alternator and get it to charge.  If the bulb is burned out or there is a wiring problem, no charge.
Chuck Taylor
1963 230SL #00133
1970 280SL #13027 (restored and sold)
1966 230SL #15274 (sold)
1970 280SL #14076 (sold)
Falls Church VA

garymand

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Re: Charging circuit on 1967 MB 250SL
« Reply #8 on: August 08, 2014, 20:42:55 »
Let us know what happens.   I certainly wouldn't go through a replacement before ensuring it will or will not work with an open bulb. 

I can't tell for sure by the diagram. The details of what is in the regulator are not universal electronic symbols. 

It does appear that the alternator field is energized through the light until the regulator closes.  But then it grounds the other side of the field, which I don't understand.   I think if the regulator is not energizing the alternator, the alternator will test as no output.  

If you can take it in to test them, and the alt or reg test bad, (I'm thinking both will test bad if the circuit requires the bulb to be good)  ask the store guy for a test light.  They have ones with a probe on one side and an aligator clip on the other.  connect the light from fuse #2 to D+  

I know D+ is clearly marked on the alt.  I think it is marked on the reg pin at the base of the reg.
« Last Edit: August 08, 2014, 22:48:16 by garymand »
Gary
Early 250SL German version owned since 71, C320, R350, 89 Porsche 944 Turbo S

ctaylor738

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Re: Charging circuit on 1967 MB 250SL
« Reply #9 on: August 09, 2014, 23:19:10 »
I remember facing the question "Is it the regulator or the alternator?" with one of my cars.  I did some research and posted the results.

http://www.sl113.org/forums/index.php?topic=5964.0

Cheers,

CT
Chuck Taylor
1963 230SL #00133
1970 280SL #13027 (restored and sold)
1966 230SL #15274 (sold)
1970 280SL #14076 (sold)
Falls Church VA

garymand

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Re: Charging circuit on 1967 MB 250SL
« Reply #10 on: August 12, 2014, 20:24:22 »
The more I looked at the schematic the more confused I got because the regulator voltage has to come through the 3 diodes and voltage is only flowing if the battery is feeding the rgulator.  The BMW diagram helped because the lamp can be seen as feeding the regulator when the switch is on.  But I was stuck on the amount of current I thought the lamp would have to carry.  I went to Wkipedia and found a really good explanation,  Basically the batter does supply the alternator through the lamp just enough based on the resistance of the lamp (which is why an LED lamp in that role will not turn off until the motor revs.  Once the alternator is "excited" (which means it is generating it's own voltage in a loop from the 3 little diodes through the regulator tto the rotor then to ground.  And this rotor current is a small percent of the output current so it doesn't burn out the lamp during exitation.)  the 3 little diodes then feed the regulator to what ever few amps of current it needs to recharge the battery and keep the electronics going. 

From WIKI:  Automotive alternators require a voltage regulator which operates by modulating the small field current to produce a constant voltage at the battery terminals. Early designs (c.1960s-1970s) used a discrete device mounted elsewhere in the vehicle. Intermediate designs (c.1970s-1990s) incorporated the voltage regulator into the alternator housing. Modern designs do away with the voltage regulator altogether; voltage regulation is now a function of the electronic control unit (ECU).

The field current is much smaller than the output current of the alternator; for example, a 70 A alternator may need only 7 A of field current. The field current is supplied to the rotor windings by slip rings. The low current and relatively smooth slip rings ensure greater reliability and longer life than that obtained by a DC generator with its commutator and higher current being passed through its brushes.
 
The field windings are supplied power from the battery via the ignition switch, [charge warning indicator], and regulator. A parallel circuit supplies the "charge" warning indicator and is earthed via the regulator.  (which is why the indicator is on when the ignition is on but the engine is not running [enough current flows from the battery through the ignition switch, lamp, regulator and rotor (field windings) to ground to light the lamp, 1/3 AMP for a 4W bulb). Once the engine is running and the alternator is generating power [excited], a diode feeds the field current from the alternator main output equalizing the voltage across the warning indicator which goes off. The wire supplying the field current is often referred to as the "exciter" wire.

The drawback of this arrangement is that if the warning lamp burns out or the "exciter" wire is disconnected, no current reaches the field windings and the alternator will not generate power. Some warning indicator circuits are equipped with a resistor in parallel with the lamp that permit excitation current to flow if the warning lamp burns out. The driver should check that the warning indicator is on when the engine is stopped; otherwise, there might not be any indication of a failure of the belt which may also drive the cooling water pump. Some alternators will self-excite when the engine reaches a certain speed.
 
Older automobiles with minimal lighting may have had an alternator capable of producing only 30 A. Typical passenger car and light truck alternators are rated around 50-70 A, though higher ratings are becoming more common, especially as there is more load on the vehicle's electrical system with air conditioning, electric power steering and other electrical systems. Very large alternators used on buses, heavy equipment or emergency vehicles may produce 300 A. Semi-trucks usually have alternators which output 140 A. Very large alternators may be water-cooled or oil-cooled.
 
In recent years, alternator regulators are linked to the vehicle's computer system and various factors including air temperature obtained from the intake air temperature sensor, battery temperature sensor and engine load are evaluated in adjusting the voltage supplied by the alternator.
Gary
Early 250SL German version owned since 71, C320, R350, 89 Porsche 944 Turbo S

jcochran@cox.net

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Re: Charging circuit on 1967 MB 250SL
« Reply #11 on: August 22, 2014, 18:38:23 »
Thanks for all of your help and please know that I replaced the alternator which seemed to solved the problem. The charging light is on again and the voltage is back to normal.

Thanks,
Jim Cochran

garymand

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Re: Charging circuit on 1967 MB 250SL
« Reply #12 on: August 25, 2014, 23:55:42 »
" The charging lamp is out on my Pagoda (battery is dead) and I was hoping someone would know of a trouble shooting procedure to find out if I have a bad alternator, regulator, lamp, etc."

Sorry, I totally missunderstood.  I thought you were saying the lamp was burned out, I should have known better that you wouldn't have been running with a burned out bulb sinece the battery would have died with no charge.
Gary
Early 250SL German version owned since 71, C320, R350, 89 Porsche 944 Turbo S