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Old 11-30-2007, 01:51 AM
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Join Date: Sep 2004
Posts: 142
Voltage Regulator breakin???


My Question:
Does a new Voltage Regulator need to be broken in or do I have a bigger issue? I noticed that after it had been running for a bit (10 minutes), the idle voltage increased but still didn't reach a voltage level I was expecting from a good alternator/voltage regulator combo. Was 12.5V increasing to 12.9V after 10 minutes at idle and 13.48V above idle. I was expecting over 14V. Do I have a larger issue at hand? The new voltage regulator has increased the voltage observed while running, but not to the level I would have expected. Am I missing something?

Any/all thoughts are appreciated!

-----Related information which may or may not be helpful-----
1978 300CD

Initial Symptom:
Car wouldn't start. Insufficient battery power even after running. Wouldn't have a sufficient charge to start the motor even after running around the block a few times ~10 miles.

Troubleshooting steps taken:
Jumpstart vehicle and test voltage at the battery. Voltage while running was below 12V. Suspect deficiency in alternator or voltage regulator. Removed and inspected Voltage Regulator. Spring loaded terminals were 7/16ths of an inch long (I read in another VR related post, if they're below 1/2in, you may want to replace the VR before replacing the Alternator).

Relevant History:
Battery is Sears Die Hard International approx 6 months old. Car has less than 300 miles since new Battery
Alternator was replaced 3 years 11.5 months ago. Car has approx 20,000 miles since rebuilt Bosch alternator/volt regulator.

Actions Performed:
Replaced Voltage Regulator, Jump Start, and Retest Voltage at battery terminal.

Since I don't have a tachometer, I have to go by feel. Since I'm not an idiot (well... complete idiot at least) I ran the engine at idle and at a rev plateau above idle where the engine smoothes out (I'm guessing ~1000rpm). Anyway, here's what I got:

Idle Voltage At first: 12.25V to 12.45V bouncing
Revved Voltage At First: 13.48V steady
Idle Voltage after 10 Minutes: 12.9V steady
Revved Voltage after 10 Minutes: 13.48V steady

I was expecting to see over 14V when running (at least while revved) but when the Voltmeter hit 13.48V, it looked like it hit a wall. I didn't see any momentary spike above 13.48V when revving it up under any of the conditions I tried. I gave the engine as much as 1/2 throttle and the volts never reported over 13.48V

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Old 11-30-2007, 02:48 PM
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0-60 in 10 minutes flat
Join Date: Sep 2006
Location: Middletown MD
Posts: 527
Sounds like your alternator has at least one dead rectifier (aka diode). If you don't want to buy a rebuilt alternator, you may be able to find just the burned out part.
1984 300TD

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Old 12-01-2007, 01:18 AM
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Join Date: Jul 2001
Location: SoCal
Posts: 1,303
I agree.

'91 MB 190E 2.3
'08 RAV4 Ltd 3.5
'83 Lazy Daze m'home 5.7
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Old 12-01-2007, 01:26 AM
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Join Date: Dec 2000
Location: Battle Ground, WA
Posts: 576
I also agree, you still have a problem. If you have an oscilloscope available you could scope the output of the alternator, and if you have a bad diode (or two), you will see that the output is missing some peaks. (The output waveform should have regular peaks, with none missing. One thing that sometimes happens to alternators is that the belt is loose or is glazed enough that it will start to slip under load - have you checked the belt tension? I was caught by that one once... It's a bad one, because sometimes after you tighten the belt the alternator will be outputting it's peak current, trying to charge the battery, and will overheat and blow a diode or two.
Richard Wooldridge
'01 ML320
'82 300D 4.3L V6/T700R4 conversion
'82 380SL, '86 560SL engine/trans. installed
'79 450SL, digital servo update
'75 280C
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Old 12-01-2007, 05:07 AM
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Join Date: Sep 2004
Posts: 142
The belt is tensioned okay, just the wacky values on the charge.

I don't have an osiliscope at my disposal. I guess I'm SOL for more troubleshooting.

Thanks for the input! Before ordering a new alternator I'll pull the old one and crack it open. If anything is obvious I'll look into replacing it, otherwise, I guess I'm getting a rebuilt alternator for Christmas!!!

I appreciate the help!
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Old 12-01-2007, 09:31 AM
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Join Date: Jan 2003
Location: Matthews, NC
Posts: 1,356
First, 13.5V is not unusual for a EURO built car. The other thing, the voltage will be low until the battery is fully charged. Also, a battery can hold down the voltage if it is bad. You really need to have the amperage checked with a fully charged battery and without a load (a/c, lights, etc) then checked when under a load (a/c on high, lights). I don't know exactly what the amperage should be for a 79 300D but it should be able to supply that much under a load and engine running at 1500 rpm or so. Very few people have a AMP meter that can read up to 100A so I suggest you drive the car for a while and see how it does. The voltage readings you are seeing could be normal.
By the way, I would much rather see a voltage of 13.5 with no problems than see 14.5 with no problems. The battery charged to 13.5 will last a lot longer. A battery charged at 15V will have a life span of maybe 1000 miles or less depending on the outside temp. It could also blow up but will at least have that green messy stuff on the terminals.
84 500 SEL (307,xxx miles)
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Old 12-02-2007, 09:58 AM
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Join Date: Jul 2001
Location: SoCal
Posts: 1,303
The voltages you are getting are symptomatic of a bad alternator, not a bad battery. However, disconnect the battery to test. If you read 10.5V or higher, you do not have a shorted cell. Note that 11.9V is what you would get with a discharged GOOD battery, and under charge this should read above 14V when above idle, until fully charged, from a good alternator. A battery that is sulfated and worn out would indicate an overly high charge voltage when charging from a good alternator.


'91 MB 190E 2.3
'08 RAV4 Ltd 3.5
'83 Lazy Daze m'home 5.7
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