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Old 08-11-2003, 01:06 PM
JohnnyV's Avatar
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Join Date: Aug 2003
Posts: 8
Post 70 AMP Alternator for 1988 300E

Problem: Battery Died

Bubba diags: I charged the battery and started the car. I disconnected the negative battery terminal and the car stopped running.

Conclusion: Charging system is bad.

Question: How do you check if the alternator or Voltage Regulator is the charging problem?

Note: Obviously the regulator is a more cost effective fix, so I want to be able to check for that first.

Any help is greatly appreciated.

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Old 08-11-2003, 02:26 PM
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If you don't own a VOM meter then a cost effective way would be to (leave the battery hooked up this time) drive to your nearest AutoZone and they will check the charging system and battery for free.
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Old 08-11-2003, 02:58 PM
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Posts: 8
Thanks Jeff.

Actually I do own a VOM, but it is temporily hidden in a box due to a recent move. What I was hoping to find out is if there was a way to distinguish the problem between the regulator or the alternator. I have already determined there is a charging system error with the bubba diags.

I will find the VOM soon or borrow one, but do you know how to check if the regulator or alternator is causing the problem?

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Old 08-11-2003, 03:58 PM
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Location: Malta, Europe
Posts: 54
i usually check regulators by connecting their input to a variable power supply and a voltmeter to its output. start the power supply at 5v and keep turning the voltage up. this voltage should be passed onto the regilator output (and on the VOM). when the power supply reaches 14.4 volts, the regilator should cut out and the VOM should read 0.00

if it does not cut out then the regulator is knackered
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Old 08-11-2003, 08:13 PM
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Join Date: Aug 2003
Posts: 8
For those interested in solutions, the best input I have seen yet is from Thomaspin on this posting. Click here

To everyone else, thanks for the valuable input. It appears some people do not get the humor of bubba diags.

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Old 08-11-2003, 08:55 PM
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Join Date: Feb 2002
Location: Seattle
Posts: 596
When my regulator went bad I noticed had no (none, zero, zip, all dark) instrument cluster test lights on when key was in position 2. My Bubba Diag was to recall a posting on eBay in which a seller noted this as a symptom of bad voltage regulator. I searched for completed items and found the post and completed my diag. New regulator and system is good now. (Old one was down to nothing on one side.) Then replaced the reg in my older W124 as a preventative measure. Both sides were down to nothing on that car. This is a 6 minute job.
1986 300E Anthracite + ECodes + MB Mileage Award
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