ADVANCE AUTOZONE CRANKING / ALTERNATOR TESTS ON DIESELS - Mercedes-Benz Forum

 
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post #1 of 5 (permalink) Old 12-03-2011, 12:55 PM Thread Starter
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ADVANCE AUTOZONE CRANKING / ALTERNATOR TESTS ON DIESELS

Hi All
Am coming late to the party with a '92 300D which has had 3 alternators since new (1 for each owner). Just had a cranking / alternator test done at Advance and Autozone who both said needed alternator! I left and tested 14 v with good load and then remembered glow plugs draw 100-40 amps for up to a minute. Retested alternator at idle, then shut off for cranking test and both passed. How many alternators are being replaced because of tests within seconds of starting? The OEM alternator on this 300D is able to supply only about 35amps at idle so battery voltage sags to about 12.4 until glow plugs shut off. I looked for this on forums and it is not model specific only diesels and sorry if everyone knows this so feel no need to tell us newbies. Later.

Dick
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post #2 of 5 (permalink) Old 12-03-2011, 01:57 PM
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What was the question again?
We do own 87 300DT with 280 k on it and very likely original alternator needed new brushes (voltage regulator) last year.
Autozone kids testing the parts is another story.
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post #3 of 5 (permalink) Old 12-03-2011, 02:04 PM Thread Starter
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I guess the question (implied) is does everyone know not to test alternator voltage, at idle, on MB diesels while glow plugs are still on (first 1-2 minutes after start)?? And it doesn't matter who uses the tester if they are not aware of the high current draw at idle on these (and probably other) diesels.

Dick
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post #4 of 5 (permalink) Old 12-03-2011, 02:56 PM
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Alternators are not fuel-dependent.
Big advantage of alternators v/s generators is that they can produce good amps at low speeds. I can observe voltage on car display on my W210 and glow plugs on or off, high beams on or off, the voltage stays the same at idle or not.
Might test the W124 for that out of curiosity.
Bottom line, tests machines for alternators do have whole program that last some time and it did happened to me that malfunctioning alternator from my Ford passed the test with flying colors, before it definitely died 2 months later.
From your description your alternator might have dead diode, or 2 and it does charge, but at some loads just not enough.
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post #5 of 5 (permalink) Old 12-04-2011, 08:48 AM Thread Starter
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I know alternators are not fuel dependent but only DIESELS use glow plugs drawing over 40 amps after starting but normally lower loads than gas engines, when running, so often built with lower output alternators. If the OEM alternator was not marginal, why so many threads on upgrading to ones in 100 amp area?
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