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testing alternator capacitor
My alternator is not charging the battery.
I pulled it and O'Reilly said it had a bad capacitor. Can the capacitor be tested through the three prong plug or do you need to tear the alternator apart to get to it? Also, what is the black part next to the three prong plug? It is showing an open circuit between the ground and the end of the wire. The insulation broke off when I removed it to test. Voltage regulator is good. I had this alternator rebuilt three years ago locally. Jeff
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1984 300SD 300,000 miles--two tank WVO setup 2.88 diff & 500SEL anti-squat rear end |
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The capacitor is nothing but a noise supressor. The alternator can live happily without it.
That would be the capacitor. |
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Is the capacitor the same as the diode? or is the diode internal?
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A capacitor is to a diode what a muffler is to a fuel injector. One is essential to output; the other just keeps the noise down.
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From what I've been reading the capacitor in an alternator
has three diodes and a ground. Ohm resistance should only flow in only one direction across the diodes. The capacitor turns AC current to DC current. Please correct me if I'm wrong. Are you saying that an alternator doesn't need a capacitor? Also, that doesn't answer my question about whether or not I need to take the alternator apart to check the diodes in the capacitor. Thanks, Jeff
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1984 300SD 300,000 miles--two tank WVO setup 2.88 diff & 500SEL anti-squat rear end |
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Quote:
You are wrong. Diodes convert ac to dc. Capacitors just smooth out the spikes. There are no diodes in the capacitor. Who ever is giving you the story about the capacitor is pulling your leg. Best you go see a good auto elec or read a book on basic electronics.
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Grumpy Old Diesel Owners Club group I no longer question authority, I annoy authority. More effect, less effort.... 1967 230-6 auto parts car. rust bucket. 1980 300D now parts car 800k miles 1984 300D 500k miles 1987 250td 160k miles English import 2001 jeep turbo diesel 130k miles 1998 jeep tdi ~ followed me home. Needs a turbo. 1968 Ford F750 truck. 6-354 diesel conversion. Other toys ~J.D.,Cat & GM ~ mainly earth moving |
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Quote:
http://en.wikipedia.org/wiki/Rectifier I don't know if a bad capaciter could cause a charging issue (like a short circuit) but it could be unplugged to see what happens. I don't recall my Alternator having a Capacitor.
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84 300D, 82 Volvo 244Gl Diesel Last edited by Diesel911; 06-17-2012 at 11:03 AM. |
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Quote:
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96 E300d |
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I can see where the OP is getting confused. The higher quality rectifier circuits do, indeed, make use of capacitors. The capacitor serves to make half and full wave rectifers generate a more constant DC output voltage. Otherwise you'd have positive (or negative) going half sinusoidal waves only. This isn't very efficient and could possibly wreck havoc with sensitive electronics among other things. Think of it like a "miss" in our engines, buy only invisible.
The alternator could very well function a long time with a bad capacitor (assuming it has one). But it wasn't designed that way.
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1974 240D "Boldie" 170K.- New timing chain/freshly rebuilt IP/replaced valve seals/injectors/upgraded stereo/new Bilsteins with Yokohamas/fresh paint and rocker panels plus lots of welds. |
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Pimpernell, Thank you for answering my question.
I am checking just continuity and not ohms resistance, correct? In the photo above, I get a continuity reading between the prong closest to the D+ and the alternator body. The other two prongs do not show continuity. Bad diode in the rectifier? The black capacitor shows neither continuity or ohms resistance. 911- thanks for the clarification regarding the rectifier. The fellow at O'Reilly told me I had a bad capacitor, not a bad rectifier. The alternator is going to the shop next week. Grumpy Ol' Layback40--Hey, I'm just trying to learn a little about checking these alternators from you nice folks without having to read an entire book on basic electronics. Jeff
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1984 300SD 300,000 miles--two tank WVO setup 2.88 diff & 500SEL anti-squat rear end |
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[QUOTE=Rooster300SD;2957056]Pimpernell, Thank you for answering my question.
I am checking just continuity and not ohms resistance, correct?" Yes, just continuity between the battery connection of the alternator and the alternator case. This does not guarantee that all your diodes are fully functional, but will show if one has shorted out, and allowing current to drain off via the alternator case to the engine block. In my case, one or more of the diodes went bad, and when the engine was off, in a matter of 5-10 hours, the battery would be almost dead. I disconnected the battery, charged it, and it had no problem holding the charge. Disconnected all the wiring to the alternator, and did the above test, and found that the alternator was draining off current. New alternator installed, and everything has been fine. Good luck!
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96 E300d |
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FYI
Quote:
Construction Mechanic Basic Volume 02 - Construction methods and practices .
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ASE Master Mechanic asemastermechanic@juno.com Prototype R&D/testing: Thermal & Aerodynamic System Engineering (TASE) Senior vehicle instrumentation technician. Noise Vibration and Harshness (NVH). Dynamometer. Heat exchanger durability. HV-A/C Climate Control. Vehicle build. Fleet Durability Technical Quality Auditor. Automotive Technical Writer 1985 300SD 1983 300D 1984 190D 2003 Volvo V70 2002 Honda Civic https://www.boldegoist.com/ |
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