diagnosing alternator output mep-803

rhurey

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My 803 is showing discharge when running, and reading ~25 vdc charging.

Read a thread about rebuilding the rectifier, but before I unsolder all that, there a way to tell if that's what's wrong? The AC tap is reading ~50 vac, which seems too related to the 25vdc on the charging circuit to be a coincidence.

Any thoughts here? The tm just says replace, but...
 

rhurey

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Sure. I'll preface by saying this was with a cheap multi-meter.

I'll have a good one tonight to look at this again.

Labels are from what it looks like the sticker on the alternator is pointing to... (Why there are two POS arrows, confusing)

NEG->POS: ~25vdc
NEG->POS: I can also get ~50vac
NEG->AC Tap: Nothing.
POS->AC Tap: ~50vac

The NEG->AC Tap does have an output on it that my scope could see:
Alternator-AC.jpg

So I'm going to measure this all again tonight w/ a Fluke meter and not some HF POS.

Measuring NEG-POS as DC on the scope showed a constant 24 V.
As AC, it produced this:
DC-As-AC.jpg
Which just seems really odd. (The voltage is off by 10X on the display because I didn't bother to adjust the display values.)
 

Guyfang

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Check the wiring on the alternator. Compare it to the schematic. Especially check the wiring to the battery charging circuit breaker on the right side of the set.
 

rhurey

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Ok, been a while on this, but finally got time to look at it more. (Plus, I plum forgot with a bunch of other stuff happening)

The wiring all looks right. I unhooked the positive from the alternator and measured from ground to the positive wires, 27V before starting. So they seem to have connection to the batteries.

Started and measured alternator output at 9.2V with nothing hooked to the + output.
 

DieselAddict

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Indeed. The alternator is a Perstolite. Parts are readily available for them inexpensively. I would recommend testing the alternator and repairing it instead of replacing it.

One of the generators I bought had no charging and it was fresh out of Tier 2 reset. I pulled the alternator apart and found diodes on the rectifier board blown apart. They had been installed from the manufacturer incorrectly. I replaced both rectifier assemblies for less than $20. It took a half hour to replace the rectifier assemblies. Photos attached.

Here are some part numbers and what I paid for them to reference if you find you need rectifier parts.

Alternator model Prestolite 8MR3005CA

0180000G98S (rectifier) $13.76
0180000F83S (rectifier) $5.83
0144300F16S (diode assembly) $5.60 (its not likely you need this one. It is included on the 0180000G98S assembly)
 

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rhurey

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I got this one with dead batteries and 76 hours on it, which makes me suspect infant mortality on some part of the alternator. (Feb 2012 reset, alternator with a 2011 QA inspection sticker on it.)

I'll be pulling the alternator off soon as I get a chance and tearing it apart.
 

DieselAddict

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That was the same date range mine was "reset". I wonder if there were a batch of bad rectifiers that came out. The alternator looked to have been rebuilt and everything looked new. Except for the parts that blowed up.
 

rhurey

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I'm hoping for 4 bad diodes. The math works for that to explain 9.2VDC given the voltage on the inspection sticker. (28.1V)

What I need to do I just realized his hook the scope up the just the alternator with the batteries removed and see what that 9.2V looks like before I pull it off.
 

rhurey

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Well, it was more than a 30 minute job, but inexperience.

So all the positive diodes were good, 2 on the negative side had no resistance at all from either direction. So replaced the negative half. (Turns out I'd ordered spare parts a few months ago.)

Re-mounted the alternator, hooked it up the way I had it diagrammed. Left the batteries disconnected from it.

Fired up the generator.

57.2V DC. Awww.... It went from 1/3rd to 2x. Well, its at least different. I'm going to bed.
 
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