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Old 11-17-2016, 06:04 PM   #1
Mini-Skoolie
 
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91 BB w/ 5.9 cummins alternator might be bad?

Ok...so today my girl went to start the bus. Woomp...woomp it went. I checked the battery and got 11.2 volts. Hmm. hooked it to her car and it fired up after a few minutes. Putting the volt meter on the battery as it was running showed me that the voltage was dwindling quickly. Checked the back of the alt as it was running and it was doing the same. Bad alt is my best guess. I unhooked the battery pos as it was running and it died. Bad alt right? Any suggestions would be regarded as gold to me right now. THanks.
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Old 11-17-2016, 06:10 PM   #2
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Is your engine an electronic model or mechanical? MY 3.9 is mechanical and only needs juice to start. Once going, it keeps going until the fuel is cut off.
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Old 11-17-2016, 06:20 PM   #3
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It is mechanical. Strange that it drains the battery as it is running. Never had a problem like this and I am stumped.
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Old 11-17-2016, 06:24 PM   #4
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I am hoping I am missing something as replacing the alt does not make me smile. Belt, placement...it seems like a pain. Its a Delco btw.
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Old 11-17-2016, 06:57 PM   #5
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Quote:
Originally Posted by Tango View Post
Is your engine an electronic model or mechanical? MY 3.9 is mechanical and only needs juice to start. Once going, it keeps going until the fuel is cut off.
My 1992 had an electric solenoid turning the fuel on and off. Also, that solenoid is controlled thru an electronic two-stage relay. Once the engine is running, the relay reduces the voltage to the solenoid, to keep the solenoid from overheating. But it seems worth mentioning that this system is there. (I removed it and installed a manual cable for reliability.)

My alternator turned out to be a common automobile type. So might not be terribly expensive.
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Old 11-17-2016, 07:10 PM   #6
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My Thomas only read 11.2 the other day but she turned right over..... once she was running I noticed the voltage reading was a little lower than I'm used to seeing..... where she'd been putting out 14.0 or 14.1 she was down to 13.6.... not terribly worried but a little concerned about the unexplained drop..... but it had been a little cooler the last few days than it was previously plus she sat for 11 days without being started so that might account for part of it too.

Not really sure why yours wouldn't start... unless maybe it had been sitting a while or was on the cold side.....
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Old 11-17-2016, 07:16 PM   #7
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if you are going to run a lot of electricals on your 12 volt system.. id recommend the Bosch SB-200 (200 amp) alternator... it can be run as a 1 wire or connected as an SI type... it puts out 80% of its power at 1200 Alternator RPM (not to be mixed up with engine RPM)... its made in many different configurations and chances are your current pulley will just bolt right on it.

-Christopher
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Old 11-17-2016, 07:19 PM   #8
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Replies appreciated! Ole Gary was started wit no problems bout 12 days ago. It has never not started. Battery is new but I did find that the neg cable to it had loosened but tightening did nada. If I do replace the alt, pointers to do that will be helpful. I am anxious to wake up tomorrow just so I can start working on it.
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Old 11-17-2016, 07:25 PM   #9
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Bosch eh? I looked at that one online and was curious. I am still wondering about the one in the bus now. I thought alts were all or nothing as far as voltage? Baah....
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Old 11-17-2016, 07:42 PM   #10
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they can be partial if the diode bridge is failing...
but when you measure the voltage at the big red wire on the back of the alternator that is a direct connection to your battery so you are seeing your battery voltage.. if you disconnected that wire from the alternator and measured at the alternator terminal you might then have 0 if the alternator has quit completely..

also to note is some alternators require a field-sense wire to "turn-on". if thats the case on yours and the fuse blew on the sense line then the alternator would fail to charge. .. or sometimes require the engine be revved up a few times to get the alternator to 'excite'.. so if you jumped the bus and just let it slow idle it very well may have never 'excited' so the voltage would drop till the solenoid closed and the bus would stall out..

-Christopher
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