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Old 08-26-2016, 02:51 PM   #1
Mini-Skoolie
 
Join Date: Aug 2016
Location: Central Minnesota
Posts: 15
Year: 1999
Chassis: Blue Bird
Engine: 3126 Cat
Rated Cap: 48
Voltage problem

I just bought a 1998 GMC Blue Bird with a 3126 cat and after about 35 miles on the drive home from picking it up the voltage dropped from 14 volts to about 12. It was still above the red mark but obviously not charging like it should. At the same time when I'd let off the foot feet and let it come down to an idle the tach would drop to 0 until I picked it up off of idle and then it would jump back up to where it should be. Once home I made sure the alternator was still spinning then shut it off. When I restarted the volt gauge was back where it should be and everything worked like normal again. Any ideas?
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Old 08-26-2016, 06:25 PM   #2
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Hello!


And hmmm... well with 12v it's almost always connections that cause random stuff.
I try and clean all connections with a dremel and a qtip of wd40.

hmmm...
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Old 08-27-2016, 07:58 AM   #3
Bus Geek
 
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Year: 1991
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Chassis: International S3800
Engine: DT360
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be sure your dashboard gauge isnt lieing to you.. I dont know about GMC's but I know internationals have ground issues going to the instrument clusters which will make the voltage gauge read off...

when you gauge drops to 12 volts, stop and place a voltmeter across your battery terminbals with the engine running and see if the voltage at your battery matches your dash gauge...

alternators often push less voltage as they warm up too.. you dont want to put 14 volts into the batteries all the time and overcharge them.. some alternators also have protectiuon against heat and lower their voltasge if they get hot.. as long as your at-the-batteries measured voltage is above 12.6 you are not depleting your batteries..

-Christopher
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Old 09-07-2016, 10:56 AM   #4
Mini-Skoolie
 
Join Date: Aug 2016
Location: Central Minnesota
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Year: 1999
Chassis: Blue Bird
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Just a follow up on this. I had the bus out and put on just about 100 miles on it this past Saturday. After about 20 miles the volt gauge dropped down to about 12 volts. When I got it home I ran a voltmeter across the terminals and it only showed 11.8 volts. I ended up pulling the alternator off and replacing it on Sunday. Problem solved. I guess it was on it's way out when it stopped charging the first time. I've just never seen an alternator stop charging and then start charging again so I figured wiring as well.
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Old 09-07-2016, 11:02 AM   #5
Bus Geek
 
Join Date: May 2009
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Chassis: International S3800
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ive had this happen before... it was likely one of the diodes in the trio... diodes are temperature sensitive esp when they start to go bad so it very well may have been trickle charging... at 11.8 with the bus running if you had any fans, A/C, lights, etc you were likely getting a little bit of juice from it..

the best alternator ive found is the Bosch SB-200 school bus alternators.. they can run SI or as a single wire or with a single battery sense lead...

they put out 140 amps at engine idle speed..

-Christopher
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Old 09-07-2016, 11:18 AM   #6
Mini-Skoolie
 
Join Date: Aug 2016
Location: Central Minnesota
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Year: 1999
Chassis: Blue Bird
Engine: 3126 Cat
Rated Cap: 48
I actually had a Delco SI-22 I believe it was 154 amp alternator laying around that I threw on there for the time being. It charges fine the only problems with it are it doesn't excite itself without running the rpm's up a bit. Also the tach drive part of the alternator doesn't seem to be working which is a minor issue
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Old 09-07-2016, 01:19 PM   #7
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Quote:
Originally Posted by QBus View Post
I actually had a Delco SI-22 I believe it was 154 amp alternator laying around that I threw on there for the time being. It charges fine the only problems with it are it doesn't excite itself without running the rpm's up a bit. Also the tach drive part of the alternator doesn't seem to be working which is a minor issue

when i first got my bus my old alternator which was so old I could barely get the numbers.. had the excite issue.. sometimes it would take me driving the bus 5-10 minutes before id get any charge...

it slowly got worse and worse so I changed it to the SB-200.. that one excites in aboiut 10 seconds and the book says it excites at 1200 Alternator RPM.. which most pulleys are 2-1 or 2.5-1 so it will excite at slow idle..

the bosch's do a "voltage check" every minute or so where they stop charging for about 1/3 of a second.. I was told if you run them as single wire thats how they test the top charge on the batteries...

if you run them with the sense wire they dont do that... so far ive been happy with it.. but if I had a "free" alternator laying in the garage i wouldve run that for sure!

-Christopher
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