73-87 Chevy _ GMC Trucks > Electrical

Stupid-simple alternator upgrade -- 55A to 94A

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rich weyand:
OK, so I got carried away.  First there was the powered subwoofer.  300W, or 25 amps peak draw.  Then there were the Hella 7" round replacement glass with Philips Xtreme Power bulbs.  11 amps.  Then the replacement bumper with the integral fog lamps.  9 amps.  Finally the Flexalite Syclone electric fan.  18 amps run and maybe 30 amps on start-up.

I have been waiting for the 10SI 55 amp alternator to die, and it did.  Lost a diode, I think.  Anyway, it won't lock in at idle any more if even just the headlights are on.  So, time for a replacement, which means time for an upgrade.    ;)

Yes, you can grind brackets and change wiring and change pulleys or go serpentine, and then run the 105 amp CS130.  Or you can just buy a 12SI 94 amp alternator, and swap it out with no other changes.

The one I got is from O'Reilly's.  It's their part number ULT 01-0266.  They stock these all over as they were ubiquitous.  $70 plus a $15 core until you bring the old one in.  It lists as for a 1984 Chevy Camaro with the 305 G-code engine as one example.

One change I did make.  I did not hook up the stock charge wire to the BAT connection on the back of the alternator.  That 12 AWG wire down to the starter is not stout enough for 94 amps.  I cut the connector off the end and put some shrink wrap over the end to keep it out of trouble.  Then I ran a 6 AWG wire from the BAT post on the alternator directly to the battery POS terminal.  I loomed it with the negative battery cable so it's not obtrusive.

Works great.  No more problems keeping voltage lock, even when I am running lots of lights and loud stereo and the cooling fan kicks on.   8)

Here's a good resource on the 10SI and 12SI alternators.

     Thanks Rich for the post, this is one that is going into the saved file under alternator upgrades.  Great job.

rich weyand:
Thanks!  Any time I look into an upgrade or change, and I don't see all the info in one post, or have to scrounge info from all over the net, I figure it's time for a post.  I tend to do quality upgrades that don't require lots of changes, use common parts, and run parts and wiring well within their design envelopes to avoid problems later on.

So I try to write the post I wish I could have found myself, with complete description of the goals, the methods, the part numbers, things to watch out for (like that too-small charge wire), and some reference material -- basically everything you need to go "Hey, I'll just do that" and you can print it out and drag it around with you to the parts store and what-not.

Thanks for the note.  It's nice to get feedback on these posts so I know it's worthwhile to write them!

that's awesome.  Thanks for doing the leg work.  I was wondering about an alt upgrade just haven't looked yet.

rich weyand:
BTW, getting rid of the stock charging wire (BAT on alternator to starter) and replacing it with a 6AWG wire from alternator BAT direct to the battery is a good idea if you are running a lot of electrical equipment on your truck.

If you pull 50 amps through five feet of 12 AWG, you get .4 volts of loss, and heat that wire with 20 watts.  If you pull 75 amps though five feet of 12 AWG, you get .6 volts of loss, and heat that wire with 45 watts!

Three feet of 6 AWG wire from BAT on the alternator to the battery only loses .02 volts at 50 amps, and .03 volts at 75 amps.


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