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That's a nice looking rig you have there. I have a dual alternator truck as well. Having one alternator die on these 2 alt. rigs does not always set the alternator light on the dash I found out. You would not have low volts typically unless both alternators conked out. That lower alternator might also be shot. Both alternators run all the time, accept during cold start when the glow plugs are energized. The PCM keeps the lower alternator off during that time (up to 120 seconds). See Y2K's description of how they work in his post right above yours.
One way you can keep an eye on them is to check for codes from time to time. P1105 will set for the upper, P1106 for the lower, if the PCM detects a fault. It will not set the SES light though, so it's not a quick alert for you. Recommend setting yourself up with FORScan and checking for codes at least monthly on these aging rigs. How-to thread below:
Another way you can test your lower alternator, is to unplug the pigtail on the upper alternator to disable it. Put a voltmeter across one of the batteries, or in your cigarette lighter, and watch volts. Start the engine, and, after the glow plugs have cycled off, if you see more than battery voltage (13+ volts, 14.4V typical), you know the lower alternator is putting out.
That's a nice looking rig you have there. I have a dual alternator truck as well. Having one alternator die on these 2 alt. rigs does not always set the alternator light on the dash I found out. You would not have low volts typically unless both alternators conked out. That lower alternator might also be shot. Both alternators run all the time, accept during cold start when the glow plugs are energized. The PCM keeps the lower alternator off during that time (up to 120 seconds). See Y2K's description of how they work in his post right above yours.
One way you can keep an eye on them is to check for codes from time to time. P1105 will set for the upper, P1106 for the lower, if the PCM detects a fault. It will not set the SES light though, so it's not a quick alert for you. Recommend setting yourself up with FORScan and checking for codes at least monthly on these aging rigs. How-to thread below:
Another way you can test your lower alternator, is to unplug the pigtail on the upper alternator to disable it. Put a voltmeter across one of the batteries, or in your cigarette lighter, and watch volts. Start the engine, and, after the glow plugs have cycled off, if you see more than battery voltage (13+ volts, 14.4V typical), you know the lower alternator is putting out.
Yes my red alternator light never came up till i was pulled over and truck almost dead.. can’t rely on that..
plan to pull the little two wire pigtail off the top alt tomorrow and see if it will still make amps and volts from bottom.. should see it fairly quickly i think
Little confused on the one point where you say the second alternator is used after 120 seconds, I had thought it was not used if ecm was happy with the top amps, or am I missing something there, if not needed it is. It used?
So drove truck 4 hrs and voltage is low again, enough to set off warning light, so I’m guess the bottom alternator is shot, plugged in wires and top started to work again and gauge is back to norm.
so what is the bottom alternator we need for this, and where can I get one from? Best price Bottom alternator
Cheapest option would be to rebuild your lower alternator yourself. That's what I did, using a kit from rebuilderinabox.com. About $60.
Another option is to purchase a new alternator from Rockauto. Buy the ACDelco 3351155 upper position alternator for $127, and re-clock it to fit the lower position. Real simple to do by removing three screws and rotating the front and back case halves until they line up correctly. Remove the slightly larger drive pulley from your old lower alternator and install it on the new one.
For a straight bolt-on replacement, I've listed two additional options, also from Rockauto. They should be clocked correctly and have the correct pulley already, for about $40 more.
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