Join Date: Jun 2008
Location: Memphis, TN
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I have gotten two rotten alt.s in a row.
If you have a multi-tester (digital is best for this as accuracy is crucial) start the truck and turn on lights, wipers, radio and ac running. Place multi tester in proper vDC range ( no more than 6 -10 V) and Neg lead on out-put from alternator and positive on the positive battery post of each battery while helper raises RPM to about 1200. If the reading is greater than 0.2 v DC you may have bad battery connections or internally coroded cables that are providing too much resistence.
The same can be done for the ground cables with the negative lead from the tester on the alternator body and the positive lead from the tester on the neg battery terminal. Again do not want to see more than 0.2 v DC.
I had an ambulance with dual alternators and kept burning up batteries (it had three). Turns our the bottom alt was smoked but the main problem was the cable to the rear two batteries was so internally coroded that it was about to catch on fire.
A quick, non-scientifc test is to carefully feel the positive cable to the batteries as the alternator is charging them. Too much heat = high resistence.
Do not over-look the batter connectors. Be sure they are tight and corrosion free. Hops this helps, sorry for the length
Last edited by eagledieselmedic; 08-06-2011 at 07:32 PM.