Alternator bad?

[lmholmes11]

I was having some troubles with a p1781 code and the tranny not engaging few weeks ago. Changed batteries and replaced the NSBU switch, all tranny problems went away and the new batteries were awesome, cranked over really fast. Yesterday I noticed that the voltage meter on the dash would start out low and get higher as I drove (never paid close enough attention to see whether it did the same thing after new batts or not) and it would crank over noticeably slower and the code was back again right after start up. I was driving with bad batteries for quite a while and as Kennedy posted on my previous thread, the alternator could go out anytime because of being over worked. I know of a few ways to see if the alternator is working properly but I just wanted to ask everyones opinion on what way is the BEST. Thanks all.

[cowboywildbill]

Since you already have replaced the battery's. Check your cables and make sure they are clean, and all of the places that they ground to. Then hook up a good volt meter to one of your battery's. Check your voltage before you start or turn the key on. It should read about 12 volts or more. Good battery's that are charged up will usually read about 12.7 to 13 volts or so. When you first turn the key on and start your truck you may see for a brief period of time about a minute or less the volt meter drop to 10 volts or so. That is normal due to the glow plugs heating up and cycling.Make sure the lights, heater fan and accessories are all off. As the truck idle's you should see an increase in the voltage to about 13.7 to 14.7 volts. Then you can try to turn on the head lights, Heater fan to high and wipers so on. The voltage should stay up 13.7 volts or more while all of this is running. If you see a decrease in voltage after the truck is running less than it was before starting , then that is a good indication that the altenator isn't charging properly. Also if you see voltage 17 or 18 volts or higher, that too is a problem. Hope that helps. It also doesn't hurt to charge the batts up before you start the process. And sometimes even a new battery can be bad. It takes a load test to really tell if a battery is good or bad. Good luck hope that helps.

[lmholmes11]

Thanks for the info! I will do as suggested, hopefuly I get to the bottom of this before winter hits!

[cowboywildbill]

Good luck with it. If you don't find a problem with your Alt. One of the next steps would be to see if the BCM Body Control Module is not functioning properly and causing too much current draw due to something staying on. You could charge your batt's up fully and then unhook them. See if they still have the same charge a couple of day's later. If they do then hook the batt's back up and check them again in a couple of day's. If you notice a significant drop in vlotage, then you may have somnething staying on and drawing too much juice. Note that with all of the systems on these vehicles it is normal to have a very small current draw all of the time when the truck is turned off. Let us know what you find.