When I disconnect my battery, ouput voltage increases.

[Strad]

Hey guys, I'm just wondering if anyone has come across this. I converted my truck to 12 volt, and its been running fine for some while now. Well, today I was having trouble with the battery starting my truck and the battery is 2 months old. I took it Advanced Auto Parts to get tested and it checked out, it just needed to be charged. Well I hooked the battery back up and started my truck. While the truck was running with the battery connected, the alternator was putting out 12.3 volts at the alternator, not enouch to charge the battery. But while the truck is running, and I disconnect the battery, THE OUTPUT VOLTAGE JUMPS UP TO 13.9!!!
Can someone provide some insight? I'm completely at a loss on how to keep it running at 13.9! Thanks!

 

[Westech]

sounds like bad alt. the voltage regulator is weak and not able to adjust to the load.

 

[rnd-motorsports]

 

[JAsher45]

yeah bad alt.. 12.3 that is battery voltage at the Alt

 

[Oldfart]

I agree the place to look is at the triode. If the battery is draining while off for short periods, one of the diodes may be leaking back excessively. While 13.9 volts may seem enough, it should be more on the order of 14.3 to 14.8 with the battery connected and charging to replace what was used during the start. There is an inverse relationship to the volts/amps. As the volts of the charging system goes up, the amps should come down. The normal at rest voltage of a properly charged 12 volt lead acid battery should be near 12.6 volts. If you can read the amps as well as the volts, a low amperage output at 12.3 would tell you for sure that your alternator was not covering the demand. (Most likely the regulator and a diode problem, but I have encountered slip brushes that were hung up and not making good contact with the slip ring that also caused low amp output.)

 

[Strad]

Well, I had purchased the alternator at oreily's so they're going to replace it for free. Its wierd that I have to rev the engine just once when i start the truck in order to get the alternator to start putting out 12.1V...
Thanks for your help guys. I'll update as soon as I install the new alternator.

 

[Oldfart]

Having to rev the engine is something that occurs because the output voltage is not high enough to close or activate the charging circuit at lower rpms. This is more common on the old style coil regulator systems. ~~~ Could it be that your converted 12v alternator has the wrong size pulley? I am not familiar with CUCV's differences, but I have encountered the wrong alternator speed on a couple of engine swaps I have done. In one case on a generator, the rpm was too high and I had the brush commutator come apart. On an alternator system, the alternator did not spin fast enough and it would not charge from idle speed up to about 1800 engine rpm. In both cases, a change in the alt/gen pulley size corrected the problem.