Fixed: alternator overcharging/runs battery dead with key off

[ShadetreeSeattle]

ford bronco 1988. 4.9 inline 6 applies to any 2nd generation and likely 3rd generation ford alternator setup. (internally regulated style alternator)

complaint was that battery (charging failure) lamp was on. upon inspection i realized it was overcharging condition, instead of no charge condition. (also activates charging lamp!) never knew it did that for overcharge as well. anyways, we left it to sit overnight and the next day i go to diagnose further, and the battery's dead. so i recharge it, and hit the books again. i read up on 2g charging systems, and when i go back outside to test and see if something's staying live when it shouldn't be i noticed that the alternator was quite warm, compared to the engine which hadn't been ran all day. unplug the regulator's connector and the draw on the battery goes away. (watched battery voltage creep back up instantly) so i scored another voltage regulator from the junkyard and compared results between the two. it seems the voltage regulator under normal conditions switches the negative side of the field circuit but only when it get's power from the green wire which is controlled via ignition switch and also if it detects that the battery voltage is less than alternator voltage via yellow wire which goes to main solenoid power distribution point. in this case, the voltage regulator shorted closed circuit on the negative side of the field circuit. with vreg removed it shows a short between one brush and aluminum heatsink (normally it would not return path untill it comes alive with the ignition on and detects lower voltage on battery than alternator output. anyways, didn't see anyone discussing this failure mode of these alternator regulators. (typically in the past i have seen them just go open circuit and quit charging) hope this helps out someone. cheers!

 

[miesk5]

Yo,

Thanks for coming back with the update!

 

[Skitter302]

Wow my 2g alt just did this too.

I washed my engine (used a ton of plastic bags to protect things) and when I was done I started the engine and went for a drive. Everything was fine for about 10 minutes and then my alt light came on and off depending on rpm. Anything above 2000 rpm and the volt gauge would jump to 18volts. Took it to O'reilly to get my entire system tested with their meter. Battery fail'd but alt passed.  So I got a new battery and headed home and everything seemed fine. No more voltage spikes. Next morning I wake up to a dead battery.  I leave the Bronco home with the charger on it while I go to work. Come home to a charged battery and do some testing an the alt tests as a pass with the engine at idle. So I do a parasitic draw test at the battery. I have a 5amp draw on my system :| Not too happy about that. I unplug the voltage regulator and the draw goes away. I replaced the alt and havn't had a problem since. I even took the opportunity to upgrade to a 3g 130amp unit. No more dead batteries for me.

 

[ShadetreeSeattle]

nice! happy that worked out for you in the long run upgrading to the 3g. the 2g's have crummy connectors on them too which overheat/fall off because the tab broke off a decade ago. the regulator itself features a solid state transistor which can fail in open or closed state. it's pretty easy to id a 'failed open' regulator as the voltage drops like a typical dead alternator does.  but a failed closed regulator will typically look great at idle (sometimes overcharged) and then ya hit the gas and it's boiling the battery.