So as some of you may have deduced, I've gotten my truck up and running again after an engine and tranny rebuild some 8-9 months ago (what can I say...life gets in the way...).

So now I'm seeing what needs to get corrected from it sitting so darn long. One thing that is new is the alternator light is now glowing dimly. Never used to do that. I have one of those voltage meters that plugs into the cigarette lighter, but I can't tell if its giving me useful information. Driving the truck home, I remember the display up around 14+ volts. Thought it was high but just shrugged it off as the battery being discharged. No alternator light on though. Once at home, I noticed that the alternator light started glowing dimly on startup and didn't go out. Also the plug-in volt meter is now at a constant 12.6 and when I turn the lights on (blower motor disconnected for AC conversion to Vintage Air) the voltage drops to 12.3 and stays there. Does this sound normal? Should I be measuring the voltage at the battery and not relying on this gizmo? Or given the light is on and never used to be, is something definitely wrong.

Thanks in advance,

Geoff

 

You put in a 3G alternator? Did you rewire the entire truck or did you just wire in 3G to existing harness?

Two things... Did you reuse the old wiring running on passenger fender in to cab? Did you put new wiring under dash (new fuse block or dash wiring)?

The reason I ask, there is a shunt in the stock wiring harness that runs along the passenger fender. This shunt ends up at the ammeter (or light, can't remember) that basically let's the gauge see two different voltages. Layman's way of explaining it. The 3G alternator wiring may have bypassws part of that harness (as it should) and now the gauge shows a discharge when it really isn't.

Second. I'd you have stock wiring harness and fuse block in cab, they are old and woefully inadequate for carrying much current. You may only have 12.3 volts at the cig lighter, with heater going. Remember, all power comes down that passenger fender.

So...you need to measure voltage at battery. Is it greater than 12.5? Then alternator is working. 14ish would be max. Less than 12 volts indicates no charge. Measure battery voltage after battery sits overnight to get you battery baseline volts. Measure more than that when truck is running and alternator works.

 

YOU GOT A BIGGER ALTERNATOR...............YOU GOT GET A BIGGER BATTERY..............

 

I should have clarified, gents: The 3G alternator is an old install that I did when I first got the truck a couple of years ago. The install was performing great to the day that I yanked the engine for the rebuild. Putting everything back together and firing it up, I see that the alternator light is now dimly glowing. Trying to find out what the heck changed?

Geoff

 

Bad or missing grounds (engine to frame and engine to cab, cab to frame, etc...)?

 

Proper running voltage is 14.5v. I would remove the alternator and get it bench tested to start. This lets you isolate the problem to either the alternator itself or the wiring. Something may have happened to it while it was off (dropped, terminal broken?) Once you know the alternator is working correctly, then you can deal with any wiring issues.

 

Good advice, Dave. Took it in and failed repeatedly in the light circuit test. Before I took it in, measuring the voltage at the battery never went higher than 12.6 volts.
A new alternator is on the way.
Thanks for all of your collect help!

Geoff

 

The volts on a good ALT should be between 13.5 and 14.5.
A fully charged battery should have 12.6 volts and you need more than that to put back in / charge the battery and why the above volts.

As you found out that cig. volt meter works so keep it.
I also liked that someone said to check all grounds and may be add more as you can never have too many grounds.
Dave ----

 

Swapped in a 3G alternator for a 97 Mercury Sable from Blamazon, and no more alternator light. My plug in voltmeter showing 14.3-14.5 volts. All is as it should be.

Thanks for your help.

Geoff

 

I'm glad the problem is solved. Good info on this thread folks.