The gist of it is that the ECM and the gauges show that my truck is producing about 12.5v at idle but the battery and the alternator's output are both showing 13.5-13.6v at idle. This is a direct repost from a different forum.

This is where my voltage gauge usually sits (for the year that I've owned it):


As of today it's been sitting like this:


When I check Torque Pro via OBDII port I get this (Torque Pro gives me this voltage based on the voltage at the OBDII port):


And the kicker is when I check voltages at the battery and the alternator I get this:


Another note, this is ONLY at idle. The second RPMs go up everything goes back to normal. It stays normal while driving as well as of this moment. Another side note is that my headlights have recently begun to dim at idle and brighten when I give it gas. I was leaving towards alternator but that wouldn't explain the 13.5v at the battery and 13.6 at the alternator. Right now I'm just confused. Every thing I've read suggests that if it is the ECM then it shouldn't be charging at all. What triggered my concern was when I first noticed that my headlights were dim as I sat at a red light. They would then brighten as I revved the engine ever so slightly to leave the idle RPM range and the gauge in the dash would go up. This suggests to me that it's the alternator going bad BUT the charging system still shows 13.5v. I'm about to try another multi-meter just to be on the safe side. Will report back later.

Any ideas?

Edit: It's a 1996 Dodge Ram 1500 5.2L engine and factory alternator from what I can determine. 128k miles on the ODO.

Edit 2: Walked outside to do some more testing and on a cold start everything reads normal. So now I'm letting my truck warm up to see if it's heat related.

Edit 3: Voltage is slowly dropping via the dash gauge while the truck runs and warms up.

Edit 4: after more testing I believe this may just be a wild goose chase and now I just don't trust that gauge in the dash as it wants to report the actual voltage instead of 14.7 all the time. As for the OBDII port I suddenly think that it is supposed to look like that. I think I'm going to install an actual voltage gauge and stop trusting the one in the dash.

I'm still open to ideas if anyone sees anything wrong in all of this.

 

The gauge in the dash is not accurate, Mine and many others do the same thing. If the alternator stops charging the idiot light will come on so as long as it's putting out (as per your voltmeter(vom)) i wouldn't worry about it.

 

as for the headlights dimming, I would probably start with checking your cables and connections to make sure they are clean. then check the wiring to the headlights for the same thing.

 

That's the conclusion I've come to after messing with this truck all morning. Your response just confirms my suspicions. I always thought it was funny that my truck was charging at the high 14 low 15v range according to that thing.

I think I'm going to relocate the grounds to the battery. I did a headlight relay mod myself a few weeks back. I used the same grounds that the engine straps use, but currently I'm considering not using them and running them back to the battery.

 

Do you have the stock pulleys on your truck still?

My truck exhibits some odd behavior at idle as well.... rather randomly.... Idle will drop, and then voltage will drop as well..... When idle drops below about 500 RPM, I see battery voltage on the gauge in the dash. The lite doesn't come on..... so, the alternator still seems to be trying to charge, I think it's just RPM is too low for it to do so effectively. Of course, it only does that when it is REALLY cold out..... and the rest of the time, the engine idles around 650-700 RPM. No clue what's going on there.

 

I believe they are all stock except for the idler pulley. The only one really having issues is the AC compressor pulley but that will get replaced in a month or so. The only real issue I had was it appeared that the dash voltmeter was below what it usually sat at. If I revv the engine above idle then it goes up to 14+ volts range according to that gauge. Now, I have measured both the alternator output and the battery and both are about where they should be with the battery topped off after start. The alternator produced 13.6v and the battery was at 13.5v. I couldn't actually get the voltage to dip at the battery or the alternator by any significant amount. I turned the brights on (sport mods, I have mine wired to run the brights in both the low beams and high beams for brights), stereo blasting (only 2 working speakers lol), dome light on, door open so that annoying buzzer would go off. Pretty much everything I could to put a load on the alternator. It stayed around the 13.5v range. So by this point I think the headlights are just a poor judgement in ground location and the voltmeter in the dash is crap. That's what I'm thinking at this point.

 

That seems like a fairly reasonable supposition. Not like dash gauges have ever been really accurate.

 

Yep, I've noticed that with the Oil Pressure Gauge. That thing does whatever it wants to. I mean it pretends to work. Sometimes it's static and sometimes it actually moves with RPMs.

 

Exactly.