Schrodinger's Alternator

Roughly half a decade back I did an upgrade from the stock 75A 2G alternator to a 130A 3G alternator.  Really helped overcome variety of issues associated with 30 years of wiring aging.

The truck's alternator is being weird.  The charging gauge is one of the few from the factory that actually work.  After driving the truck for 17ish years I know when the alternator is properly charging.  For the past two months or so the gauge has been reading low, low enough that about a month back the battery should have left me stranded.  I finally found my meter and actually checked it, 12.85 volts with the engine off it was 12.74 volts.  The funny part is that I took it to two separate stores and found that it is charging properly, 14.2 volts.

I haven't put it back in the truck yet, but I checked my wiring (because its my wiring) and everything looks good.  Continuity where it should be, the 150A fuse I put in the system isn't blown, everything looks good.

So, what is possibly going on here?  Some crud in the connections?  Something else?

Check all of your grounds, that seems to have a magical ability to cure random electrical weirdness.

You can measure the voltage drop from the hot lead of the alternator to the positive battery terminal with a meter, and then measure the voltage drop between the negative terminal of the battery and the alternator's case. 

Possibly a bad battery.  If it's putting out 14+ on a bench and only 12-and-change on the battery... I suspect the battery.

Ground is a distinct possibility as well.  Put one end of jumper cables on the battery negative and ground the other end to various parts on the engine/frame. If you get a voltage increase, bingo.  Bad ground.

In reply to Curtis73 (Forum Supporter) :

Possibly a bad battery, but based on past experience I would have expected the battery to strand me by now.  

Do a voltage drop test on both positive and ground sides of the charging circuit. 

Check the brushes, I just had an alternator bite the dust on the Mercedes, put out 12.5 or something, something scuffed the armature in the alternator and wiped out a brush. It should be over 13.5v charging.  You might have a stuck brush also. 

If it bench tests at 14.xx volts but you only see 12.xx on the gauge, the issue is in the wiring somewhere between the alternator and the gauge.  Assuming you checked the voltage at the battery terminals?  If so, your problem is between the alternator and the battery.  Is this alternator externally regulated?  When it was bench tested, did it use the same regulator and wiring?  If internally regulated, then thats a non-issue.

First thing I would do is take a pair of jumper cables and do the following:

With the engine running and a multimeter across the battery terminals....

Connect a jumper cable between the negative battery terminal and the engine.  Closer to the alternator the better.

Voltage goes up?  Your issue is in the ground cable, either engine to chassis or battery to chassis.

Voltage stays low?  Connect the jumper from the output stud of the alternator to the positive on the battery.

Voltage goes up?  Your issue is in the positive wiring somewhere between the battery and the alternator.

At that point you can shut the engine off, disconnect the battery, and start measuring the resistance between various points on the troubled side of the circuit.  Eventually you will find some wires with resistance.  Replace those and you will be in business. 

Remember that V=IR.  Just a little resistance in the charging connections will knock your voltage down. 

I had an issue with my first superduty where the alternator plug was bad and it wasnt seeing proper sensing voltage so it wasnt charging correctly.  You can also try running a jumper from the battery positive straight to the sensing terminal on your alternator and see if the situation improves.  

You could try temporarily swapping another battery but I would be surprised if thats your issue.  

Are you basing this concern over what the gauge in the dash of a Ford truck is telling you?

Yeah.  Stop that right now.

So I cleaned the contacts on the plugs, I degreased the alternator (had a small transmission leak at the cooler,) degreased the rest of the engine cause I was in there and why not spray myself with water when its in the high 50s out, took apart all heavy gauge lines and cleaned the contact points, put it all back together and tested it.  Now I'm at 14.44v at the battery.

In reply to Streetwiseguy :

I was basing this concern on ~250k miles of daily driving this truck over 17+ years of ownership and what that gauge has told me through 4 or 5 dead batteries and 3(maybe) dead alternators.

Ctrl + Alt + Del

Yaayy... fixed it