I've posted a couple of times about the weird electrical problems I've been having with my 1993 tracker, mainly my battery going dead overnight.  I've slowly been ruling out possible causes and have come to the conclusion that something is not quite right with my charging system.

I took the alternator off and had the local O'Rileys check it out on their machine and it tested good.  The lug broke off the main power connection while I was reinstalling it, so I replaced that and thought I'd found the problem, but the battery is still going dead.  Took the Tracker back to O'Rileys and had them check the charging system out with the vehicle running, and their tester shows no power coming from the alternator.

The odd thing is if I hook a voltmeter across the battery posts and let the vehicle run the voltage slowly creeps up to around 16 volts over about a 5 minutes time period, so it looks like the power is coming from somewhere.

Has anybody else had a problem like this?  Could anything other than a bad alt cause this problem?

Might be a problem with the voltage regulator.

You're getting output from the alternator - that is evidenced by the rising battery voltage, but, not enough to properly charge the battery - literally any poor connection in the charge circuit can cause that, or, a defective alternator or battery.

The negative side of the circuit is fairly simple - the alternator case grounds to the engine block which should be connected to the battery negative by a piece of heavy cable, usually bolted somewhere near the bell housing - typically there will also be a second connection through the chassis (engine to chassis, chassis to battery negative)

The positive side is only slightly more complex - that will run from the alternator output, most likely to the fuse box, possibly through a fuse, and then to the battery positive.

Check both sides and make sure all connections are clean and tight.

  Very simple and coherent explanation of the alternator wiring fordem. Well done.

Battery has been replaced twice, and it tests good.  I only have two cables going to the negative side of the battery.  The heaviest gauge cable connects to the body.  Haven't been able to successfully trace the smaller-gauge cable but it runs down under the body somewhere.

The heavy-gauge wire off the starter connects directly to the clamp on the positive side of the battery, no fuse or anything in line.  I've used a voltmeter to verify continuity between the lug on the alternator and the positive clamp.  Checked the ground resistance between the negative battery terminal and the outside of the alternator casing.  Get as close to zero as my meter reads so there should be good positive and negative connections back to the alt.

Seems to me that for the voltage to be going as high as it does, but with insufficient current to charge the battery that the problem has got to be either in the alternator or in the regulator.

If the heavy ground cable is running to your body move it! Ground it to the engine! I would find out where your other ground goes. It prob connected to the body! If not put one on! It also wouldn't hurt to ground the engine to the chassis. And chassis to the body! Bad grounds are prob the biggest misdiagnosed electical problem out there!  As for the testing of the alternator testing! Have you tested it while it is under load?

I will check the ground out tomorrow and make sure they're all good.  I've got some heavy gauge wire left over from wiring a welder outlet so I can make up some more grounds if need be.

Since I'm looking at possibly replacing the alternator anyway, does anybody know of a list of GM vehicles that used the 12SI alternators?  I've been looking through the mid-90's vehicles on rock auto's site but none of the descriptions on the alternators there say whether it's a 12SI or not.

I run a Delco 12si from an 84 Camero with a V8, 94 amps and appears to be woking well.
Norcal

Battery has been replaced twice, and it tests good.  I only have two cables going to the negative side of the battery.  The heaviest gauge cable connects to the body.  Haven't been able to successfully trace the smaller-gauge cable but it runs down under the body somewhere.

As has already been suggested, connect the heavy cable to the engine/transmission - this is the starter return - if it is not connected correctly (or at all) it will typically cause problems starting rather than charging the battery - but it is one possible cause of your problem.

The heavy-gauge wire off the starter connects directly to the clamp on the positive side of the battery, no fuse or anything in line.  I've used a voltmeter to verify continuity between the lug on the alternator and the positive clamp.  Checked the ground resistance between the negative battery terminal and the outside of the alternator casing.  Get as close to zero as my meter reads so there should be good positive and negative connections back to the alt.

Lets not get confused between the starter and the alternator - from the factory, the starter main feed always connects directly to the battery, the alternator output may or may not connect directly and more often than not, does not.

Also continuity is one thing, conductivity is another - connect a digital volt meter set for volts positive lead to the battery positive lug, negative lead to the starter's battery connection and then crank the engine.  You've got continuity but I'll bet the voltage drop across that piece of cable surprised you.

Seems to me that for the voltage to be going as high as it does, but with insufficient current to charge the battery that the problem has got to be either in the alternator or in the regulator.

If I'm not mistaken the regulator is inside the alternator - and you said you already had that checked - it is possible to have one or more bad diodes in the diode rectifier bridge that will create a situation where the alternator can develop the required voltage and still be unable to deliver sufficient current.

A good battery going dead overnight is more likely to be the result of some sort of drain, a lamp left on or similar - the last time I had it happen it was my power amp not turning off when the head unit was shut down.

Connect a lamp in between the battery positive lug and the lead - if it lights you have a load drawing current, pull the fuses one by one until you find which circuit is causing it - some drain is normal, ECU, clocks, head unit radio presets, etc.

Try disconnecting the alternator and then leaving it overnight. A short circuit diode could be causing a current drain through the alternator. I think 16V across the battery is too much and would also point to a suspect regulator or a bad ground.

Try disconnecting the alternator and then leaving it overnight. A short circuit diode could be causing a current drain through the alternator. I think 16V across the battery is too much and would also point to a suspect regulator or a bad ground.

X2
Great suggestion

Well, I've got a 12SI alternator that should be in tomorrow.  All of my grounds and wiring checked out.  With the alternator disconnected putting an ammeter between the battery and the positive cable I get around 0.01 amps with everything turned off.  And the best part is it can sit overnight without draining the battery, but I only get about 6 starts out of the battery before I have to put the charger on it.

Was also rather irritated to find out that my lower alt bracket is a cast piece so I can't just cut it and re-weld it to work with the 12SI.  I'll either have to make something from scratch or spend another $24 to get a Trail Tough bracket.  Got a new mig welder for Christmas that I haven't used yet, so need to put it to the test anyway. 

And just for the heck of it, I'm probably going to beef up my grounds while I'm in the engine compartment.  Since whatever bracket I use will probably only make use of 3 of the 4 bolts I'll probably use the 4th bolt hole as an additional ground point.  Anybody see any problems with that?