I have a new alternator and a new battery. Both installed about 7 days ago. I drive the truck every day. The battery is dead today. Who is the next culprit/suspect?

 

Could possibly be the voltage regulator.

 

Well, assuming original configuration of a 76XLT the external voltage regulator is suspect.

When it's running what voltage is read at the battery?
Have you disconnected the battery? When you do is there a spark?

Many issues come up. Grounds being bad, or a poorly grounded component, and shorts can cause most of this. I hate electrical problems myself because there seems to be so many things that can go wrong.

I was chasing issues on my 75 and replaced the alternator with a one wire internally regulated one so I could ditch the mess of wires I had somewhat attached to the VR. It looked to be a new start solenoid and VR when we got it, so it had been a problem before. The wiring is still an absolute train wreck, but it now charges correctly, and doesn't drain the battery while sitting.

 

Well! two of you suggest the voltage regulator. They are cheap and available, so i will give that a try. (the one on the truck now is surely the original one).

BTW, what does a VR do? Please don't tell me it regulates voltage! Give me info. I can use!

 

The VR senses voltage in the electrical system and feeds a small current to the alternator to energize the alternator windings. As system voltage drops below target the VR feeds more current to the alternator to increase its output and bring up the system voltage.

As mentioned, checking with a voltmeter is a very good and simple test. After you thoroughly charge the battery back up and replace the voltage regulator start the truck up and bring it to about 2000 RPM with lights and accessories off. Check voltage across the battery posts. It should be between about 13.2 and 14.5 volts if the alternator is working correctly. Turning the lights on should bring it down a little but still in the same range.

If the alternator is not working properly voltage will be under 13 volts with this test.

 

Another vote for Voltage Regulator, or like somebody else mentioned, that you may have a perfect working charging system, but you may also have a short in the wiring that is draining your battery.

Unk Bob

 

Blue and White,

Thank you so much! And thanks to the rest of you. Buying a new VC today.

 

Before spending money on parts do an Amp draw test. If you have a DVOM (digital Volt Ohm Meter) it should have an amp setting. Disconnect the the positive lead on the battery and set the meter to Amps with the Key off and everything shut off radio lights etc that may use power, check the amp draw between the battery and the Positive cable. If you have an measurable draw (There may be a minute amount if you have a newer radio) then disconnect the VR if it drops you have your culprit. If it does not, time to go searching for other things.

 

Matthewq4b,

I do not have a digital meter. Only an analog. Is there another test I can do for the VR with an analog meter?

 

If your Analog meter has an Amp setting you are good to go.
Cheap DVOMS that have amp capability are available pretty much any where for $20 and less. That is all we need for the stuff we do on vehicles we do not need $200 plus meters for these old trucks. It really should be one of the first things to have in a tool box.


If you drained a battery overnight or in a 48 hour time period you have a minimum draw of 1 or 2 amps.


If you have a 12V test light you can use this to check to see if you have substantial draw. Follow the same procedure. I did forget to mention that you need to make sure the battery is charged before doing this.


Disconnect the positive battery cable and put one end of the test light on the cable and the other on positive battery terminal. If the light lights up at all you have excessive draw. Then you can start unhooking stuff (like the regulator) to see what causes the light to go out and or dim. When it does that is your culprit.


If you do not have a Test light you could fab one up out of a couple pieces of wire and a 12V bulb. The smaller the bulb the more sensitive it will be to detecting amp draw. Just solder or hook up the wires to the bulbs terminals.


Standard instrument panel light bulbs work good as you can twist some smaller wire around the terminal leads to jerry rig a temp test light wth out soldering.
With test lamps Even stroe bought ones always verify their function first by connecting it to a known power source (Battery positive and negative) to insure they are functioning.


Alternatively you can use the Analog meter but this will detect draw of just a couple milliamps. For example a modern radio that has KAM draw (keep alive memory) to remember radio presets will trigger the Analog meter.


The procedure is almost the same with the analog meter, Make sure the battery positive cable is disconnected, Then check the Battery voltage between the positive and negative with the Analog meter on the appropriate volt setting . Take note of it. Then hook the meter between the Positive cable and the positive battery terminal
Note the Voltage , if the voltage is the same you have some amp draw, this will not tell you how much just that you do. If the Voltage is more than 2v lower than what was noted for battery voltage you have draw but it is so minimal to not be an issue.


With the analog meter you could try disconnecting the VR and see if it makes a difference.




Regardless of what method you use If the VR is not the culprit start disconnecting the wires one by one on the Battery side of the of the starter solenoid. Eventually One wire is going to cause the DVOM the Analog meter or the test light to read zero or not light up. You then know that some component that circuit has failed in such a manner to cause the draw or the Harness/ wiring some where is leaking to ground.


Once you identified the suspect wire disconnect all the other wires from the battery side of the starter solenoid and hook the wire up that had draw Make sure EVERYTHING electrical is turned off in the truck. You can then go in the truck Key on the ignition see what it powers by turning stuff on then off, and what fuses it powers (Note not all items go through the fuse block head lights for example) Check what fuses have power by checking for voltage at the fuse block on individual fuses. You can then start pulling fuses to see what one causes either the test light to go out the DVOM amp draw to fall or the Analog meter voltage to fall. This will then identify the circuit that is causing the issue and what devices are on it. You can then replace the fuse and start disconnecting individual devices one by one, to see what one causes either the test light to go out the DVOM amp draw to fall or the Analog meter voltage to fall. If all the devices on that circuit are disconnected and it is still showing draw you have a potential wiring fault. If that happens let us know, as there are tests to trace where in the wiring the fault is.


All this assumes that the wiring is in the factory layout and has not been butchered with a bunch of added on/in after market stuff. If it has the testing procedure could be more complicated.

Hope this helps.

 

Matthewq4b,

Wow dude! YOU THE MAN!. Your explanations make total sense to me. Thank you so much. I'm all over it. Thanks again for taking the time to explain.

By the way, this is a one owner truck: my dad and then I. All the engine & wiring, etc., is bone stock from factory. All I have done is body work trim.

 

No problem, and glad to help.

I have just never been one to go throwing parts at stuff in the hopes that it fixes it without knowing for sure what the problem is. It likely is the VR but it only takes a couple minutes of testing to know for sure.

Electrical issues can drive you nuts. But if a logical methodical approach is taken to them it is one of the easiest things to trouble shoot and repair.

 

Matthewq4b,

Just purchased a full set of shop manuals on E-bay. should have done this long ago.

Anyway, long story short, the testing procedure in the shop mauals is pretty much exactly as you explained. The VR was burned out at one terminal. $21 bucks at Auto zone. Awesome!!

Again, thanks for your time.

Chris

 

Matthewq4b,

Well, I was wrong! I thought I had the VR isolated as the trouble. Using your method, I have traced a 4.4 amp draw from a wire that plugs into a three prong harness connector next to the starter solenoid. It runs into the firewall and into the cab. The wire is Black with a yellow stripe. It's about 12 gauge. Have not traced where it goes yet. I ran out of daylight!

Now, I am just learning a few things as I go, so help me understand: a "Draw" is the DC positive current flowing through a completed circuit, grounded, and getting "used up", right?
Or, it is a "Short" where the positive current is making contact with ground somewhere and flowing back to battery ground, Right?

I don't understand the "short" scenario, wouldn't the place of contact (broken insulation), cause a spark/heat/FIRE?! how come my truck does not burn up overnight while the battery is draining? Or just blow a fuse?

 

That is the main cab power feed.

Now start removing fuses one at a time to see if you can isolate the circuit.