[w100 360ci]

ok so my truck isnt really putting out enough to charge the battery.... in the day with no lights on and the motor rapped up perdy tight it will and at nite just in town the lights and heater will pull the battery way down and you have to rev it up to bout 3 grand to get them to brighten up but it just does that brighten them up not enough to charge the battery...and even at an idle just my heater going will drain the battery... i think it might be the voltage regulator

 

[Megunticook]

What's your ammeter reading in the instrument panel? Also, can you get hold of multimeter and determine the voltage in your charging circuit? Both of those info. will help diagnose this.

In any case, the first thing I would do would be to check all the wiring in the charging circuit and make sure everything's clean and tight, including both ground wires from the battery. Make sure the alternator and voltage regulator are solidly grounded, too (clean then wire brush or sand the firewall around each VR mounting hole down to shiny metal--add a thin layer of dielectric grease if you want to prevent any future corrosion--and replace rusted mounting bolts with new ones; make sure you have a ground strap or heavy-gauge wire from one of these bolts to the block, although my truck ran without one for 10 years so it's probably not essential, just good insurance).

Lights dimming at idle is normal for these trucks, by the way, unless you install relays.

Let us know how you make out.

 

[cannonfodder]

ok so my truck isnt really putting out enough to charge the battery.... in the day with no lights on and the motor rapped up perdy tight it will and at nite just in town the lights and heater will pull the battery way down and you have to rev it up to bout 3 grand to get them to brighten up but it just does that brighten them up not enough to charge the battery...and even at an idle just my heater going will drain the battery... i think it might be the voltage regulator

My 78 got like that after the voltage regulator ran away and pinned the amp gauge. I tossed the amp gauge in favor of a voltmeter and it was cured. You couldn't believe the difference. No more dull headlights,radio quitting or heater fan slowing down at idle anymore. Worked for me. Maybe yours is the same.The guys on here will tell you the ampmeter is a curse.

 

[Megunticook]

I'm in the minority but I kept my ammeter--just rewired the main charging circuit, bypassing the troublesome firewall connectors, and made sure my ammeter connections were shiny clean and tight. It's typically the loose/corroded wiring connections, especially at the firewall, that cause high resistance, overheating, voltage drop, etc.

But I gather that some of the later model ammeters had some plastic in them and were just plain cheap. If you have one of those, probably best to ditch it.

 

[bherder]

The fact that when you rev the enginge, everything brightens up, tells me your alt is putting out... So the first things I would do is (As others have said) is to check/clean your connections on the alt, voltage reg, battery, bulkhead and of course, your grounds. Make sure your fan belt is tight enough... If you decide to buy a new voltage regulator, don't buy a cheap-o one. Trying to save a few $$ in this case is not worth it. We've all seen enough posts here where someone went through a couple cheap-o ones, only to still have problems and then finally bought a good one and everything was fine.

 

[w100 360ci]

i no the lights will dim at idle...but not go dead... and i no the altinators putting out...just not enough...i have an aftermarket gauge and it says with no lights on reved up plus 30.... lights on reved up plus 12ish...lights adn heater 0 and a discharge at idle...infact a discharge with anything at idle

 

[w100 360ci]

oh and i also had my offroad lights short out like the wire burnt up and headlights and everything quit before all this happend so i shut the truck off quick and cut the wire adn all the power came back to everything... i lost power to everything when this happend the only thing working was the coil....even stuff like my cb wired right to my battery quit but as so as i cut the wire it worked... thats why i was thinking i fried the voltage regulater

 

[88D250]

If you put POWER TO THE FIELD WIRE ON THE ALT. If it charges normal it is a regulator. If not it is the alt. Just DON"T GROUND THE FIELD ON THE ALT. or the alt will be bad for sure.

 

[w100 360ci]

ok theres 2 fields...what are there functions

 

[B-300]

There is 1 field winding with 2 connections on the alternator. 1 of the wires goes to igntion on and supplies power to the field winding. (small red on vans.. it is also connect to 1 of the regulator terminals so the regulator can do its job)
The other field wire connects to the regulator (green on older vans).
The regulator measures the voltage at the red wire and regulates how much current to ground thru the green wire.... If the regulator isn't grounded well the system will behave badly.

 

[Speed Dragon]

I thought both field wires ran to the VR?

BTW a slight discharge reading is normal at idle on these trucks.

 

[bherder]

I thought both field wires ran to the VR?

They do, but one is tapped into ignition 'on' ... along with connecting to the VR ..

BTW a slight discharge reading is normal at idle on these trucks.

True... Whether a 40 amp alt or a 100 amp alt ... Even with my 100 amp alt, at idle with just the headlights on, shows about 12.5 volts, which I'm guessing (Or if I remember right ) on an amp gauge would show a slight discharge... 1000-1200 rpm or more and everything springs to life... 14 volts.
Since no alt or VR system is perfect, this is the pulley ratio 'give n' take' ....

 

[Megunticook]

They do, but one is tapped into ignition 'on' ... along with connecting to the VR ..

On mine the red field wire goes to the "mother splice," that big ol' mess o' red wires that join in the harness along the firewall. And right, one wire from that splice does go to the VR, and one to the ignition (don't have my wiring diagram in front of me, but the splice also includes a lead from the ignitor as well as the ballast resistor--but I'd have to double check that).

[True... Whether a 40 amp alt or a 100 amp alt ... Even with my 100 amp alt, at idle with just the headlights on, shows about 12.5 volts, which I'm guessing (Or if I remember right ) on an amp gauge would show a slight discharge... 1000-1200 rpm or more and everything springs to life... 14 volts.
Since no alt or VR system is perfect, this is the pulley ratio 'give n' take' ....

Interesting--I hadn't noticed my ammeter needle leaning left at idle, unless my idle's out of adjustment. When running at factory spec (750) seems fine. I'll have to check the voltage, though--got me wondering.

 

[w100 360ci]

foudn the problem... the ignition field was not hot

 

[Speed Dragon]

Interesting--I hadn't noticed my ammeter needle leaning left at idle, unless my idle's out of adjustment. When running at factory spec (750) seems fine. I'll have to check the voltage, though--got me wondering.

If there's nothing electrical running, it will be fine, but with the lights on or blower motor it will show a bit of discharge.

 

[bherder]

Interesting--I hadn't noticed my ammeter needle leaning left at idle, unless my idle's out of adjustment. When running at factory spec (750) seems fine. I'll have to check the voltage, though--got me wondering.

I wouldn't read into that too much there Meg ... It may be so slight you've never noticed it and/or remember you're dealing with a 35 year-old gauge too


But as Speed says, yeah with the engine running and nothing on, you probably won't see anything, but with lights/blower on at idle, you'll more than likely see the needle just a tad on the discharge side. And thats pretty much been my experiance with every Mopar I've ever owned..