Electrical charging problems

I'm lost here, I have my 75 dodge powerwagon, it charges great when idling in park and neutral and when driving. But when I am in gear idling in traffic my battery slowly gets drained. Am I missing something here? What can I do to fix this?

Does it seem to have the factory pulleys? Or does the alternator have an oversized one?

Other than that is to determine what amperage the alternator is, whether the alternator is performing up to capacity (no problems, blown diodes) and whether the wiring has a problem.

First thing I'd do is download a late shop manual, like the 72 or 73 over at MyMopar and read through the section 8 on testing the charging system.

We can step you through some tests.

Part of this, depends on how fast the engine idles, how big the alternator is, and how good shape the battery is, and how much "junk" you have turned on,

I do have factory pulleys with new belts (tension on pulley is acceptable), it has a 42 amp alternator from autozone (duralast), engine idles at 900 rpm at park/ neutral and 600 rpm in gear, battery is 2 years old, and all I have for accessories are my general lighting (headlights, driving lights, tail lights, blinkers, cab clearance lights, and an after market radio) even before installed the radio my jalopy had this problem. I downloaded a 75 dodge truck service manual but it does not read anything about my situation.

Where did you find the 75 truck manual?

What does your "new" alternator look like? What you want is the later "squareback" and not the old style "roundback"

You want the type on the right which is better for low speed.



A "quick check" you can do is to disconnect the green field wire at the alternator and use a clip lead to ground that alternator terminal. See if the idle / slow speed voltage picks up. If so, you might have a voltage drop problem (poor connections) in the field circuit.

With this ground lead in place, turn off the engine, and turn the key to "run." Measure voltage at the remaining blue field wire at the alternator WITH the blue hooked up. It should be "close to" same as battery.

A 42 amp alternator is going to have some seriously low, low output at idle, especially at 600 rpm.

I wasn't even really impressed with the 78A square back when I had one, but it would probably be adequate.

The first thing I would do is look at the voltage at the back of the alternator vs at the back of the battery. There shouldn't be more than about 1/2 volt difference with the engine running. If it is, the wiring is suspect.

I would replace the wires in your charging circuit with some 10 GA wire. The factory ones are very small.

If you have a NON-A/C small block, it's super easy to convert to a Delco 12SI which will charge extremely well at idle.

goldduster318 said:

A 42 amp alternator is going to have some seriously low, low output at idle, especially at 600 rpm..

Click to expand...

Actually the original 46A squareback, so I'm talking about "as Chrysler produced it" and not some pirated, mexicanated, chineseoated "rebuild" puts out better at low RPM than some of the larger rated alternators, and absolutely better than the older roundbacks. In other words the output curve is not linear.

Be sure to check all your grounds;including the one from the motor to the frame.Loose or bad grounds are battery killers.

Pretty typical of the mopar alternators. They tend to not have sufficient output at idle. Check voltage at alt charge stud at idle.

A different alternator would likely help with electrical requirements at idle. I use the Denso's off a toyota in a lot of cases. Takes a bit of work on brackets/spacers.

300 rpm drop in gear is a big change. I know we have smog stuff to deal with sometimes. The tune up is a bit off with that much drop or there may be a vacuum leak somewhere.

67Dart273 said:

Actually the original 46A squareback, so I'm talking about "as Chrysler produced it" and not some pirated, mexicanated, chineseoated "rebuild" puts out better at low RPM than some of the larger rated alternators, and absolutely better than the older roundbacks. In other words the output curve is not linear.

Click to expand...

They really aren't linear on any alternator curve I've ever seen. Pretty much a logarithmic curve. In any case, with the pulley ratio which is let's guess 2.5:1, the alternator is spinning at 1500 rpm at 600 rpm idle and 2250 at a 900 rpm idle, which probably moves the speed up the curve enough to make it work correctly.

One thing I do know is it's definitely not 46A @ 600 rpm engine speed. Probably more like 25 IF everything was in perfect order. I'd still guess there's a lot being lost through the harness that doesn't get delivered to the loads or battery.

67Dart273 said:

Where did you find the 75 truck manual?

What does your "new" alternator look like? What you want is the later "squareback" and not the old style "roundback"

You want the type on the right which is better for low speed.

A "quick check" you can do is to disconnect the green field wire at the alternator and use a clip lead to ground that alternator terminal. See if the idle / slow speed voltage picks up. If so, you might have a voltage drop problem (poor connections) in the field circuit.

With this ground lead in place, turn off the engine, and turn the key to "run." Measure voltage at the remaining blue field wire at the alternator WITH the blue hooked up. It should be "close to" same as battery.

Click to expand...

I downloaded it from allpar.com and then a buddy of mine had a service manual that belongs to his uncle, it's the same downloaded book (word for word) from online, and the te alternator is a round back.

I you want to insure no problems, I have a new 100A alternator (photo) for an older Dodge truck that should fit and would look tough in your truck. I bought it on ebay. It is much bigger than the photo appeared, so won't be going into my little A-bodies. I think it was put on tow trucks and such, and would be great to run a winch while idling. If passing thru Sacramento, I'll sell it cheap just to free shelf space. PM and I'll send photos and PN.

Otherwise, a squareback alternator is easy. I grabbed some almost new ones at PickNPull for $10 on half-off days. I probably have more alternators in the shed than cars now, but hard to resist when they so clean and cheap.

Before that, do you really have a charging problem? Does your battery run down over a week of driving. If not, sounds like "no problem, man". Perhaps you are just worried by the ammeter showing slightly in the "D" when idling in gear. They fixed that after ~1978 when they took ammeters out of cars and just put a "battery" idiot light. By the time that lit in my 1982 Aries or 1982 Chevy S-10, there wasn't enough battery left to crank. Ammeters were confusing drivers and much cheaper without them.