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Tech Question

Clay Cooke

I have an ’84 Dodge Ram with a 440. I swapped in the engine over this last summer, and ever since then I have had problems with the charging system. Power is getting lost somewhere, and I dont know where. The problem seems to be a loss of power while the vehicle is sitting, but could also be a charging system problem. I have a dual field alt. bumped up to output 100 ampers, and having a large stereo system, I have a battery isolator which has the alternator keep both batteries up, but doesn’t let them discharge each other. It seems as if each week I end up having to charge the “main” battery to be able to drive it when ever I want to. I have checked the usual culprits, alt., regulator (installed MP constant output unit, didn’t solve anything), ground, and nothing seems to work. When I first was playing with the system I never got constant voltage on the wire from the alternator to the battery, and think that might be my problem, but am not totally sure. Can you suggest some of the possible problems, or any things I can check out to see what the problem is?

Thanks…..

Clay,

I get tons of mail on electrical problems, so, you’re not alone. More so than any other type of problem, electrical problems must not be “shotgunned,” in other words, you must really troubleshoot the problem; not just throw parts at it. Of course, that shouldn’t imply that you should skip a visual inspection for corroded terminals, etc., but even these will be caught if you do it “by the book.” (MY book!)

You MUST have one piece of test equipment for this: a digital multimeter (DMM.) A pretty good one can be had for around a hundred bucks; my choice of brands is Fluke. (What a name for a test equipment company, huh?)

The swap to the 440 isn’t the problem, but it could be something you did during the swap. So, let’s analyze this step by step. First, you need to determine if it’s a charging problem, or ignition-off draw problem (IOD.) Charge the main battery, then disconnect the negative lead. Get a test lamp (12v.) with a medium size bulb (say, a dome lamp bulb,) and connect one of the test lamp leads to the battern neg. terminal, the other to the cable terminal you disconnected. At the most, the bulb should glow very dimly. If it’s very bright, definitely you have a major IOD. Start by removing fuses one at a time, disconnecting the dual-batt isolator, etc., until the source of the current draw is located. Then, repair / replace the faulty item.

If the bulb is completely off, it’s NOT an IOD problem. But, what if it’s dim? Substitute your DMM for the bulb (on current) function, and measure the draw. If, say, it’s 100mA (1/10 of one ampere) it would take about 30 days to completely discharge a large, beefy battery. Therefore, I’d say 100mA is MAX, and I’d be unhappy with much more than 20 or 25mA. If it’s up there, use the same procedure described above to locate the problem source.

Now, what if the IOD is normal? We’ll if the battery is good, and you fully charge it (using an external charger) you should be able to come back a month later and crank ‘er up! But, who wants to park it for a month for a test? So, here’s what you do: charge it up; wait 24 hours. Measure the voltage acoss the battery (plus to minus.) It should be 12.60 v. or better, with all lights, accessories, etc. OFF. If it’s lower, and you’ve proven that there’s no substantial IOD, the battery is no good.

Now, if it’s okay (12.60 volts or better) you know the battery and load circuits are okay. So, next we have to test the charging system. Here’s the easy way to do that: Start it up, and turn on every electrical accessory: high beams, heater blower, stereo, rear defroster, etc. Measure the battery voltage again, with the engine at fast idle (approx. 1200 RPM.) The voltage will vary with temperature, but should always be OVER 12.85 volts. If it’s lower, there is in fact, a charging system problem.

There are really only four possibilities there:

1. Alternator not properly driven (belt slip.)

2. Alternator low output (bad diodes, windings, brushes, etc.)

3. Alternator not being “told” to produce more output (regulator or reg. wiring NG)

4. Output wiring NG…output voltage, and, therefore, current, not “making it” to the battery.

Let’s start with the last one, since it’s easy to check. Disconnect the battery neg. again, and, using you DMMs lowest resistance (ohms) range, measure the resistance from the battery positive to the alternator output stud. It should be less than one ohm….MUCH less. (Hold the two leads together to see the lowest reading the meter will give. The batt-to-alt reading should really not be more than 0.2 ohms more.) If it is, locate the fault, and correct it.

To check items 1 thru 3 quickly, do this: disconnect the field terminal(s.) Jump either terminal to ground, and the other to battery positive. The light should get real bright, and the voltage across the battery will be 15 volts or more. Yes? The alternator and belt are okay. Check the regulator and wiring, including battery voltage on the blue wire. No? Well, by now it should be obvious!

As far as “constant” voltage, don’t worry about it. It’s the AVERAGE that counts. Sort of like in horsepower production, the “area under the curve” is what gets the job done!

Let us know how you fare. Rick E. last update 5/27/97

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