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Phantom electrical issues.


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Well, Been working on my 70 240, and going slowly through the electrical system. Even before I replaced the fuse box, and put the 60 amp alternator on, the car would start with a jump, run just fine, and start up if tried again quickly after stopping. But leave the car for a little while, and the battery drains. Went through a battery testing this as well (battery ended up being bad as well).

Would love to know what people suggest for going through and chasing phantom issues.

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To be perfectly honest, it's not so much a phantom issue but a lack of knowledge in how to diagnose.

If a battery keeps running down, two of the most likely culprits are a bad battery or a bad charging system. A faulty voltage regulator or alternator can discharge the battery.

Note: My preferred way to do an initial test of a bad regulator or alternator is to hook up an ammeter to the battery (Search online to learn how to use one.) and disconnect first the alternator and then the regulator. If the current draw drops, you found your culprit.

After that, I would look at any electrical "add ons" in the car: alarm system, stereo, etc.

Next comes the "faulty switch". This can be the glove box door not closing fully, leaving the light on, corrosion in a switch causing contact even when "off", etc. This takes more patience since you have to pull fuses or loads (lights, buzzers, stereos, etc.) and watch for a current drop to find the circuit. Oh, by the way, expect a draw when the car doors are open. The dome light will be on.

Now, if you are replacing the alternator & regulator with an internally regulated 60A alternator, you may already be curing your battery drain.

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Wait, you did the swap and I want to know, did you do anything to your existing T plug? If you did not cut off your existing T plug and wire it correctly I'm positive you have an alternator that is calling or pulling for power while the car is off. This is very common with the alternator swaps because the wiring to activate the new 60 amp alternator is different than that of the old alternator. Your wiring should be like this. Top of the T the part that runs horizontal is your "sense" wire. It you have 12 v constant. I typically cut that wire, put an eyelet on it and jump it direct to the battery stud. The bottom portion of the T plug, should be your switched power and should have a diode inline with it. If not your car will start and charge but typically that style alternator will backfeed out of that terminal and will not allow you to shut the engine off without the diode inline. Diodes are cheap, you can find them at radioshack for less than $5 a pack. Hope this helps. If its not a true "drain" on the system. Then you obviously have a bad battery. I'm going to assume you checked that first but if you have not checked that, please do as any starting or charging issues diagnosis should follow 3 rules. Suspect in order. Battery, cables, components in system. ALWAYS in that order. You don't know how many times a week I see someone griping about how their car won't start or won't charge and they SWEAR that battery is good/new, or those cables are good and I find otherwise. Good luck.


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You have what is known as a "parasitic draw."

The easiest way to fix it is to take it to a qualified auto electrician and say "My car has a parasitic draw." Two hours, $160 bucks or so, and you're done.

Being as this is an internet forum, I know you will choose to diagnose it yourself.

First thing you have to do is to learn to use your voltmeter to measure amps. Correctly. Most meters have a 10A limit, and if you exceed this, a little 10A fuse inside the meter itself will blow.

All you have to do ONE SINGLE TIME is have the meter set on Amps, and touch the leads to 12V and ground, AS IF you were intending to measure voltage, not amperage, and you've blown the little fuse, because the meter "looks" like a dead short when in amp measuring mode.

So the first thing you must do is TEST your meter, and the fuse within it. Take a tail lamp bulb, hook one end to ground, and put your meter between B+ (battery positive) and the other contact on the bulb.

You meter should show somewhere between 100 milliamps (.1 amp) and 500 milliamps, depending on the bulb you chose.

Ok, so now you've confirmed you've got a working meter.

You measure AMPERAGE by putting your meter IN SERIES with the circuit being measured. think Xmas tree lights. One goes out, they all go out. If the little fuse in your meter is blown, they ALL GO OUT.

You measure VOLTAGE by putting your meter in PARALLEL with a circuit. Xmas tree lights, but the style where one goes out, the REST stay lit.

Part II

Parasitic draws drain a battery down over time.

The "rule of thumb" is that 50 milliamps (.05 amps) is an ACCEPTABLE DRAW for most vehicles.

A 50mA draw will drain your battery in 3-4 months of sitting.

A 100mA draw ill drain your battery in 1-2 months.

200mA or more, about the same as a glovebox light, will drain your battery in a week or less.

So based on how fast your battery drains, you have SOME IDEA how big the drain is.

50mA or thereabouts is generally required to keep the radio from forgetting its station programming, memory in the ECM, etc. Newer cars generally are 25mA or less, the early '90's cars were the worst with 50-75mA draws considered "normal" A Z car should be faily low as it has very little "keep alive memory" in its ECM, etc.

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Part III --- Actually measuring the draw.

The way you MEASURE your parasitic draw is to disconnect one battery cable, set your meter on AMPS, 10A or maximum setting, and hook one test lead to the battery post, the other to the battery terminal.

It helps to have some alligator clips to do this. I have a special test lead, with an inline fuse holder, with one battery jumper cable clamp on one end, and an alligator clip on the other. It's about eight feet long, and in a moment you'll understand WHY.

I put a 5A fuse in the inline fuse holder. That will blow BEFORE my meter fuse, should something go wrong --- like forgetting, and turning the ignition on while I have the meter inline.

Ok, so let's say you've done this, and your meter is showing 75mA. You've got a parasitic draw. Excessive.

Now be sure you have all the doors shut, courtesy lights off, etc. when you make your measurement.

So now, with your eight foot lead, get yourself and your meter INSIDE the car, passenger side, and start pulling fuses.

When the parasitic draw goes to zero, or near zero, you've found the circuit with the draw.

On the Z-car, you could also BEGIN by pulling fusible links to narrow down the scope of circuits.

Let's say you've pulled all your fuses and fusible links and STILL have a parasitic draw of 75 mA.

What is the one thing that is DIRECTLY connected to the battery without a fuse or fusible link. That's right. The alternator.

(I'm not sure, the Z may actually have a fusible link here....you can research this instead of me.)

What else is DIRECTLY connected to the battery. Usually someone has tagged in fog lamps, power for stereo amplifiers, etc. directly to the battery post. These can also be the source of your problems.

Now here's a finer point. DEPENDING on your meter, whether it's autoranging or not, you may have to change ranges from 10A, the maximum, down to a LOWER range to see 50mA or less.

So you've got your meter down on the 100mA range, and open the car door, and POOF! there goes your meter's fuse because the 200mA draw of the courtesy lamps exceeds that range.

So either disconnect your meter, or move it back up to the 10A range before you open the car door, turn on accessories, etc.

Unless you like running down to Radio Shack trying to find some obscure fuse for the INSIDE of your voltmeter. When you do, tape a spare to the bACK of your meter for the NEXT TIME!

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Part IV

There's one OTHER way to determine which circuit in a vehicle is the source of a parasitic draw.

Remember the equation, V = IR? Voltage = current times resistance.

Well, fuses have SOME resistance. It might only be 1/10 or 1/100th of an ohm, but there's always SOME.

So if you have a 100mA (100 milliamps, .1 amp, 1/10th of an amp) parasitic draw across a fuse with .001 ohms of resistance

I * R = V

.1 amp * .01 ohms = .001 volts. One millivolt. One thousandth of a volt.

If you put your meter, set on millivolts, ACROSS a fuse, you can easily measure a one millivolt difference. Meters are actually real good at measuring low amounts of voltage.

All the OTHER fuses should show zero volts.

Why zero? V = IR. If the current flowing through a fuse is ZERO, then there will be no voltage "drop" across it. Zero times anything is zero.

This provides a meter-safe, FASTER way to determine which circuit has the draw than putting your meter inline and pulling fuses one at a time, the old standby method.

And I have to be honest, I've never actually done it this way. I've only read about it, and been told (by auto repair professionals) it works just great.

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