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Do I Just Not Know How To Use a Multimeter?

Posted By Half-dude 7 Years Ago
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Half-dude
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I'm trying to troubleshoot a battery drain issue with my 55, You leave the car a couple weeks with the battery plugged in the battery will drain, sometimes just one week will drain it, and according to the GEN light slowly getting brighter and brighter while driving at night the generator evidently isn't charging enough to keep up with the headlights I guess.


Anyway I'm trying to get a base reading with my multimeter and I looked on youtube to find out how to take the readings. My initial readings with the car running was at about 6.10v with the motor idling, and then when I turned the headlights and heater fan on it'd go down to 5.79v. I went for a drive and came back and the reading was about the same but here's the thing, I went to start it a few minutes later in my driveway and the battery is dead. Question is, if the battery is dead why is it still reading 6.7 volts on the meter? OR am I just completely misusing the meter?

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miker
Posted 7 Years Ago
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I don't really have an answer, but the charging voltage should be up around 7.2 volts above idle. I don't know the exact specs Ford used, but the old rule of thumb was 2.4 volts per cell, 7.2 on 6v and 14.4 on 12v. With a generator you'll need 1500-2000 rpm maybe to see that. Secondly, the voltage at the battery at speed and under load should remain at 7.2. So turning on the lights and heater and running the motor up to say 1500, you should still see the 7.2volts. If the gen won't maintain that voltage under load, you're slowly running the battery dead.

If you don't get that it could be a number of things. Voltage regulator adjustment, generator in need of maintainence (brushes first) or connections at the gen/regulator/battery. It takes very little corrosion to push a 6 volt system down, could even be hidden corrosion on a fork or spade connection crimp.

This is one of the reasons I believe in new wiring in almost any vehicle at 50+ years of age. Insulation breaks down, switches fail, grounds fail due to corrosion between body panels, etc. I've rewired several of my friends cars after the meltdown. Fortunately they got lucky and just burned the wiring not the whole car.

FWIW, just the modern radios in my old cars, with a station memory wire, will run the car down in two weeks or so. I've got battery shut offs on everything. With a completely disconnected batttery, they will start fine 6 months later.

miker
55 bird, 32 cabrio F code
Kent, WA
Tucson, AZ
charliemccraney
Posted 7 Years Ago
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A bad battery can show a good voltage when there is no load but as soon as a load is applied, the voltage drops and nothing happens.  If the battery is bad enough, even a low load like simply turning the ignition or an interior light on is enough.
Set the meter up so you can see it while attempting to start the engine, or have a friend try to start it, and see how much the voltage drops.


Lawrenceville, GA
Half-dude
Posted 7 Years Ago
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Thx for the input guys, I've got some stuff to look at.
Gene Purser
Posted 7 Years Ago
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If the battery still shows 6V when you try to start the car, check your connections at the battery and the grounds to be sure they are tight and corrosion free. There is also a chance that the solenoid circuit is open if it doesn't click when you try to start it. 
Half-dude
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I went ahead and bought a new battery.

One thing that came to mind, I thought about changing the battery cables too, the negative had gotten a little melted on one side from laying against the exhaust manifold and the positive (-) is the original and the casing is kinda cracked up. But one thing came to mind when I was at the store looking at all the gauges of cable. I think when I bought the old negative cable way back when I didn't pay attention to the gauge I got. The wire says it's 1 gauge. A website says these fords should have gauge 00 or gauge 0000.

Would having the wrong gauge negative cable cause any of these draining symptoms?
Dobie
Posted 7 Years Ago
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Nope, but when they get old they cause other problems like hard starting. 6v systems operate at twice the amperage as 12v systems so they need the biggest cables you can stuff in there. You have a parasitic draw somewhere. You can find it by disconnecting electrical devices one at a time. When the draw stops you've found the culprit.
miker
Posted 7 Years Ago
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Dobie's right, the main problem with undersized cables is voltage drop under load. Mainly starting load, but as you add lights, heater etc. you can have that problem in a running condition also. On 6 volts it doesn't take much to start to show up on the ignition side while running.

The restoration guys sell the right cables, a tractor store might have them that will work, or you can make your own with welding cable of the right size. Battery to starter solenoid, solenoid to starter on the hot (negative on a 55 if I remember). Ground side to engine or frame, and another between the engine and frame. It's been so long since I converted my 55 bird I don't remember.

A good multimeter set on the amperage scale will show the parasitic draw. Isolating it is another matter, disconnecting one thing at a time, and maybe finding more than one. If it's minimal, just use a battery shut off switch when parking more than a couple days. My buddy just called tonight wondering why his battery was dead. The car's been parked 3 weeks and the modern radio "memory" lead killed it. He forgot to shut it off.

miker
55 bird, 32 cabrio F code
Kent, WA
Tucson, AZ
Half-dude
Posted 7 Years Ago
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miker (8/10/2017)
Dobie's right, the main problem with undersized cables is voltage drop under load. Mainly starting load, but as you add lights, heater etc. you can have that problem in a running condition also. On 6 volts it doesn't take much to start to show up on the ignition side while running.



Well maybe that explains why the GEN light would slowly illuminate over the course of an hour or so when driving at night with the headlights on eh? I posted a topic about this awhile back and no one knew the answer.

So when it comes to gauges, the lower the number the bigger the wire? Kinda like how 6volt wiring is bigger then 12v wiring?

I did the parasitic leech test on my 55 the other day, where you disconnect the ground wire and run two leads one on the ground cable and the other on the ground terminal. I believe if I was reading it right it displayed like .4 miliamps which is hardly anything right? Not sure what it could have been though these old cars should have no draw at all when off if the doors are closed and the clock isnt hooked up.


Are those battery cutoffs hard to hook up? Is it a switch between the positive battery cable and the solenoid?
Ted
Posted 7 Years Ago
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You should be pulling zero amps when hooking up the amp meter between the battery and the ground cable.  As the charging system is not at 7+ volts when checking it at 1500 rpms, then the voltage regulator is a good place to start for both the inadequate voltage and the continuous loss of voltage when the car is not running.



Lorena, Texas (South of Waco)




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