Resistance Across Positive & Negative Cables

After disconnecting the battery, should there be any conductivity across the positive and negative battery cables?

Thanks!

Joe

Reply to
Joe Colella
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If your heater blower motor switch is turned on, (or anything that is a load) it will show conductivity across the leads. Make certain every switch is turned off. If your car has a computer, you might have to disconnect it's cable. Dave '72 mach1

Reply to
nospam

According to Ford's Technical Service Information for a 93 Mustang, when no electrical loads are present, i.e., all the interior and exterior lights and all other accessories are turned off, a battery drain test (using a clamp-on DC ammeter) should read less than 0.05 amps (or less than 50 milliamps).

Well, I get 3.8 amps!

What the heck could be drawing 3.8 amps???

Thanks!

Joe

Reply to
Joe Colella

If all everything is turned off, you can start pulling fuses one by one until the current draw ceases. I've seen a faulty brake light switch cause problems before. Make sure your doors are shut and courtesy lamps are turned off. Dave '72 mach1

Reply to
nospam

Dave,

Pulling fuses one by one makes perfect sense.

Back to the resistance...

When I measure the resistance (-)COM electrode to negative cable and (+) electrode to positive cable, I get 7 ohms. (Of course, when the two electrodes touch each other, I get 0 ohms and when they are not touching themselves or anything else, I get infinity ohms.)

However, when I measure the resistance (-)COM electrode to positive cable and (+) electrode to negative cable, I get infinity ohms.

Since the battery is completely disconnected and conductivity knows no direction, what component could allow conductivity in one direction, but not in the other???

Joe

Reply to
Joe Colella

When I measure the resistance (-)COM electrode to negative cable and (+) electrode to positive cable, I get 7 ohms. (Of course, when the two electrodes touch each other, I get 0 ohms and when they are not touching themselves or anything else, I get infinity ohms.)

However, when I measure the resistance (-)COM electrode to positive cable and (+) electrode to negative cable, I get infinity ohms.

Since the battery is completely disconnected and conductivity knows no direction, what component could allow conductivity in one direction, but not in the other???

Joe

Reply to
Joe Colella

'Conductivity knows no direction' is not always true. A diode conducts only in one direction, as you probably know. Which makes me suspect your alternator. Disconnect the alternator and check the current draw again.

Reply to
Happy Traveler

Diodes

Reply to
Mike Hunter

yes, in the newer cars, clocks, radio etc could be 2K to 100k ohms. or less.

no in the older cars, but it should not be more than 100K ohms I would guess.

Reply to
chumley

Measuring resistance across the cables won't tell you anything. You need to measure current draw with everything turned off and the doors closed. Less than .05A is acceptable. One very common culprit is the voltage regulator. Shorted diodes in the alternator are also common. I recently had a vehicle that the factory radio was causing a 3A draw, and another where the EATC was continually powering the Blend door actuator. Pulling the fuses one by one should narrow down which circuit the draw is on.

Reply to
Tom Adkins

a diode put in for reverse polarity protection

3.8 amps is bad, something is getting hot too, 40 watts worth, check your alternator connector, there is a design flaw in the 5.0 alternators that will toast,cook, fry the connector side inside the alternator over time, especially with a weak battery.
Reply to
chumley

There are two connectors on the alternator; one on the back near the bottom and one on the top. The one on the top certainly does look fried.

Is changing the connector a simple cut and splice job?

Joe

Reply to
Joe Colella

I found two connectors on the alternator... one on the back near the bottom and one on the top. The one on the top certainly does look fried.

Is replacing the connector a simple "cut and splice" job?

Joe

Reply to
Joe Colella

When I removed the altenator, the draw dropped to 0.03A, but the I still have "good" conductivity between the positive and negative cables, in both directions, with the battery disconnected.

Is this acceptable?

Reply to
Joe Colella

See if you can remove it, on my 93 5.0 the inside of the alternator was burned, the connector part. I think I replaced both sides at an alternator shop, and the alternator, and one must use silicone grease in the connector to keep heat conduction high. Should be easy splice, but heavy duty, but it needs to carry 20 amps or more peak when starting. Problem is that the lands on the PCB holding the diodes are not big enough to carry lots of amps for a long time, so when battery gets low, more amps are pulled out of the alternator more of the time. (I think the top one has two wires, and the bottom one has one, from memory about 10 years ago, could go look in the driveway but it is hot outside)

Reply to
chumley

Thanks... hot here, too!

Reply to
Joe Colella

You certainly found the problem. Measuring resistance with any kind of electronics (radio, engine management computer, cruise control, ABS controller, clock, etc, etc) connected is practically useless. The main reason is that the ohmmeter uses a very low voltage, at which the circuits are totally out of their normal realm of operation. The detailed explanation will be very boring for those with no interest in electronic circuits... So unless you can assure that all of those devices are disconnected, current draw at the actual battery voltage (and the proper polarity!) is the only meaningful test.

Reply to
Happy Traveler

A diode would do that (that's an example)

your multimeter actually applies voltage whenver you use the Ohmeter setting, so yes, something as a diode would give you that kind of reading

Reply to
El Bandito

When I measure the resistance between the positive and negative cables (with battery disconnected) I get 7 ohms.

When I make the same measurement with the meter probes reversed I get infinite ohms.

What component could allow conductivity in one direction, but not in the other??? ______________________________________________

The alternator, through its internal array of diodes.

Rodan.

Reply to
Rodan

When performing a parasitic draw test, it is important to allow sufficient time for all modules to "go to sleep". This can take up to 45 minutes (and even more) depending on vehicle and option load.

In the shop, we prefer to use an inline ammeter... inductive meters leave too much to chance for our liking for this purpose. We prepare the car in advance - open the hood for BJB (battery junction box) access and disable the light if present (after ensuring it does go out when the hood is closed) as well as opening the doors and tripping the latches (to fool the car into thinking the doors are closed.

Once the modules time out, remove the fuses on by one - do not reinstall a fuse until the problem is located.... Reinstalling a fuse may turn a module on and make the task more time consuming.

"Good" cars will have parasitic draw in the neighbourhood of 20 mA - as long as no aftermarket devices are in the loop.

Reply to
Jim Warman

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