Resistance Across Positive & Negative Cables

Page 1 of 2  
After disconnecting the battery, should there be any conductivity across the positive and negative battery cables?
Thanks!
Joe

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
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

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

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.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
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
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
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

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
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

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

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.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

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
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

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
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

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)
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Thanks... hot here, too!

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
I found the culprit drawing 3.8A on my 93 Mustang GT...
The 3-wire connector on the top of the alternator was fried as was the component into which it was plugged.
Once I removed/disconnected the alternator, the draw dropped to 30mA.
I have fully charged my battery, spliced on a new connector and installed a new alternator. I hope I am ready to go.
My plan forward is to monitor the condition of my battery each morning and after any long non-stop trips and if I notice a drop in voltage or amperage, try to determine the cause.
Any other suggestions, anyone?
Thanks, again, for all your help!
Joe

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Joe Colella wrote:

Install a voltmeter gauge, to know battery condition before startup and battery/charging condition going down the road.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Joe Colella wrote:

Did you slice the connector pigtail with crimp connectors or did you solder them? They should always be soldered and covered with shrink tube.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
First, crimped, then soldered, then shrink tubed, then taped.

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
"Joe Colella" wrote

You mean you didn't put them in a non-conductive plastic box, with at least 1/2" clearance all around the wires, fill the box with epoxy, put that box into another corrosion-proof stainless steel box, fill that box with silicone, bolt the assembly to the fenderwell with grade 8 hardware, and finally use some case-hardened steel rods, bent to shape and welded, to build a protective cage around it all?
Kids today... can't do anything right!
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Joe Colella wrote:

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
--
Don't drink water, fish have sex in it!

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Joe Colella wrote:
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.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
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.

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
I found the culprit drawing 3.8A on my 93 Mustang GT...
The 3-wire connector on the top of the alternator was fried as was the component into which it was plugged.
Once I removed/disconnected the alternator, the draw dropped to 30mA.
I have fully charged my battery, spliced on a new connector and installed a new alternator. I hope I am ready to go.
My plan forward is to monitor the condition of my battery each morning and after any long non-stop trips and if I notice a drop in voltage or amperage, try to determine the cause.
Any other suggestions, anyone?
Thanks, again, for all your help!
Joe

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Related Threads

    Motorsforum.com is a website by car enthusiasts for car enthusiasts. It is not affiliated with any of the car or spare part manufacturers or car dealers discussed here. All logos and trade names are the property of their respective owners.