Checking grounds, continuity, does DMM lead orientation matter???

[WalledSonic]

Ive tried changing DMM leads, i made sure all connections are bare metal, etc etc. Ive found that how you orient your +/- leads on the DMM can change your measured continuity. Ive checked grounds all over the electrical system, with varied results. If i place the + DMM lead on the battery - and the - DMM lead on some grounded location, i get full continuity. If i place the - DMM lead on the battery - and the + DMM lead on some grounded location, i get a variety of values, usually 1.2 to 3.7 ohms, depending on location.

WHYYYY??? Please, only reply if you have a logical response. Thank you.

 

Maybe the fact i have the battery grounds still attached to the batteries is a problem. Going to test the same ground cables with no batteries attached this weekend to see what if i can repeat these measurements.

[WalledSonic]

@snafu , have you seen this before? Boggles my mind, makes no sense to me. 

[ShadeTreeMechanic]

I would suspect that there is a small amount of voltage that is throwing the reading off. I would expect to see 0.1 like the leads are connected together. Instead its either flat zero one way and greater than zero the other way. Try measuring millivolts at the same location to see if there is any voltage present.

ps. I assume that the battery in your meter is good.

[WalledSonic]

Little update, disconnected the grounds from the battery terminals and have now confirmed my ground run for the rear bank is good. I guess the battery does influence the DMM reading. 

 

[ShadeTreeMechanic]

Glad I could help. Are you checking connections because a problem developed or just verifying everything is good? Your Lithium is looking good in the back there too.

[snafu]

Hmmmm . . . no, I have not seen that before.  However, I'm not sure exactly what you're testing here.  You say the "grounds are good."  In what respect?

One cannot use a DMM to measure continuity from one ground point to another in an effort to determine a "good ground."  It doesn't work that way - especially not with high powered car audio systems.  Rather, one has to use the DMM to measure the difference in voltage between the ground of the accessory in question and the ground of the charging system (case of the alternator) with the system playing music at it's maximum volume.  This measured voltage between these two points is referred to as voltage drop and in and of itself somewhat meaningless.  However, the formula works as follows:

Voltage Drop / Running Voltage = %age of Voltage Drop

That %age must be less than 3% in order for the ground to be classified as "good."

[WalledSonic]

I guess power under load versus with a DMM signal are two different things, along with how impedance of a woofer may change when measuring with just the IM-SG versus with the AMM-1 (under load).  

I saw your video recently testing voltage drop.  I will try that out soon.  Thank you for chiming in.

[snafu]

You're welcome Mr. SOTM Winner!