Hi all! 10/23/09
I checked-in to the list today at lunch time and saw a few replies about my suggestion
to use an ammeter instead of a test light when testing for a parasitic drain on a battery.
I learned this lesson the hard way almost thirty years ago.
You see, I was trying to diagnose why a certain Oldsmobile was having the Battery
run-down every 3 or 4 days.
Following the conventional wisdom of the era, I had a test-light installed in series between
the battery and a disconnected cable.
The Lamp glowed brightly!
I proceeded to follow conventional wisdom (again) by pulling one fuse at a time until the
test-light went out.
Eureka! That fuse was for the Rear Suspension Load Leveling feature, really nothing
more than Air Shocks, a Compressor, Height Sensor and Module.
OK, now what?
I re-installed the fuse and the light was brightly lit again.
Thinking the most logical failure would be the Module itself, I disconnected it.
Lo and Behold! The light went out! Yay!!!
After replacing an expensive Module I retested again with the Test-Light.
To my horror it was brightly lit!
Disconnecting the new Module made the light go out again!
After my steaming bright red face explained this to the shop owner, he handed me
the phone number to a brand-new (to me) service. A technical hot-line for Auto
repair mechanics.
Now remember this was almost thirty years ago, electronics were just beginning to
to become "the force" that they are today.
The consultant politely listened to my dilemma and then asked one solitary question.
"Why are you using a test light"?
He explained that this particular Module needs to have FULL voltage and current in order
to settle the rear suspension to the proper height before it will shut-down and go to sleep.
With a test-light in series this could never happen. He suggested I retest using an Ammeter
and wait for five minute to see what would happen.
Well, needless to say, the system leveled and shut-off.
No more drain!!!
I've used an Ammeter ever since for this test!
After seeing today's replies I decided to put together a quick blog showing this phenomenon
on a typical car we had in the bay.
It is a 1999 Chevy Malibu, 3.1L engine, Auto Trans.
It was not in for any sort of Battery problem so I knew it would be a perfect candidate.
I was able to do these tests and take the pics in-between jobs and waiting for parts.
In the first pic you'll see I have test-light in series with the Battery.
You'll also observe that the Light is lit brightly even though the flash of camera almost
drowned it out.
After 15 minutes IT WAS STILL BRIGHTLY LIT!!!
(remember to click on the image for a larger view)
Hmm....
Now that's interesting!
Now let's see what happens when I use the Ammeter in my Fluke 88 DMM
The drain started out high, about 300 mA (0.3 Amps) then within about a minute fell
to what you see, 62 mA (0.062 Amps).
I now know for sure that this car DOES NOT have an issue with parasitic drain on the
battery.
If I had *trusted* the test-light method I would have wasted a great deal of time and
parts money and maybe even risked the trust of a customer.
Michael was asking about using an inductive Amp Clamp to get a quick idea about the
possibility of too much parasitic drain.
First you would not be able to trust an Amp Clamp that measures anything over about
10A.
The one I use is this:
It is a *Low Current* probe capable of measuring very low Amperage's.
I use it primarily for use with an oscilloscope to measure Fuel Injector, Fuel Pump, Ignition
Primary events.
It is fast enough and accurate enough to give me good results.
To use it you connect it's leads to your DMM or Oscilloscope and convert it's output to Amps.
The problem with using it for parasitic drain testing is it's "nose size".
I can't fit it around most battery cables...
If you would like to peruse this tool more you can go to http://aeswave.com/Products
/Product.asp?i=59 for more info.
All in all there are many myths that can get you trapped in a diagnostic boondoggle that
does nothing but cost you time and money.
Hopefully I've been able to clear up at least one of them with this post...
Master Nimrod,
Mike.
