The Best Way to Perform a Parasitic Draw Test

   #1  

Uwe

Benevolent Dictator
Staff member
Joined
Jan 29, 2014
Messages
30,490
Reaction score
21,750
Location
USA
VCDS Serial number
HC100001
Last edited by a moderator:
   #3  

DV52

Verified VCDS User
Verified
Joined
May 16, 2014
Messages
3,706
Reaction score
4,125
Location
Melbourne, Australia
VCDS Serial number
C?ID=194404
^^^^^ Very good video (IMO), but just a note of caution - professionals (like the folk @ RT and Charles-the humble mechanic) correctly invest in good test equipment because it makes commercial sense to do so.What this means is that when these types of videos are made, there is an implicit assumption that what the meter reads is actually correct - and in most videos, the reading is correct (to the accuracy of the meter)

However unless Joe-average is an enthusiast, he/she generally has less financial justification when choosing which multi-meter to buy. But this notwithstanding, Joe-average generally has the same unshakable degree of confidence in the reading shown on the meter screen and on lots (most?) domestic multi-meters this confidence is misplaced .

The particular issue when "domestic" multi-meters are used for parasitic leakage analysis is that the test numbers are usually very small and this exacerbates the meter inaccuracy issue (which is further complicated by the noisy electrical environment in which the test is performed). This reading inaccuracy (@ small volt-drop numbers) makes the current draw number from the reference table in the video somewhat fungible (in my bitter experience)!

So, if I may interpose a further point of complication to an otherwise excellent video - certainly do all the multi-meter tests shown in the video as a precursor to measuring the volt-drop across the fuse, but consider also that multi-meters sometimes tell deliberate lies (it's not their fault)! Even if your preliminary multi-meter tests are successful - just because the meter screen says 3 milli-volts, doesn't mean that the volt-drop is actually 3 milli-volts!

Don
 
Last edited:
   #4  

Boki Ar

VCDS Distributor
Joined
Oct 27, 2014
Messages
1,374
Reaction score
1,418
Location
Arandjelovac, Serbia
VCDS Serial number
C?ID=57300
^ Exactly, which is why we should always remember to compare the measured value with the table. ;-)
 
   #5  

Bruce

Active Member
Staff member
Ross-Tech Employee
Joined
Jan 30, 2014
Messages
1,477
Reaction score
2,712
Location
Near Philadelphia, PA, USA
VCDS Serial number
--------
- just because the meter screen says 3 milli-volts, doesn't mean that the volt-drop is actually 3 milli-volts!

Don
Don, I could not agree more. Can't tell you how many times in my career a technician working with me gave me a reading number just because the meter said so... And then when I shorted the leads together - where the meter should read 0volts - I showed that the meter had an inaccuracy.

Bad leads can change your numbers guys. Bad meters can change your numbers. Your meter itself should be checked regularly if you are going to do millivolt measurements.

Voltage drop tests can be a help but be skeptical of results... Learn your gear and know what it can/cannot do for you. Practice doing voltage drop tests on known good systems before you have the confidence that on a screwed up system, the measurement you take is the Gold standard of measurements. It likely is not!

Oh, and one more thing - the shorter the leads to the meter - the more likely the measurement will be accurate.

Have fun out there!
 
   #6  

Uwe

Benevolent Dictator
Staff member
Joined
Jan 29, 2014
Messages
30,490
Reaction score
21,750
Location
USA
VCDS Serial number
HC100001
^^^^^ Very good video (IMO), but just a note of caution - professionals (like the folk @ RT and Charles-the humble mechanic) correctly invest in good test equipment because it makes commercial sense to do so.What this means is that when these types of videos are made, there is an implicit assumption that what the meter reads is actually correct - and in most videos, the reading is correct (to the accuracy of the meter)

However unless Joe-average is an enthusiast, he/she generally has less financial justification when choosing which multi-meter to buy. But this notwithstanding, Joe-average generally has the same unshakable degree of confidence in the reading shown on the meter screen and on lots (most?) domestic multi-meters this confidence is misplaced .

The particular issue when "domestic" multi-meters are used for parasitic leakage analysis is that the test numbers are usually very small and this exacerbates the meter inaccuracy issue (which is further complicated by the noisy electrical environment in which the test is performed). This reading inaccuracy (@ small volt-drop numbers) makes the current draw number from the reference table in the video somewhat fungible (in my bitter experience)!

So, if I may interpose a further point of complication to an otherwise excellent video - certainly do all the multi-meter tests shown in the video as a precursor to measuring the volt-drop across the fuse, but consider also that multi-meters sometimes tell deliberate lies (it's not their fault)! Even if your preliminary multi-meter tests are successful - just because the meter screen says 3 milli-volts, doesn't mean that the volt-drop is actually 3 milli-volts!

Don
TL;DR: Don't use a $15 multimeter from Harbor Freight for this. :thanks:
 
   #9  

JulianHicks

Verified VCDS User
Verified
Joined
Jul 8, 2015
Messages
161
Reaction score
96
Location
Kent, UK
VCDS Serial number
C?ID=265735
This technique is all very well but not all fuses have the connectors exposed for external measurement with a meter.:(

If you look at the video, most of the small fuses at the top don't have the external contacts although there are some pink ones that do.

Certainly, in my A8, ALL the small fuses on both sides of the dash don't have the contacts for measurement.

Julian
 
   #10  

Jef

Ross-Tech Employee
Staff member
Ross-Tech Employee
Joined
Jan 29, 2014
Messages
2,553
Reaction score
4,251
Location
HQ
This technique is all very well but not all fuses have the connectors exposed for external measurement with a meter.:(

:rolleyes:

If one does run into this issue, then I think the correct solution would be to buy new fuses with test ports. Fuses are pretty cheap. The ones without test fuses are still good, toss them into an empty Altoids tin for later use.
 
   #11  

Jack@European_Parts

NostraJackAss
Joined
Jan 29, 2014
Messages
20,715
Reaction score
8,809
Location
Montgomery, NY, USA
VCDS Serial number
C?ID=57337
   #14  

Zenerdiode

Verified VCDS User
Verified
Joined
Jun 26, 2014
Messages
1,007
Reaction score
1,559
Location
Newcastle, England
VCDS Serial number
C?ID=24330
I did some testing a while ago. Riveting stuff (yeah, the hours fly by :rolleyes:)

This was current measured by direct connection (i.e. not with a non-intrusive current clamp, the meter was in series with the negative battery clamp.)

Skip to 3:30 where the 'fun' starts. You hear some relays in BCM2 dropping and the current drawn drops accordingly. What astonishes me is the final quiescent current value is around 7mA! Each flick to 10mA is in unison with the alarm LED flash too. In that 7mA, the alarm antennas are still 'listening' for the key fob press, the door handles waiting for a touch to open; and every device inside the car that can be woken up by a button press - still needs to have its circuitry powered. 7mA. My.

 
   #15  

Uwe

Benevolent Dictator
Staff member
Joined
Jan 29, 2014
Messages
30,490
Reaction score
21,750
Location
USA
VCDS Serial number
HC100001
What astonishes me is the final quiescent current value is around 7mA! Each flick to 10mA is in unison with the alarm LED flash too. In that 7mA, the alarm antennas are still 'listening' for the key fob press, the door handles waiting for a touch to open; and every device inside the car that can be woken up by a button press - still needs to have its circuitry powered. 7mA. My.
That's good. ~5 AH per month. Car should still start after sitting un-driven for a goodly fraction of a year.

But what I'd like to know is why the heck it sucks down almost 10 amps for over 3 minutes, after you've locked it? That's the equivalent of 3 more says of sitting un-driven!

-Uwe-
 
Top