Tests pour vérifier les Ground and Low Reference

Salut !

J'ai trouvé ce petit texte dans un GM Techlink. Je le mets à titre de référence.

Testing Ground and Low Reference Circuits
A test lamp can light with up to 200 Ohms in a ground or low
reference circuit, so it is not recommended to be used for this
type of test. Instead, use a Digital Multi-Meter (DMM) when
validating ground or low reference circuit continuity. Updated
information about testing ground and low reference circuits is
now available in the Service Information (SI).
TIP: For 2007 and later vehicles, click the Circuit Testing link
in any SI diagnostic procedure, then select Testing Ground and
Low Reference Circuits.
When using a DMM, there are many vehicle conditions that
can affect the ground and low reference continuity testing. If
these conditions are not met, a ground or low reference circuit
test can fail on a good circuit, causing longer diagnosis time and
incorrect component replacement.
Any current flow through a ground or low reference circuit
while being tested will skew the DMM continuity reading, or
display a reading higher than when there is no current flowing.
Performing a ground or low reference circuit continuity test in
reference to the vehicle’s battery negative terminal will have the
highest risk of failing a good ground or low reference circuit test
than any other ground reference point. The best ground test
points are a control module’s housing (if the housing is metal
and grounded), door jamb striker (if attached to metal), under
dash metal framework, engine block, or body ground studs
(other than where the negative battery cable is attached).
A typical DMM ground or low reference circuit continuity
reading can be as high as 100 Ohms with the ignition On and
drop to 15-25 Ohms after the ignition is turned Off. The reading
will drop below 10 Ohms after 30-40 seconds, and then below
5 Ohms after 60 seconds. Once the vehicle completely goes
to sleep (generally 3-10 minutes) the reading will drop below
0.3 Ohms.
Conditions that may need to be met to ensure a valid con -
tinuity reading on a ground or low reference circuit include:
• Ignition Off
• Key out of the ignition switch
• Retained Accessory Power (RAP) Off
• Battery charger set at a 2 Amp or less charge rate
• No scan tool communication with a control module
• All access doors closed
• Headlamps Off (Auto headlamps disabled)
• Any delay lighting Off
• HVAC afterblow Off
• Any accessories Off that can work when the ignition is Off
• Wait up to 60 seconds (after all other listed conditions
are met)
TIP: Activating the door lock function with a key fob can
greatly reduce the time to achieve a valid ground or low
reference continuity test result.
– Thanks to Dave Nowak