Testing Ground and Low Reference Circuits
Because a test lamp can light with up to 200 Ω in a
ground or low reference circuit they are not recommended to be used
for this type of test. A low impedance test lamp can light and
indicate the circuit is good when there is as much as
20–30 Ω present, while a high impedance test lamp can
light with up to 200 Ω present in the circuit. This is why
the Digital Multi-Meter (DMM) is used when validating a ground or
low reference circuit continuity.
When using the 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. This may cause 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 vehicles 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
would be a control module housing (if the control modules housing
is metal and grounded), door jamb striker (if attached to metal),
under dash metal frame work, 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 Ω with the ignition ON, and drop to
15–25 Ω after the ignition is turned OFF. The reading
will drop below 10 Ω after 30–40 seconds, then below
5 Ω after 60 seconds. Once the vehicle completely goes to
sleep (generally 3-10 minutes) the reading will drop below
0.3 Ω.
The following list of conditions may need to be met to ensure a
valid continuity reading on a ground or low reference circuit:
• |
Key out of the ignition switch
(when not equipped with keyless entry and remote start)
|
• |
Retained Accessory Power (RAP)
OFF (open and close the driver door after ignition OFF)
|
• |
Battery charger set at a
2 A or less charge rate |
• |
Scan tool not communicating
with any vehicle control module (in some cases it may need to be
disconnected from the DLC) |
• |
Headlamps OFF (auto headlamps
disabled) |
• |
Any accessory that can work
when the ignition is OFF |
• |
Wait up to 60 seconds (after
all other listed conditions are met) |
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.
|