Engine Electrical System

Diagnostic trouble code: P0646 “Undervoltage in control circuit of A/C
compressor relay”
Item
No.
Operation procedure
Inspection
results
Follow-up
procedure
1
Connect the diagnostic tester and adaptor
and turn the ignition switch to “OFF”
position.
Proceed to next step
Yes Proceed to Step 4
2
Pull out the A/C compressor relay, turn
the ignition switch to “ON” position and
check if the voltage between the power
supply terminal of relay (i.e. 30# and 85#
pins of the relay) and the negative grid of
power supply is about 12V.
No Proceed to next step
Yes
Repair or replace
the wiring harness.
3
Check if the power supply circuit of relay
is open or short to ground.
No Proceed to Step 2
Yes Replace the relay.
4
Inspect with multimeter to check if the
voltage between the control terminal of
A/C compressor relay (i.e. 86# pin of the
relay) and the negative grid of power
supply is about 3.7V.
No Proceed to next step
Yes
Repair or replace
the wiring harness.
5
Check if the circuit between the control
terminal of A/C compressor relay (i.e. 86#
pin of the relay) and 70# pin of ECU is
short to ground.
No
Refer to diagnosis
help.
E-59
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