Pinpoint Test C: DTC B1255 Or DTC B1257 — A/C Ambient Temperature Sensor Open Circuit Or Short To Ground
Refer to Wiring Diagrams Cell
55
, Automatic Climate Control System for schematic and connector information.
Normal Operation
Under normal operation, the A/C ambient temperature sensor receives a ground from the electronic automatic temperature control (EATC) module through circuit 3059 (YE/LG). A 5-volt
reference voltage is supplied to the A/C ambient temperature sensor from the EATC module through circuit 3061 (LB/OG).
Possible Causes
An open in circuit 3059 (YE/LG) or 3061 (LB/OG)
A short together in circuit 3059 (YE/LG) and 3061 (LB/OG)
A short to ground in circuit 3061 (LB/OG)
A/C ambient temperature sensor
Electronic automatic temperature control (EATC) module
PINPOINT TEST C: DTC B1255 OR DTC B1257 — A/C AMBIENT TEMPERATURE SENSOR OPEN CIRCUIT OR SHORT TO GROUND
B7 CHECK MODULE CONNECTION
Carry out the Electronic Automatic Temperature Control (EATC) Module Cold Boot
Process.
Carry out the EATC on-demand self test.
Operate the system.
Does the concern return?
Yes
INSTALL a new EATC module. REFER to
Section 412-01
. TEST the system for
normal operation.
No
The system is operating correctly at this time. The concern may have been
caused by a loose or corroded connector. CLEAR the DTCs. REPEAT the self-
test. TEST the system for normal operation.
Test Step
Result / Action to Take
C1 CHECK THE AMBIENT TEMPERATURE SENSOR RESISTANCE
Ignition OFF.
Disconnect: Ambient Temperature Sensor C132.
Measure the resistance between the ambient temperature sensor terminals.
Is the resistance within the specified values for these temperature ranges: 10-20°C (50-68°F),
37,000-58,000 ohms; 20-30°C (68-86°F), 24,000-37,000 ohms; 30-40°C (86-104°F), 16,000-24,000
ohms?
Yes
GO to
C2
.
No
INSTALL a new ambient air temperature sensor and bracket.
CLEAR the DTCs. REPEAT the self-test. TEST the system for
normal operation.
C2 CHECK THE EATC MODULE OUTPUT VOLTAGE
Ignition ON.
Press the AUTOMATIC button.
Measure the voltage between ambient temperature sensor C132-1, circuit 3061 (LB/OG) and C132-2,
circuit 3059 (YE/LG).
Is the voltage between 4.7 and 5.1 volts?
Yes
INSTALL a new ambient air temperature sensor. TEST the system
for normal operation. If the code returns, GO to
C7
.
No
If diagnosing DTC 1255, GO to
C3
. If diagnosing DTC 1257, GO to
C5
.
C3 CHECK CIRCUIT 3061 (LB/OG) FOR AN OPEN
Ignition OFF.
Disconnect: EATC Module C228a.
Measure the resistance between EATC module C228b-17, circuit 3061 (LB/OG) and ambient
temperature sensor C132-1, circuit 3061 (LB/OG).
Yes
GO to
C4
.
No
REPAIR circuit 3061 (LB/OG) for an open. CLEAR the DTCs.
REPEAT the self-test. TEST the system for normal operation.
Page 11 of 45
2006 Town Car Workshop Manual
12/30/2011
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