Ford 2003 Mustang Workshop Manual page 3312

Pinpoint Tests
PINPOINT TEST A: THE HORN DOES NOT SOUND
A1 CHECK POWER SUPPLY TO HORN RELAY
Disconnect: Horn Relay.
Measure the voltage between the horn relay connector Pin 2,
Circuit 460 (YE/LB), harness side and ground; and between the
horn relay connector Pin 5, Circuit 460 (YE/LB), harness side and
ground.
Are the voltages greater than 10 volts?
A2 CHECK HORN RELAY
Check the horn relay; refer to Wiring Diagrams Cell 149.
Is the horn relay OK?
A3 CHECK HORN SWITCH SIGNAL TO HORN RELAY
Measure the resistance between horn relay connector Pin 1,
Circuit 6 (YE/LG), harness side and ground under the following
conditions:
Horn Switch Position
Horn Switch Position
Pressed
Released
module.
Horn switch.
Test Step
Resistance
Less than 5 ohms
Greater than 10,000 ohms
Result / Action to
Take
Yes
GO to A2 .
No
REPAIR the circuit in
question. TEST the
system for normal
operation.
Yes
GO to A3 .
No
INSTALL a new horn
relay. TEST the
system for normal
operation.
Yes
GO to A4 .
No
GO to A5 .