2004 F150 Alarm

[bigler45]

I bought a boat with a 2004 F150 and the boat needed some work. While going through some things I tried testing the alarm by turning the key to run with the lanyard disconnected and it didn't sound. I ohmed out the lanyard switch at the ignition wiring and it checks out. I took off the buzzer and hooked it up to the battery and it made a bunch of racket so put it back on, then tried to unplug the temp switch and jump it and still no alarm. With the lanyard off and the temp switch still jumpered, I measured pink wire on the 10 pin plug and it was 13v.

I'm assuming this pink alarm wire is supposed to be driven to ground to activate the buzzer? Does it get driven to ground by some relay or ECU directly? Is there anything else I can check?

Thank you!
Michael

[fairdeal]

The pink wire goes directly back to the engine ECU,
which provides a ground when it wants the horn to sound.

I tried testing the alarm by turning the key to run with the lanyard disconnected and it didn't sound.

To be clear, there is turning the key to "run" -
aka "key on"
and then,
there is the further turning of the key to "start"
aka "crank" - activating the starter motor.

With the lanyard removed, and the key on,
the ECU is receiving a "stop signal" via the white wire.
But it doesn't yet sound the horn.

If it simultaneously senses rotation of the crankshaft -
from the starter motor turning it -
THEN it will sound the horn.

[bigler45]

Thanks for the reply. So, the same would go for the thermoswitch? Is there a way to test this without potentially starting the motor? Maybe disconnect plug wires while cranking?

[fairdeal]

You can test the thermoswitch itself directly:

heat it up to >180°F (maybe as high as 194°F)
and it should change from no continuity, to continuity

You can test that it prompts an overheat alarm
by simply jumpering the black and pink leads on the engine harness together
(the leads that the thermoswitch connects to)
to simulate a closed thermoswitch

but the motor will have to be running,
and there's a built-in delay of over a minute, after starting,
before it will alarm

[bigler45]

Understood, thank you both for the clarification. What I saw didn't specify that the motor had to be running for the thermoswitch jumping or cranking for the disconnected lanyard switch. I will have to wait a few days until I have the chance to turn it over.