2006 Kenworth W900 low pressure light works but buzzer does not

(Not a mechanic - but)

Check that the pressure switch you replaced is the same - it should have a stamping or marking somewhere that says . . . (it's often on the brass nut part)

NO (Normally Open) or
NC (Normally Closed) -

A NC switch will have the buzzer working, until pressure is supplied at 60 PSI or so. Then it becomes an 'open' circuit - removing power to the buzzer.

A NO switch will be the reverse - the buzzer will be off till the pressure hits 60 PSI or so, then it will 'close' the circuit, supplying power to buzzer.

That's all I got.

 

Sounds like someone has put the wiring and/or low pressure switch in the wrong port in the manifold.

The ports for the buzzer and light will always have pressure.

 

I'm a little confused. My buzzer and light are a module. The buzzer didn't work, but the light did. I changed the module because the fan light didn't work. When I changed it, the buzzer now works. I know that the part I put supersedes the one that was originally in mine, so it definitely covers later years than mine. If mine has the buzzer in the same assembly as the lights and the buzzer is not serviceable, wouldn't yours be like that too? 1994 for your information.