2000 Pete 379 3406e water/temp light & buzzer won't shut up

temp buzzer and warning light are a peterbuilt system. the cat ECM doesn't control them.

they're pretty crude. they're just thermal resistors and temperature switches with no failsafe. so when the wiring breaks or deteriorates and the resistance changes so does the gauge/buzzer/light behaviour.

the cat ECM uses a better quality thermal resistor and in some early 40 pin ECMs they use a 3 wire 5v supplies sensor. this ensures the cat ECM data is accurate. it also has fail safes. it will log codes in the event of open circuit or short circuit. I take the ECM temp as gospel and disregard the truck OEM sensor data.

given you've replaced the truck OEM sensors with no change its likely the wiring has greater resistance causing the gauge/buzzer/light to be incorrect.

 

depends if the switches are normally open or normally closed. if theyre normally closed the a broken wire will bring on the light/buzzer. if theyre normally open then you either have your signal wire shorted to your return or shorted to ground.

 

Before I got too involved I would disconnect 1 wire from the low air pressure sender, and see if that is it. I have had them cause identical symptoms as you describe. I am not sure about a 2000, but on older 379s the air pressure sender breaks continuity as air pressure reaches 70 psi. If they start to stick the light flickers and the buzzer sort of buzzes intermittently until the sender goes completely.

It may not be the cause, but it certainly is worth checking before spending much time chasing anything else.

 

Glad you found it!

 

I got 96 KW T600 with 3406e. My fan won't come on automatically. Temp gauge reads out fine, but I see thru scanner my Temp sensor reads out -1F. By the thermostat housing there are 2 sensors on top (2wire and 1wire sensor), then one sensor on right side and another from front between engine and fan. Does anybody know which one is which and where is my temperature sensor that controls when to kick on the fan? I appreciate your inputs guys here.... good stuff