I drive a 2004 Freightliner Columbia. I have a switch that says SHDN OVRD. I am guessing it must be something to do with overriding the computer to start the truck if needed.

I don't have a manual for the truck. Can someone explain what this switch does?

That is the Shutdown Override!

It is for:

When drivers call for a shutdown, that will do absolutely nothing but make truckers look worse than we already do!

Press 3 times and keep on trucking!

Actually, what it's for is this:

When the ECM detects low oil pressure or high coolant temp and decides it is best to shutdown the engine, the shutdown light and buzzer will come on indicating that an emergency engine shutdown is imminent.

The shutdown override switch can be used to delay the emergency shutdown, giving you time to pull off the road and park safely rather than being "dead in the water" in traffic.

I just about pissed my pants. That is a good explanation.

Sorry, was not sure if he was being serious, or just following through on the April 1st theme! Figured I would play on the thought of the day!

Pressing and holding the switch overrides the engine protection system which splitshifter already described.

It's also important to remember that the ecm records this action as a start and end time, and can point to the operator as the cause of engine damage... in the event that someone chooses to get home with it instead of the side of the road.

Thanks for the info. I havn't had problems yet but now I know what to do if I hear the buzzer and see some lights to get to the side of the road. I have a Detroit in this truck

My truck before this had a Cummins and it would just go into limp (idle) mode automatically so you could get over to the side of the road.

It depends on how the options are set on that Detroit. Mine was set to shutdown by the factory and that's exactly what it did on I-35 upper level in Austin TX one day, the button did nothing. I got 75% off the road and up against the left side divider wall. Of course it was 5 pm, so I wasn't moving anyway, but after that, neither was anyone else.

So, I plugged her in and set it to derate. Never again had any trouble like that, but if I had, I could have got somewhere safe at 200 horsepower.