1999 Freightliner Classic DDEC 4

Unplug your fan solenoid and see if your gauge settles to normal.

 

I thought engine fan on those were controlled by the ecm.

 

Don’t know about your gauge. I do know for sure there was a bulletin on the fan. I did the upgrade on my 99. It involves a wire splice into the harness near the solenoid. Easy fix. Did it in parking lot at Youngstown Freightliner. Had the problem for years before I heard about the bulletin. Fan would come on for no reason. Wintertime, stopping to pay a toll for example. No one else seemed to know anything. Finally Youngstown came through with an actual upgrade kit. They even loaned me a fancy crimp tool. I mentioned it to them a few years back. No one knew anything. Lol. My 2000 has a pdmi box in the fuse panel that controls the fan, and a few other things. Eliminating a few sensors. Memories a bit rough. See if you have a pdmi box. If not, maybe you need the upgrade kit if it can be found.

 

Yes.

They are, but that signal goes through the Coolant Level Sensor Module on the Freightliners, which interfaces with several other outputs

 

Most, Not all had mechanical Gauge from factory.

 

Sticking thermostats will cause mechanical gauge to be in accurate/or all over the place. Possibly causing a ecm coolant temp sensor to see a overheat situation and cycle the fan.

Or you have a head gasket/liner issue about to show its face.

 

See if it's a mechanical gauge, possibly fan control too, but shouldn't be on that engine.

A pic of the right rear of the head, just below the valve cover, and a pic of the thermostat housing would verify