99 Detroit series 60 coolant temperature

I don’t know about the coolant temp warning light, but the other problem could be the harness. Mine had a broken wire. It started only while pulling a hill, probably because the fan kicks on and causes the wire to shake. Eventually it would shutdown when it got wet.Id unplug it, run till it shutdown again, then plug it in!!! I replaced 3 coolant level sensors over 4-5 yrs. It was so intermittent, each time I thought it was fixed ( again) . Finally figured it out. New pigtail plug for sensor, about 12” long. Problem fixed. Maybe it’s also causing the other fault on your Truck. Mines a 99 in Freightliner. Either way the new pigtail should fix the check engine light. Meanwhile a paper clip jumping the plug while it’s unplugged may turn light off, not sure though if the wire is broken. It’s a common problem.After you repair pigtail plug and connect it back to level sensor on the tank, make sure to keep wire away from head ( heat) and zip tie up good to minimize movement

 

I had same issue, till it eventually shut off with no check lights at all. Cleaned all the grounds, culprit was ecm ground at frame. 2 small wires near battery box. Also check Small inline fuse for ecm on hot side off battery

 

I'm having the same issue, my fan usually kick in around 200 degrees, but I replaced the fan clutch and my fan is still kicking on well before the normal temp. Any suggestions?

 

Mines doing it again. This time, like before, when it’s raining hard, or after a Truck wash. Trying to spray water on wires and sensor plug to simulate the problem. Must be pigtail again. Usually it shuts dow, unplug it and run till it shuts down again. I guess it dries out. Then plug it in, good to go. Now it’s doing it for no reason it seems. Thought I had it fixed, maybe I did. Going to replace pigtail metro connector again. This time I’ll cut the original wires back as far as possible. I was told they know for breaking where they run along the valve cover.Any ideas out there? I think the fan coming on makes the wire move a bit, and over time break. That’s what I thought I’d figured out anyway.

 

Update: replaced pigtail, still had problem. This time it was the level sensor. New sensor, Problem fixed.

 

New problem, same as OP, temp doesn’t go high as it should. New thermostats radiator. Never worried about it, since radiator was old, and I had 180 thermostats. Now I have 190s. Never gets over 180. 2000 Freightliner Classic 12.7. It was stating under 150 in cold weather. I thinks it’s causing bad fuel economy. Though Trucks running great. 4.5 mpg. Still breaking in fresh overhaul.

 

Did you figure this out?

 

No, not really. I think it just needs a winterfront when it’s real cold. I think it’s the same as all Freightliners. My last one was the same way. I should probably be using a winterfront while idling also. I don’t know what the conditions called, but idling cold will glaze the cylinders. Friend of mine just put a winterfront on his 12.7 Coronado 12.7, says his mpg’s are up. Since getting the new radiator, my fan rarely comes on at all. Except when the a/c is on when idling to bring the pressure down. Summertime I’ll lock the fan on, when idling.