First of all, I have never seen a wiring diagram for a Freightliner, Columbia.

The truck I'm driving has an unusual problem. Something has changed since it was first built. During normal driving, the coolant sensor must be left "UNPLUGGED" in order to keep the truck running. During rain, I don't have a problem until some of the 4-wheelers start passing me, and putting the fine mist into the air. Then, the "ENGINE PROTECTION" light comes on. After about a minute or two, it flashes and shuts me down. At that point, I open the hood and plug the sensor back in, and while it's raining and wet, it runs fine. Then when I get to dry roads, things dry out, and it shuts me down again. Open the hood and unplug the sensor and I'm good to go again.

I'm guessing that over a few years, road salt has gotten into a plug in the cable harness and corroded something. When it's dry, it doesn't conduct, but when it gets a little water in, the salt conducts like it's supposed to???

Any ideas where to start looking for this? The shop has had the truck apart twice now, and have not found it.

you have a short somewhere. but you can have anyone who has one of those hand held diagnosis comp's for a detroit, & they can reprogram it so the engine WILL NOT shut down on that particulair sensor. the engine sut down light will still come on , but the engine will not shut off. what I find strange is that under normal conditions that it will start & run unplugged.. sounds like it is near the e.c.m wiring & not at the water tank sensor.

It could even be a bad ecm. Best way to torubleshoot would be to run new wires right from the ecm plug to the sensor (using a new plug on the sensor end) bypassing that part of the orignal harness. If that works you know the harness has a bad spot. If it doesn't work and a new sensor doesn't help then it's in the ecm.
I know it may not be feasible to do this yourself especially if you don't have a pinout diagram of the ecm and new pins and plug to build the new harness. It would be the best method though.

Couple things you can check. Pull the plug out of the ecm and check for any corrosion on the pins in the ecm or the plug. It could also be loose or corroded inside the harness plug at the sensor. If you don't see anything obvious just looking at the connectors you would have to pull the pins out to check the crimps.
If you got a diagram of the ecm plug you could also use a mutlimeter to check for resistance on the individual wires from sensor to ecm. Is this a Detroit and if so is it DDEC 2,3,4 or 5? I might have some diagrams for 3 and 4.

I'm about certain that it is not the sensor itself. By unplugging it, it takes it out of the equation. I suggested the shop check the plugs and such for signs of green corrosion. The fact that it gets wet and changes says that at some point, road salt got in and damaged something. But, I thought the ECM was waterproofed... And, don't they have engine oil flowing through them to maintain them at a designed temp regardless of the ambient temps outside?

Company is talking about down-sizing again. Maybe they'll get rid of this truck. Doesn't sound like they want to get rid of me, though. Still, maybe I'll simply retire and not worry about it anymore. Also, I understand I may have a chance at becoming a machinist at a local company that is still working overtime. They seem to have a problem getting people that don't do drugs, and working with open, rotating equipment like that is no place for someone on anything. Guarranteed about 20 hours of overtime every week, at about $28/hr. They do dredging in about some 30 surrounding states, and all the equipment comes back here for repair. One of the welders figures he can get me in there after I machined a new start mechinism for his scooter. What I didn't tell him is... It's not compatible with any other design currently being made... Repair parts available only from ME...:clap:

Sounds like you may not have to worry about it much longer either way since it's a company unit. The ECM and the plugs are all made to be waterproof but you know how that goes. It could even be as simple as someone once poking a wire with a test light that's causing the problem. Or could be a software glitch that acts up when plugs get yet.
ECMs do not have anything running through them for cooling. Older models used to be bolted to a cooling plate that had fuel running through the plate for cooling purposes. That may be what you're thinking of.
Best of luck in whatever you do.

Actually, back when I had my own truck, my machanic (who spent six months at Cummins, learning everything about their engines) said that they do have engine oil circulating through them for temperature purposes. Maybe, that's only N-14's. I don't know that much about the rest of them. Cummins was the only one I really got into because I owned one.

Odd. Here's a troubleshooting diagram from Cummins Quickserve for the N14.
-->Diagram<--
It plainly shows only fuel lines going in and out of the ECM cooling plate.
Maybe the oil thing is something I've never heard about. I'll ask my boss about it today.

That's incorrect, the N14 has DIESEL FUEL flowing through the ECM cooling plate... NOT engine oil.

Using fuel to regulate ECM temperature has been fairly common over the years; Detroit, CAT, and Cummins have all used it at one time or another, ISX uses charge air for cooling, others just use ambient air.

...apparently, your mechanic didn't learn as much as he thought at cummins?

Well, I don't know what the shop found when they worked on it, but I certainly know they worked on it. I got up to Manchester, TN, and was sitting on the side of the road. A couple of times, when I put the clutch in, the engine seemed like I'd switched the key off. Then, the last time I put the clutch in, it died and would not re-start.

I was looking around by the transmission to see if there was anything that might have come unplugged. Nothing. On one of my trips to look from the front, alongside the engine, I looked up and noticed a big round plug hanging over the back of the engine cover. (rocker arm cover) Looked like it has a couple of dozen pins in it. You nearly have to put your head down on the engine to see where it plugs in to the firewall. Found out the shop manager was the one that plugged it in... And didn't twist the "twist-lock".

Coolant lever sensor was plugged in, and the engine ran fine until I got to MO. Engine Protect light came on, engine shut down, and would not re-start until I unplugged the sensor. Made the rest of the run with it unplugged. But, when I got back to the yard, I plugged it in and the engine stayed running...

I do know what I have to do to keep it running and make my deliveries. My name is not on the truck, only inside as the driver. I'll let them worry about fixing it.

my company 06 columbia daycab has been in the shop at least 10 times within 12 months for the check engine codes.truck would be at the shop for a week, theyd call saying it was ready, go get it fire it up and what do you know they didnt do anything....so it seemed. the mechanics replaced sensors wiring harness, injectors, filters, radiator, batteries etc. still had issues. they bypassed sensors and it seemed to be fixed... few weeks later same thing. would fire up idle for a few minutes cut off.so freaking fustrating. i had to restart the truck so many times over the year that the starter doesnt want to work half the time lately. im sure itll be going to the shop for that soon too.