Help with LMTV cooling issues

[Fraxinus]

Hey everyone,

To start: I've been lurking on this board on & off for a year now, and wanted to say THANK YOU for the knowledge you all share about these damn cool vehicles.

Background: I've got an LMTV M1078 that I'm working to convert to a wildland fire rig. Current bed will come off, and a lower-profile/ lighter bed will be created. Should hold about 715 gallons and remain under curb weight all plumbed out & full of water.

My issue: the in-cab thermostat never registers a temp. Sometimes the needle shakes like it's going to, but then returns to resting at bottoms. Over the winter I wasn't worried about it, but I recently had a 70 degree day where I took it out for a drive (25 min or so) hoping to get it up to operating temps & burn off the cobwebs. Temp gauge still never moved. I kept checking the coolant hoses going to the top of the radiator, and they stayed cold until about that 25 minute mark, when one finally started getting warm to the touch. At that point, I tested the heater & got very hot air almost instantly. That worried me and I stopped the test drive.

I've only worked on pickup trucks & UTV/ ATVs before, so an LMTV is a new world for me. I've been researching trying to figure out if this is normal or I should be worried. I've learned that the cooling system is cooling both engine & transmission, so there's a lot of thermal mass there. I've also learned that a common failure point is for the fan to run all the time. As soon as I start the truck, the fan is running... BUT it continues to run if I use the "fan off" switch on the dash. I also get a few other dummy lights about heating air intake & check engine when I flip the fan-off switch. If anything, just idling in the parking lot it seems to run faster when I switch it OFF from the dash. At this point I'm a bit stumped as to the next steps to troubleshoot.

Other information... On the last drive I did (back in Jan in a 50-degree spell) I had a coolant leak from a hose up top going into the radiator. I thought maybe I got an air bubble in the system, but my understanding is that the cooling system should "burp" on its own up to the expansion tank. I had to add fluid to the expansion tank twice after I fixed the leak. Maybe there's still an air bubble?

I am supposed to be driving this truck across the state in the next week to get the new bed worked up. If something is stuck open or fan is stuck on causing it to run cool I'm fine making the drive & fixing the issue after. But what I need to figure out is if something is stuck closed and hot coolant is never getting to the thermostat... in which case I'm home until I get this fixed.

Any and all thoughts appreciated. I'm sure this has been covered, but wasn't able to find it searching... so I appreciate you all bearing with me if this is basic stuff.

Thanks so much,
-Tony-

 

[NDT]

The fan will freewheel when the engine runs. If the fan clutch is engaged, the fan makes HUGE noise going down the road. What do you think, is the fan super noisy? If so, there are numerous threads here on fixing it, or refer to the Technical Manual.

 

[Ronmar]

What year truck/what engine? Sounds like you have an A1/3126 engine?

When you turn off the main switch, do you hear a click and woosh of air from under the passenger dash? If so that is your fan clutch solenoid operating. If you reach under the truck after you hear that woosh, the fan clutch should be engaged And you won’t be able to rotate the fan by hand. If the truck is still full of air, and you turn on the main switch(don’t start it), it should power that solenoid which will send air to the fan clutch which releases the clutch. So you should then be able to reach in under the truck and rotate the fan freely.

where the upper radiator pipe connects to the engine, there is a yellow elbow bolted on top of a block on the side of the Thermostat is right where the elbow bolts onto the block. It is a bypass type thermostat(search it up on utube). Coolant always flows past the thermostat, and either bypasses it to be recirculated back thru engine until it warms enough to open the thermostat at which time it closes the bypass port as it opens the port to the radiator. Yes the system with the expansion tank at the high point is self purging of air.

The heater gets it’s flow from the block below the thermostat so it gets hot water as soon as the engine starts making heat. The temp sensor for the gauge is also in that block(i think it uses a sensor there for the A1?). I don’t recall if the A1 also has a high temp switch that activates an over temp light in the dash Like the A0 has...

An IR thermometer will help you get an idea of what temps it is running until you get the gauge issue resolved, but your description of your test drives doesn’t sound unusual, except you may have a bad gauge, gauge control module, temp sensor or a wiring issue somewhere. probably the thing I most dislike about the A1/A1R is the integrated gauge control module...