m939 Heater Core

SLOrazorsedge said:

I just had my 2010 rebuilt M936A1 blow the heather core into the cab. Spent hours looking in the TM for a R&R procedure and nothing...thanks for your thread and instructions on the R&R. I guess I'll see how bad it is and try a radiator ship first too.

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Yup, same thing happened to me and after I wasted all that time, I figured I would help as many have helped me here by posting on ss.

Patracy does have a deal on his though

Thanks again Juan, took me about 1/2 hour and gouging my thin skinned left forearm to remove the heater core. I can see what the issue is, the heater core is installed with sticky backed foam strips on the bottom and top. That simply is inadequate to isolate the road and engine vibration. Once I have the core repaired, looks like the solder on the bottom edge end cap was leaking I'll insulate with rubber pad top and bottom.

Thanks Patracy, if the core is not repairable I'll give you a shout. It does look brand new, even all the bolts to the housing were new cad plated.

I just started digging into mine today....as far as I can tell it is getting coolant but my blower will not turn on. Anyone by chance have a diagram for the blower switch? I have an extra power plug hanging from my dash that I'm suspecting has something to do with the blower.

Ronny,

Welcome to ss!

The blower motor is 12v on low and 24v on high. See the attached wiring diagram from the p2p software available here on ss at the top of the 5 ton forum in a "sticky". Also the newer 23 series manuals have excellent troubleshooting flow charts here on ss

The blower motor is available generically at NAPA. You would have to splice in the military packard connector but otherwise it is an identical fit. Search the 5 ton forum as there has been some threads on the aftermarket motor.

The blower motor electrical is totally seperate system than the water flowing to the core so one would have nothing to do with the other as far as working.

I just brought my M923A2 home, heater failed with 400 of the 1200 mile trip to finish at -20 C. The info here is awesome . Thanks everyone for the posts.

Welcome to the green envy madness. I just got my heater core out of the radiator shop. They sweated the tanks off cleaned the core and re-soldered it back together looks good as new for $55. I was right, the lower seam gave out likely from vibration and rattling around inside the sheet metal can. BTW, it was 18F this morning in central Calif only 6 miles from the Pacific Ocean , thanks Alaska!

I stuffed mine in tight with a dense sticky taped foam from Home Depot. I think it was for doors and windows but universal application. It was maybe 1 inch or slightly wider.

Suprman said:

My truck was rebuilt in 2010 and a new heater box installed then. They dont seem to last long. Not that they are hard to change its just annoying to have coolant pouring into the passenger footwell.

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Worth mentioning that there are provisions to close-off coolant flow to the heater core if anyone finds themselves with one that starts leaking. Just look where the coolant lines are connected to the engine.

jesusgatos said:

Worth mentioning that there are provisions to close-off coolant flow to the heater core if anyone finds themselves with one that starts leaking. Just look where the coolant lines are connected to the engine.

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Very true!
I will also add that mine were steel. When I installed my coolant filters I changed the shut off valves because the handles were twisted off and the stems bent. What I found was that they were almost rotted through and very pitted at the hose barb. Definitely would have failed at some point. If you have steel shut offs, pull the hose and check for rot.

I replaced them with take offs from a scrap m931 and they were brass.

Well reinstalled the heater core today with new heater hose and clamps. Similar to Juan I used actual rubber door/window sticky back molding 1" wide. That will hold up better than the sticky back crush foam that was OEM. During the install I discovered the round felt air dam insulator from the air inlet flap valve came loose and fell down into the blower motor vanes. The blower doesn't blow now. I ran the switch on both low and high, no bueno. So I either have a smoked fan motor or smoked a fuseable link. Interesting that I have not found a fuse block/panel on any of these trucks. Didn't have time for further diagnostics today. Anyone have similar experience?

There is no fusible link. The power feeds 12 or 24 from the fan switch. I suspect either the switch or the motor is fried. See if you have power at either wire on the fan switch? There are some circuit breakers on the firewall bit I do not recall where the 24v comes from on the switch. the 12 is straight from the battery if I remember correctly.

The generic motor at napa is a Balkamp 655-1320. Just save the packard connector and splice in. The other wire is just a ground. OE I think is only one wire (hot).

Thanks brother, I won't be able to get to the diagnostics until next week. The Napa store is right around the corner from me :0)

Printed off a couple of the diagrams from the TM today and got the power running again to my blower motor. Started to smoke as soon as I turned it on so I ended up pulling the motor out to find it was completely seized up. Walked over to Napa and ordered the replacement so once that comes hopefully my heater core is good and everything works. Thanks for the part number.

Ronny, how did the Napa blower motor work out? Mine is frozen up too after the flapper felt fell down in it and locked up the blower vanes.

It hasn't come in yet- they only have about 6 in stock in the nation and they were all on the west coast. They had 3 sitting in Phoenix so that is where mine is coming from. I'm about to give them a call today and see if it is in yet and I will let you know how it works out.

Any luck with NAPA on your heater blower motor? Did it work out okay? I'm holding off until I hear how you made out brother.
'

SLOrazorsedge said:

Any luck with NAPA on your heater blower motor? Did it work out okay? I'm holding off until I hear how you made out brother.
'

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Purchased a NAPA blower motor and it worked out great! perfect fit and runs great!

I got mine in on Tuesday and it works fine. I will have to redo my grounding when it isn't 10 degrees out but I'm happy to at least have heat. I think I might have to cover up a bit of the front of the hood though it takes absolutely forever for the engine to get to temp running it as-is. I should say for whatever reason the fan assembly on mine was completely seized on the shaft of the old motor and I ended up having to pop it into the drill press and bore the shaft out to get it off. It mated up to the new motor just fine though.

Yes they do take some time to warm up in cold weather. I have seen cold weather grill covers for the 800 trucks but not for 939s

I have seen a full front/hood mask with headlight cut outs on some m939 GL pics from Alaska.

They also have a M939 grill only roll up cold weather vinyl cover. Check wsucougarx build thread as he shows one installed.