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M939 Cab heater not blowing hot air

nayrbrellim

nayrbrellim

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So I replaced the blower motor under the squirrel cage and now have air coming out, I guess when I did this I never really checked to make sure it was getting hot as it was still warm out. Well, evidently the air isn't getting hot or warm even.

Any good advice about where to start first?

thanks
 
nayrbrellim

nayrbrellim

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I was reading something about that in the TM but just wasn't sure. I don't think the heater has worked since I acquired her so this is the first time air has come out of the vents. I'll need to climb up there and check that.

Is there a reason somebody would close it?

thanks,
 
Mullaney

Mullaney

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nayrbrellim said:
I was reading something about that in the TM but just wasn't sure. I don't think the heater has worked since I acquired her so this is the first time air has come out of the vents. I'll need to climb up there and check that.

Is there a reason somebody would close it?

thanks,
Click to expand...
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Yes Sir! In the summer it makes a LOT of difference when the hot water is turned off.

AND if you don't have an add-on fan in the truck -- you can use your heater blower to stir the air in the cab without the water being on. Worked great in the days of driving a school bus. Sitting at a stop light, that air circulation is pretty nice stuff!
 
nayrbrellim

nayrbrellim

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Mullaney said:
.
Yes Sir! In the summer it makes a LOT of difference when the hot water is turned off.

AND if you don't have an add-on fan in the truck -- you can use your heater blower to stir the air in the cab without the water being on. Worked great in the days of driving a school bus. Sitting at a stop light, that air circulation is pretty nice stuff!
Click to expand...
Awesome, maybe I'll get lucky and that's all it will be.

thanks
 
Ajax MD

Ajax MD

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nayrbrellim said:
Awesome, maybe I'll get lucky and that's all it will be.

thanks
Click to expand...
Which engine do you have? If you open the coolant valves to the heater box and still don't have hot air, it most likely means that coolant crud is blocking the valves and/or their fittings. If both hoses going to the heater box aren't hot, then you don't have flow. The supply hose will be hot but the return hose will be cold.

I have the NHC-250. While performing a hose replacement and coolant flush, I popped the hoses off of the shutoff valves and found them packed tight with black crud. A few minutes with a sharp scribe to dig it out was all it took to get the coolant flowing again.
 
nayrbrellim

nayrbrellim

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I have the NHC 250

Sounds like a flow prob one way or the other either off or dirty - thanks guys - know what Ill be doing this weekeend.
 
Last edited:
M

Micmada

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nayrbrellim said:
I have the NHC 250

Sounds like a flow prob one way or the other either off or dirty - thanks guys - know what Ill be doing this weekeend.
Click to expand...
Could be a stuck heater core or the core needing the bleeding valve for air opened (if so equiped).

My problem is I always have heat. The fresh air hose going to the blower is also hanging 4 inches above the blower instead of capping as I think it should (?). So I think I get hot engine air in no matter what. I think the previous owner did not want to fix heater core issues so he simply moved away the fresh air intake.

i also have sometimes windows and gauges steaming up when I switch on the fan when it rains. It still eventually defrost/defogs the front, but the back is damp. I put a small Audi auxiliary coolant pump to push coolant through the core, and now it smells a bit sweet. The motor seems to be churning quite a bit of bubbles in the beginning (I suppose I put it a bit higher than the manifold but still below the reservoir level). I need to inspect the core, I guess.
 
Mullaney

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Micmada said:
Could be a stuck heater core or the core needing the bleeding valve for air opened (if so equiped).

My problem is I always have heat. The fresh air hose going to the blower is also hanging 4 inches above the blower instead of capping as I think it should (?). So I think I get hot engine air in no matter what. I think the previous owner did not want to fix heater core issues so he simply moved away the fresh air intake.

i also have sometimes windows and gauges steaming up when I switch on the fan when it rains. It still eventually defrost/defogs the front, but the back is damp. I put a small Audi auxiliary coolant pump to push coolant through the core, and now it smells a bit sweet. The motor seems to be churning quite a bit of bubbles in the beginning (I suppose I put it a bit higher than the manifold but still below the reservoir level). I need to inspect the core, I guess.
Click to expand...
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Sad but somewhat true for a lot of M-9xx Series trucks.
Almost wonder if back in the olden days if a woodstove would have been possible. :-(

Sweet smell in my past experiences have turned out to be a leak of some sort.
Spending time and effort though... Sealing every hole in the firewall might help solve it too.
 
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