939 heaters

[clydizzel]

For all you guys pushing snow or anything similar how are you keeping your windows clear? My heater/defroster only barely keeps the 2 fronts kinda clean while the sides and back freeze up. Im getting a rattling noise from my blower also. Question is should my heater work better or have any of u guys added a different type of heat? Im considering removing mine and installing one of these http://dcthermal.com/catalog/sd24-3500. Does anyone have an input on this or have a better idea? Thanks in advance

 

[Alexsha]

35 amps is a bit much to put on your alternator. Might want to upgrade that.

 

[74M35A2]

I'll be watching this one closely. The heater in my M925A2 also completely licks. It performs exactly same as you just mentioned. I changed my thermostat as my engine temp would drop on the freeway in the winter. Now it holds a solid 180F, even at 65mph and 10F ambient temp, which helped keep the heater warm somewhat, but it just does not have any volume to it when blowing. Mine also makes a metallic "clunk" noise when the fan speed is changed.

WildChild467 has an M35A2, and he kind of made his own heater setup, by moving the heater core and blower assembly inside his cab completely. I have been in his truck when the heater is on low, and you have to roll the window down, reduce the temp, or shut the heat off. Something about his is using an A3 cab with A2 accessories or whatever, but it works amazing and I am envious. So by observing this, I am going to blame our cold air inlet unless somebody says otherwise. Almost needs a poll. Mine sucks so badly that I bought a 60k BTU multifuel heater to mount on the toolbox and blow into the back window with an 8" duct.

I have also seen what appears to be factory option of a 30k BTU fuel fired heater on our trucks, which is mounted at the heater core location under the hood, blowing inward.

The 24V heater you linked would work good for no warm up required, but the CFM seems low, which is almost the same problem we are having already.

But, I would like to hear if anybody in a snow laden area does have a 900 series where the heater works well enough to turn it down for comfort. I so far don't think it exists, but the answer could tell if it is a design issue, or maybe something to do with the clunking in our heater boxes (like a diverter valve flopping over, blocking off flow).

 

[Alexsha]

I wonder if the cold air inlet could be re-routed to in the cab. Then you'd be heating air that's not so freezing.

Also: many M939 trucks with the original heater cores need a good flush on the core. I need to do mine.

 

[74M35A2]

Because the fan and heater core are outside the cab, it would probably be easier to move the entire thing inside like I mentioned. But then you are on permanent recirc which is not going to do well for defog (defrost maybe OK).

When comparing to a gasoline car, whose coolant is 198F, that is only 20F warmer, and any car heater can cook your pants off, but the CFM out of a car blower seems so much stronger than the 900 series, or at least my truck anyway.

I tested the liquid flow of my heater core, and it was not plugged, flowed well in both directions. Like the original poster, I don't want to put a ton of time into diag, if the design just sucks. Would rather remove and install something that actually works well and has some performance margin in it where you have to turn it down to stay comfortable.

Anybody have a 900 series in the snow belt where the heater works (meaning need to take your coat off inside)?

 

[Wildchild467]

When comparing to a gasoline car, whose coolant is 198F, that is only 20F warmer, and any car heater can cook your pants off, but the CFM out of a car blower seems so much stronger than the 900 series, or at least my truck anyway.

I tested the liquid flow of my heater core, and it was not plugged, flowed well in both directions. Like the original poster, I don't want to put a ton of time into diag, if the design just sucks. Would rather remove and install something that actually works well and has some performance margin in it where you have to turn it down to stay comfortable.

We should check the voltage at your blower motor. When you turn on your blower motor, doesn't the voltage gauge drop some? I would like to troubleshoot a little more with yours.

One thing that helps out my heater a lot is my cold weather front cover. My heater does not work well without it. If I put the cover on, it is night and day. I also do not have the rubber floor mats in my truck that the 900 Series have. I don't know if that contributes at all.

Does the blower motor in your truck now spin as fast as the one you took out? Did you use the same fan blades from the stock fan?

 

[74M35A2]

We should check the voltage at your blower motor. When you turn on your blower motor, doesn't the voltage gauge drop some? I would like to troubleshoot a little more with yours.

One thing that helps out my heater a lot is my cold weather front cover. My heater does not work well without it. If I put the cover on, it is night and day. I also do not have the rubber floor mats in my truck that the 900 Series have. I don't know if that contributes at all.

Does the blower motor in your truck now spin as fast as the one you took out? Did you use the same fan blades from the stock fan?

Squirrel cage carried over. The NAPA blower motor I installed spins much faster than the one it replaced. The original was seized.

 

[rivercreek]

74M35A2, you mentioned you replaced your original motor with a NAPA blower motor - you wouldn't happen to have that NAPA part # would you by chance?

 

[Alexsha]

NAPA: BK 6551320

The only difference is the new blower has a ground wire. The original grounds to the body. You'll just need to wire that ground to the frame and use the connector off the original motor.

 

[gimpyrobb]

Have any of you looked into getting a heater core with more than 2 rows? If memory serves me right, the stock heaters have 2 row cores in them, but there is room in the housing for more. I think it was the moderator Stretch that put a second core in the box and said it helped a lot.

Just going from memory, I've been known to be wrong before.

 

[doghead]

I would simply add ducting to the intake of the fan and draw the inside cab air through the fan(recirculate).

 

[Plugugly]

I haven't used my truck when it's super cold yet, I haven't had the need. But I do know the heater with the soft top was going to be an issue, it just wasn't confidence inspiring when it was in the 30's. I have a Big Buddy heater for ice fishing that I have been planning on securing in the cab, just in case. It has a battery driven fan and should put out plenty of heat for the space.

 

[Swamp Donkey]

One thing that helped my heater tremendously had nothing to do with the heater. Sealing off the back of the soft top helped keep the heat in. This was discussed in another thread recently but I had already done this after the first couple weeks of winter. It's not so much that the back let's cold air in, as it let's hot air out.

While we have milder temperatures in Georgia we dipped down in the mid teens last month...and will again tomorrow. While I still had to wear a jacket that morning, I could comfortably run the heater on low. I think the problem with the side and rear windows staying fogged has more to do with the lack of a front facing vent to blow on them more the anything. Even luke warm air can clear windows if it's blown directly on them.

I think the stock heater system is poor by design. A small fan, with a small core, with no recirculate function and no front facing vents on a cool running diesel don't make for impressive warmth.

 

[Baradium]

I have also seen what appears to be factory option of a 30k BTU fuel fired heater on our trucks, which is mounted at the heater core location under the hood, blowing inward.

That's the arctic heater, and as you said it mounts in place of the coolant based heater. All the 939s I've personally seen in Alaska have one. The only exception is *my* 923 and that is because I bought it from WA and barged it up. Maybe one of the Alaska members other than myself bought an original AK truck and will chime in on it. The only thing I've heard about it is that it's rare to not want to crack a window when it's running to keep the cab cool enough. That said, I have a NOS arctic heater kit coming for mine.

I've been thinking about setting up the coolant based heater in the cab somewhere so I can run a lower output heater when the temps are just cool, instead of using the oil fired one with the windows down.


Also, my original heater doesn't clunk that I've noticed. I'll have to play with it when the temperatures get above zero again and see what I come up with. As far as it only being a 20 degree difference in coolant temperature, when it's really cold out, that 20 degrees can be a big difference in how well your heater seems to work!