After replacing my heater core and sealing up the housing around it, I remembered how inadequate the heaters are in our trucks. So I came up with two solutions from spare parts laying around. During this operation the heater box was out of the truck to replace the core and it made it much easier to do the modifications.
Solution #1: I found a blower motor out of a 84-86 Jeep Grand Wagoneer (w a/c.) Compared to the stock motor in my truck the casing was longer. I imagined that this would allow for bigger magnets and armature inside which would equal more power. The motor mounts exactly the same as the ford part with two long bolts running thru the motor.
Solution #2: There was a heater fan assembly laying around from my '75 f-100 parts truck. I compared the blower motor wheels. The 70' was metal and small but the '75 on the other hand was larger, made of plastic, and had about 2-3x more fins on the wheel. The old metal wheel would almost fit inside the plastic one.
The choice was obvious, use the bigger one. Although the bigger blower wheel won't fit physically in our stock heater box, a little trimming on the box with a dremel tool will allow it to fit thru the hole perfectly. But be sure to remove the little metal clips that the fan assembly screws into before cutting with a dremel.
Next would be to mount the jeep motor onto the ford metal mounting plate which then attaches blower motor assembly to the heater box. I used the 75' ford mounting plate with a little cut off to match the contour of the heater box. I suppose you could use the factory one off your truck that would work also. Mount the motor on the plate with the two nuts, then attach your plastic blower wheel to the motor shaft. Insert the assembly into the heater box. Now you have to position the assembly in the heater box to assure maximum air flow.
To do this attach low power to the motor (don't just hook it to the battery- I used a 2/10/50 amp battery charger for this) make sure you have a good hold of the motor there is allot of torque there and don't get hurt. Move the fan assembly around till you get the highest airflow possible thru the heater core. When you achieve this mark the location of the motor assembly relative to the heater box for future reference. It's now time to drill new mounting holes for the assembly. Make sure that the holes are far enough outward that once the screws are installed they wont hit the blower wheel inside.
Once the holes are drilled remove the blower fan and drill the appropriate size holes to mount the little metal clips that the screws go into. I ended up using three of the original screws and clips to eventually mount the assembly.
OK so now we have our mounting holes ready and the fan wheel oriented top to bottom / left to right. There is just one more adjustment to make. If you notice by comparing the old blower wheel and the new one, the plastic one isn't as tall as the old one. So we need to adjust how far the blower wheel sits on the motor shaft. Do this again by inserting the blower assembly into the heater housing / lining up the mounting holes then connecting low power to the motor.
It will be a trial and error operation, moving the wheel on the shaft till the point you can hear it rubbing on the heater box metal inside until you move it back on the shaft a tiny bit at a time till there is no more interference. Once this is done, mount the blower assembly to the heater box and you are done with that part of the upgrade.
Now comes the time to keep your wiring from burning up your truck. We have upgraded the motor size, and installed a plastic lighter wheel assembly. This baby is gonna spin like never before, and is going to use more power. You will need to install a relay for the highest fan speed. To do this you will need a 20-30amp automotive realy. Connect as follows:
Relay Switch- One fused heavy guage wire directly to the battery positive side. The other switched connection to the blower motor positive wire.
Relay Coil- You will need to cut (cut it long) the orange wire from the back of the factory heater fan speed switch. The side that goes towards the switch will be connected to the relay coil, the other side of that cut wire will need to be insulated with electrical tape or a wire nut. The other relay coil connection will go to ground.
That's it. Try it out. Your heater will blow like never before.... I found the new medium speed was like the old high speed as far as air flow. The new high speed is like a leaf blower. Have fun and hopefully next winter will be just a bit more bearable.
On a side note, the heater core for a 75' non a/c is the exact same as a '70 application, why they don't list it that way is beyond me.
