Help, setting up blower motor for custom cab, dropping one motor leaves only on speed

[MoCoNative]

I need some help from someone that understands this stuff.

Background, I'm building a cab for my John Deere tractor. I used a complete cab from a larger John Deere. I had to narrow and shorten the cab to make it fit on my machine. In this process I took what was an integrated AC/Heater unit in the cab roof and made it into just a heater. It originally had two blower motors with twin squirrel cages on them in the headliner area of the cab. When I narrowed it, I can only fit one of thes large double units back in the space where I built a cutom enclosure for the heater core.

The unit has a working switch with two speeds. I'm trying to reuse the orginal wiring harness just ease building my own, plus it will look good reusing the original switches and such.

The heater blowers hooked up with four wires, two to each blower. I thought would be as simple as hot and ground for each motor but it is not. If I hook up the one blower to a ground wire and a switched hot from the switch I get one speed, high. If I hook up each motor like it was originally, I get high and low. Here is the catch. The hookup for the second heater is a hot lead from the battery/fused relay to the heater, the second wire goes back to the switch.

What I think it does, with both motors hooked up, the resistance of the two motors makes them run at half speed. At full speed, the second motor gets full power from the hot lead, and the return wire becomes the ground through the switch. Either motor hooked directly to the battery runs at full speed. At half speed the power must route from the power of the one motor through both???? that makes it run slower????

I'm stumped, I tried different combos of hooking the wires up. Of course jumping where the second blower is just gives me high on the one again. Obviously no resistance in a jumper. Tried a small bulb there, still probably not enough reistance. Mixing the wires up and jumping alternate pairs gives me no function or high only.

So how do I trick this. I'm assuming I need a resistor for the second blower motor to mimic it's pull. How do I determine the size and rating. Are we taliking ohms resistance here, watts, votage drop, what do I need.

Thanks,

 

[MoCoNative]

Well I think a resitor will do the trick, measured resitance through the motor and it was 1.1 ohms or so. I'm going try one of two resistors. There is a fairly generic resistor for Fords, for two speed fans, but I have no clue what the resistanc eof it is. The other is a ballast resistor with 1.2 ohms reisistance, both of these I can get at NAPA cheap and easy.

Would a ballast resistor be a good choice here. The resistance will be adequate to mimic the second blower motor I think and give me a low speed. Will it take the load though? I think it will, I just don't know for sure how many watts they are rated for, and if too many watts is a bad thing.

 

[MoCoNative]

Guess I picked the wrong spot to post this question.

I tried the 1.2 ohm ballast resitor from NAPA, seems to work in my test so I guess I will use it. We'll see how long it lasts, hopefully it holds up. Gives me tow blower speeds so that is nice. A little more metal work and sanding an I can paint this cab and get it mounted to my tractor.

 

[Bogo]

No, you just dropped into the no clue zone for most mechanics. It's not like electrics are hard, they just can't wrap their minds around them.

Anyways, using resistors is the way. You may have to experiment with the resistance value, but it should be fairly low. As for the watts rating of the resistor, Amps * voltage drop over the resistor is a minimum. Amps * system voltage(12VDC or 24VDC depending on tractor model) will easily provide enough. Amps is the number of Amps the motor draws. Might be on the outside of the case, but may not be as it is likely an "auto part". In all likelihood it will be under 10 Amps.

While you are in there switch a bunch of the heavy loads like the lighting to using relay control. Also loom the wires. That should mitigate the two main causes of electrical problems John Deeres have. They are notorious for electrical fires in their cabs. They often push the current spec for the switches.

 

[MoCoNative]

This being a custom cab deal, I have stripped some of the wiring and functions it had out. It was a 1970 Hinson cab for a 4020 Tractor. It had AC and all all that good stuff. I have stripped it down to just the one heater blower, wiper motor and lighting.

The tractor I am putting it on is an 870. It only has a 20 or 30 amp alternator on it, just a little thing. So I know I am limited on power. I got some LED flood lights to add to the front of the cab, and I am replacign the rear PAR36 lamps original to the cab with LEDs also. This should make the lighting load minimal, as it only has two headlights in the grill stock, and some tailights.

I hope by eliminating the light load, it will leave enough extra to run the heater blower, a primary requirement. Wipers are secondary, and I'm only putting it back on cause I can. The switches seem to be okay, a litte dirty, the thing was sitting in a field, but I think they are going to work. During my testign they have worked okay.

It was orginally wired with several wires going down the A-pillar to the original tractor loom. I now will only need hot wires to run the wiper and blower, and a dome light. I'm going to tie the lights into the light switch on the dash in the field position.

Right now it has two 15 amp relays in the loom that mount in the roof. I'm just going to reuse them, but they each have a seperate heavy guage wire running down the old loom. I thought I would just tie them together and then onto a source of switched hot. Is this a bad idea?

I'm also adding an ATV winch to the PTO driven snowblower to rotate the chute, as the rear window will not allow me to reach the hand crank now. This I was going to run to the battery. But I would like a way to shut if off when the vehicle is off. I thought about a selenoid.

Would it be wise to run a heavy lead from the battery to a selenoid? Have it come on with the ignition switch and from there go to the winch and the hots for the cab features? This would make everything turn off when the key is off, and effectivley relay it correct? The breakers in the orginal loom are curcuit protection enough I would think.

Am I on the right track, or should I put in fuses. The heaters are weird cause one blower essentially gets power from one relay, and the other got power from the other relay with the wipers pulling power from one also. I figured if it works, I would just go with it, but I don't see a need for two wires running all the way to the switched hot, but now you have me thinking, comments?

 

[Bogo]

I'm not familiar with that cab at all. If it has relays, use em. Also fuses are good. Fuse everything at the supply, ie battery, or fuse panel. That way if the insulation wears through and a wire shorts, it only blows the fuse, and doesn't cause a fire.

A solenoid is a type of relay.

 

[MoCoNative]

Do you think running all this through a selenoid is a good idea? I know it will take one to handle the current for the winch, would simplify the wiring perhaps. I would only have to splice into one switched hot lead, and it would only carry the load of the selenoid, not any of the accesories that way.

There are no places to tap into the factory fuse box, it only has a handfull of curcuits.