Better ways to control a blower motor?

**JeepsAndGuns** · 10-26-2014

Ok, electrical gurus, I'm looking for a better way to control a blower motor.
So I bought my GF a 94 jeep cherokee to drive back and forth to work. I noticed the blower motor switch was backwards, high was low, and low was high. So one day while I had the dash apart to swap guage clusters, I pulled the hvac control and found the switch was simply installed upside down. I found the electrical connector looked like it had gotten hot at one time, as it was a little melted, but still looked ok enough to use. I'm thinking someone replaced the switch in the past. It also had a slight electrical gremlen, where every now and then, when you start the jeep, the hvac, radio and a few other things wouldnt work. Turn it off then back on and it would work. I traced the problem down to the ignition switch. I got a new one and found one of the wires on it had gotten hot and kinda burnt one of the contacts inside the switch. I looked at the wiring diagram and found its the wire that powers the blower motor/hvac and a few other things. Google found this was a common issue with these models and many have died a early death from fires. The wiring is just not up to snuff for the blower motor. From what I gather, as the blower motor ages, it pulls more power then everything melts. I have read a few reports that even after replacing the blower motor and fixing the burnt connectors, they still get hot. The factory wiring is just not good enough. There are a couple fixes, some using like 3-4 relays, but I was thinking surely there is a better way.
So I then remembered my 99 grand cherokee. Its a limited model and has the automatic zone control hvac. The blower motor on it is controlled by a module. Two large wires going in it from the battery, then two large out to the blower motor. It then has one smaller wire that controls it from the hvac contoler. The original blower motor module had some issues, I had to replace mine. But the new updated one seems to be working great. So I was thinking something like that would be great, as its infinate/variable speed. Just need a way to control it. I was hoping it was something simple like a 0-5v or 0-12v control from a potentiometer (aka, the speed control knob on the dash), but according to the service manual, its pwm. Could I even be able to control something like that without the factory control module?
But, it doesnt have to be that module. If anyone knows of a vehicle that uses something simillar but is easyer to control I could get from the junkyard, that would be fine too.

My perfect end goal, would be to use the original wiring to trigger a relay to send power on some larger guage wires straight from the battery to the controller/module, then from the controller/module to the blower motor. I would like to get rid of the factory wiring and resistor pack.
So, whats yalls opinions/ideas?

**dave w** · 10-26-2014

My thought is use a Pulse-width Modulation (PWM) http://en.wikipedia.org/wiki/Pulse-width_modulation to control the motor.

https://www.youtube.com/watch?v=WxGWFhhxnuE

dave w

**Six_Shooter** · 10-26-2014

You would really need to adapt the entire system from the donor Jeep to make it work, easily.

While I won't say it's impossible to make what you want to work, it's just not that easy, and in reality not really worth the effort IMO.

Your best bet is to use a relay for the high setting, and allow the resistor pack to do the rest. GM did it this way for many years.

In reality you're still not changing the total current draw, the motor still pulls the same amount of current regardless of the control circuitry, so you would still need large current carrying capacity devices and wired directly to the battery anyway.

In most cases the only relay needed is on the high setting, since this is where the highest current draw is. If you bypass the resistors you will actually lose your speed control and every speed would be high. ;)

**steveo** · 10-26-2014

first, bust out the multimeter and see how much juice this blower motor actually draws. then you'll know what you need to do. if we're talking 10+ amps, i'd give up linear control for robustness, and go two relays, one resistor, and one three position switch. off, medium, and high.

**JeepsAndGuns** · 10-27-2014

Originally Posted by dave w

My thought is use a Pulse-width Modulation (PWM) http://en.wikipedia.org/wiki/Pulse-width_modulation to control the motor.

https://www.youtube.com/watch?v=WxGWFhhxnuE

dave w

I started reading up on PWM after I posted this. I am starting to think this might be the way to go. After watching that video and looking up the pwm controler they showed, that seems like a nifty little thing. If it could be up to the task of it, use it to controll one of those hella pwm relays. I will need to figure out amp draw on the blower motor to see if one will be enough or if I would need two. Only thing I would wonder, is if that pwm controller could drive the two pwm relays? It says max output of 6.5 amps. Surely two pwm relays would take that much power to run them.
http://www.jameco.com/webapp/wcs/sto...gDrillDownView
http://www.amazon.com/HELLA-H4177300.../dp/B000VU5FPE

Then all I would need to do is find a potentiometer with a knob and mount it in place of the original switch and I now have variable control. In theory though. But "on paper" its sounding like a really nice idea/plan. Seems like it might be easyer/cheaper than the blower motor control module.

**dave w** · 10-27-2014

The MOSFET (http://en.wikipedia.org/wiki/MOSFET) used in the Jameco PWM kit is the current limiting component. Using a MOSFET with a higher current rating might be an option? Using a high current MOSFET will eliminate the need for a solid state relay. With some homework, a Do It Yourself PWM circuit could be designed to use a higher current MOSFET without using the Jameco kit or using a solid state relay. http://www.555-timer-circuits.com/pwm-controller.html

Here is a 30 amp controller, no homework needed.
http://www.electronickits.com/kit/co...or/CKMX066.htm

dave w

**lionelhutz** · 10-27-2014

Dave's got you covered. Just use a circuit already built like the one you linked or the one he linked depending on the current of the fan motor.

Don't bother feeding a relay like the Hella with a PWM kit. It would work but why bother when there are PWM motor controllers already available with the correct current rating.

Just putting a bigger MOSFET into a kit might cause a problem because the gate driving circuit might not be capable of switching the bigger device quickly which leads to more heat in the device. There could also be ringing in the gate drive due to changes in the circuit capacitance causing extra heating of the MOSFET.

**JeepsAndGuns** · 10-28-2014

Originally Posted by dave w

Here is a 30 amp controller, no homework needed.
http://www.electronickits.com/kit/co...or/CKMX066.htm

Now I like that. I cant imigine that the small blower motor could pull more than 30 amps. But I will test it first. (forgot to bring home my multimeter with amp clamp)
I wonder if the potentiometer could be removed and mounted sperately from the main board? Not knowing exactly how big it is, I'm not sure about clearance where I would want to mount it. Now if I could mount just the potentiometer where the other controlls are, then I could mount the board in another spot with more room. Then just run some wires to connect them. Now knowing anything about the controler, could this be done, or does the potentiometer have the be mounted directly to the circuit board?

**stegey** · 10-28-2014

can i make an innocent observation ? why not just keep it the way it is ? these models do have blower motor problems , but it sounds to me like you just need to check the integrity of the wires and connections and make the resistor for the blower motor are good and the blower motor itself , they make updated resistors and a wiring repair kit that goes to it from JEEP , just sayn i can't count the number i have soldered in new wires for the resistor , replaced th resistor and never had another complaint from the customer and i don't work at a dealership .

take it for what you want or have fun tryeing to re design it , i'm thinking toggle switches and the accommpinieng resistors.

**dave w** · 10-28-2014

Originally Posted by JeepsAndGuns

Now I like that. I cant imigine that the small blower motor could pull more than 30 amps. But I will test it first. (forgot to bring home my multimeter with amp clamp)
I wonder if the potentiometer could be removed and mounted sperately from the main board? Not knowing exactly how big it is, I'm not sure about clearance where I would want to mount it. Now if I could mount just the potentiometer where the other controlls are, then I could mount the board in another spot with more room. Then just run some wires to connect them. Now knowing anything about the controler, could this be done, or does the potentiometer have the be mounted directly to the circuit board?

With some good soldering skills and wire, the potentiometer could realistically be a few feet from the board. If your serious about using PWM, consider the option of a Slide Potentiometer.

dave w

**1project2many** · 10-28-2014

The pre- made motor controller circuit is the best answer if you're determined to go this route.

Chrysler products use a "chemical fuse" built into the blower resistor to limit current if the blower motor draw is excessive. When the fuse opens only high speed operation is available. Bypassing or replacing the fuse restores all speeds but subjects rest of system to higher current draw. Corrosion and misshapen terminals create localized hot spots and eventually another component can fail. We usually get 3-400k miles out of our Caravans and during this time we'll replace the blower "chemical" fuse once (25A ATC type gets installed), replace 2-3 blower motors, and generally one connector at the blower resistor or blower motor. If you choose to use relays you'll want one for each speed. It's not that difficult to mount these and run a small harness but there is some added complexity. You could keep the OE blower switch intact by replacing the potentiometer of the PWM controller with specific, fixed resistances for each speed selected at the dash.