Blower only works on high speed
This is a common problem with all makes and models. The vehicle feeds full power to the fan switch and the switch routes that power to a resistor block. The resistor block 
reduces the voltage to reduce the speed of the blower motor. The resistor block accomplishes this task by running the voltage through one of three different high resistance coils. Think of these coils like a filament in a light bulb. The coils actually heat up, thereby reducing the amount of voltage that can flow to the blower motor. When a resistor coil burns out, you lose that fan speed. To repair the problem, simply replace the resistor block. It’s usually located in the ductwork (to keep the coils cool). Unplug the electrical connector, remove the screws and replace the unit. Replacement blocks are usually available from any auto parts store or the dealer. Prices range from $15-$40 dollars, unless you have a variable speed blower motor. Those require a new solid state controller. You guessed it, they cost a lot of money.
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© 2012 Rick Muscoplat
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The blower resistor assembly on the Ford Bronco incorporates a diode through which current flows to the resistor trio. This diode is usually the culprit in failure leading to high speed function only (the resistance of the lower speed coils stops and/or reverses current flow through the failed diode).Simply replace the diode if you have the necessary skills.
Great explaination about the diode, Ananymous. So, how do I know which direction to install the diode? And / or what is the risk of just jumpering the defective diode with a solid wire? Is the diode a safety element, or a necessity-for-functionality component? In my case it is in a 1985 Ford Ranger.
Replaced this resistor in my 2002 caravan. The resistor is behind glove box and was a piece of cake to replace. Great site and great advice….thank you!!!
The diode is there for circuit protection and you MUST install a new one in the same orientation as the old one. If the old one is “smoked” and you can’t tell which end of the diode has the “bar” then you will need to find the wiring diagram for your vehicle. You need to get the same diode that was originally there or at least an acceptably rated replacement. They are not normally hard to change but you must pay attention.
I have a 2001 mercury mountaineer monteray. The a/c only works on high. When you say the variable speed blower motor are you talking about a a/c that you can set the temp. because in my car I have that it’s like a thermostat. The mercury dealer here said that a resistor would not work for my car I needed a new circuit board. Any help is appreciated.
Nope, not talking about the variable temp control. This refers to the variable blower speed control. If you can set your blower speed to any setting, as opposed to only 3 speeds, then you have the variable blower control and you need a new circuit board.
hello…
i love your blog first of all!!
I am having an issue with a 2001 ford expedition.
The air/heat does not blow hard? i can feel it so i know its working but when its on high it not working well… i have been reading your blog and came across this and it make sense but i have also read stuff about it possibly being a fuse? can you help?
you can contact me @
ryans4020@yahoo.com
My problem is on a 2004 dodge quadcab 1500. I have a blower speed on high but not on 1,2,3? Any advice?
I found this forum while investigating problem with blower only working on high speed (2002 Voyager). I found the resistor block behind the glove box, all the resistors check out OK. But the “diode” mentioned above is open (won’t pass current in either direction). I put it in quotes because someone on another forum said it was a temperature fuse. I don’t know why you would need a diode in a DC circuit (or is it a diode used as a temperature fuse, set to go bad at a certain temperature, which is 128 deg C according to the guy at the other forum?). Anyway, the temperature fuse concept makes sense to me since prior to it going bad it had bad air flow due to clogged filter, meaning less cooling to the resistor pack.
Car makers usually place a diode in circuits with motors to prevent backfeed surges from the motor windings on shutdown. Remember, a motor is like an electro-magnet. When it shuts down and the magnetic field collapses, you get a surge of current that can damage control circuits. As far as being a thermal diode, it’s rated for a certain current. A clogged cabin filter reduces air flow and the motor draws more current. That would blow the diode.
Fan motor on Chrysler – Voyager 2001 and 1999 Sebringonly works in high speed, low speeds don’t work. I had the same problem, and I fixed it for under 2 dollars. I took the resistor block out of the car, (it’s located behind the glove box). You’ll have to squeeze the sides of the glove box to drop it down so you can get to the resistor block that is directly behind the glove box. 2 screws holds the resistor box into the plastic blower box. Disconnect the two wiring blocks (with clips), that are connected to the resistor block. Once you have the block in your hand, you’ll notice that the resistor block has a small thermal fuse in the center – it looks like a small 1/2 inch silver mini ‘crayon’. Notice which end is the pointed end, because you have to put the new part in place with the exact same direction. Radio Shack sells part number 270-1322 – about 2 dollars. Thermal fuse Rating is 10 AMPS, temp rating for 128C / 264 degrees farenheit. I took the old part out, and soldered the new one in place (paying attention to which direction the ‘point’ of the fuse was facing), and then replaced the resistor block in reverse order of removal.
Worked great.
Art – many thanks for the tip. The nearest Radio Shack is about 7000 miles from me (South Africa) but thankfully a colleague was coming out here and he got the part for me ($1.69). Rick, at least on the Chrysler Voyager this is definitely not a diode (essentialy zero resistance in both directions); it could be confused with a diode because it looks like its directionally sensitive. In a sense it is; the instructions on the back of the part say that the temperature sensor is located opposite to the “nose cone” end.
John,
Thanks for the update/clarification on the temperature sensor. I usually just replace the entire block and have never replaced just the component. I’m sure all the other blog members will appreciate your clarification.
I have a 1995 Taurus SHO with exactly the same symptoms: electronic temp control, high fan only, shuts off if I turn off air conditioner. But I'm a novice at electronics and they want over $180 for a new variable blower control module.
I have pulled the module to where it is accessible. There is nothing obviously broken or loose.
Can anybody tell me EXACTLY what part of the unit/circuit board to look at (like, where are the resistors or diodes) on the circuit board? Where do I start?
Thanks!
Frank,
You're in WAY over your head thinking you can fix a variable blower control module. Even the auto electronics rebuilders won't touch these things.
Looks like I have the blower motor resistor block problem in my 2001 ford escape. Blower motor only runs on high. Where is the resistor block located?
It’s behind the dash on the passenger side
I’ve got a ghost in the blower motor of my 2005 Chevy Cobalt. The blower motor only runs on high, but runs on high whether the fan switch in on or off. I replaced the blower motor resistor and the blower motor relay to no avail. When I first start the car in the morning, the fan is not blowing, but starts cutting in and out very quickly. I noticed this morning that when I touched the brake, the fan came on, and when I stopped touching it, it went back off. A few moments later it came on and stayed on.
I suspect that the brake pedal is causing a short that is sending full power directly to the blower, bypassing the resistor, but have no idea where to start searching for the fix to this problem. Any advice would be appreciated.
You can email me directly at wyrickken@hotmail.com.
Get a wiring diagram. I’m pretty sure the blower motor has power to it at all times. The circuit completes on ground side. Check the connectors and harness to see if you’re getting ground with the switch off. Then trace backwards until you find out where it’s shorting to ground.
Have problem with my ac 300rh lexus 2000. I replaced the compressure, put verione,and ac resistor still the ac still blowing hot air
I have a 2006 Dodge Caravan SXT.
My blower motor has quit.
After spending time on your site (Rick, and thanks) I have the blower motor resistor out. I have tested the blower motor with 12v and it works fine. I have power coming in on one of the dark blue wires into the resistor block.
Now, I don’t know how to test the resistor block out.
I have continuity on all of the little coils if that means anything. I’m sure they are OK. I guess it would be the center diode that I would have issues with. Can you actually test that?
Or should I just replace the whole resistor.
I am handy with soldering, so I am interested in doing the repair.
I just need to isolate the problem part.
Any help you can send my way, I’d appreciate it.
Thanks, Bob
You test a diode with an ohmeter. You should get continuity in only one direction. Diode = 1-way check valve. I would just buy a new resistor block myself. But if you want to repair it, knock yourself out. Let us know how you did it and how much it cost.
Thanks for the response, Rick.
I appreciate the fact that you read these posts and answer them.
Excellent!
I understand that the diode is one way and after testing it, there is NO continuity either way.
So, I am hoping that is the issue and will get a diode at Radio Shack today and give it a quick try.
I guess part of my question had to do with the three coils (resistors?).
I have continuity on them. Since they seem like coiled up wire, I wasn’t sure if there was some other way to test those.
I’m guessing since there is continuity, they are in good shape.
I’ll update how my diode replacement goes …
Thanks …
Bob
Success!
I got the diode from Radio Shack.
I tested it out with alligator clips and the blower worked just fine!
So, I soldered it in place and we are good to go.
Soldering was a challenge with the tight space in the resistor and not wanting to get the diode too hot, but I got it done. I used a small pointed vice grips to hold the two leads together and also work as a heat sink.
Anyway, thanks again for the great site and the help.
On to the next issue, the little tiny green lights in the buttons to operate the windows, door locks, etc. Some of them are not working. Is there a post or place on your site that discusses this?
Thanks again …
Bob
The bulbs are most likely LED’s. So if you want to do surgery, you can probably just solder those in too. Please post the Radio Shack part 3 for the diode. I’m sure other readers will appreciate the homework you’ve done.
I have a 2008 Ford Escape and am looking to replace the Blower Motor Resistor. Is it in the same spot as the 2011 model above?
have a 2005 cobalt, fan working on 4 only, replaced the resistor and it started smoking. Help!
Bad resistor, high resistance in the connector or wiring. Check for melted plastic on connector. Check for ground from HVAC control head
I HAVE A 2009 FORD RANGER AND MY AIR BLOWER ONLY RUNS ON HIGH, I ALREADY REPLACED THE BLOWER MOTOR RESISTER BUT IT STILL DOES THE SAME THING, WHAT ELSE CAN I TRY ?
The blower motor in this vehicle gets power from the blower motor relay. But it gets ground from two places–from a ground point at right rear of engine compartment, and through the blower speed switch.
If the fan switch is OFF, but the mode switch is in any position EXCEPT off, power flows to the blower motor, through all three resistors, and then to ground at right rear of engine compartment. That’s what gives you ventilation air flow. As soon as you turn the fan speed switch to LOW, power flows through blower motor and two resistors, then to ground through the fan speed switch and terminates at a ground point on the left side of the engine compartment. MEDIUM power is same except it runs through one resistor. On high speed, ground goes directly through the speed switch to ground.
So somewhere the blower fan is getting full ground and my guess is that you’ve got a short between the violet/orange and the black/grey wire that’s providing ground to the ventilation speed.
OR, you’ve got a short to ground in splice 129 located under the dash near the blower motor. Follow the wiring harness from the blower motor to a larger harness and open it to see if the violet wire is rubbed through and touching another wire or touching metal ground.
Hi Rick, my turn
2000 Jimmy 4.3 litre…no heat no blower at any speed. It has automatic climate control.
Relays and fuses ok.
Took the 2 connections from the blower control module off blower motor. Test light in power, other end to negative on batt. Test light lit.
Test light on ground connector from module, other on positive at battery, no light.
Connected power wire from blower module back to blower motor and ran a jumper from blower motor ground connection to neg battery…fan came on even with no key in the ignition.
My research leads me to beleive because I have Automatic climate control I don’t have a resistor, only a blower control module.
Based on my diagnostics, I beleive it is the blower control module.
Thoughts?
Thanks in advance!!
This is a really complicated system. The HVAC head sends pulsed voltage to the controller to tell it what speed to run the motor. The controller varies the speed by pulsing voltage on and off to the motor in inverse proportion to the signals it receives from the control head. The motor grounds THROUGH the controller, not through the brown wire in the motor connector. If I understand you, you jumped power from the purple wire in the motor connector to the motor and provided battery ground and the motor ran with the key off. Right? Did it run on high? Can you change the speed of the motor manually?
The only real way to test this system is with a scope. You’d see if the controller is receiving pulsed signals from the HVAC head on the purple/white wire with the ignition off. If it’s not, but the controller is still providing power on the purple wire to the motor, then the controller is definitely bad. But if it IS receiving pulsed signals, then the HVAC head is bad. The HVAC head has two fuses, one of which is hot at all times.
My best guess is that you’ve got a bad controller. But before you throw a new one in, check the current draw on the motor when you hot wire it. If the motor draws more than 18 amps, it’ll burn out the new controller too. In that case, you’ll have to replace both the motor and the controller.
you jumped power from the purple wire in the motor connector to the motor and provided battery ground and the motor ran with the key off. Right? YES Did it run on high?YES Can you change the speed of the motor manually? NO
Unforunately I don’t have a scope…checked for continuity on the module…seemed fine although without a schematic I’m grasping any straws.
I have in the last day removed the unit to get part number, etc. REALLY hard to find…everything I research just talks about the resistors. $215 at dealer, $100 on Amazon???
So I put the unit back in and provide battery ground. Fan runs again with key out engine off. I start the car and let it run…to see if anything happens..perhaps I can run a switch to the cab and my wife can use that…high only, but heck, its heat! 5 minutes in, as I kind of suspected, the fuse in the engine compartment fuse block blew. Ok, can’t by-pass. I reconnect everything from module to blower motor, fuses/relays good and the thing starts as its supposed too!!!!!!!!!!!!!!!!! Speeds work, different levels, only when the key is like its supposed to etc. Un believable!!!!
…so my wife calls from work this evening…blower works fine…except it only blows cold. Seriously.
Ok, now we’re talking gremlins. We now know that the blower motor controller isn’t fried. That leaves two possibilities:
1) Bad power or ground connections at the HVAC head controller
2) Bad ignition switch
This vintage GM vehicle is known for having intermittent ignition switch electrical problems. If the HVAC controller isn’t getting full battery voltage, it will misbehave.
So start by checking for full 12.75 battery voltage on the brown wire at the HVAC controller connector. Then check for good ground on the black wire. Ground goes from the control head to a splice in the dash behind the ash tray. From there the ground terminates on the right side of the dash. Clean that ground connection point. Then recalibrate the HVAC control head by disconnecting the negative battery terminal and touching it to the positive cable. That’ll drain capacitors in the HVAC controller (and the PCM). Then insert the ign key and turn to ON, but DON’T start the engine. Keep it in that position for at least 40 seconds. Then start the engine. Move the temp control from cold to hot and see if it works now. Also, check the hoses to the heater core. They should both be hot (one perhaps hotter than the other). If one is cold and the other hot, you’ve got a plugged heater core.
…do I leave the positive cable connected to the battery when touching with the disconnected neg wire to drain and reset? Sparks? Is this safe?? Thanks Rick
Well, when you disconnect the negative from the battery, you’re taking the battery out of the picture. But if you’re more comfortable, remove both terminals and touch the cable ends together.
It was my belief that since that fan was once again going, I was going to jump ahead to touching the positive/negative terminals (no spark…hahaha)…low and behold, heat!!!!!; everything 100% perfect.
At this point, my years of putzing around on my cars has taught me not to ask any questions…don’t wake a sleeping bear. It works now and my wife is toasty on her way to work. Thanks so much Rick!!
My best guess at to what happened is that your control unit experience “logic lock” which is exactly what it sounds like. The computer locked up and gave you a “blue screen of death” without the blue screen. The HVAC controller somehow managed to start communicating again with the blower motor controller, but it wouldn’t command the blend door to the HEAT position. If this happens again, do the battery terminal routine before you do any other tests.
Congrats on fixing it. Glad to hear that I could help.