Diagnosing Blower Motor Problems

[Jeremy airjer W]

I wrote this up back in January and finally got around to moving it to the auto forum. This should help if you ever run into this problem.

The diagram is generic and loosely based on GM's setup and more specifically a Silverado. Most manufacturers have a similar setup and the only major differences would be a ground side control (instead of the switch supplying power to the resistor it would be grounding the power from the resistor if this is the case you test light would need to be connected to power to test the switch) and late model Chrysler minivans (they use a module that looks for a difference in a/c voltage from the switch)

If the blower works on high only than likely the resistor assembly is the culprit since power to the blower essentially bypasses all of the resistors and/or the resistor assembly entirely. The reason why none off the other speeds will work is that the resistor or the circuit for the speed four setting has burned out. The lower the setting the more resistance you need to drop the voltage to the blower. The first speed setting requires all four resistors to drop the voltage enough to turn the blower at its slowest speed. Speed 2 needs to turn the blower a little faster so it uses three resistors. Speed 3 uses two resistors and speed 4 uses one.

In some cases the fan will work on high and speed setting 4. The likely cause for this is the speed 3 resistor or circuit has burned out. Speed 4 still works because it doesn't use the lower speeds resistors to control the speed of the blower. If the speed 2 resistor burns out than speed 1 and 2 will not work but 3, 4 and high will continue to work. If the speed 1 resistor burns out than speeds 2, 3, 4, and high will continue to work.

This leads us to how do we know if its the resistor or the switch or the blower. If the blower works on high than we know the blower works. That leaves us with either the switch or the resistor assembly (there are many other possibilities) wich can be easily tested with a test light (or a voltmeter if you choose) The first thing to do is locate the resistor assemble. They can be behind the glove box, under the dash close to the blower motor or under the hood, again usually close to the blower motor. It will typically have 5 to six wires going to it. We'll start by unplugging the connector, turning the key on and connecting your test light to ground.

1.Turn the switch to high and backprope all the wires. Two of them should illuminate the test light. In the diagram below this would be wire E & F. If you don't have a digram then hold the test light on one of the two wires and move the switch to another speed setting. the wire that continuously illuminates the test light regardless of the switch position will be the constant power for high blower speed and will not need to be rechecked in the following steps.

2. Turn the switch to the next lowest setting. Backprobe the remaining wires to see if one of them illuminates the test light. Repeat until you have checked all of the speed settings.

3. If the test light has illuminated a different wire for each setting then we can reasonably assume that the switch is functioning as intended and the source of the problem is the blower resistor.

4. If you find one or more settings (but not all the settings) on the switch that does not illuminate the test light on any wire than we can reasonably assume that the switch has failed.

There are a few other things that can cause blower motors not to work as intended. The switch or the resistor assembly are the two most common problems.

[CrappiesNeyes]

I installed a new resistor (98 plymouth breeze) and it cured the problem for approx 10 minutes and now the fan only works on high again. I assume that the blower motor is pulling too much load and this caused the resistor to burn out again? Or is it possible I got a "bad" new resistor? Thanks in advance.

CnE

[fishing tech]

The resistors go bad for a reason, they overheat due to to much of a draw from the blower motor. Replace the blower motor and another resistor.

[Jeremy airjer W]

The breeze is a ground side control systym. This means that the power goes through the blower first. The resistors and the switch are on the ground side. The only current that the resistor sees is what is necessary to get through the resistor. Its hard to explain in words.

This is a tuff call. If the part was O.E. I would be a little concerned that there may be another problem. If it where aftermarket I would lean towards it possibly being defective. If it where a customers car I wouldn't have any problem trying another.

[Shack]

Go figure, I read this post yesterday and after work, my blower motor is not blowing on any speed. It has to happen on one of the coldest days.

I have a Linc. Town car as my shooter and it has auto temp control. I figured it was a blower motor or the ATC head. Well a little Cub parking lot diag. and I got it fixed for now. I searched and found the elec. motor housing cover for the blower motor and banged on it once with a quick square I had in my trunk. Started blowing right away! At least I know it’s a blower motor and not ATC head. Plus the blower motor is located under the hood by the fire wall and is very easy to get at. Some are behind the glove box area.

Had to share this, because I have not had a blower motor go out in one of my Veh. in many years, but after reading this thread and with in minutes, mine was failed.

Also figure I would share this , because it could get you some heat for the ride home from work and help a person figure out what is wrong. Just give the blower motor a rap and it will buy you some time. What happens is the brush's stick because or wear or somthing. The bang free's them up for a short period of time, before they stick again, that or something sticking inside of the blower motor. The blower motor will have to be replaced as soon as you can.

Good luck!!!

[Jeremy airjer W]

LOL, Glad I could help!!!!

Good point about tapping on the blower if it does not work at all. There have been many blower motors diagnosed with a good tap by this guy!!

Another thing I was going to mention is that If any of you are looking for diagrams or specific repair info head to your local library. Many libraries offer ALLDATA for free. ALLDATA is on of the two (Mitchell is the other) main automotive resources for independent shops.