HVAC fan motor question

[drocker]

I took my fan out to see if I could oil it. It has been making a lot of noise since the cold weather. When I got the fan out the blades were dirty and so was the motor where you could see in the vent tube hole. Blew the motor out, washed the fan blades, oiled the motor and reinstalled. Here is the question I have. When I hooked the power wire back up the fan I decided to run the fan. On low the motor didn't turn at all. I was looking into the vent hole and could see the the windings. On the other 3 speeds the fan worked fine. So is the fan supposed to turn when on low? If so what could be the problem with mine?

 

[schiker]

The motors can go bad but there is also a resistor mechanism that helps control the speeds. Don't know much about it but have read fan speed issues and that seems to be the culprit.

 

[BigBlueChevy]

based one what your describing it sounds like what Schiker said, the resistor. Chances are its starting to go bad and will require replacement. A good and cheap test is to turn the fan on low. Then turn it on full blast. At full blast the system bypasses the resistor. So if it only works on HIGH then chances are the resistor is shot.

And of course, my understanding is that its NOT something you will enjoy replacing...

 

[WarWagon]

That old shot bearing motor is likely drawing too many amps anyway. New ones from China are like $25.00. Your low speed resistor could be shot or the wire fell off/loose with the repair. Also possible shot brushes take more power than low to spin the motor.

Put a volt meter on the blower to know for sure. Voltage on low with the motor not spinning means bad motor. No voltage is a wire or resister pack.

Also possible the oil you added got on the brushes from being thrown from the bearing. This could also cause low speed trouble.

 

[drocker]

When I had the glove box out, up on top all the tubing for the HVAC there was a bracket with what looked like your typical relays. There was 4 of them. The power wire going to the fan looked to be coming from one of these relays. Fan works on all 3 upper speeds and each time you move up a speed the fan speed increases. It is just when it is on low that the fan doesn't turn at all. I wonder if those 4 relays I saw were for each of the fan speeds.

 

[drocker]

That old shot bearing motor is likely drawing too many amps anyway. New ones from China are like $25.00. Your low speed resistor could be shot or the wire fell off/loose with the repair. Also possible shot brushes take more power than low to spin the motor.

Put a volt meter on the blower to know for sure. Voltage on low with the motor not spinning means bad motor. No voltage is a wire or resister pack.

Also possible the oil you added got on the brushes from being thrown from the bearing. This could also cause low speed trouble.

Damn, I didn't even think to do that. I should of hooked the multi tester to the power wire and test it. I can at least get to the power wire and test it tomorrow.

Also, those 4 relays I talked about earlier may be for the HVAC doors that need to open and close depending on where the air flow is going.

 

[ak diesel driver]

The resistors everyone mentioned actually stick into the airbox they are literally fan cooled. It's held in with with 2 screws IIRC. It's mounted just to the left of the blower and is a real pain to get out and back in.

 

[drocker]

Okay I went and tested my voltage at the power wire. Wire was purple in color. I had 11.45 volts on low, medium low and high. I had 8.48 volts on medium high. Motor would only spin on the upper 3 speeds and not turn at all on low. So I am not sure if the resistor is bad. Maybe the motor?

 

[ak diesel driver]

Pull the resistor pack and check the resistors

 

[drocker]

^ any suggestions on how to test the resistor pack when it is pulled out.

 

[ak diesel driver]

the resistors are just little springs they are either good or not. Also I just went and looked at my 94 and it appears that the resistors are hiding underneath the pcm unlike my 96 which it is over to the left of it.

 

[drocker]

^there are springs and resistors. Springs look good just dont know how to test the resistors. I am trying to google how to test them.

 

[WarWagon]

^there are springs and resistors. Springs look good just dont know how to test the resistors. I am trying to google how to test them.

Test the resistor wires and the thermal fuses. The circles in the picture indicate where you put the ohm meter with the two probes. You should have "0" Ohms on the brown with red letter thingies - these are the thermal fuses. You will have different ,but, not open resistance reading for the coils of wire - these are the resistors. The resistors are in series with the thermal fuses. So one of the terminals with both a resistor wire and fuse wire is simply making a bridge to put them in series. So if the thermal fuse pops, from the resistors getting too hot from no airflow, no current goes through the resistor. Motor would also stop...

As you measured 11.45 volts on low... This is across the motor terminals? (Motor positive to a ground on the motor. DO NOT ASSUME THE MOTOR GROUND IS GOOD!) If you are 100% sure the motor is not running - look in the vent tube hole. Motor not running with 11.45V across it - it is shot. Again bad brushes or high friction in the bearings. Hit it with a hammer on low speed - if it starts running it is brushes. (Who cares non serviceable anyway.)

You do have 2 relays - a high and low relay. The low speed relay can have a burnt contact to prevent low speed. Otherwise high speed relay contact failure would prevent Med speed.

The yellow wire on the resistor pack needs to make it to the Purple wire going to the blower motor for low speed to work. It goes through all 3 resistors and 3 fuses. With both relays off you have med speed.

 

[drocker]

^thanks. Let me go pull it out again and test it. So don't go anywhere. Should be back in ten minutes

 

[drocker]

Everything tested 0.00 ohms except reading that is shown in red in the picture. Motor does run on the 3 upper speeds, it is just the low speed that doesn't work. Where are the relays you mentioned?

 

[WarWagon]

Everything tested 0.00 ohms except reading that is shown in red in the picture. Motor does run on the 3 upper speeds, it is just the low speed that doesn't work. Where are the relays you mentioned?

Forget the relays...

Reading in red is open. This fuse has opened up. Resistor pack is defective.

You can get a new fuse at an electrical supply shop - good luck changing it out as solder would blow the fuse...

Edit:
2 relays is for a: off, Low, MED, and High 3 speed system. I do not have a 4 speed diagram handy. It may take 4 relays...

 

[drocker]

Thanks

How about just replacing the whole blower motor resistor. Only like 20 dollars from local auto part store.

 

[ak diesel driver]

The dry bushings in the fan probably took out the resistor from too much load. That's what I would do replace the whole resistor.