Blower motor runs only on low

[sidewayzz]

1979 chevy with air. Changed switch in dash.nothing changed. Took blower motor out hooked direct to battery it runs fast there. Is that round piece a resistor? When i bypassed that and pluged into the wire feeding the motor ,it wont run at all. What am i missing,bad resistor? Bad motor? Bad switch in dash,i replaced that one with one i had on a 81 truck. I noticed the 81 with air didnt have that round piece.thanks in advance, steve

 

[89Suburban]

Sound like a bad motor ground to me. Should be a ground wire from the motor flange to the firewall. Check it and clean the connections.

 

[sidewayzz]

Cleaned it real good. Still wont run on anything but low.still scratching my head! Thanks for the help

 

[sidewayzz]

changed that spring relay/resistor in the ac box. still doing same thing. Any ideas at all. Thanks in advance

 

[highdesertrange]

have you checked the relay? not the springy thingy(resistor) but the actual relay. highdesertranger

 

[sidewayzz]

are you referring to the round looking object that is inline with the wire that plugs into the blower? If so no I havent. I took that piece out of the circuit and plugged straight to motor.

 

[highdesertrange]

no that round thingy is a condenser, like on vehicles with points. I have a relay sitting on my work bench at the ranch. it's a few miles away. I will try to get a pic tomorrow. you know shop manuals tell you how to trouble shoot this problem, even some chitons and Haynes. have you tried this? highdesertranger

 

[87ChevyR10]

I have the same issue, but my blower only turns on when put on high. It used to blow at the other 2 positions, but stopped. Not sure if it's the switch or what.

Any ideas?

 

[sidewayzz]

If the relay is that square plug that bolts onto the AC box,yes I did change that out with one of a 81 truck,still have same issue. I have cleaned all the grounds real good still no change. Put another blower motor,switch in dash,relay in ac box.Still get same results. Broke wire some where between switch and blower motor ?? Just dont know what else to change. Thanks for all the help.

 

[chengny]

Did you check the ground wire that comes out of the relay? The relay needs to have the coil grounded for it to move - and switch speeds.

There should be 5 wires to the relay:

Red - Battery voltage in
Purple - Battery voltage out to blower- for high speed
Dark blue - reduced voltage - medium speed
Orange - reduced voltage - low speed
Black - Coil ground leg


Check the black wire for continuity to ground.

BTW - You also have another switch in the blower control circuitry that I don't see as having been checked/changed.

When the HVAC system is operated in the MAX AC mode there is an override switch that commands the blower to go to high speed.

It is in the vacuum control section of the control head - away from the normal blower select switch. Try unplugging that.

 

[chengny]

Here are the diagnostic charts - they include both problems as described above (i.e. only low speed and only high speed)

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[highdesertrange]

you guys are great. even the diagnostic flow chart. highdsertranger

 

[sidewayzz]

Thanks for all the info and chart,I did find the problem with mine. It was a bad relay (the second one I tried). Replaced that last night now it is working. Thanks again. Now on to the lift kit.

 

[chengny]

I have the same issue, but my blower only turns on when put on high. It used to blow at the other 2 positions, but stopped. Not sure if it's the switch or what.

Any ideas?


That is textbook description of a bad resistor block/stack.

What happens is, the resistor coils burn up and no current can pass through the resistor stack and over to the relay. This is only true for the low, medium 1 & medium 2 voltages and their associated speeds. The high fan speed is not affected because it's voltage does not pass through a resistor and get reduced. When the fan switch is set to high, battery voltage is applied directly to the motor windings.

I can almost guarantee you that a new resistor stack will get all your speeds working.


It just drops in the hole in the evap casing as indicated below - I think about $15 -20.

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[smoothandlow84]

Ok...so now I have a fan speed issue. No high speed, but all others work.

Switch, replaced....
Resistor replaced...
Relay replaced...
Ground cleaned, checked and its good.
Blower fan...yeah it works too


I suspect that the relay that I just purchased is faulty. I ran power directly from the battery to the fan....it works and at full speed.

With the relay plugged in, key on , I have 12 volts between the purple wire and the red....when the fan is switched to high speed....no power. I also ran a conductivity check on the relay with a meter. I have conductivity between alternate contacts, but NOT between the red (power in) and the purple. Is it the new relay thats bad? This is the second relay that has been swapped in. The first relay went bad and I had no speeds whatsoever. Now the new relay wont allow high speed...wtf?

Also...when I replaced the speed control switch (the tip of the lever broke off)...I noticed that the metal contact for high speed position was cooked and discolored. Im wondering if possibly a bad wire somewhere under the dash?


As always any input and guidance is appreciated.