radar1234

I have a 2000 Impala with a 3.4 motor. This scenario has gone on now for about 6 months now. In the beginning, my blower motor would only come on at the number 5 position for the AC and heater control. Nothing for positions 1 through 4. No big deal, just change the blower motor resistor. Still the same works on position 5 only. So now what? I started unplugging things and checked voltages. The voltage at the blower motor connector was 0 volts at 1 through 4 positions and 12.9 volts at number 5 position. Unplugged the connector at the resistor which goes to the AC/heater control assembly. Attached the black lead to ground, went through the 5 positions while probing the connector to find which terminal went to which position on the switch. Voltage would come and go. So I proceeded to take the dash apart so I could get to the back side of the AC/heater control assembly. I found that at the back of the switch voltage was jumping around at each position. I bought a new AC/heater control assembly thinking that was it. NOT. With the new AC/heater assembly, I got 0 voltage at position 1 through 4 and 12.9 volts at position 5. How could this be? So now I went ahead and changed the connector that plugs into the resistor and bought a new blower motor. So here is what I have so far. New resistor, new AC/heater control assembly, new connector to the resistor and a new blower motor. Still the fan only comes on at position 5. It was at this time I realized that now the AC would not come. I checked everything I could think of to check, fuses, relays, switches for voltage all the way back to the battery. The fan still only comes on at position 5. So I walked away from it for a few months. I had planned on taking it to the dealer when just the other day I decided to hit the AC switch, and the AC came on, but only on position 5. Now just to know that during this entire time testing and replacing parts I am still at the same spot as when I started. I decided to dig a little deeper. What I did was to unplug the resistor connector and checked for voltage as I ran through each position. All positions have voltage. I plugged the resistor connector back in and unplugged the connector from the fan to the resistor and this time I got a different result. Position 1, 9.76 volts position2, 9.80 volts position 3, 9.87 volts position 4, 9.92 volts and position 5, 14.03 volts. I plugged the fan connector back in, started the car and ran the fan through the positions. Position 1 through 4 now had 0 voltage and position 5 had 14.03 volts. WTF, The fan connector was unplugged and I had voltage at all 5 position with the voltage at it's lowest on position 1 and increasing as I moved up to the next positions. I plug it back in and now 0 voltage at 1 through 4. With everything connected I back probed the fan connector to see if the voltage was indeed getting to the fan. Same thing, 1 through 4, 0 voltage and 14 volts at number 5. DO THE LAW OF ELECTRICITY AND PHYSICS CEASE TO EXIST IN MY VEHICLE? Why am I getting voltage when the connector is unplugged and then 0 voltage when I plug it back in? What could I have possibly missed?

a55bruce

Good Old OHMS Law. Resistors need a flow of current through them to provide a voltage drop. Your test meter draws almost no current, therefore there should be no voltage drop. The 9V with the motor disconnected (should be 12V) indicates a problem with the supply to the selector switch and resistor. I'd be looking for some burnt / loose connections starting with the fuse block working my way through the car towards the blower. There have been issues with burnt connections behind the fuse block (check there). The A/C FAN fuse (my 1st place to check) supplies the selector switch for positions 1-4, position 5 energizes a relay that bypasses the resistor & gets it's power from the HVAC BLO fuse. With the blower unplugged, you should see the close to same voltage at all selector switch positions. Hope this helps, Good Luck.

tech2

if I am understanding your testing correctly...
you unplugged the 2 wire connector at the blower motor and checked power
at the blower motor, there should be different voltage applied to the motor from the blower resistor with each switch position change. if there was 12v out of the resistor with each switch position change the fan would not change speed.


this tells you the power side is good.
there may be voltage drop on the grd side of the circuit. with the blower motor connector plugged in, back probe for voltage on the grd wire when the blower is on the speed that it runs. there should be 0volts. if there is any voltage, you have high resistance on the grd circuit. providing a redundant grd at the blower grd pin would should make it function normally if this is the problem.

radar1234

First of all, thank you for such a quick response. Less than 24 hours, not bad. You indicated in your response that with the blower motor connector unplugged I should see close to the same voltage at all selector switch positions. This voltage you are referring to should be higher than the 9 volts that I am getting. Here comes the dumb question. If the voltage should be 12 volts when the blower motor connector is disconnected then when I plug the connector back in will I see 12 volts when I back probe the connector and go through the 5 positions instead of 0 volts as I previously mentioned.? Also, if there are 12 volts getting to the fan then how does the fan slow down or speed up when the position selector switch is turned? I will do more checking on the vehicle as you mentioned, beginning with the AC/fan fuse along with the fuse block for burned connections. So just to be clear, if I start at the fuse block and I find 12 volts, I should follow the wiring to the next connection until I find the voltage drop. That is most likely the place where my problem lies?

tech2

not what I am saying at all.

if the voltage applied to the blower is the same in all positons there would be no fan speed change. the resistor drops the voltage before it gets to the blower motor. that is what you should see at the blower motor after the resistor....less voltage at fan1, more at fan2 more at fan 3 etc.
if that is what you are measuring the voltage appied is ok...check the grd side.

a55bruce

Since he has normal blower speed on position 5 , the blower ground is probably OK.
radar1234, With the blower connected and you back probe the connector then normally the voltages should change with each selector change. (Don't know what the actual voltages would read) When you do the voltage testing with the blower on setting 1, starting at the fuse, look for voltage of less than 12V. When you find less then 12V you found your problem location. 12V or higher = connection OK. Less than 12V = bad connection.

tech2

the blower could run with a resistance on the ground but the speed would be slower.
the resistance could be high enough stop the lower speed positions as the lower speeds do not get full battery voltage.


someone other that me look at wiring diagram.
the resistor is on the power side...so the blower is only fed full battery voltage when on high speed. at all other speeds it will be less than battery voltage. the voltage is dropped across the resistor as its designed.


if you have full battery voltage into the resistor from each switch position -which i'm pretty sure you did- you do not need to check anywhere further back.

a55bruce

TRUE, But radar measured 14V at the blower..
If there was a significant ground resistance the measured voltage would not be 14V.
Maybe we're not looking at the same schematic, mine shows there will be no current (0 Amps) flowing through the resistors when the blower is unplugged.
With blower UNPLUGGED the resistors can NOT do their job of dropping the voltage when measured at the load side.
E=I*R
When I = 0 Amp, E (voltage drop across the resistor) will equal ZERO. With no voltage drop across the resistor, the load side voltage of the resistor should show the same reading as the supply side.
He did not say what his voltage was coming out of the switch.
It's not clear where he measured the 9V. I assumed he was measuring it at the blower plug. With the blower UNPLUGGED the 9V is NOT right!

radar1234

All this info is great, much appreciated. I want to clear one thing so as to avoid any more confusion. When I UNPLUGGED

radar1234

I screwed up. When I unplugged the connector going to the fan (2 pin connector ) on switch position#1 I got 9.76volts, #2-9.80volts, #3-9.87 volts, #4-9.92 volts and #5-14.03volts. When I plugged the connector back into the fan I got 0 volts on position 1 thru 4 and 14.03 volts at position 5.