Blower Schematic and HVAC Operation

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Blower Schematic and HVAC Operation

Postby zeke » Sun Apr 26, 2020 10:53 pm

With the ignition switch turned on, power flows thru the fuse, thru R1, thru the normally closed contact of the relay and motor runs on slow speed.

Systems with A/C have 4 speeds total of which 3 are user selectable. When the fan switch is in MED1 or MED2 position, power also flows thru the resistor pack. Now that R1 is in parallel with R2 or R3, the resistance is less and motor speeds up. The resistors are located in the air stream to keep them from overheating.

The total equivalent resistance of a set of resistors in parallel is found by adding up the reciprocals of the resistance values, and then taking the reciprocal of the total:
1 / Rp = 1 / R1 + 1 / R2 + 1 / R3 +... The total is always less than the highest individual resistance. Less resistance means means less voltage drop across the parallel resistors and more voltage to the motor therefore increased speed.

The high speed operation is a little different. The fan switch energizes a relay coil in the high position. This disconnects the fan motor from the resistors and connects it to full 12 volts thru the normally open contact.

Back in the 70's, the largest glass fuse made was 25 amps so high speed was done this way so that more current coud be drawn directly from the battery connection at the starter. Overload or short circuit protection is provided by a fusible link.

If the fan does not operate you'll need a test light.

Pull connector off of fan motor and jump 12 volts from battery positive directly to it to ascertain motor's condition. Check to make sure the motor has ground.

If motor runs, check for power at fuse block and at brown wire on the 3 pin connector behind dash.

A/C cars have an extra switch for powering the compressor circuit and blower circuit. Next check for power at the fan selector switch and then the output of it.

The fan selector switch is a common issue and can be dissassembled to clean the contacts.

The resistor pack does not usually go bad. Check for power in and out.

If the fan works on all lower speeds but no high speed, the relay has probably failed. Relay is located on air box under hood or on inner fender. Check for power on the orange coil wire with fan switch in high position. Check for ground on black coil wire. Check for battery power on the red wire. The fusible link at the starter is a common problem and should be replaced with the proper fuse link wire if found to be open or corroded.
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zeke
 
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Location: Southern Ontario, Canada

1979 Chevrolet Monza 2+2


Re: Blower Schematic and HVAC Operation

Postby zeke » Sun Apr 26, 2020 10:56 pm

Mode selector switch controls when compressor circuit is active and feeds power to the fan switch.
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temp_select_switch_hbody.jpg
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User avatar
zeke
 
Posts: 2581
Joined: Fri Jul 21, 2006 3:27 am
Location: Southern Ontario, Canada

1979 Chevrolet Monza 2+2


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