1973olds98

My switch work but the fan shuts off when i put the switch it all the way up...any ideas

1973olds98

i realized my swith does work ..low,,med,,high,,,,but then there is another setting if u push the switch all the way up...You can hear the fan barely on..Should there be only 3 settings for the speed of the fan..

Lady72nRob71

Not sure about the '73s but the prior years used a relay and inline fuse for the highest speed. Look for a fuse holder on the firewall.
Of course this is a WAG riding on the assumption the car has factory AC.

jaunty75

On my '73 Custom Cruiser, which has the base A/C system (not the Comfortron), the fan switch does have four positions. I found that on my car, the switch contacts are a bit dirty, and I have to jiggle the switch at each position to get the blower to run, but it should blow at a high speed when the switch is in the topmost position.

My guess is that, if your blower works in the first three positions, then it will probably work in the highest-speed position, too, if you just jiggle the switch. If that doesn't do it, you will probably have to disassemble the dash to get at the switch and clean the contacts. If it were me, I'd probably live with the three speeds I have and wait until I had some other reason to go through the trouble of taking apart the dash.


The service manual doesn't show a separate fuse for the highest-speed setting. In fact, the treatment of the heater wiring system in the service manual (on page 1-4, Figure 1-3) shows only a three-position blower motor switch. I'm guessing that that's because it's a one size fits all discussion, and the difference between a three-position switch and a four-position switch is only in the number of positions, so there was no need to show a separate diagram.

1973olds98

OK,,I will wiggle it abit and see what happends,,I can live with the 3 settings,,,So your saying your fan works at the top most level,,,

jaunty75

Yes.

1973olds98

dang it,,i hate when things dont operate like they should...i may go ahead and take it out..It looks like i can take it out easily

MDchanic

'73 has the same basic system that's in at least '68 to '72 (if not earlier and later...). The big diagram in the manual shows the heat-only system; the A/C system is illustrated separately in the A/C section.

Heat-only systems have 3 fan speeds, all handled through the switch and the resistor block.

A/C systems have a 4 speed fan.

• The lowest speed comes right off of ignition power (brown), and has an interconnect to an engine temperature sensor that's attached to the vacuum heat valve.
  The idea was that the fan is on low all the time, unless you turn it higher, but that if it's cold out, it doesn't come on until the engine warms up.
  Most of these valves have been replaced with non-factory units that don't have the temperature sensor, and the sensor has either been eliminated or ignored.
  If there's no sensor, then either the wire (black with white stripe) is just hanging, the "Low Speed Relay" isn't activated, and there is no Low speed,
  or, the wire is connected to ground somewhere, and you've got the fan on Low all the time, regardless of temperature (which is the way Chevys always are).
• The 2 middle speeds come off of 2 terminals on the switch (yellow and blue), which go to the resistor block.
• High goes from a terminal on the switch (orange) to the pull-in on the SPDT High Fan Relay, pulls the relay closed, disconnecting power from the resistor block (and the three low speeds) (black with white stripe) and connecting a wire straight from the Horn Relay / Key Buzzer / Power Lug on the firewall, through a 30A fuse, (black with orange stripe) through the High Speed Relay, to the fan (purple).

If you're missing High, it's almost always a blown fuse, bad fuse connections, other bad connections, or a bad High Fan Relay (its probably NOT the switch).
The relays can be opened and cleaned up, and should work fine, and can also be replaced by any run-of-the-mill 30-40A SPDT relay.

The speed switch has an extra orange wire on it from the rest of the control unit - don't worry about it.

Just check your connections and you should find it.

- Eric

jaunty75

You are right. My apologies. I didn't think to look in the A/C section. I figured a blower motor is a blower motor.

Look on pages 1B-14 and 1B-15 in the service manual. The diagram is Figure 1B-16 on the bottom of page 1B-14, and a detailed description of what happens when the blower switch is in each position is in the second column on page 1B-15.

Lady72nRob71 was correct, too, on this. There is a separate, 30 amp in-line fuse coming directly from the junction block to power only the highest fan speed. The blower motor resistor is completely bypassed when the fan speed is in the high position. The fuse in the fuse panel controls only the lower three speeds.

I would suspect this fuse. WHERE it is in the line, I don't know. You're going to have to trace the line from the junction block (black/red stripe) to the fuse. On the other side of the fuse, the wire is black/orange stripe.

1973olds98 really should have the service manual in front of him as he troubleshoots this. I'd bet money that if he finds that fuse and checks it, it will be blown. With luck, it's an easy thing to find and is easy to replace.

MDchanic

HA! A common misconception, mon frère!

The A/C blowers are much more powerful, draw more current, (hence the need for a high speed relay), and actually have a small rubber tube that diverts air from the fan to blow through the motor to cool it.

Having converted a non-A/C A-body to A/C 28 years ago, I am well versed in their mysteries...

- Eric

Lady72nRob71

I like it when a WAG turns into fact...
With the engine stopped, key on, engage the highest speed setting while listening for a click from the relay. Go from med-high to high.
If you hear it, it will help you locate it. the fuse should not be too far away. Look for wiring hacks also.
If you do not hear the click, then the relay may be faulty.