Heater Fan doesn't blow on low

[dmjlambert]

I'm a little slow, still replying to your "Blower motor fan relay maybe?" Yes perhaps. You can (kind of) see in the diagram HO455 posted that the resistors send power from the 14-gauge dark blue wire at the high-speed relay. Voltage normally passes from the resistors through the normally closed contacts of the relay to the fan. I could not follow what Richard was saying but I think he was trying to say there is not a normally closed contact in the relay. If that is what he meant I'm afraid that is not the case. It is a single-pole-double-throw SPDT relay.

At the relay purple is common, dark blue is normally closed, fat orange is normally open, skinny orange is relay energize signal from switch, and black is ground for the coil that energizes the relay.

Fat orange is always hot 12V from the CIG terminal on the fuse panel. The relay is only energized for fan on high.

When the relay is not energized, power flows from the switch to the resistors to the relay NC terminal to the relay common terminal and out to the fan on the purple wire.

When the relay is energized a small amount of current flows from the switch to the skinny orange wire to energize the relay, and a large amount of current flows from the fat orange wire connected to the CIG terminal to the NO terminal on the relay, then out the common terminal of the relay to the fan on the purple wire.

You can test before buying the relay by disconnecting the relay connector and use an alligator clip jumper wire to jump from the dark blue to purple wire on the connector. Then notice your fan works on low and medium. The high position will give you low motor speed.