Heater Motor on High Speed Only

Usually if the fan only works on high, it is the resistor.

has new resistor still only works on high

has new resistor still only works on high fan

The most common cause here is a bad resistor, but since you are confident that isn't the problem, it is likely the switch. However, I did have a situation once in a dodge caravan. The fan motor itself was causing a voltage problem and ruining the resistors.

You can use a simple test light. Unplug the wire to the resistor. Should have a 5 or 6 prong plug. Turn the key to the run position. Probe each pin with the light as you change the setting on the switch. If it lights up at each setting that means it is working and the resistor is bad. If not, then the switch. If the resistor is new and not working, or if there are any melted connections, then the fan motor is the likely culprit.

I had the same high blower problem on my 2004 Grand Marquis with EATC. My diagnosis took a while but I finally fixed mine. All comments here are for an EATC system ONLY. The problem and solution are simple but hard to get to. What caused my high blower problem is that an important resistor in the blower resistor module under the hood loses connection with the EATC. This resistor sends back the blower voltage which tempers the EATC speed control. Without this signal, the EATC control range is offset too far to the high side to allow control. If you can take out the blower resistor module under the hood, the repair is easy. The actual failure is that the resistor module connector pins are soldered to the board with very little solder. Over time, stress and flexing crack the solder joints making an intermittent contact. The fix is to scrape off the yellow green coating to expose the entire bare copper pad around the connector pins then resolder with a lot more solder to give it strength. ALL the connector pins will fail in time so do all 5 pins while you have it out. Here's a picture with the two big ones done and the three small ones ready to resolder.