Your terminology is really confusing, ("it only lets me select 3 and 4"), but I think I get your meaning that you CAN turn the switch to positions 1 and 2 but when you do the fan stops blowing any air. You CAN select those speeds, but the motor quits running on those two settings. I have to be sure so we're talking about the same symptoms and observations. You wouldn't believe how many times we have the wrong things in mind when we use our experience or training to come up with a plan of attack.
Based on those symptoms my first thought would have been the switch but since you replaced it already, we'll move on. By the way, did you replace the switch in an attempt to fix this problem or was it for something else that happened in the past?
Logic rules out the fan motor because it does run in some speeds. GM likes to use a relay but that is only for the highest speed which draws more current than the switch can handle on its own. Since the fan runs on the highest speed, we know the relay is okay.
After the switch, the next most common problem is the resistor assembly which you already replaced too. Did any of the symptoms change with the new one? There are two different resistor designs. One uses four different-value resistors, and one is switched in depending on the speed selected. A failed resistor will affect only that one speed, and all other ones will still work. The other design uses three identical resistors. All three are switched in for the lowest speed; two for the next speed, etc. That style can be wired in two different ways, with the common terminal going to the motor or to the switch. The operation will be the same but with a failure, the symptoms will be different. With the common going to the switch, like COULD be what's happening with yours, with a failed resistor, all of the speeds higher than the bad resistor will work and all of them lower, or before that bad resistor will be dead. If the switch is turned around and the common terminal goes to the motor, only the speeds lower than the bad resistor will work and the higher speeds will be dead.
Resistor assemblies also have a thermal fuse built in. They blow when they get hot from too much current or when there is insufficient air flow to cool the wire-wound resistor. Too much current is caused by a tight motor. You would have to measure that fuse with an ohm meter to see if that is what failed. Most fan motors get their current through a section of the ignition switch, and a lot of manufacturers have been having more switch problems than decades ago because they're making them cheaper and we have more toys and unnecessary computers that draw a lot of current and overheat the switch contacts and connector pins. A degraded connection will add undesirable resistance to the circuit which lowers the available voltage. It's backward to normal electrical theory, but a decrease in voltage will cause a motor to draw MORE current which can lead to repeated failure of that thermal fuse in the resistor assembly. That's why it would be helpful to know HOW the old resistor failed, (or if it actually had failed).
The next conclusion, if the switch and resistor are okay, is there are two broken wires between them. That is not likely except for two possibilities. One would be the wire harness is mispositioned and has fallen down onto a sharp metal bracket. Vibration will cause the wires to be cut on that bracket, but the clue would be one speed quit working first and the second speed quit months or years later. A more likely suspect would be the terminals in a connector overheated and the plastic body started to melt. That heat will migrate over to a second terminal and cause it to be degraded too.
If you can say with absolute certainty that both speeds quit at exactly the same time, I would be fairly confident you're going to find the problem to be in your new switch or resistor. If the two speeds quit within a few days or weeks of each other, I'd be leaning toward the switch. If you feel those aren't the cause, you're going to need a test light or a voltmeter to find the breaks in the circuit. I'll try to find a wiring diagram to walk you through the test steps.
Tuesday, December 20th, 2011 AT 11:42 PM