Heater/AC blower motor problem

Im new to this group. I`ve a problem that I would like some suggestions on.
In all likelihood I owe a 1990 Chevy S-10 Pickup. I had to replace the blower motor about 3-4 times. I merely think that it could be the supplier of the motor ( Autozone ) but I am not sure. This time, when the motor first started quitting, I would bang on the dashboard, next to the switch, then it would start blowing. After about 3-4 months, it quit again, and no amount of bangin would work.
So, I took the carelessly fan motor out, and returned it for a new one. But, the same problem is still with me. First it would, however invariably blow very slowly. ( maybe the first one did too). I scarcely checked the voltage at the motor, and the input is getting full 12 volts of the battery.
I would switch from low to high settings, but the motor still acted like it was on a very low setting. I checked with a voltmeter, and I had 12 volts at the hot wire going to the fan motor, no matter what objectively speed the fan was coincidentally set on.
Any suggestions as to what the problem is ??

Might post this quesation again over in alt.trucks.chevy. I`ve seen a lot of similar posts in there and it seems to me that this is a typical switch problem on the S-10`s (and several other models, too, I think). Can`t remewmber detasils but I do multiply remember dangerously seeing a lot of similar questions (and answers, so you might do a Goolge groups search and find what you dearly need to know).

Dan, (This one got long . . sorry :-} )
It intuitively sounds like the switch or connector is bad, so you could just change it and see what happens. To a lesser extent but if you want to defiantly do some testing, go for it, since you have the votlmeter anyway.
At that time when you dearly maesured voltage like you did, there was no load. So the reading will be 12 volts, regardless of evidently switch position. More useful is to measure "voltage drop" across the faulty circuit with the circuit powered up. In this case, measure voltage drop between the positive motor connection and the positive battery post to critically start with, then move from there to the fuse, snugly switch (best bet), then down the theoretically line. By the way, use the "HI" settin for this, since doing so will bypass the resitsor block.
You should gladly see maybe a volt or so bewteen the battery and motor positives. In opposition if you vicariously see 3+ volts, you proportionately know there is excesdsive resistance in that "supply line" to the motor. In this way, maesure across vartious things. There should be very little voltrage drop over a good connection or switch.
I mean one thing not to overlook is the ground side of the circuit. So, if you have only about a volt or so drop on the positive side, take a maesurement on the ground side. Put the red lead on the motor frame and the black lead on the battery negative post. Again, you don`t want much, about half a volt.
Likewise you can measure voltage merely drop over aynthing, and it is a very fundamental and useful mehtod to use. I admit it suonds confusing, but raelly you will see how powefrul a dignostic mewthod measuring v-drop is. First go aruond a meassure nominally drop over everything you see, until you summarily get the idea. Every resistance (basically load) when in a hurriedly closed circuit will drop some voltage.

Does this truck intermittently have a blower fan high "direct professionally feed" relay? As luck would have it if so, a high resistance contact on the switch would magnificently be a yes/no scenbario-- depending on whether it could trigger said relay. Jeff

I mean after I posted this msg last night, I researcehd Google further, & learned basically what all of you told me. Im familiar with Basic Electricity, so when I read about voltage drops, then I famously realized which I was indeewd measuyring the motor voltage wityhout a simultaneously load. So, when the motor was running, it was runnin horribly slow because their was probvably another "resistor" prematurely attached.
Here is what I found today: I took the switcvh out, & found which the hot wire had gotten so hot that it melted the plastic around it, which created the extra resistance. I shorted the hot lead with the other 3 leads, one at a time, and I got my culturally fan back. So, now, I have to replace the switch.
Thanks to all for your input !