So grandpa's 2008 f150 blower stopped working and I thought my daughter and I would attempt to fix it,

I understand this isn't an IDI issue but I thought my friends here my be able to splain it in a way I can understand,

not exactly sure what the problem was since I was reading if the resistor is bad it should only operate on high, we pulled the motor off,

We tested the motor direct on two different batteries and it did even try to spin, I figured we had a bad motor but wanted to try to diagnose or troubleshoot a little more just to say we did,

hooked a multimeter to the fan plug and got 12.3 v on all fan speeds,
changed the multimeter to 10a setting and it read a constant 1.3 at all fan speeds

I was expecting to see a change in the multimeter as the fan switch was moved to different speed settings,

running out of daylight we went a bought a new motor, hooked it to the plug and it works and changes speeds as expected,

Was I using the wrong setting on the multimeter ? the motor only has the single two prong plug,

 

i never messed with one. just replaced them when they stopped working. and i have had them die high only, and also low only.

 

You can't test a resistor voltage drop with out a load attached to it.

​​if fan ran the same in all three settings, then maybe you wired it wrong or the resistor pack is shorted.

 

I’m no help because I myself don’t fully understand how it all works, but your original thinking was correct. If fan works only on high it’s the resistor. If fan plays out on the other three settings as well as high it’s the motor.

On my 09 I’ve had two blower motors go out. I misdiagnosed the first one and changed the resistor lol my bad. Changed the resistor on the sidewalk in town, then changed the motor at home in a snowstorm. These have to be the easiest trucks to change a fan motor in.

I’m a little smarter now, and keep a spare fan motor on my shelf. From rock auto of course, the dealer wanted like $400 for one. The first one I changed out cost me $136 from the same dealer several years prior.

When testing the new motor before installing, always make sure the fan is on the lowest speed. Otherwise when it’s on high it startles a guy and he drops it, causing the squirrel cage to explode…

 

The multimeter 10A setting is supposed to be used in-line with the load to see how much power the fan is consuming. If you just jammed them into the fan plug, youre shorting out the circuit and I'm surprised you didn't blow out your multimeter fuse. That is, of course, you moved the red lead plug from the (volts/ohm/hz) socket to the (10a) socket.

 

I agree that if you have leads set in 10a mode and simply connect it across 12v (or even way less) and that source can supply enough current, you're gonna pop the fuse inside the meter. Small glass or ceramic cartridge fuse, usually size AGX-2 and usually 630ma ( 0.630.amp). If you want to measure more current than 10a, a clamp-on meter is best.
Not expensive and very handy to have, can even measure starter current, any current in truck that you want to see.

 

and that's why I'm only a junior mechanic,

I did have the multimeter **** turned to 10a but I failed to see ( getting dark and getting in a hurry) that my multimeter had a different sock to plug the lead into for 10a ( I actually noticed this when I was putting everything away yesterday )

thank you for the comments,

 

Good thing you didn't have the lead in 10a socket on meter....didn't blow anything then!
A clamp-on Ammeter really is safe and handy.
It simply clamps over the single wire you're measuring with no connections required.