AFM Test

[Techno Viking]

So,

I hooked up a multimeter to test the resistance of my AFM, and the numbers were within the limits of what they should be. The only thing is, there was more resistance halfway though opening the door than there was at full open....is this a bad sign?


Thanks in advance,

Casey B

[jlevie]

The only bench test you can do on an AFM by resistance checks is to look for worn spots in the resistance trace, which will cause a jump in resistance as the vane moves. To completely check the AFM it must be powered like when in use and voltage plotted as a function of vane angle.

[nomansland92]

it should be a smooth transition to min to max. As you described yours its sounds bad

[e30chronic]

check out "the944.com" its a porsche site that has a how to on checking afm's

[CoClimber]

On my AFMs, the resistance will not increase smoothly because throught the range there are trim resistors that will affect what the multimeter reads. As Jim said, the only good test is to power the AFM and look for a smooth change in voltage as the vane moves.

I could go into a more technical explination as to why the resistance check won't work but it would require diagrams and I'm way too lazy for that!

[lateracer]

+1 ^

My AFM resistance gets jumpy when measuring ohms across terminals with no power. Powered with 5 volts and measuring voltage instead of resistance, it reads smoothly across the range.

I'm not one to respond to a thread just to say "Yeah, what they said" but there seems to be a misconception that measuring resistance on an unpowered AFM is a good test.

[CoClimber]

I think the Bentley manual says to measure resistance. Love that manual but I disagree on this point. There were probably some earlier AFMs without the trim resistors. If that was the case, it explains why the manual says to test this way.