’92 Nissan Pathfinder - Fan Circuit Noise

Ok, I wasn't sure where to post this one, so I thought I'd put it under General. I have posted this on "Nissan-specific" boards as well, with no success. So, I thought I'd try the electrical experts (since I suspect an electrical related issue). So here goes...

In my 1992 Pathfinder, we get an intermittent high-pitched whine or squeal coming from behind the dash where the temperature controls are. You know, the kind of high-pitched "frequency" noise a TV or computer monitor will generate sometimes. Well I can make it go away by adjusting the fan speed selector. Turning it "off" will make the noise disappear, while adjusting the speed up or down will sometimes change the frequency of the sound and eventually make it go away. Then it comes back, and ..... well you get the picture.

So, is there something in this circuit that could cause this - maybe a relay or something? I do not think that this is a "rotational" noise coming from the fan. It definitely is an electrical frequency kind of sound.

Any thoughts on this on Folks?

Thanks in advance.

hi,

the fan may be wearing out.....the brushes specifically.  you could try adding small values (.1uf to 1uf) across the leads of the motor.  this may help suppress the noise.  or replace the blower motor with a new one.  just my two cents.

mark

Thanks for the quick reply Mark.

So, worn brushes could be to blame, eh? Interesting - I would not have guessed that, given the symptom. What do you mean by "adding small values (.1uf to 1uf) across the leads of the motor"? While I kinda know my way around with stuff like this, I have no idea what you're referring to here. Could you please elaborate?

Thanks again,

Dwayne

I appreciate the feedback and will be following up on Ween's suggestion of the brushes. I just thought I'd dig a little more with the relay thought. So, if it is a relay, does anyone know where I would find it? I realize this is a vehicle-specific question, but I thought I'd try...

Thanks

me again,

the fan motor should have two leads on it.  basically a positive and negative.  connecting a capacitor across those two leads should, in theory, act as a filter to reduce the interference.  the reason for the range in values is that it would be hard to determine which value will work best.  go to a radio shack / electronics store, and ask for mylar capacitors......

http://www.radioshack.com/family/index.jsp?categoryId=2032267&cp=2032058.2032230&allCount=102&fbc=1&f=PAD%2FProduct+Type%2FMetal+Film+Capacitors&fbn=Type%2FMetal+Film+Capacitors

i'd start with two packages each of the .1uf and .22uf.  as you parallel the capacitors, the values simply add up.  hope this helps.

mark

Yeah, I think I ruled out the relay yesterday. Turns out it is actually mounted on the inner fender under the hood - nowhere near the noise.

One thing I noticed in my vehicle wiring schematic is that there is a resistor in the blower circuit (I assume to control fan speed). Is it possible that this could be the source of the sound, 'cause it definitely seems to be coming from inside the dash, in behind the controls...

Dwayne

Hey WEEN!!!

When you get a few minutes, would you mind elaborating on the brushes thing? How would failing brushes make a sound? I would love to understand the science of this a little better.

Thanks.

Your fan speed controls do probably use resistors, but I've never had a noise situation caused by them.  Dimmer switches used in homes are considered to be sources of noise, but that's a much higher voltage- and AC, not DC.  You should look elsewhere.

hi,

as the motor rotates the brushes and commutator make contact.  the current passing through them causes a spark (electromagnetic interference).. as the motor i.e brushes and commutator wear out, the gap between them increases causing a bigger spark.  the capacitor across the leads of the motor suppresses the spark.  the frequency and amplitude of the noise change because the varying voltage across the motor.

mark