The brushes aren't for voltage regulation, they pass current to the armature. Regulation is done by an electronic module on the back of the alt (the one with the heat sink that includes the connector socket - the gray thing is the rectifier that converts AC to DC).
It is true that when Denso alternators go, it's usually the brushes, although the symptom of them going is intermittent charging. I'm not sure if low voltage is an early stage of that, but since it is rpm-dependent, I tend to think the problem may be the regulator. The brushes are cheap enough, though, ($12-$13 including shipping) that it probably wouldn't hurt to try replacing them.
Looking at this site:
http://www.nationsautoelectric.com/altparts.html
This looks to be your alternator:
http://www.wai-wetherill.com/products/sresult1bom.cfm?ManufName=NIPPONDENSO&item=100211-718
And if so, this would be your brush set - the 39-8200 brush assembly:
http://www.wai-wetherill.com/products/partimage.cfm?wai_num=39-8200
There are a couple of these brush sets on ebay, but this one comes with the seal, and looks to be of better quality, and it's no more expensive, in fact is a buck cheaper:
http://cgi.ebay.com/ebaymotors/ws/eBayISAPI.dll?ViewItem&item=230329760285
This is the regulator, the IN434:
http://www.wai-wetherill.com/products/partimage.cfm?wai_num=IN434
A heavy duty version also available, the IN434HD:
http://www.wai-wetherill.com/products/partimage.cfm?wai_num=IN434HD
Here's an old thread that documents the rebuilding process - to see the pics you have to look at an archived version:
http://web.archive.org/web/20040530...orum/showthread.php?threadid=27270&forumid=10
Another page on rebuilding a different, but similar, Denso alternator:
http://www.fixinrams.com/id12.html
If the regulator is bad, brushes of course won't help. There are other parts that can go bad, too, including the copper slip rings, the bearings, and the diodes in the rectifier. It's possible to buy those parts, too, but by the time you do, you have probably spent more than a rebuilt one costs. Also, the copper slip rings, which are the surface that the brushes rub on, are not easy to replace. Depending on how scored they are, you might just smooth them with some fine wet-or-dry sandpaper or fine emery cloth.
If you buy a remanufactured alternator, stay with a Denso alternator rebuilt either by Denso themselves or sold by a reputable reseller like Beck/Arnley. Rock Auto has both for good prices:
http://www.rockauto.com/catalog/x,carcode,1277704,parttype,2412,partGroup,18
autohausaz also carries the Denso-remanufactured one, but for more $$ than rockauto.
Let us know what you decide and how it works out.
