There are two things to check for the low speed.

If the fan low speed does not operate but high speed does and
If vehicle is air conditioned, turn on the AC.  If the fan comes on the
circuits are good and the sender in the radiator is bad.  It has a two stage
switch in it.

If the AC fan does not come on then the resistor pack is probably bad.
Found usually behind the headlights on most vehicles.

If not AC equipped check the resistor pack and the switch in the rad.
jimt

On 6/5/05 11:03 AM, "Detroit Bus" <detroit.bus@GMAIL.COM> wrote:

> Hello from 90 degree NYC:
>
> Why does the Bentley wiring diagram for an 87 show two positive posts
> on the radiator fan motor, when my fan motor has just one positive
> post and one negative post? Both hot leads are combined in one
> connection at the positive post on my motor, 251959455M. My vanagon
> has A/C.
>
> How does the fan motor know whether to go high speed or low speed? My
> low speed option isn't working and i'd like to fix it.
>
> I get 12.1 volts on the low speed lead and 12.6 volts on the high
> speed lead  when I jump the radiator thermoswitch (red/black wire and
> red/blue wire, respectively). If the single positive post is correct,
> does this mean I need a new fan motor? Or should the low-speed voltage
> be higher than 12.1? High-speed works fine: the relay clicks and the
> fan spins very nicely.
>
> I've searched the archives and apparently there was something about
> this on Ken Wilford's site at one time which I can't find.