Fir those symptoms there are 2 likely possible culprits with the 90.
The fan motor ground wire connection may be bad.
The low speed fan resistor may be bad.

If you remove the lower front grill you can access the rad temp switch
low on the driver side of the radiator. Pill the wiring connector
straight out. The wiring will have 3 female pins. Briefly jumper between
any 2 pins with something metal. Then do the another 2 pins, then the
last combination of pins. 1 pair should do nothing, one pair should make
low speed come on, and one pair should click on the high speed relay and
cause high speed fan. What did you get?

Mark

Jack Reynaert wrote:
> When I run the AC on my 84 Westy, the Aux. fan on the Radiator runs as long as the power is on to the AC in back (regardless if the compressor is on).
> When I'm stuck in traffic, and the temp gage rises, the Aux fan does not kick-on by itself.  So, I turn on the AC, without compressor, and the fan kick's on and the temp gage goes down.  I suspect the temp sensor on the radiator is bad, and plan to change it.
>
> Now, my question is related to the Aux radiator fan on the 90 Westy we are just about complete with the mechanical overhaul (including new 2.2 GW motor, brake system, radiator, condenser, LED upgrades, dash rebuilt with AWESOME GW Kit, new everything that attaches to the GW motor as well).
>
> We took it for a ride today, and even at full operating temperature, the aux fan didn't kick on.  Put 12v to the fan motor and it worked fine, fuse #1 fine, power to the red/white wire to the switch etc.  However, when we turned on the AC, the aux fan didn't turn on (like it does on my 84)... is that normal for a 90?
>
> Any suggestions on likely causes (relay #53 or temp. sensor lower drivers side of radiator?)
>
> Thanks for any insight!
>
> Jack R.
> N. of Detroit
>