I had the same problem with my '84 Westy. It turned out to be the
radiator temp switch was broken for the low speed. Replacing it with
a new "normal" threshold switch fixed the problem completely. I was
just about to add a dashboard switch to override it but this became
unnecessary after the low speed came back into service.

Peter

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At 6:41 AM -0800 4/1/03, mark drillock wrote:
>Your experience may be normal if what you were hearing was low speed
>fan. If high speed fan was coming on you have some kind of problem.
>
>In the radiators of all water-cooled Vanagons are 2 temp switches to
>turn on the radiator fan when the coolant leaving the radiator is still
>too hot. These switches fail as do other parts of this circuit most
>notably the large resister used for lower fan speeds on later models and
>models with A/C. It is common for the low speed to stop working and high
>speed then comes on more often. This will result in more frequent large
>swings in the temp gauge at low road speeds or in stop and go traffic.
>While driving it is often difficult to hear the fan on low speed but
>high speed can be startlingly loud, at least on models with the larger
>fan motor like your 90.
>
>The 83+ radiator temp switch is a integrated dual temp switch. One
>switch comes on at a low temp for a low fan speed and the other switch
>turns on at a high temp for high speed fan operation. There are
>different temp range switches available and 2.1 models with A/C use the
>one that activates the fan at lower temps. Behind the driver's side
>headlight lives a large resister that is used to limit the current to
>the radiator fan when low speed fan operation is needed. On 2.1 models
>with A/C this resister also provides the middle fan speed that comes on
>whenever the A/C is turned on. These are the models most likely to loose
>low speed fan operation due to this resister being burned out from
>constant summer A/C use.
>
>
>Mark

At 8:34 AM -0500 4/1/03, Vanagon man wrote:
>I installed a toggle switch on my dash that is connected to the high speed
>fan wire.   It is a 3 pole switch, with a constant hot, a ground, and then
>one wire spliced into the hot wire for high speed on the fan.
>
>Thus, when I am in a long drive thru, traffic jam, etc., i flip the switch,
>and I keep the temps low the whole time.  It took about an hour for me to
>install.

At 2:07 PM -0500 4/1/03, David Brodbeck wrote:
>I added two LEDs to my fan circuit, so I can see when (if) the fan
>switches turn on.
>
>The high-speed one is on the coil side of a relay and works as expected.
>The low speed doesn't use a relay, though, and at highway speeds the LED
>will glow.  Clearly, the fan is windmilling and generating enough voltage
>to light the LED.