Page 37 of the 1988 Volkswagen Vanagon and Transporter Owner's manual:

WARNING LIGHT

For a bulb check the red light in the coolant temperature gauge will flash
for about 3 seconds when the ignition is turned on.

If the light flashes while driving, the engine may be overheated because of
low coolant level or V-belt damage.
Move the vehicle a safe distance off the road, stop the engine and depress
the emergency flasher switch.

The fan should be operating when the warning light is on (listen for fan,
located in front of the car behind grille.)

If a visual check shows that the coolant level and V-belt are normal, but
the warning light continues to flash,
contact your nearest dealer to have the cause located and corrected.

Todd
'88 Westy