Just to thank all of you who helped, and to let you know the overheating
was caused by three factors: a poorly functioning  radiator, improper
wiring of the fan switch, and air in the coolant system .

We had figured out the fan switch and improper bleeding last weekend, but
it did not cure the overheating. So he ordered a new radiator from Bus
Depot, and Ron had it on our doorstep the second day after the order. We
got it installed Saturday morning. Yesterday, with a six person load, A/C
on and 94F air temperature, it ran nicely with the needle right over the
coolant indicator light. Coolant level is constant in the expansion tank,
so no leaks.

Initial diagnosis was confused by the fact that when the guy bought the
bus, he had his mechanic flush and refill the coolant. Turns out the
mechanic had no idea about proper bleeding procedure, and the resulting air
in the system had the coolant level light going on and off all the time,
sometimes in concert with overheating and sometimes not.  This was the same
 mechanic who had jury rigged the fan switch so it would not go into the
high mode, because it "made too much noise and I thought it must be out of
balance".

All in all, the episode makes a good case for the old John Muir creed: come
to terms with your ass for it bears you.  Another argument for doing your
own work!

Thanks again,

Tom F.