Heat stuck on, '89 100Q

[Frank Martin <fmartin@mr.oa.ithaca.edu>]

Hi Folks.

The climate non-control seems to have gone haywire in my 100Q. It is putting 
out heat at ALL times, except when on the "Lo" setting. I thought that perhaps 
it was the control valve in the supply line to the heater core, but this seems 
to be working - the vacumn line (to the valve) coming out of the firewall is 
simply not causing the valve to close, *unless* the Lo setting is selected. 
So, it's either full open or shut.

One clue may be that the little temperature light has flashed on occasion, 
upon starting the system. I understand that this is a general system-problem 
warning (?).  Also, at least for a few months the temperature has seemed to 
take big swings, causing us to become "thermostat jigglers."

I wonder if the problem is with the interior temperature sensor?  Just not 
sending a signal that the interior has indeed warmed up?  Could anyone give me 
a clue as to where this is located, or offer any other suggestions as to the 
cause of this problem?

(For $.02 I'd convert the whole schmear to good old-fashioned cables and 
levers.)

Thanks much.

Frank M.
'89 100Q
'88 Mazda 323 GTX