The fairly obvious answer is that the 40-year-old switch has gotten "old" and no longer closes at the originally specified temperature. Some options are:
1. Remove the switch from the engine
. Get a verified thermometer. Attach an ohmmeter or other indicator across the switch. Put the switch in water
on the stove
and see when it closes. This is probably the least costly way.
2. Replace the switch first. This saves you work
messing around with the old switch.
Neither one is a sure-fire optimum solution. In case 1, you’ve spent time diagnosing the problem. If it’s not the switch, you still need to find the problem, and you’ve wasted the diagnostic time/effort.
In case 2, you’ve bought a currently unneeded spare and still have to find the problem.
It depends on whether you have more time/curiosity than money
, or Vice versa.