Thermo timing switch flat head screw on side ???

[micbrody]

I have been bench testing a used Thermo timing switch and fairly sure it is not testing as it should.
I noticed a rubber plug on the side of switch. When I removed plug , there is a tiny flathead screw.

Does anyone know purpose of screw?
Calibration ? Disassembly of switch?

[micbrody]

I removed screw entirely and saw no obvious way to disassemble.
I then tested device with ohmmeter under hot and cold with screw removed.
(From memory - I could have backwards but not important:)
With screw out, I tested from W terminal to ground:
It was essentially closed circuit (2.7ohms) in ice bath or 160 deg water.
I inserted screw all the way in and it became an open circuit (144.5 ohms) - (this is the part I just don’t remember - whether it was closed with screw in or open circuit .
What’s important is the screw all the way right flipped the circuit status vs no screw - regardless of ice bath or 160 deg water.
I loosened screw about one turn and the circuit would flip between hot and cold water.
I then used two cups: one with water in the mid 80’s and the other about 105 deg:
A total of two full unscrewed revolutions calibrated per specs per a Thermo timing switch chart I found (below 86 deg circuit closed; above 104 deg F circuit is opened .

[micbrody]

Why it gets out of calibration: who knows.... maybe oxidation somewhere???
Maybe the contacts were stuck together within the switch, and by removing screw is adjusted some internal spring that freed up connection.
And as to how long this will work, who’s to say.
But I will install it in near future and report back.
Potentially a fix for an expensive part of fuel injection system

[micbrody]

One last addendum:
I noticed the hole and screw had dark coating of something - either carbon or engine oil might have seeped into hole with screw.
Potentially the calibration screw creates the ground from internal apparatus to brass body of entire sensor- thus maybe that can be one mechanism as to why switch stops working.
I did clean the screw and hole (carefully) with deoxit spray on a thin q-tip multiple times. I did get black stuff off.
Recalibrated (again about 2.5 turns out from tight); interestingly the ohms on closed circuit is now 1.3 ohms.

[Daddio]

I recently ran across a TTS from a fuel-injected Volvo 140 motor (circa 1970-75) in my parts;
it carries the same part number as the one in a 1981 2-liter Fiat FI motor.
another source for FI parts that may be in your nearby U-pull.
Volvos (and other Eurocars) from the 70s and 80s used Bosch D-jet, K-jet, L-jet and LH-jet, which all use the same cold-start/run components.