Re: How does the thermal timeswitch work
Posted: Sun Apr 16, 2023 7:18 pm
I can give you my understanding of how the thermo timewitch works from observations from my CIS car.
Obviously you have a MFI car but I think the timeswitch is the same ( or at least performs the same function ).
The timeswitch is made up of two elements.
Firstly there is a bimetallic switch between the connection 'W' and the shared ground.
When the thermoswitch goes below a preset temperature the switch is closed, and when it goes above it is open.
The cold start valve solenoid is generally wired between the starter solenoid positive and the thermoswitch. (On some CIS cars it is also switched with the throttle switch, so that the cold start valve will only operate when the hand throttle is used [ or foot throttle operated sufficiently ).
Therefore you only get the cold start working when the thermoswitch is cold.
The second element is that the switch has an integrated heater coil between connection 'G' and ground. This is the element that gives the time effect as after a number of seconds of cranking the thermoswitch will heat up, the bimetallic switch will open and the cold start vavle will be inhibited.
This gives the following operation.
Cold start on a cold engine, but only for a time limited number of seconds of cranking.
No cold start on the restart of a hot engine.
That's my understanding of the operation but happy to stand corrected.
If you wanted to test your thermoswitch I would suggest.
1) Measure the impedance between 'W' and ground when cold ( should be zero ohms ).
2) Heat up thermoswitch ( hot air gun etc ) and check the impedance goes high ( open ).
3) Let it cool back to room temperature, and the impedance should go back to zero ohms. Apply 12V between connection 'G' and the body ground and time how long it takes for the heater to switch the bimetallic strip and the measured impedance to open again ( maybe 10 secs or so ).
Not sure why the D911 part can take a constant 12V feed, I suspect those notes on the part are incorrect as I can't see how that can ever work.
Hope the above helps in some way and that you get it all sorted.
All the best
Andrew
Obviously you have a MFI car but I think the timeswitch is the same ( or at least performs the same function ).
The timeswitch is made up of two elements.
Firstly there is a bimetallic switch between the connection 'W' and the shared ground.
When the thermoswitch goes below a preset temperature the switch is closed, and when it goes above it is open.
The cold start valve solenoid is generally wired between the starter solenoid positive and the thermoswitch. (On some CIS cars it is also switched with the throttle switch, so that the cold start valve will only operate when the hand throttle is used [ or foot throttle operated sufficiently ).
Therefore you only get the cold start working when the thermoswitch is cold.
The second element is that the switch has an integrated heater coil between connection 'G' and ground. This is the element that gives the time effect as after a number of seconds of cranking the thermoswitch will heat up, the bimetallic switch will open and the cold start vavle will be inhibited.
This gives the following operation.
Cold start on a cold engine, but only for a time limited number of seconds of cranking.
No cold start on the restart of a hot engine.
That's my understanding of the operation but happy to stand corrected.
If you wanted to test your thermoswitch I would suggest.
1) Measure the impedance between 'W' and ground when cold ( should be zero ohms ).
2) Heat up thermoswitch ( hot air gun etc ) and check the impedance goes high ( open ).
3) Let it cool back to room temperature, and the impedance should go back to zero ohms. Apply 12V between connection 'G' and the body ground and time how long it takes for the heater to switch the bimetallic strip and the measured impedance to open again ( maybe 10 secs or so ).
Not sure why the D911 part can take a constant 12V feed, I suspect those notes on the part are incorrect as I can't see how that can ever work.
Hope the above helps in some way and that you get it all sorted.
All the best
Andrew