How does the thermal timeswitch work

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AndrewSlater
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Re: How does the thermal timeswitch work

Post by AndrewSlater »

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
1966 Porsche 912 Slate Grey, red interior - first owner owned for 41 years
1974 Porsche 911 2.7 (The Manhattan project) viewtopic.php?f=28&t=51455
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gridgway
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Re: How does the thermal timeswitch work

Post by gridgway »

Thanks Andrew, that makes sense. As a supplementary question. how should the squirter solenoid be wired? Is it W to the solenoid connection with the body to earth? If so that doesn't give enough volts for it to activate.
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AndrewSlater
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Re: How does the thermal timeswitch work

Post by AndrewSlater »

Hi Graham,

I hope I am not leading you astray as I don't have an MFI car only a CSI and the implementation is different.

However after a little googling, the wiring diagrams seem to show a relay is used to control the solenoid for the cold starting unit.
The thermo time switch grounds the coil of the relay (via the 'W' connector), activating it which passes a 12v feed to the solenoid.

On the wiring diagram I found the relay is item 52, solenoid is item 12 and the thermoswitch item 14.
https://www.flickr.com/photos/7184021@N07/6695660137 It is a 1971 wiring diagram but should be pretty similar.

There is a discussion about it here
https://www.early911sregistry.org/forum ... lectricals

If you scroll down to post #7, this shows the physical location of the relay on a '71 car on the rear console.

Hope this helps ( and is accurate )
Regards
Andrew
1966 Porsche 912 Slate Grey, red interior - first owner owned for 41 years
1974 Porsche 911 2.7 (The Manhattan project) viewtopic.php?f=28&t=51455
1973 VW 914 1.7 Olympic Blue - ( gone to a good home )
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Re: How does the thermal timeswitch work

Post by gridgway »

Thanks Andrew, really appreciate the effort. Will have a detailed look later
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Re: How does the thermal timeswitch work

Post by gridgway »

Stole a couple of mins to have a look and that's helped a lot thanks. I have a feeling that on the engine change we've tried to drive the squirter solenoid directly off the TTS, but now that I can see that it drives a relay to then drive the solenoid (which is all present in the car) I should be able to bring the squirters to life.

Really appreciate the help!

Graham
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Re: How does the thermal timeswitch work

Post by AndrewSlater »

Glad it was helpful - sounds like you on track to have it sorted ;)
1966 Porsche 912 Slate Grey, red interior - first owner owned for 41 years
1974 Porsche 911 2.7 (The Manhattan project) viewtopic.php?f=28&t=51455
1973 VW 914 1.7 Olympic Blue - ( gone to a good home )
gridgway
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Re: How does the thermal timeswitch work

Post by gridgway »

So I am pleased to report that my work here is done! Thanks to all who helped and especially Andrew whose googling and investigative skills are far better than mine.

So with the TTS out, I bench tested it and it worked as expected giving about 5 or so seconds of pass through power when cold.
Then I figured out the relay from the wiring diagram using the numbers on the relay to mark the relay socket and trace through the wires. I chose an unused relay on the board in the engine bay (but the one in the right place for the squirter relay). Then I tracked the wires and worked out how to wire it. I tried it out using a test lamp before using it to activate the squirter solenoid. I also tested the squirter solenoid separately and got a healthy click from it.

Then I tested it out for real and the cold starting was loads better - huge amounts. Still took a couple of goes to get it to catch, but previously it took 8 or so goes.

It took an age as I checked, rechecked, double rechecked and finally checked again. Getting electrics and fuel wrong together doesn't sound a good plan.

All in all another good job done. Took it for a test drive, got it warm and yup, still like a scalded cat! In an unusual move, I'm going out for the day to Cadwell on Sunday, I think I'll take it out. More miles in one day than the whole of last year! At this rate, I'll change my mind about selling it!!
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Re: How does the thermal timeswitch work

Post by camperco »

A little advice based on our experience. Make sure that all of the feed pipes are in really good condition before re-activating the cold start system. That is not dry & cracked. Also the supply pipe from the solenoid valve can foul against the MFI pump drive belt. We've seen the fire damage this can cause.

Take the squirters out of the intake pipes and make sure they have the "innards" present. That's a little flat brass plate with a tiny hole in it. Also make sure they are the real thing and not bits of windscreen washer pipe joiners used as a substitutes. We've seen that too and the puddle of fuel on top of the engine.

In short its a pretty dodgy system that can fire fuel all over the engine, take great care.
Angus Watt http://www.greatworthclassics.co.uk

Greatworth Classics & VW Camper Company

Its a feature - not a fault sir..
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Re: How does the thermal timeswitch work

Post by gridgway »

Thanks Angus, that's very helpful and yes I agree it's a system that goes wrong with bad consequences!

I have had it apart and replaced all the feed pipes with new. I have also had the squirters out and all have the innards present and they have a surprising amount of back pressure so they are limiting the flow well. No windscreen washer pipe joiners are present!

Good point about the mfi pump drive belt, I will double double check the routing, but it's a way away from it.

It's quite frustrating in a way as clod starting without it is really painful and with it working is super easy, so it does indeed work!
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