How does the thermal timeswitch work

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gridgway
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How does the thermal timeswitch work

Post by gridgway »

My MFI squirters aren't working so cold start is a bit of a pain.

A specialist has looked and checked the solenoid on top of the fuel filter housing and that works. If you activate that manually, the engine fires up a treat. They say it's the cold start switch. That looks to be the sensor in the housing part number 911 617 117 00. Firstly is that correct?

Secondly, how is it supposed to work and is it testable/fixable before I try and buy a replacement?

Thanks
Graham
gridgway
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Re: How does the thermal timeswitch work

Post by gridgway »

Mind you, is it a timeswitch? When the temp is low enough, the squirters are activated from the starter circuit, so when it cranks? Or only for a certain time when it cranks?
gridgway
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Re: How does the thermal timeswitch work

Post by gridgway »

To bump this to the top, I'm still trying to work out how the fuel control mechanisms work for the 1973 2.4S install.

What I think I have is:

1 squirters - fuel is fed from the top of the filter housing to the squirters for cold start. 12v comes from the starter feed, goes through the temp switch in the top of the housing to the solenoid at the top of the filter housing. Fires in fuel on cranking when the engine is cold enough. So will fire fuel whenever the engine is cranked when cold.

2 enrichment within the MFI pump. Determined by the hot air from the LH heat exchanger to the pump with that disk thingy to enrich the pump.

3 stop/start - there is a solenoid on the end of the pump that needs 12v from the ignition and it stops the pump supplying fuel when there isn't 12v.

Is any of that right does anyone know?

Cheers
Graham
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hot66
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Re: How does the thermal timeswitch work

Post by hot66 »

sounds right ,

you've got the fuel overrun cut off microswitch activated by the throttle linkage. Cuts fuel off via the solenoid at the end of the pump when the throttles are closed via the speed switch ... activates when the rpm is above 1500 and deactivates below 1300 rpm ?
James

1973 911 2.4S
1993 964 C2
2010 987 Spyder
1973 MGB Roadster

Its not how fast you go, but how you go fast ;)
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hot66
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Re: How does the thermal timeswitch work

Post by hot66 »

the thermo enrichment .. it is rich when the pump is cold and the discs have not expanded. As the engine heats up , the discs expand and push the rod into the pump which moves a lever that starts leaning out the mixture.

If the heater hose fails or the circlip pops off the end of the thermo rod the car goes ( instantly in the case of the circlip popping off ) into an over rich rich condition and is undrivable .. useful to know if this ever happens whilst driving
James

1973 911 2.4S
1993 964 C2
2010 987 Spyder
1973 MGB Roadster

Its not how fast you go, but how you go fast ;)
Ian 2.2S
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Re: How does the thermal timeswitch work

Post by Ian 2.2S »

Years ago I bypassed the thermal switch with a little manual push button hidden under the dash.
Ian
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Re: How does the thermal timeswitch work

Post by gridgway »

Would anyone know the parameters of the thermal switch? At what temp does it go open circuit?
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Re: How does the thermal timeswitch work

Post by gridgway »

Ian 2.2S wrote: Tue Apr 04, 2023 12:18 pm Years ago I bypassed the thermal switch with a little manual push button hidden under the dash.
Ian
I can see why that would be a good idea with the switches being £260. I need an hour or two on it to check exactly what's what as cold start takes an age, so I'd love to fix it.
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Re: How does the thermal timeswitch work

Post by jwhillracer »

Ian 2.2S wrote: Tue Apr 04, 2023 12:18 pm Years ago I bypassed the thermal switch with a little manual push button hidden under the dash.
Ian
That was a common racer’s mod on MFI cars back in the day. I have it on our hillclimb car.
As well as the obvious cold start use, it overcomes the problem of fuel evaporation from the injection pump on a hot engine, and refusing to re-start hot.

JW
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gridgway
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Re: How does the thermal timeswitch work

Post by gridgway »

I've had a little look. The switch with the engine cold is closed circuit at 0 ohms so that's a good start. One of the wires goes to the solenoid on top of the fuel filter console so that's a good start. The other wire goes to earth which puzzles me as I expected it to go to a switched positive effectively from the starter being activated. So I need to find the other end of that wire to see what's going on. I'll consult the wiring diagram to see where it goes. It could be just a wiring problem not a switch problem. I will need to test the switch to make sure it goes open circuit when warmed up if I fix the feed wire. Otherwise I'll always squirt when turning over even with a hot engine.
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Re: How does the thermal timeswitch work

Post by Nine One One »

gridgway
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Re: How does the thermal timeswitch work

Post by gridgway »

I think I am a bit punch drunk on links! I've got a wiring diagram which I'll track through. On first glance it looks to have the compoanents on that I can see on my engine. Need to look at it afresh tomorrow!
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Re: How does the thermal timeswitch work

Post by hashman »

Ian 2.2S wrote: Tue Apr 04, 2023 12:18 pm Years ago I bypassed the thermal switch with a little manual push button hidden under the dash.
Ian
This is exactly what i did on mine also.
Glad to see it wasn't just my cheap arse ways..... :lol:
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gridgway
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Re: How does the thermal timeswitch work

Post by gridgway »

gridgway wrote: Tue Apr 04, 2023 7:16 pm I've had a little look. The switch with the engine cold is closed circuit at 0 ohms so that's a good start. One of the wires goes to the solenoid on top of the fuel filter console so that's a good start. The other wire goes to earth which puzzles me as I expected it to go to a switched positive effectively from the starter being activated. So I need to find the other end of that wire to see what's going on. I'll consult the wiring diagram to see where it goes. It could be just a wiring problem not a switch problem. I will need to test the switch to make sure it goes open circuit when warmed up if I fix the feed wire. Otherwise I'll always squirt when turning over even with a hot engine.
It looks from the wiring diagram that the other wire is supplied with 12v from a relay, so it could well be earthed when not switched to 12v. I should measure it when cranking to see if there are any volts.

Graham
gridgway
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Re: How does the thermal timeswitch work

Post by gridgway »

Hopefully there are still people with the patience to follow this and help as I have been diagnosing further but still stuck. Here is a snippet from the wiring diagram that I think is right for the installation...
Image
It shows the thermal time switch (TTS).

The yellow wire (called G in wiring colour code) is the positive that comes when you crank the starter. I have now checked this out and lo and behold it does (tick).
The Red/black wire (called W in wiring colour code) goes to the squirter solenoid. I have used a sep 12v battery and the solenoid clunks (tick). It has one terminal and the body is earthed.

So the switch passes 12v to W in reference to earth to activate the solenoid. This should only be for a short period to prevent flooding (hence it being a timeswitch).

I have taken out the TTS to test it and found a handy page on Pelican where someone shows their test setup here http://forums.pelicanparts.com/911-engi ... stion.html

My switch works as his does using the test config he describes. The test light comes on for a few seconds then goes off - perfect.

However there is a difference. The test light is connected between the two terminals of the switch G and W. For my installation (as shown on the wiring diagram) the 12v is supposed to come from W and earth. If you do a test in that config, you get a weak light that doesn't go off.

Then if you go to design911 to look up the part it says "this switch has a design change and might require minor modifications (Direct feed from battery)". I don't know what that means.
Here https://www.design911.co.uk/p/porsche-9 ... 161711700/ If you look at the PET picture it gives you, the TTS is bolted into the chain cover not into the housing on the top of the engine. It makes me think the part was redesigned for later cars.

The squirter solenoid can't be wired across G and W is it uses its body as the earth.

And just to add, I don't think the squirters have ever worked since the engine was transplanted in, not that something has failed. I don't know what the arrangement was in the car it came out from.

I know one answer is to bypass the TTS and run a wire to a switch or button in the car, but I really don't want to do that! Single wires added in is my pet hate!
I could just live with the squirters not working, but it takes an age to start when its cold.

I am chasing my tail a bit on this one. All help welcome!
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