CIS Thermo Time Switch ????

[patkeefe]

As part of my rewiring project, I am checking my Thermotime switch, 78 SC. I have continuity across the switch (It is in my bench vise, cold). I also have continuity across either terminal and the switch body. Neglecting any effects of the 3 watt heater built into the switch, I would imagine the switch should be open circuit when warm.

So when I heat it with a propane torch, and look at the ohmmeter as I heat it, it should go to open circuit when it reaches the 45C threshold, i.e., warm start conditions. It doesn't.

Can I assume the switch is bad? Also, which terminal is 15 and which is W on the switch. One has a big connector and one is smaller?

TIA
Pat

[spuggy]

Pat, I can't answer your questions off-hand (although your test sounds valid to me), but I'd mention that the side connector on mine is covered, and the nipple on the top has one of those bare funky square connectors. So I'd guess that the side is "hot" and the other (the top) isn't...

But I'd go further and say "don't bother".

My understanding, this runs the cold start valve (CSV) injector for a maximum of 30 seconds on a cold start.

I ran my car last winter with it disconnected (hard pipe for the intercooler prevented getting to it to re-connect it), and it started just fine without it. Even with ice on the windows.

It's connected now, and it makes absolutely no difference that I can detect.

[boyt911sc]

Pat,

The black/red wire is attached to the larger connector at terminal W. The smaller connector with yellow wire is hooked up at terminal G. This yellow wire is also connected to CSV and terminal 15 (starter). Hope this could help.

Lastly, I really appreciate your generosity for allowing me to visit your garage and spent that time explaining the operation of your turbo engine. The 250 plus mile drive under pouring rain was worth the trip. Thanks.

Tony

[Early_S_Man]

This diagram may help ... Terminal 15a goes to a special second set of contacts in the starter solenoid that replaces an external relay.

[patkeefe]

Tony:
Thanks, that helps a lot. BTW, stop by anytime, I'm always happy to try and help out.

Warren, Thanks once again. The current flow diagram helps a lot. My Bentley has only the old style wiring diagrams.

Spuggy, I didn't care about cold start, until I blew my airbox up last week. That kind of got my attention

Pat

[Mysterytrain]

Pat,
It is my understanding that one of the connections [W ] on the tts is the switched ground for the Cold Start Valve and the other [ G] is 12 volts for the internal heating element. G comes from the starter solenoid contacts. When the engine is cold the thermotime switch provides a ground path for both the CSV and the tts internal heating element. Above the temperature set point the switch is open and there is no ground for the CSV or the internal heating element. Additionally, when the engine is cold, the tts is providing a ground for the heating element and heating the internal bimetal switch. When the switch gets hot enough it opens and the ground connection is lost.

The testing a cold tts should show a zero resistance between the tts case and the W terminal. You should also see some small resistance between the G and W. That should be the resistance of the heating element. Now, if you apply 12 volts to G and then repeat the measurement between W and the case the connection should open. [might take a few seconds] . Physically heating the tts case should also open the ground connection.

[patkeefe]

Thanks Ron:
I'll try to do a power on check tomorrow. I'm also going to fix my AAR...thanks to the great repair write up you did on the AAR a while back.

Still, when the TTS is hot, it should be open circuit. The heating element is only providing heating if 15 is energized (starting), if I read that correctly?
Pat

[Mysterytrain]

Correct, when the tts is hot the ground switch opens and the Cold start valve won't work nor will the heating element. Those Germans were pretty slick, if the engine was warmish then it wouldn't need much fuel enrichment and the heating element wouldn't need very much time to generate heat to open the ground connection. Basically, the heating element is there to prevent flooding the engine.

[patkeefe]

Slick is right, this is another example. The switch is only 45C, which isn't exactly scalding hot. I don't know how long it would take the 3W heater to overcome 45C internally, but I would guess the CSV could squirt for maybe 20 revolutions of the engine before it flodded? Maybe I'll just wire a switch in the car to energize the CSV by hand, and scrap the TT switch altogether.

Pat

[tom1394racing]

I used this test procedure originally posted by Warren to check one of these recently.

The Thermo-Time Switch could be bad, and should be tested prior to trying to wire anything to it!

The 'G' terminal is the heating coil, and should measure 15 - 25 Ohms to the body [ground] of the switch.

The 'W' terminal is the switched ground signal for the cold-start valve, and should measure less than 1.0 Ohm to ground [switch body] when the switch temperature is less than 113°F, and should be open [no continuity to ground] when switch temperature is above 113°F.



I wired the W terminal on the switch to an ohm meter and placed it in a pot of water on the stove. The resistance switched as the water heated up past 110 deg F.

[boyt911sc]

Originally Posted by patkeefe

Maybe I'll just wire a switch in the car to energize the CSV by hand, and scrap the TT switch altogether. Pat

Pat,

Replacing the defective TTS and installing a new one will only take a few minutes. This is a more prudent course of action than by-passing this TTS switch with another switch. I have no doubt you could install the switch considering your excellent craftmanship and skill. The final decision is yours but I beg to disagree with your proposed action. Just my two-cents.

Tony

[patkeefe]

Tony, you're right. I'll get a new switch.
Tom, you have an email.

My ohmmeter was all over the place when I heated the tts on the bench vice, but it never went to open circuit.

Pat