Thermo-time switch questions (911.617.117.00)

[bmcfarland]

Does anyone know what typically fails in the thermo-time switch? I have tested a couple of used ones and think the contacts may be fused.

I say that because using the test method shown in the attached diagram (let me know if it has any flaws), the W terminal grounding circuit shows good. When I apply 12V volts to the G terminal heating element, it does not open the contact (W stays grounded) after a time period lasting much longer that 8 seconds. Also used a hair dryer to heat up the switch body. Same thing - W stays grounded indicating the bi-metal strip is not flexing and contact is closed when it should be open.

I very well could be missing something obvious and fundamental. Let me know if I am.

Used switches may be readily available for a reason. Before I invest in a new one, just hoping someone might be able to identify any errors in my test method and shed some light on what typically fails.

Thank you!

[r lane]

Originally Posted by bmcfarland

Does anyone know what typically fails in the thermo-time switch? I have tested a couple of used ones and think the contacts may be fused.

I say that because using the test method shown in the attached diagram (let me know if it has any flaws), the W terminal grounding circuit shows good. When I apply 12V volts to the G terminal heating element, it does not open the contact (W stays grounded) after a time period lasting much longer that 8 seconds. Also used a hair dryer to heat up the switch body. Same thing - W stays grounded indicating the bi-metal strip is not flexing and contact is closed when it should be open.

I very well could be missing something obvious and fundamental. Let me know if I am.

Used switches may be readily available for a reason. Before I invest in a new one, just hoping someone might be able to identify any errors in my test method and shed some light on what typically fails.

Thank you!

here is a cross section. My understanding is this; The coiled link is grounded and power is supplied to it. It heats up forcing the bimetal link to open. That link would have sent a ground signal to the cold start valve under cranking until the bimetal link opens. Bob

[wgirardot]

I did this exact bench test on 2 thermo-time switches. 1 from my existing car, and another a used spare I bought ( seller claimed tested and good ). On both tests according to your test diagram they passed the 1st test. But on the second test, they both failed ( light didn't go out at all after waiting quite some time ). I used a battery charger instead of an actual battery; could this explain failing 2nd test? Please, anyone advise

Bill

[bmcfarland]

Thank you. Bill. I appreciate your post. For my bench tests, I used a battery and separately tried a charger (one that does not sense if a battery is connected, or if it is LI or LA). My results were the same as yours, thus the reason I am questioning the test method.

Also checked resistance, but the acceptable resistance ranges in the Bosch catalogs are for Bosch Thermo-time switches, not VDO. And I prefer a bench test that more directly reflects operational conditions.

[boyt911sc]

Ben,

Set-up #1 works because you needed a ground to lit up the TEST LAMP. Using set-up #2 to energize the heating element will not work. Why? The wire going to G(+) has no power. All you need is to re-arrange your wiring connections and you will be able to test a TTS reliably.

Below is the wiring diagram of my TTS & CSV Tester.





Tony

[bmcfarland]

That’s what we were looking for, Tony. I used your method. It works. Thank you!

To avoid confusing others, attached is an updated sketch showing Tony’s testing method. If you don’t mind, Tony, please double check my work and everyone else please disregard the one I previously posted.

[boyt911sc]

Ben,

I wish I had your computer skill. Imagine replacing the test lamp with the CSV in your illustration and adding the starter/solenoid too. This will be a very helpful illustration for some people intimidated by schematic diagrams. Nice work.

Tony

[bmcfarland]

Thank you, Tony! My car is a ‘72 so when I started troubleshooting, I marked up a ‘72 wiring diagram to help me understand the enrichment system and how it works. However, more than happy to take a stab at sketching out what you describe (taking out the two, 14-pin connectors might get us closer).

I’m also interested in understanding how I got the test wiring wrong in my first attempt. If you zoom in on the Thermo-time Switch, the way it is shown seems to indicate the G terminal and heating element is an isolated circuit that grounds to the casing, thus the reason I initially thought applying power to G terminal fired up the heater.

[wgirardot]

This is awesome. Your second corrected diagram verifies that both my thermotime switches test good on the bench ! Thanks


Bill

[bmcfarland]

Yes, sincere appreciation for @boyt911sc and others like him who really know their stuff and willingly share it with others through active participation in the forum!!!

[bmcfarland]

Originally Posted by wgirardot

This is awesome. Your second corrected diagram verifies that both my thermotime switches test good on the bench ! Thanks


Bill

To Bill, Tony or anyone else that has performed this TTS test: At garage temperature of 85°F±, the test light turned off in about a second. I placed the TTS in the freezer (0°F±) and it turns off in approximately 2 seconds.

Do these durations sound right?

Thank you!

[wgirardot]

Both of mine turned off in about 1-2 seconds at ambient temp in garage of about 75 degrees. I’ll trust it now to work as designed regardless of whatever the ambient temp outside, hot or cold


Bill

[bmcfarland]

Thank you, Bill. Sounds like our bench test results are similar, signaling the cold start fuel enrichment operation is very short in duration. To reinforce that, using a voltage tester I separately checked the duration that power gets sent to the enrichment solenoid after the key is turned to start. It was also about a second.

So it appears the fuel enrichment system is working as designed. Additionally, I am experiencing a quick and smooth cold start when I use the cold start procedure Porsche recommends in the ‘72 Owner’s Manual (hand throttle full up).

However, one final issue to figure out: I am getting some fuel carrying over and leaking from the exhaust system, but apparently not enough to flood the engine. Closely checked the entire fuel system. No leaks.

Does anyone have any theories, possibly the cold start solenoid? I have applied power to the solenoid terminal. It clicks. Is it possible that the solenoid is engaging, but may be worn and “sloppy” thus sending too much fuel?

[wgirardot]

I am going thru entire cis understanding as well. These Bosch/Porsche/VDO components are incredibly reliable. My car just isn't happy until it is fully warmed up ( I guess just the nature of the beast ).

Bill

[bmcfarland]

Thanks again to Bill and Tony for their input. As mentioned above, the cold start fuel enrichment system components all tested good. The car typically starts quick and smooth when I use the cold start instructions in the ‘72 Owner’s Manual (hand throttle full up). However, I am experiencing some fuel carrying over into the exhaust system, but not enough to flood the engine.

As a final check of the cold start enrichment system to definitively rule it out as the source of the fuel carry-over problem, I installed temporary tubing on the enrichment solenoid outlet and collected approximately 50ml of fuel. That sounds about right.

So my fuel carry-over issue is not related to cold start fuel enrichment system, I will expand the trouble shooting to other areas, and wanted to bring a reasonable end to this thread which started as a thermo-time switch testing query.

Thank you!

[bmcfarland]

. . . For additional comparative reference, these are the multimeter resistance numbers I measured on a switch that tested good using the 12V function test mentioned above:



Note: In multiple publications, Bosch provides in-spec resistance values but specifically states they are for Bosch thermo-time switches, thus do not apply to Porsche-VDO switches. I unsuccessfully tried to secure similar resistance range data from VDO and separately from Porsche Classic. Both claim the information is proprietary.

[garment]

One other thing to mention for those having cold start issues. The wiring to the area of the engine where the thermotime switch lives sees a lot of action - heater tube for those running heat has to come out when adjusting valves, for example, and the connections to the thermotime can get rubbed/jostled easily. In my case, the wiring at the point of the right angle spade connectors looked ok, but def was not. I replaced both spades, and no more cold start issue - signal restored to the cold start valve, clearly.

Good advice for all 40-year old wiring, I suppose.

[30KFT]

Great information! Thanks to all. My VDO TTS dated 7/77 matches all the info listed in bmcfarland's chart EXCEPT my TTS unit is stamped as 12V/3W. That is quite a lot lower wattage than 40W listed in the chart. Is the 3W unit the proper TTS for my application? Application is a 11/75 3.0 turbo w/stock CIS.
Thanks, fellows-

[wazzz]

I may be totally wrong, but I think the 3W you mention is the heater power, since there is a heating element inside this switch.
The 40W mentioned in the table above might be the max load that can be switched with this unit. So under 12V that should allow up to 3.3A.
Nothing sure, just guessing.

[30KFT]

Thanks, Gilles,
30W seemed like a lot to me. Nevertheless, I tested mine using bmcfarland's schematic and found that my unit is within spec. I have mixed emotions about that since it is a wretched job to get to it. It seems weird to be disappointed when something turns out to be working. It must have been changed by a PO since it is dated 7/77.
tat