jwarner6

Greetings!

I've been working on my newly acquired '82 928 Automatic for a few weeks now making small improvements as I go. I will provide the various details of this purchase in another posting as they are numerous and I don't want to clog up this post anymore than I have to. So for my first official post, I'm going to try and resolve an issue I'm having with the Thermo Time Switch. Here goes:

I have three Thermo Time Switches. None are known-good as far as I know. One of them, of course, is currently in the car. The Cold Start Valve is not getting 12v when the engine is cold and the TTS is used. If I connect the W pin of the connector to ground, the CSV will fire and the car will start. Testing the switches according to the manual produces identical results for all three switches. So either all three switches are identically bad or the manual is wrong and the switches are fine. And that is basically my question.

Here are the details: All three switches when tested cold (below 86F) produce 0 ohms between the W pin and the switch body according to spec. At the same time all three switches will show 34 ohms between the G pin and the switch body. The manual says this should be 0 ohms just like the W pin. The third test is resistance between the two pins and they all show 38 ohms there which is within spec. I've tried putting two of the switches in the freezer to encourage the G pin to give me a 0 ohm reading and it had no effect.

So, do I have three bad switches or is the manual wrong? I'd be very interested in knowing what results others get when testing the G pin cold or if anyone knows with certainty they're bad. I'd also be interested in theories as to how the G pin influences the function of the switch since it somehow is influencing the ground signal from the W pin.

So there you go, my first official post in this new ether land I've been exploring for a few weeks now. I hope I've provided enough detail (something I tend to do too much some times) and one of you 928 Gurus can eliminate this question from my To-Do list!

Thanks in advance.

Jeff

Randy V

Welcome, Jeff!

There's several folks here with expertise on the old L-Jet system. I'm sure they will provide guidance.

Dennis Wilson

Your ohms won't vary as the circuit works on an on/off heated switch basis. Try heating the switch to make sure you lose the circuit once it reaches about 100F. Not sure about your model but the CIS thermotine switch should not show a circuit to ground from either pin. I do remember an earlier thread that discussed the negative side contact going to ground internally on the later models.

Dennis

Dennis Wilson

Jeff,

Forgot to mention that (IIRC) the cold start valve should get a constant 12 volts (15 circuit?) and the thermotime switch completes the ground when cold. i.e. you may have a bad wire or bad connection at the fuse/relay panel.

Dennis

jwarner6

One of the switches I've been testing came out of a CIS engine. It doesn't appear to be any different and tests out the same as the other two. If I understand what you're suggesting then any resistance would be considered "off". So that would imply that maybe the G pin is turning off the circuit since I'm seeing resistance there if that has something to do with how the thing works. So that makes sense if the test in the book is right and all three switches are bad. Maybe the W pin has the circuit turned on but the G pin is over riding it and killing the circuit. This is what I think is happening.

jwarner6

Okay, but I'm making the cold start valve work by bypassing the switch and taking the W lead directly to ground. So that would imply, I think, that at least the cold start side of the circuit works along with the W side of the TTS plug. There may yet be a problem with the G side of the circuit but if I could just decide if all three of these switches are bad then I could just pony up the bucks for a new one and then see what happens. My FLAPS has said that I could test a new one at the store but I'd have to pay for it first because they'd have to order it. That's why I'm trying to figure out if it's just a fluke that I've got 3 identically bad thermo time switches or if the testing criteria is possibly wrong and it's something in the circuit instead.

Dennis Wilson

Jeff,

The thermotime switch is really just a simple sytem that, when cold, a bimetal switch completes the circuit for 5 to 8 seconds. The wiring is wrapped around the bimetal switch and works as a heater and shuts the circuit off once the temp reaches about 100F. If the ambient temp is above 100F or the engine is warm, there should be no ciircuit and the cold start valve should not operate. What normally causes the TTS to fail is due to the heater portion of the wiring burning through and shorting the hot side to ground. Again, test your switches by heating them to make sure they shut off the circuit when warm. If you are continuously operating/bypassing the cold start valve, it may be the cause of your driveabilty problems.

Dennis

jwarner6

The CSV only runs for as long as the starter is engaged so I don't believe it's introducing enough fuel to cause any problems except for possibly during a hot start condition. Since this setup is only temporary, future potential hot start conditions aren't a problem either. The car will not start with the bypass removed and the TTS connected. Unless of course you give it a shot of ether. The book says the G pin on the switch should show 0 ohms below 86f. At 32f it still shows 34 ohms instead of 0 so it's already off way before it would ever get to 100f. According to the book, both pins should be connected to ground below 86f.

jpitman2

I thought I had an issue with the TTS once, and Wally came in with the readings all the pins should give (search time), which were on the money. My problem actually was the aux air valve was not opening properly, and needed some internal TLC - now works fine to lift idle when cold, where before it would stumble and stall.
jp 83 Euro S AT 51k

Dennis Wilson

Jeff,

You are correct that it runs off the 50 circuit (starter). The 86 F is probably correct for your fuel system. The ohms are staying at 34 since this is a heater circuit as well as completing your ground. Does your connection to ground go open above 86F?

Dennis

jwarner6

Hmmm... So far the aux air valve seems to be doing a good job of correcting but I haven't ruled it out and will efinately pull it next time I yank the AFM. Thanks for the thoughts.

Jeff

jwarner6

Some how you and I keep getting our wires crossed so to speak. I sure do appreciate your sticking with me though. You ask if the connection to ground is breaking (going open) above 86f. The W side of the switch may very well be opening above 86f which would shut off the fuel but the manual says the the G side of the switch should also be grounded until it gets to 86f and it's not. This testing of the switch is done either in or out of the car. The results are the same either way. The 34 ohms I'm seeing are resistance inside the switch between pin G and the switch body (ground) even when it's ice cold. The book says it should be 0 ohms below 86f. So according to the book the switch is bad. The wierd thing is I've got three switches that test exactly the same. That's why I'm questioning whether or not the test in the book is right or what others are seeing if they test pin G to ground when the engine is cold.

Dennis Wilson

Jeff,

Below 86F the circuit is complete (grounded) so the cold start valve sprays. When above 86 the circuit should be interupted (no ground) and the cold start valve should shut off. The attached page may explain it better and there are simplifed testing instructions. Also on the picture the #5 contact is shown as closed so the CSV is powered. When heated, contact 5 opens and there is no power to the CSV.

Dennis

jwarner6

Awesome! Thanks a bunch. Is that a Chiltons or Haynes? It's always helpful to have multiple sources. According to that test my switches fail unless there's no power to the TTS in the first place. No, wait a minute, that's still not right because when cold both pins are grounded. Seeing the internals of the switch makes all the difference in the world. Now I undestand what it's doing. I think. I'm guessing the G pin is the supply side and according to the text I'm guessing it gets it power from the fuel pump relay. But what doesn't make sense is that it looks like the W side is grounded by default unless there's either heat from the engine or 12v from the G pin. If that were true then it aught to work even if it's cold and there's no juice. AARRGGHH!! It would be nice if the pins were labled in the diagram. Still wish I had a new TTS in hand right now. Back to the shop to do some more testing and dig out the schematics.

Thank you so much for sticking with me and helping to clear this up. I feel like I'm getting closer and stupider at the same time.

Jeff

jwarner6

Here are the tests I've been using from the service manual. Thanks to Sharknoob.

_ v _
w ..g


notch on top w is on left g on right


g to ground is .....

under 86* 0 ohms
over 104* is 100-160

w to ground is

under 86* 0
over 104* 100-160

w to g is

under 86* 25-40
over 104* 50-80


I got 34, 0 and 38 respectively at under 86 degrees F on all three switches.