Captain Obvious

Chuck, I normally make it to this show, but I don't think I'm going to make it this year. I'm nursing a recently attained sports injury and am putting most of life on hold for a little while. Else, I would take you up on your offer.

It's clear from the scratch marks on the little metal tabs of that switch that someone has opened that thing up in the past. I'm sure it's all related. Maybe the wire's solder joint was weakened by that process, Maybe they took it off completely during that process and didn't do a good job of putting it back on. Maybe the headlight side rocker inside the switch was burned beyond recognition so they swapped the one from the running lights over. There might not even be a rocker on the running light side if that's the case. Maybe they took that wire off intentionally to use a separate mounted switch somewhere else. Just thinking out loud as to why that switch and wiring is the way it is now. I think you have identified the "what". I'm just musing about the "why".

Glad to help. All that research done, however, I have little confidence of exactly what style (year) will arrive when you order from rockauto. The most important takeaway is that it probably doesn't matter. So, about the gas tank... Didn't you already have the tank done? Why is it costing you more money?

My pleasure. Glad to help. Steve and Dave know the system inside and out, but at least I can do that much.

Haha! Actually, I stand by my answer. The OP was asking about the brake light switch, not the T/S switch. I was responding to the question about the brake light switch when the OP raised the possibility that the issue resided there: "I wonder if the brake light switch under the dashboard is the issue? I can see that the switch has two wires coming out of it. Is one for the left side and one for the right?" Adding that clarification to my answer... It's not the (brake light) switch. The two wires are not split like that. Both wires work both sides. If either side works, it's not the (brake light) switch. I'm just trying to save him the 23 bucks on the brake light switch he linked to at MSA. >> (link to the page here) << That's not what he needs. And in any event, I'm glad you guys are on it. He'll be fixed up in short order!

I'm back and had a few cycles to look into the EGR systems. Well guess what! They made a change in the thermal vacuum valve between 77 and 78. Short story is that (I would claim) they are interchangeable. Longer detailed story goes like this: The 77 TVV is a bang-bang digital switching device. Should be closed up to about 120F and stay open at any temp above that. (EC-11 and EC-12 of the 1977 FSM) The 78 TVV, on the other hand, is a more gradual opening device and it should also be closed up to about 120F and open above that, but there is a THIRD state for the 78 TVV where it will close again if it gets too hot. The 78 TVV will close at temps above about 200F, while the 77 version will stay open. (EC-11 of the 1978 FSM) Note, however, that in normal operation and a 180 degree F thermostat, the TVV should never see temps that high. This an overheated state. It seems that for whatever reason, they decided it was a good idea to cut off the EGR system if the engine got too hot. So as long as you don't overheat your engine, the 77 and 78 thermal vacuum valves should operate pretty much the same and be interchangeable.

It's not the switch. The two wires are not split like that. Both wires work both sides. If either side works, it's not the switch. I'd look for rust or corrosion inside the bulb socket.

Well I tested my original EFI temp sensor and my new replacement (with the offset tab) in a pot of water on the stove yesterday. The results are that I believe the replacement is close enough to the original to be considered a suitable replacement. I've got graphs and charts and numbers and stuff that I will post when I get more time, but unfortunately I'm headed out of town now and won't be able to get to it for a couple days. In the meantime, I can tell you that the sensor I bought was AIRTEX / WELLS5S1523 (SU4115). Interesting that the pics on rockauto don't have any tab, but the part I have definitely has the offset tab.

Yeah, it looks like the one I tagged was for 78. I don't know what the difference is, or if it matters. My instincts agree with yours in that it doesn't matter, but unfortunately I'm headed out of town now and won't be able to dig into it for a couple days. If you haven't figured it out, I'll investigate more when I get back. How many vacuum tube ports are (were?) there on your busted one? Two or three? The only thing I can think they would change would be a fine tune of the switching temperature. And if that's the case, I don't think you would care.

Haha!! Understood!

As it should be. The EGR runs off a ported vacuum source in the throttle body that is only active at a relatively low pedal position. Because of this, the EGR is only activated at "light cruise". It is completely disabled at both idle and anything beyond that light cruise position. In other words... The EGR is designed to incur no performance impact.

Haha!! Well SOMEONE had to ask, right? It could be BOTH, you know!!

Woah, I think my buddy paid less than $200 for his glass. Not sure if it was NOS sitting in a warehouse somewhere, or if someone is still making them new. But I'm positive he didn't pay anywhere near $600!

Am I sure? Not really, I guess. I bought one off rockauto a couple years ago because it was cheap and it came from the same location as something else I was buying so the shipping was free. If you look at rockauto, they have about ten different offerings for this sensor and some of them have the center tab, some have the offset tab, but most of them have no tab at all. The temperature curve would matter more to me than where the tab is. I'd just file the tab off and be happy. But if the temperature response is wrong... That would be an issue.

So, about that thermal switch thingie... Yes, it's all related to the EGR system. When the engine is cold, it blocks the vacuum to the rest of the system (thereby disabling it), and once the engine warms up, it will allow the rest of the system to operate. So, it depends on how much you care about the EGR system, but I can give you one other piece of data to help make your decision... Here's a replacement thermal vacuum switch on rockauto for $4.15. It's a mfgr close out. Buy two. https://www.rockauto.com/en/moreinfo.php?pk=3647541&cc=1209260

Oh, he's gonna make it. He has to bring me my brake booster!

Sweet. Glad that worked out. So, I've been meaning to ask... Is CRS your initials, or an acronym for a mental condition?

I wouldn't make any assumptions about the glass availability. A buddy of mine just replaced his this past winter and paid a very reasonable price for a brand new one. I don't remember the cost, but I do remember that I was surprised it was as low as it was. More expensive than a used one from a junkyard, I'm sure, but still way less than I would have expected for new.

Well that sucks. Did the shop that did the work at least acknowledge that they screwed up? You think that Z in the scrapyard has a drain plug?

Yeah, I also presume the tab location was to try to prevent improper connection. Which makes it odd that my car has the centered tab on both the temp sender and the thermotime. It makes much more sense to have those two devices keyed such that it would be difficult to connect them wrong. Maybe a PO switched mine around somehow. In any event, the reason I'm messing with mine right now is that I removed my cold start injector years ago, and (since it's now useless) I'm removing the thermotime as well to simplify things. In fact, I'm replacing the whole thermostat housing with one from a 260 because it's simpler and has fewer holes.

Yeah, don't be chicken about reaching out for someone local with experience. If I were closer, I'd be happy to help.

Zcon in Atlanta is middle of October. 16th through the 21st. Good luck with the Z today. Hope you get her fired up!

If the power brake booster is dry and clean inside, I would be interested in that. The U-Pull yard near me charges $19.50 (with core) for a booster. If he's anywhere near that and it's clean inside, I'll take it. I'll pick it up when we meet in Atlanta at ZCon!

Well that's interesting. My car has no offset on either the sensor or the connector. They both have the tab centered. The connector is a new one I put on when I rebuilt the entire harness, but I'm assuming the sensor is original. Now... Adding to the confusion, is that my new sensor (which I haven't installed yet) has the offset tab. That means I'll have to make some change as well. I'll either have to change the connector over to one that has the offset slot or modify something like you mentioned. However, I think it would be a lot easier to grind the male tab off the sensor body instead of trying to grind a second slot into the connector housing. Even easier might be to pull the connectors out the back of the connector shell and get a replacement from a junkyard. Most of that style of connector that Bosch used is interchangeable between bodies. Been a while since I looked at the details of those connectors, but maybe cut a connector off any car that has the slot where you want it, take the old pins out, and snap your existing pins into the body?

Would I believe it? Absolutely. Haha! The sensor shell is what they call a "thermal well". It's essentially a dry cavity that is inserted into whatever fluid you are trying to measure. The well keeps the sensor segregated from the fluid. That sensor actually makes it's seal at an angled face on the body about halfway down the shank. The threads are on the dry side. As for the EFI temp sensor, I'm messing with one right now. I had bought a new sensor a while ago and I'm just now putting it in. The new sensor has an offset tab on the body, but my original one has the tab centered. And the groove in the mating connector on my car is in the center as well (matches the old sensor). Does your car have an offset groove on both the connector and the sensor?