1969 | 318 | 904 - reverse lights not working

[MaximRecoil]

They used to work so the wiring configuration is correct.

I've bench tested the bulbs in their housings with a battery and they both work fine.

When they are installed in the valance panel, they both have a good ground (perfect continuity between the housings and the rear bumper).

I've tested the power connector for 12 volts when the engine is running and the transmission is in reverse and there's no voltage on either connector.

The old neutral safety switch was flaky (the engine would always start in neutral, but sometimes it wouldn't start in park), so I replaced that with a new one (Standard NS-11), and it works as it should with regard to only starting in neutral and park, but it didn't fix the reverse lights.

Assuming that whatever mechanism inside the transmission that activates the switch for the reverse lights is working, and assuming the new NSS isn't malfunctioning, that only leaves a broken wire and/or a bad connection at a terminal somewhere.

I'd like to check continuity of the wiring, but I don't know where the terminals for the power wire for the reverse lights are. One of the three terminals in the NSS plug must be one of the relevant terminals, but which one? I'm guessing there's a relevant terminal in one of the three bulkhead connectors too, but I don't know which one.

[Nacho-RT74]

You can jump out both outer ends of the NSS plug with key in RUN or ACC. It should light on. If not, problem is on wiring

One of the wires gets 12 volts from fusebox, the other runs to reverse lights. Both are black, one traced and one solid. They use to become white into the cab, same one solid and one traced. Then on rear body harness becomes violet.

Solid color wire is tipically the one what is energized, althought it could be reversed and will work anyway.

Center wire ( brown with yellow trace ) is the one running to starter relay for safety ground in N or P

[MaximRecoil]

You can jump out both outer ends of the NSS plug with key in RUN or ACC. It should light on. If not, problem is on wiring

That's interesting. On the NSS itself, the two outer terminals/prongs normally have continuity with each other (i.e., normally closed), and pressing the metal pin/button on the bottom of it breaks continuity. So if shorting the two outer terminals of the plug sends power to the reverse lights, that must mean that the NSS's metal pin/button is always pressed in except when the transmission is in reverse.

I'm willing to bet that the lights won't come on when I short the two outer terminals of the plug together, because both my old NSS and my new NSS are working properly with regard to the switching function of the two outer terminals/prongs (I tested them both with a continuity meter), so both of them should have already been shorting the outer two wires together when in reverse.

One of the wires gets 12 volts from fusebox, the other runs to reverse lights. Both are black, one traced and one solid. They use to become white into the cab, same one solid and one traced. Then on rear body harness becomes violet.

I forgot to mention that I tested all of the fuses with a meter and they are all good. It definitely seems to be a bad wire, which could be the wire from the NSS plug to the reverse lights, or the wire from the NSS plug to the fuse box (do you know which fuse it is?), or whatever wire feeds the fuse.

Solid color wire is tipically the one what is energized, althought it could be reversed and will work anyway.

Center wire ( brown with yellow trace ) is the one running to starter relay for safety ground in N or P

Yeah, I know that wire. I've disconnected it before and plugged a dedicated ground wire in its place to bypass the old flaky NSS.

[Nacho-RT74]

That's interesting. On the NSS itself, the two outer terminals/prongs normally have continuity with each other (i.e., normally closed), and pressing the metal pin/button on the bottom of it breaks continuity. So if shorting the two outer terminals of the plug sends power to the reverse lights, that must mean that the NSS's metal pin/button is always pressed in except when the transmission is in reverse.

I'm willing to bet that the lights won't come on when I short the two outer terminals of the plug together, because both my old NSS and my new NSS are working properly with regard to the switching function of the two outer terminals/prongs (I tested them both with a continuity meter), so both of them should have already been shorting the outer two wires together when in reverse.

Correct, the NSS is normally closed. The "roostercomb" into the tranny gets a nylon skate with a depression. The skate keeps the NSS contact pressed except on the depression when allows the NSS to be released and close the circuit. You can even play with the old NSS on the air plugging it into the NSS plug and press and release the contact.

The center pin at the same time being metallic provides the ground too for the starter relay. The "roostecomb" gets two metallic areas which matches with P and N to provide the ground via the NSS center pin

I forgot to mention that I tested all of the fuses with a meter and they are all good. It definitely seems to be a bad wire, which could be the wire from the NSS plug to the reverse lights, or the wire from the NSS plug to the fuse box (do you know which fuse it is?), or whatever wire feeds the fuse.

Sure I can tell where is the wire for 3rd gens, but not on 2nd gens, but if we take appart to check the wiring diagrams, if you look on back of the bulkhead it should be a solid white wire and THE ONLY solid white wire running throught the bulkhead. There are not other white wires running through the bulkhead than the back up wires. The traced one will be the one coming back from NSS.

If you don't care to laydown the dash and reach the cavity where the white wire is you can check it straight there. Remember with key in RUN or ACC

[Nacho-RT74]

Cavity D... on wiper plug harness. On diagram shows is the same wire/fuse coming from Radio plug ( spliced from there up to bulkhead )

[MaximRecoil]

Thanks for the information and picture. For starters, I discovered today that my new NSS sometimes doesn't start in P, same as my old one, so it must be a problem with the "rooster comb," i.e., its metallic area in the P position must not be making good contact with NSS's center pin.

As for the reverse lights, the first thing I did was unplug the NSS plug from the new, installed NSS and then plugged it into the old NSS to short the two outer terminals together. Then I turned the key to the run position. As I suspected, the reverse lights didn't come on. Then I tested continuity between each of the outer sockets of the NSS plug to the reverse lights' power plugs, and there was no continuity. Then I noticed, up by the bulkhead connector, there was a black 2-prong plug that was disconnected, and it was part of the NSS harness. It was obviously the problem, because that plug feeds power to the reverse lights. I don't know why I didn't notice it before.

I plugged the black connector back in and then started the engine, put the transmission in R, and still no reverse lights. That was strange, because they should have worked at this point. Then I repeated my first step (unplugging the NSS plug from the installed NSS and plugging it into the old NSS with the key in the run position), and the reverse lights came on. That means that everything is working correctly, except, apparently, the new NSS isn't engaging the rooster comb properly. I know that the new NSS's switching function of its two outer prongs works fine, because I tested it with a meter before installing it, so the rooster comb must not be releasing the NSS's center pin when the transmission is in R.

Since I know for a fact that the old NSS used to work with the reverse lights (I'm guessing that the only reason it stopped working is because that black plug by the bulkhead connector got disconnected at some point), and since the new NSS didn't fix the sometimes-doesn't-start-in-P issue anyway, I'm going to put the old NSS back in to see if that fixes it. I need to wait until my mechanic friend is home to try it, because I don't have a 1" socket for the NSS.

[Nacho-RT74]

it could be a linkage adjustment problem.

BTDT

and about remove the NSS. Thats a piece not really torqued, so an adjustable angle plyer like this could make the job:

[MaximRecoil]

Wouldn't the linkage adjustment only affect how well the gear that the transmission is in corresponds with the shift lever indicator that points to either P, N, R, 2, or 1? In other words, if the transmission is actually in reverse (evidenced by pressing the gas pedal and the car goes backwards), shouldn't that inherently line up the rooster comb correctly regardless of how the shift lever linkage is adjusted? In my case, the shift lever indicator corresponds perfectly with the actual gear that the transmission is in. Also, the reverse lights used to work, and nothing has changed with the linkage adjustment. I don't know when they stopped working because I don't drive that car at night very often. Since I suspect that the plug by the bulkhead connector coming undone (it doesn't fit very tightly into its socket) is what originally caused the reverse lights to stop working, the first thing I'll try is putting the old NSS back in.

It's hard, or impossible, to get at my NSS with a wrench (due to various things being in the way), and I think it would be even harder to get at it with Channellock pliers. By maneuvering around the transmission lines and other obstructions, I can just barely get at it with a socket and short extension.

Edit: I just came across this comment on a YouTube video, which might be the same thing that's going on with my car right now:

Having an issue swapping out the neutral safety switch on a 727....parts store part, looks just like the one I have now, though plastic plunger may be slightly longer. Car will start fine with it but reverse lights will not kick on with the new switch installed. Old switch is fine but was leaking, possibly at the seal, so I figured just put a whole new one on rather than just a seal......the old one has a seal that is thicker, metal reinforced.....the new one is just a rubber seal. Any ideas?

[Nacho-RT74]

The spot for the P and N ground provision is so small that could be a linkage adjustment. Compare with the reverse WIDE spot area. Aside this, linkage is old and could got some play along its assembly.

BTDT. Just a small allmost unapreciable adjustment could fix the P ground

Yes, there are two NSS switches, but they are noticebale diff outside being shorter and I think thats for laters. If the one you got seems to be the same outside with the old one, I bet is the same also on the inner side.

About the plunger lenght... if there was some problem on this with reverse light, it could be anything but not they won't turn on. Remember the NSS is normally closed and the depressed area on skate at "rooster comb" it will alow to release the contact to close, while will keep pressed on the rest of track. I could think a shorter plunger won't reach the skate and will keep reverse lights on, but even if the plunger is longer, will reach the skate and open the circuit except on the reverse depression area. i can't think on what was happening with that guy at youtube.

Plunger as far I recall gets a preload spring if eventually reachs the track and you still have to get the NSS tight to seal with transmission housing, so who cares if its a tad longer?  Will seat over the skate the same.

On the other side, I can't recall have removed my NSS with a socket ever LOL. I'm allways on a lack of tools and tipically use whichever I have on hands. NSS can be actually installed and removed with hands, so the tool will be just to tight or get it loosen. So maybe half or 3/4 of turn will make the job. Just saying.

[MaximRecoil]

Yes, there are two NSS switches, but they are noticebale diff outside being shorter and I think thats for laters. If the one you got seems to be the same outside with the old one, I bet is the same also on the inner side.

My NSS plus won't fit on the short one anyway (which is from the 1980s).

i can't think on what was happening with that guy at youtube.

Whatever happened with him is the exact same thing that happened with me. I reinstalled the old NSS today and that fixed the reverse lights. My new NSS looked identical to my old one, and it tested good with a meter (perfect continuity between the two outer terminal prongs, and pressing the center pin/button broke continuity, i.e., a perfectly-working NC SPST switch), but for whatever reason, it wouldn't activate the reverse lights when it was installed in the transmission.

On the other side, I can't recall have removed my NSS with a socket ever LOL. I'm allways on a lack of tools and tipically use whichever I have on hands. NSS can be actually installed and removed with hands, so the tool will be just to tight or get it loosen. So maybe half or 3/4 of turn will make the job. Just saying.

When I first crawled under there the other day, I had a 1" wrench with me, and I couldn't even get it onto the NSS, due to transmission lines, the bottom of the transmission itself (which juts out, preventing you from getting a wrench on it from directly underneath the NSS), and the underside of the floor pan directly above it, being obstructions. I may have been able to find an angle where I could get a wrench on it, but I didn't need to, because my mechanic friend has a thin-wall 1" socket that made it easy.

[MaximRecoil]

About the starter sometimes not cranking in park: I'd always figured it was a random occurrence, but recently I noticed a pattern to it. It only happens when I park somewhere that's completely level so that the car doesn't roll forward or backward at all when I put it in park. I tested it today by finding a level spot to park, turned it off, and sure enough, nothing happened when I turned the key to start it. Then I got out and pushed the car forward a couple inches until the rear wheels locked due to the transmission being in park, got back in, turned the key, and it started right up.

The reason it doesn't happen very often is because when I park the car, the ground usually isn't completely level, so it rolls forward or backward an inch or two until the rear wheels lock.

I don't know much about the internals of a 904 transmission (nor any other type of transmission, for that matter), but the parking pawl must be connected to that rooster comb thing, and the rooster comb's park-position ground contact must only be making good contact with the NSS's center pin when some level of force is being exerted on the parking pawl.

[John_Kunkel]

Check for worn/missing plastic bushings in the shift linkage. Missing/worn bushings can cause slop which makes gear selection inconsistent.

[ACUDANUT]

Or, just watch where your backing.