Dome Light Curiosity

[ckurtz2]

Sounds like I will be the source of amusement tonight for some of you.

Decided to tackle why my dome light was not working. First made sure the switches at the doors were pushing in and out. Sprayed some deoxit in there and they freed up pretty nice and looked shiny. 

Next went to the dome light and measured voltage. I saw I was getting 12V between both terminals at the light bulb so I thought that was odd. I then measured voltage and then pushed in the door switches. Volts dropped to zero like I had hoped they would. Ok, so this made my brain all fuzzy why the bulb was getting 12V, but wouldn't turn on. Bought new bulb. Still didn't work. Then... 💡went off in my head. The lights require a push to turn on, and my entire lense is missing. So I think I just wasted several hours of my time, because the dome light requires a push to turn on?

Can someone completely explain to me how this mechanism works? The dome light assembly itself. Wondering if there is a way to jump things to see if my light would work, even if I can't push the lense.

Also judging by the photos I provided, do I pretty much have to buy a new assembly? Kind of disheartning considering the price for a new one. Thanks!

 

Edit*** Now I am even more confused, because I just read that the push button on the dome light has no correlation with the door switches. The door switches turn on the light when the door is open and the dome light is on the off button. When the dome light is in the on position, it is just perminantly on.

Edited November 21, 2022 by ckurtz2

[Captain Obvious]

I'm confused... It looks like you only have half the switch in there? Like the whole top half is removed and missing? Like someone has totally hammered your switch to pieces?

Take a look at this thread for some other pics:
https://www.classiczcars.com/forums/topic/65118-dome-light-repair/

 

[ckurtz2]

@Captain ObviousThe more I read into this the more I get confused too. Haha, yes, mine is definetly goofed beyond repair judging by that extremely helpful pic you just shared. Not sure what happened to mine. Pretty sure the lense probably broke way back in the 90s and the switch has just been festering with rust/oxidation ever sense. Any remaining part of the lense literally crumbled into a million pieces when I touched it. I saved this car from sitting 30 years outside so it has been full of awful surprises (somehow very little to no rust).

       Anyways to try and explain my issue better. I put a multimeter prong at both ends of the socket for the light bulb and I was able to read around 9-12V. So the fixture itself was getting battery voltage. Now my simple way of seeing if the door jamb switches were working was to hold the multimeter prongs at the fixture like before. Then I actually used my foot to push in the door jamb switch (disconnect the ground). The volts at the fixture then would drop to zero. Did this for both door jamb switches, and both times volts went to zero. So I can conclude the door switches are ok and providing ground for the fixture? 

    However, I still could not get the dome light to turn on. So I bought a new bulb and still nothing. 

So I am a little confused what I am missing here. It seems like the bulb is getting voltage, but it won't light up. Then I realized my dome light is all messed up, because I can't use the on off button it provides. I was wondering if somehow the dome light assembly could have something to do with my problem.

 Let me know if this helps.

[SteveJ]

Have you looked at how the circuit is designed? Here it is from the 74 FSM, but all S30s are the same for the dome lamp.

Measuring voltage at the light does not mean it will light up. It means it will light up once you give the circuit a path to ground. Corrosion on the body around the door switches can prevent the path to ground as the door switch grounds at the body.

[ckurtz2]

@SteveJYes, I believe I understand it. I wasn't just measuring one end of the bulb socket and then connecting the other end of the multimeter to a ground. I physically had One end of the multimeter to the positive, and one end to the ground portion of the socket, and it read voltage. I then while doing this would push in the door switch (which disconnects ground as you already know) and the voltage would go to zero.  I am pretty sure that is how the circuit is designed?

This is why I am so confused. Voltage reads fine at the fuse too. I think I am just going to get another interior light assembly, and just clean up grounds, but I am still mystified by the electrical magic. I have the FSM for a 77 280z that correlates to my car so here is also a great photo of the circuit. Maybe someone in the future will find this forum and get to use it too.

And to confirm to remove the door switches I just use a screwdriver and pry them out from the front? they seem like they are on there pretty good. Thank you for bearing with me!

[SteveJ]

So did you remove the bulb and measure resistance across it or do some other test to verify you have a good bulb? With a bad bulb, you would still see voltage across the bulb. You didn't mention anything about measuring current. Also what is the resistance between the negative side of the dome light and chassis ground? How about between battery positive and the positive of the dome light? (Of course, those measurements are with the bulb out.)

Was the bulb in your photo the bulb you used, or was there any chance you used an LED bulb?

There is one other factor that would allow you to see voltage, but the circuit may not work. That would be corrosion, such as around the door switch. Think of it this way:

Your battery is like a water tower. It's full and ready to supply water. You can verify this by using a pressure gauge (voltmeter). The pipes are all connected, and you open the valve (close the switch). However, you don't see much water coming out (no light). What you didn't realize was that the pipes are old, and a lot of calcium had built up, blocking the pipe (corrosion). If you had a flow meter installed (ammeter), you could measure the low flow. The resistance tests I asked about above are also like testing the pipes, though with the meter, you can get a false low resistance reading.

Good voltage does not equal good current flow. 

[ckurtz2]

 

Yes the bulb in the photo is the one I used. No LED

So I tested to see If I got voltage at the bulb socket with the bulb removed. I did. I then tested the bulb by a continuity test and I got it as continous, which I assume is proper. I then used a brand new bulb in the circuit and it still did not light. Maybe I need a basic lesson on how these festoon bulbs work. 

To answer your other questions. The input for the push button (B wire in diagram that goes right to ground) showed continious between it and the chassis. The other female spade connector (BR in the diagram) showed 0.01ohm with the chassis when the door was open and then was not continous when I closed the door. Thus I believe it is functioning, but maybe with some resistence. I removed both switches and they both were disgusting. I cleaned them up at the door and also where they connect to the harness. Circuit still doesn't work. Should I also clean the area on the door where the switch pushes into? I provided photos of the before and after. So between LW and B and LW and BR I read voltage. Also between LW and chassis ground I get voltage. I agree with you I think it has to be corrosion at this point blocking the flow, but I am confident in the door switches. I can't test between battery positive and  the LW wire, because I don't have long enough multimeter leads. I have the $12 autozone special for a multimeter haha. Looking to upgrade to something if you have any recommendations, I have been dealing with enough electrical problems to justify it. 

 

Edited November 21, 2022 by ckurtz2

[Racer X]

You need to check the grounds  for continuity from the wire to the chassis.

If there is no continuity at the grounds, you could put a thousand volts into the wire from the battery side and the light will not illuminate.

All grounds need to be bright and tight.

Edited November 21, 2022 by Racer X

[ckurtz2]

@Racer XFrom the B wire and BR wire I get continuity from the wire to anywhere on the chassis. In this case I put one end of the probe on the wire and the other on one of the bolts that mounts the center console to the chassis (I had to open the lid).

 

If you want other odd thing I noticed. I purposely shorted the B and the LW wire to see if it would spark (I figured the worst that would happen is I would blow the fuse). Well, nothing happened. Not sure if this helps anything, I know its an extremely stupid way of testing, but at this point I was merely curious.

Edited November 21, 2022 by ckurtz2

[Racer X]

1 hour ago, ckurtz2 said:

@Racer XFrom the B wire and BR wire I get continuity from the wire to anywhere on the chassis. In this case I put one end of the probe on the wire and the other on one of the bolts that mounts the center console to the chassis (I had to open the lid).

To check continuity of a connection, that connection must be intact.

Testing as you described doesn’t do that, it only tests the circuit through the wire, back through the system to the point on the chassis where you touch the probe.

 

To check the integrity of a ground (or any connection) ensure the ground is clean, bright and tight. Then set the dvom to ohms, and using sharp probes, pierce the installation of the wire about 1/2” from the connection with one probe (polarity doesn’t matter here), then pierce the finish of the chassis about 1/2” away from the connection with the other probe. There should be some resistance, the lower the better.

Edited November 21, 2022 by Racer X

[heyitsrama]

You don't need to spend big pesos on a new dome switch... 💸

 

[ckurtz2]

Well, I got the dome light working with both doors!

I can't give an exact answer on what needed fixing, except that I cleaned every ground possible, and it still wasn't working. Then I blew the dome light fuse when scrubbing the fuse block. Replaced the fuse and came back about 30mn later and it was working. So mission accomplished, but still lost as to what was wrong. The fuse was fine until the moment it blew (tested it multiple times for continuity, also the metal band was visibly fine).

Thank you to all above who took the stab in the dark to help me. I appreciate any and all advice.