TheBMan Posted December 14, 2020 Share Posted December 14, 2020 My 72 MC has the ZJ9 Lighting Group which includes, among other lights, an underhood light and a trunk light. Neither of these lights work. My question is - and I've looked for the answer which I can't even find in the original factory owners manual - how to you turn those lights on and off? I've tried the courtesy light position on the headlamp switch which doesn't work and they don't come on and off with the doors. I have tested the bulbs and they are good. I've also tested all fuses, which are good. I feel like it's a silly question but I seriously cannot find the answer! Quote Link to comment Share on other sites More sharing options...
Leghome Posted December 14, 2020 Share Posted December 14, 2020 Most of the under hood and trunk lights I have seen are operated by a mercury switch in the light itself. Youi raise the lid it comes on and lower it goes off. If they are not coming on when the lid opened then the bulb is bad, broken wire or not connected. 2 Quote Link to comment Share on other sites More sharing options...
TheBMan Posted December 14, 2020 Author Share Posted December 14, 2020 @Leghome - this makes perfect sense. The bulbs are good, fuses are good, wires are connected. The housings themselves, especially the underhood light assembly are pretty corroded. I'm guessing the innards are junk. This car is mostly original (numbers matching, too). Just needs some TLC to get it back to 100% proper working order. Thank you! Quote Link to comment Share on other sites More sharing options...
Rob Peters Posted December 14, 2020 Share Posted December 14, 2020 Remove the assembly, clean any rust from the housing, trunk/hood mounting position and screw so you have bare metal touching metal. This is how it provides a good ground to complete the circuit. The wire going to the assembly should always have 12VDC going to it and you should be able to test who's with a volt meter rob Quote Link to comment Share on other sites More sharing options...
Leghome Posted December 14, 2020 Share Posted December 14, 2020 you can try and clean where the bulbs make contact in the socket. I have seen them corrode. Rob posted as I was typing Quote Link to comment Share on other sites More sharing options...
72-CLASSIC_RIDE Posted December 14, 2020 Share Posted December 14, 2020 I believe there is an In Line Fuse in between the Wiring from Bulb contact to connection on Bulk Head. I would suggest using a light tester probe under hood lite by removing bulb, with Hood up touch Light tester probe on the mercury switch and see if it lights the tester, then check in line fuse 1st. Is this a Custom Model? They came standard with the Light Group. Good luck with your project. Doug 1 Quote Link to comment Share on other sites More sharing options...
TheBMan Posted December 14, 2020 Author Share Posted December 14, 2020 @72-CLASSIC_RIDE - yes, it is a Z03 Monte Carlo Custom and included the ZJ9 Lighting Group (straight from the build sheet). I'm going to check the inline fuse, test the connection at the mercury switch as you suggested. Per @Leghome's comment, the bulb sockets are clean, no corrosion. And per @Rob Peters's comments, the trunk light has a steady 12v supply but the lamp doesn't work (bulb tests good). The underhood has no voltage at the socket which leads me back to @72-CLASSIC_RIDE's comments about the inline fuse and connection at the bulkhead. I'm going to check these things but it may not be today. I'll follow up as soon as I do! Thank you very much! 1 Quote Link to comment Share on other sites More sharing options...
TheBMan Posted December 14, 2020 Author Share Posted December 14, 2020 Sooooo... pretty easy diagnosis. See attached image. Wire was tucked up behind the brake booster. Looks like it got cut at some point and exposed the wire, shorting it out and burning the fuse holder. There is a constant 12v at the bulkhead connection so I'd say I just need to either a) fix the wire and fuse connector or b) replace the assembly. I'll probably just get a new assembly since it comes with the wire and fuse holder. The current one is pretty rusty and the wire is brittle. I feel bad for all of the time and advice you guys gave. Had I just looked at this first, it would have been a no-brainer. BUT, I really wasn't aware they were mercury switches so I was stumped on how to turn them off and on. That's how this all started. LOL Regardless, thanks again. Quote Link to comment Share on other sites More sharing options...
72-CLASSIC_RIDE Posted December 14, 2020 Share Posted December 14, 2020 Not sure you will be able to locate a NOS / OEM replacement. Aftermarket ones are sold on eBAY and you will have to check and make sure the in line fuse is included in the harness along with at least minimum length of harness to connect. It is possible to disassemble the Mercury Switch from the Housing while housing still attached to vehicle. There is a Snap Ring like retainer right behind the Housing that holds the Bulb/Mercury Switch in place. Remove Snap Ring retainer/Mercury Switch and it's plastic housing will slid back and out, allowing you to inspect/replace. If taken apart, make note of pieces during disassemble so that you can reassemble correctly, if needed. I usually have to take pic's................ I also have a 72 Monte Custom. I have been keeping a Spreadsheet on those 72 Customs still serviceable in order to get a idea of just how many are still roadable. Unlike the 70/71 SS, GM never distinguished with production numbers the difference between the Base Model Production and Custom, there is NO WAY to determine just how many were produced. That Spreadsheet is located on this Site under 454 SS thread, The 1st Article 72 Custom Numbers? has the most up to date info for the Custom and is the Thread I started. Feel free to look it over and then decide if I can ADD your Vehicle to my Database. You will see the pertinent info I included along with YOUR VIN# that helps prevent me from duplicating them by cross referencing the VIN's. If you have a Build Sheet, include a pic of it and if legible, can copy the info from it. Otherwise, you would need to copy those Options for me. Let me know if this is possible. I will say anyone can view this Spreadsheet and you can also make a copy for your own use. Only active Members can make a copy........... Doug 1 Quote Link to comment Share on other sites More sharing options...
72-CLASSIC_RIDE Posted December 14, 2020 Share Posted December 14, 2020 The Trunk Lamp has a Harness wire that runs along left hand side under trunk inside ledge Rear Harness with that single wire running out and up the L/H Trunk Lid support into rear Trunk Lid and connected to Lamp Housing. I would locate/isolate that wire back to Harness connection and see if there is power to it. If yes, might be a ground issue. If not, double check connections as I suspect it is a loose connection for trunk lid. Doug 1 Quote Link to comment Share on other sites More sharing options...
Leghome Posted December 14, 2020 Share Posted December 14, 2020 It is amazing reading these comments and remembering that I had that info in my memory bank. That being said the older we get the more info we have stored the longer it take to retrieve it. That is my story and I am sticking to it, LOLO LO LO LO L OL OL OL Lloll😀🤣😂😝 1 Quote Link to comment Share on other sites More sharing options...
TheBMan Posted December 15, 2020 Author Share Posted December 15, 2020 14 hours ago, 72-CLASSIC_RIDE said: The Trunk Lamp has a Harness wire that runs along left hand side under trunk inside ledge Rear Harness with that single wire running out and up the L/H Trunk Lid support into rear Trunk Lid and connected to Lamp Housing. I would locate/isolate that wire back to Harness connection and see if there is power to it. If yes, might be a ground issue. If not, double check connections as I suspect it is a loose connection for trunk lid. Doug @72-CLASSIC_RIDE Hey Doug, I PM'd you my Custom info for your database. And yes, there is constant power at the harness connector in the trunk for the light. I ordered a new lamp assembly. Thanks for everything! Bryan Quote Link to comment Share on other sites More sharing options...
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