Drakeule Posted February 10, 2022 Share Posted February 10, 2022 In looking at the schematic for the dome light, it can be activated by the pillar switch or either door jamb switch. Upon examining those jamb switches, it appears to me that the only thing they do is interrupt the grounding when the door is closed. However, the schematic clearly shows that they function as a hot (6V) interrupter (which is what I would expect.) Interrupting a ground doesn't seem to make any sense, as the dome light is already grounded...In fact ,the jamb switches shouldn't even work! I thought that the jamb switch would have 2 contacts, and that one would be hot; when you open the door, it connects the circuit, thus giving the dome light voltage. The way I see it now, if you connect a positive to the door jamb and open the door, you create a short condition-like hooking pos. and neg. on a battery together! Can someone make sense of this for me? Link to comment Share on other sites More sharing options...
37_Roadmaster_C Posted February 10, 2022 Share Posted February 10, 2022 If you can please post a 1940 wiring diagram. I could not find one, but in 1939 and 1941 the dome light has an isolated ground and the door switches and the post switch ALL ground the ground side of the dome light. This is how dome lights were wired for virtually ever by all manufacturers. I am sure there were some variances, but not many. The reason for this is it is a VERY easy way to allow many switches to light the dome lamp without complicated wiring. 1 1 Link to comment Share on other sites More sharing options...
Ben Bruce aka First Born Posted February 10, 2022 Share Posted February 10, 2022 Robin is correct, I believe. My 1950 is that way. Light should be isolated from ground. Switches then provide a ground. Ben 1 Link to comment Share on other sites More sharing options...
neil morse Posted February 10, 2022 Share Posted February 10, 2022 On my '41, the dome light is only wired to be activated by the pillar switch. The jamb switches only activate the "courtesy lights" (two on the side of the front seat frame and two on the back of the front seat frame). I have to check the wiring diagram to refresh my memory, but I'm pretty sure both of these circuits function the way described by Robin and Bruce. The switches provide ground. 1 Link to comment Share on other sites More sharing options...
neil morse Posted February 10, 2022 Share Posted February 10, 2022 Here's what the pillar switch for the dome light looks like on my '41. You can see that it just provides a ground through one of the screws that holds the switch onto the pillar. (Photo copied from Ebay.) 1 Link to comment Share on other sites More sharing options...
EmTee Posted February 10, 2022 Share Posted February 10, 2022 (edited) Having the dome light switch provide ground to the lamp rather than positive voltage is safer because if the dome switch or its wire becomes shorted to ground somehow all that happens is the light comes on and if not noticed it will eventually kill the battery. There's voltage present, but current is limited by the dome bulb filament. Shorting a positive voltage risks burning-up the harness and possibly the car. If the bulb blows then there's no voltage at the dome switch. Edited February 10, 2022 by EmTee typo (see edit history) 1 Link to comment Share on other sites More sharing options...
Drakeule Posted February 10, 2022 Author Share Posted February 10, 2022 All: Thanks for the info. This is the only solution that makes any sense! I will attach the schematic ASAP. The schematic seems to (on second look) show a grounding switch like you guys state. However, it appears that it fuses (because of a rear cigar lighter, which my sport coupe doesn't have) and then goes to the "+" side of the Ammeter. Certainly not a ground ! Link to comment Share on other sites More sharing options...
EmTee Posted February 10, 2022 Share Posted February 10, 2022 The circuit should still be fused to protect the part of the circuit feeding battery voltage to the bulb. I think you're correct; eventually everything sources battery voltage from the ammeter. 1 Link to comment Share on other sites More sharing options...
Drakeule Posted February 10, 2022 Author Share Posted February 10, 2022 All: Link to comment Share on other sites More sharing options...
Drakeule Posted February 10, 2022 Author Share Posted February 10, 2022 Sorry for smallness of schematic. Not sure how to enlarge it. However, you may get the picture from this. Link to comment Share on other sites More sharing options...
Matt Harwood Posted February 10, 2022 Share Posted February 10, 2022 (edited) I recall when I was re-wiring the courtesy lights in my 1941 Limited that the bulbs were special bulbs that didn't ground to the housing but used separate ground and power wires. The door jamb switches were the grounds and power came from some unswitched +6V source (I pulled it from the same wire as the power divider window, but your Roadmaster will have a different source). Like Neil's car, the overhead light is switched only, with the door jamb switches activating the floor courtesy lights. I suspect yours should be the same. Here's an extended look at how I did mine: Here's a bigger view of the wiring diagram for a 1942 Roadmaster sedan, which should at least point everyone in the right direction (the image above is just too small to read). Edited February 10, 2022 by Matt Harwood (see edit history) 1 Link to comment Share on other sites More sharing options...
neil morse Posted February 10, 2022 Share Posted February 10, 2022 It looks from the diagram Matt posted that your coupe has the dome light activated by the door jamb switches -- the top diagram of the three immediately above. The sedans are different, like mine and Matt's, with the courtesy lights on the door jamb switches and the dome light only on the pillar switch. But all variations are the same principle -- the switch just provides the ground. Link to comment Share on other sites More sharing options...
Drakeule Posted February 10, 2022 Author Share Posted February 10, 2022 Thanks all. Sorry for the small diagram. Matt: my 40 Roadmaster does not have floor courtesy lights. Like Neil said, it must be like the top (of the 3) small diagrams that you posted. The '40 70 wiring diagram differs a bit from the general one for your '42. Link to comment Share on other sites More sharing options...
37_Roadmaster_C Posted February 10, 2022 Share Posted February 10, 2022 1 hour ago, Drakeule said: Thanks all. Sorry for the small diagram. Matt: my 40 Roadmaster does not have floor courtesy lights. Like Neil said, it must be like the top (of the 3) small diagrams that you posted. The '40 70 wiring diagram differs a bit from the general one for your '42. I think you have a special case and YOU are correct!! Your schematic is a little out of focus and a bit hard to read, but I am fairly sure in the upper right corner the dome light is indeed grounded at the socket and the pillar switches switch battery power to the bulbs. If you could repost a clear picture of the wiring diagram I will confirm what I think I see. There is a first for everything!!!! Link to comment Share on other sites More sharing options...
Drakeule Posted February 10, 2022 Author Share Posted February 10, 2022 Link to comment Share on other sites More sharing options...
Drakeule Posted February 10, 2022 Author Share Posted February 10, 2022 37 Roadmaster: I was able to resize, but this program out thinks me! My scanned correctly oriented copy insists on coming out turned 90 degrees (even though it looks OK in the computer file view!) I think it's readable, although you might get a crick in your neck if you look too long... However, I really don't care what my schematic says because in the real world, those jamb switches do indeed complete the path to ground. Therefore, the bulb wire (not the casing) must be negative and the bulb casing is hot. So much for factory diagrams. And I thought I was through with jack up schematics with Italian cars.... Link to comment Share on other sites More sharing options...
EmTee Posted February 11, 2022 Share Posted February 11, 2022 (edited) 21 minutes ago, Drakeule said: However, I really don't care what my schematic says because in the real world, those jamb switches do indeed complete the path to ground. I don't understand why the schematic you posted doesn't match your car... Does your dome bulb socket have one or two contacts (not counting the shell)? Edited February 11, 2022 by EmTee (see edit history) 1 Link to comment Share on other sites More sharing options...
32buick67 Posted February 11, 2022 Share Posted February 11, 2022 From my National Service Manual, years 1930-1941. 1940 and 1941 wiring diagrams if they help you. Link to comment Share on other sites More sharing options...
37_Roadmaster_C Posted February 11, 2022 Share Posted February 11, 2022 Ok, there is something strange going on here. The factory diagram should be correct unless someone changed something. Do the dome light assemblies have one wire or two? If one then the switches switch battery positive. If two wires then the switches switch ground. Either way will work, but ground switching is the way MOST dome light circuits work, but the 1940 diagram clearly shows that battery positive is switched to the dome light. The battery positive also powers the cigar lighter as you mentioned. Now a stupid question because I am lazy.... What was the original question 🙄? 2 Link to comment Share on other sites More sharing options...
Drakeule Posted February 12, 2022 Author Share Posted February 12, 2022 Roadmaster: Something strange going on here...Ya think? My original Q was if this was a hot (positive) interrupted system, why do the door jamb switches just ground out when the doors are opened? There is no other explanation other than it is a ground interrupted system; switches ground out, thus completing the circuit. Believe me, after running new wires throughout the entire roof area with a miniature fish tape, breaking continuity of the small wires (got to be thin, because the fatter ones you can't get through whatever opening you find) I have had to take a break and affix the half smile of the Buddha on my face and contemplate on why I insist on getting this car back to original working order. Again, I care about what will make it work, rather than blindly following a schematic. As Groucho Marx opined: "I don't want to be a member of any club that would have me as a member." 1 2 Link to comment Share on other sites More sharing options...
37_Roadmaster_C Posted February 12, 2022 Share Posted February 12, 2022 I think you have an excellent outlook on this. I agree that the best thing is to do whatever works! I would still drop the dome lights to see how many wires connect to them. If it is only one you are going to have a problem isolating the ground as most dome lights ground through the mounting screws. I think you understand how things work so you will make it work. Keep us posted on what you find and do 😀. Link to comment Share on other sites More sharing options...
Drakeule Posted February 12, 2022 Author Share Posted February 12, 2022 Roadmaster: It does appear the the center of the dome light bulb connects to a single wire with a strange little connector that I have never seen; it seems to be a tiny hinge (?) It also appears that the casing looks like it may have had a wire soldered on top. So, I think there were 2 connectors. If the casing truly does ground through the mounting screws (and it shouldn't, if this thing works like I think it does), I plan on using nylon screws-if they come that small! 🙄 Link to comment Share on other sites More sharing options...
37_Roadmaster_C Posted February 12, 2022 Share Posted February 12, 2022 Sounds interesting. The next thing I would check is to see if there is power to the wire that is connected tom the center of the bulb. If there is, then I would ground the housing to see if the light lights. If it does you then need to isolate the housing and connect the housing to the door grounding switches and you will have dome light when the door opens. Good luck!!!! Link to comment Share on other sites More sharing options...
Oldtech Posted February 12, 2022 Share Posted February 12, 2022 Circuit diagrams can change. even back on my '17 the horn shows the switch on the 6v side. That is not how it was wired. Power to the horn and the button grounded it. Link to comment Share on other sites More sharing options...
Drakeule Posted February 13, 2022 Author Share Posted February 13, 2022 Well...I was able to successfully hook up all switches (both jambs and the pillar) and make them work with the light frame out. Next, I noticed that the mounting frame that the chrome frame screws into was stood off from the body by what looked like wood or cork. The whole thing kind of looks like an industrial ice cream sandwich. Ah ha, says I, there's your floating ground (not shorted to the body.) Wrong... Just as soon as I attempted to screw in the tiny screws into that frame; a short. Now, I am almost certain that once upon a time, the mounting frame was insulated from the body. But no more.... So, I am going to mount that frame with nylon screws. So there, there's the %$#(*& floating ground for ya'. Still an enigma. Link to comment Share on other sites More sharing options...
Buick35 Posted February 13, 2022 Share Posted February 13, 2022 I think it's weird that my dome light on my 35 Buick only works when the passenger door is closed.The pillar switch is also on that side.Maybe it's getting a ground with the door shut? Link to comment Share on other sites More sharing options...
Bloo Posted February 13, 2022 Share Posted February 13, 2022 On 2/11/2022 at 4:39 PM, Drakeule said: Something strange going on here...Ya think? My original Q was if this was a hot (positive) interrupted system, why do the door jamb switches just ground out when the doors are opened? There is no other explanation other than it is a ground interrupted system; switches ground out, thus completing the circuit. Oh there's one other explanation. The car might have the wrong door switches. The diagram clearly shows switching on the hot side. 99% of all dome light switches only switch the ground. I'll bet they were originally 2 terminal switches (to switch on the hot side), and somebody changed them. 3 hours ago, Drakeule said: Wrong... Just as soon as I attempted to screw in the tiny screws into that frame; a short. Now, I am almost certain that once upon a time, the mounting frame was insulated from the body. But no more.... Are you sure? It wouldn't need to be if it matched the diagram and switched the hot wire. On 2/11/2022 at 4:39 PM, Drakeule said: Believe me, after running new wires throughout the entire roof area with a miniature fish tape, breaking continuity of the small wires (got to be thin, because the fatter ones you can't get through whatever opening you find) Yikes. I'd rather have it switched on the hot in that case. For one thing, you only have to get one wire through there, not two. For another, if you switch on the ground, one of those wires that got drug through the roof is hot all the time. Link to comment Share on other sites More sharing options...
Drakeule Posted February 13, 2022 Author Share Posted February 13, 2022 Bloo: Ya' know, hind site is truly 100%, and would agree with you except for the fact that I am about certain that the jamb switches are indeed the correct ones. Bob's (and several other sources I looked at) all say that those switches are correct for that year ,inclusive of other years. Like Roadmaster posted previously: If you can please post a 1940 wiring diagram. I could not find one, but in 1939 and 1941 the dome light has an isolated ground and the door switches and the post switch ALL ground the ground side of the dome light. This is how dome lights were wired for virtually ever by all manufacturers. I am sure there were some variances, but not many. The reason for this is it is a VERY easy way to allow many switches to light the dome lamp without complicated wiring. Link to comment Share on other sites More sharing options...
Drakeule Posted February 16, 2022 Author Share Posted February 16, 2022 All: For the "what it's worth dept.", I finally did what I said I would do: I re tapped the dome light switch holes to 6-32, and used nylon binding screws to secure the frame. They work like a charm; both jamb switches and the pillar switch. Still a mystery of how it was supposed to work, but oh well.... BTW, a miniature fish tape (available on line or from any hardware store), does the trick. Hint: take out you sun visors for intermediate access along the path down to the kick boards, use 16 or even 18 gauge wire, as it's really hard to fish tape the larger stuff through the roof. I think that I would rather work on dirtiest brake-fluid laden brakes, than do this again! 4 1 Link to comment Share on other sites More sharing options...
Buick35 Posted March 9, 2022 Share Posted March 9, 2022 I was thinking of replacing my dome light bulb with a six volt led bulb but not sure if it will have enough clearance. Now it's like a small camp fire in a big forrest,cheerfull but not very bright. 1 Link to comment Share on other sites More sharing options...
32buick67 Posted March 9, 2022 Share Posted March 9, 2022 I used these, not made in the USA, but very few electronics are now unfortunately. https://www.amazon.com/BA15S-12smd-Reverse-Lights-Lumens/dp/B071154DHZ Fit fine in the dome light, and wasn't too bright (glass dome diffused nicely). It took a few weeks to receive them. Just be sure to order the BA15S 1156 for the single filament style replacement. BA15D 1157 is for the dual element brake/tail combo type bulb. Another option https://www.ledlight.com/6-volt-negative-ground-automotive-led-lights.aspx Link to comment Share on other sites More sharing options...
Drakeule Posted March 11, 2022 Author Share Posted March 11, 2022 LED lights are the coming thing; Alfa Romeo guys (my other car) like to replace dial lights with these. I understand that it has made Xmas lighting affordable (VS the scene in Xmas Vacation where Clark plugs his house lights in and the substation goes into A Fib!) 1 Link to comment Share on other sites More sharing options...
Buick35 Posted March 11, 2022 Share Posted March 11, 2022 Thanks,I ordered some last night.I still don't know why on my 35 Buick sedan when I open the passenger side front door the dome light goes out.Is that normal? Link to comment Share on other sites More sharing options...
32buick67 Posted March 11, 2022 Share Posted March 11, 2022 27 minutes ago, Buick35 said: Thanks,I ordered some last night.I still don't know why on my 35 Buick sedan when I open the passenger side front door the dome light goes out.Is that normal? Can you please post a picture of your door switch? I think I might have an idea of what is happening. 1 Link to comment Share on other sites More sharing options...
Buick35 Posted March 11, 2022 Share Posted March 11, 2022 Thanks,I'll post a picture soon.Greg Link to comment Share on other sites More sharing options...
pont35cpe Posted March 11, 2022 Share Posted March 11, 2022 I don`t think there are door(jamb)switches on a `35, unless they were added. No idea why the opening/closing of the passenger door is involved. Link to comment Share on other sites More sharing options...
32buick67 Posted March 11, 2022 Share Posted March 11, 2022 Based on a review of wiring diagrams, 1932 appears to be the first year for factory door post switches which turn the dome light on when the door is opened. Link to comment Share on other sites More sharing options...
Buick35 Posted March 11, 2022 Share Posted March 11, 2022 2 hours ago, pont35cpe said: I don`t think there are door(jamb)switches on a `35, unless they were added. No idea why the opening/closing of the passenger door is involved. My 35 has only one switch ,The one in the pillar post.Thanks Link to comment Share on other sites More sharing options...
Buick35 Posted March 11, 2022 Share Posted March 11, 2022 2 hours ago, pont35cpe said: I don`t think there are door(jamb)switches on a `35, unless they were added. No idea why the opening/closing of the passenger door is involved. I don't either,if a wire was shorting out it seems like the light would go on with the door opened.It only acts up on the one door.Maybe a bad ground somewhere?Wierd. Link to comment Share on other sites More sharing options...
Buick35 Posted March 11, 2022 Share Posted March 11, 2022 1 hour ago, Buick35 said: I don't either,if a wire was shorting out it seems like the light would go on with the door opened.It only acts up on the one door.Maybe a bad ground somewhere?Wierd. I ran a wire from the opened door frame to the car body and the light worked.So I'll install a small ground wire between the two.Sorry if I hijacked the op.Greg. Link to comment Share on other sites More sharing options...
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