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1940 Buick Accessory Power Connections


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I recently purchased a 1940 Buick 46C convertible (see attached pictures). As part of some trouble shooting and to better understand and appreciate my new Buick, I'm trying to determine how the accessories should be electrically hooked up. The three accessories I want to know more about are the heater, defroster, instrument light switches under the driver's side of the dash (see picture below). I have checked the 1940 Buick Shop manual electrical diagram and it does not show these three switches on the drawing. What I am trying to determine is should the power to these three switches be on full time or should they receive power when the ignition switch is on? Currently, the switches have power when the ignition switch is off. I have drained my battery a couple of times when the switches were not in the off position. I prefer to connect the power to these three switches through the ignition switch, but need guidance on what other 1940 Buick owners know on this subject.


Don (2Buicks+1more)

1969 Buick Riviera

1952 Buick Super 4dr

1940 Buick Special Convertible







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Hi Don:


I recently installed a new front wiring harness (from the dash forward) on my '41.  The switches you're asking about are in a different place on the '41, but I assume the wiring is the same or similar.  On the '41, the heater and defroster switches also do not appear on the wiring diagram in the manual -- I guess because they were options?  Anyhow, I just replaced the wiring as it was with the original harness, which had a "hot" wire from the right side (switched) contact on the ignition switch to the heater switch, and then an extension from the heater switch to the defroster switch.  So the heater and defroster fans only will come on when the ignition is on.


The instrument light switch on the '41 is wired so that the "hot" wire on the switch comes from the tail light contact on the headlight switch.  This is shown on the wiring diagram.  So the instrument lights can only come on if the tail lights are on, i.e., when the headlight switch is pulled out either one click (parking lights) or two clicks (headlights).


@Matt Harwood has warned against powering too many accessories through the ignition switch because it can create a problem if the current is being drained by the accessories when you need maximum current to start the car.  While I'm sure that can be a problem in theory, I don't think I'm risking anything with the set up I have for the heater and defroster (which I'm pretty sure is the way they were wired from the factory).  How likely is it that I would be trying to start the car with both the heater fan and the defroster fan running?  Not very, I figure.  Anyhow, that's how I've done it.  Maybe Matt can chime in here.  The heater and defroster on his Limited may be wired differently than on my Super.


Also, someone with a '40 also may be able to tell you exactly how the factory wiring was done for these switches.


By the way, your Special looks great -- hope you are enjoying it!



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Just a clarification because this has been bugging me since I posted the other day.  I had a feeling I was not remembering this exactly right, and in fact it was a little different (although it amounts to the same thing from an electrical standpoint).


With regard to the heater and defroster, I did not actually run the power for those two switches directly off the ignition switch.  I looked at the chart that Rhode Island Wiring Services provided with my harness.  It's a bit different than the chart in the shop manual, and it includes the heater and defroster switches.  If you look at the chart, you can see that is shows a separate "hot" wire going from the ignition switch to each switch.




However, the actual harness provided by RI Wire reproduced the original wiring, which ran a single "hot" wire from the left hand pole on the fuel gauge to the heater switch, and was then patched onto the defroster switch.  This achieves the same result without running the power directly from the ignition switch.  You can see that the left hand pole of the fuel gauge is a very busy place, since that's also where the power runs out to the starter switch on the carb, and also where the power runs to the directional signals.  It is also directly wired to the ignition switch, so it the "location of choice" for everything you want to power up only when the ignition is on.




You can also see on this chart that the instrument lamp switch is powered, as I mentioned, off of terminal #3 on the headlight switch, which is the terminal that provides power to the tail lights.


So I may have given you more information than you needed at this point, but I wanted to make sure I was giving you the right stuff.



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The simplest fix would be to teach yourself to turn everything off before exiting the car.

This is also the safest and it will not confuse a later driver or purchaser.  One of the biggest problems new owners have is figuring what alterations were made in the past.

In 60 years of driving my own and many other vehicles I have never had a battery discharge while parked except for one unit.  An MC7 highway coach.  If we left it parked for a week the battery would be dead.  After much searching, testing and pondering over the wiring diagram I found that the sending unit for the electric speedometer was neither fused or wired through the master switch.  A five minute fix and all was well.

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I thank you for your comment, but I'm trying to determine if the wiring is correct for the accessory switches. The electrical wiring diagram I have for the 1940 Buick, does not show or list those three switches on  any drawing in the  1940 Buick Shop Manual. If I use Neil's wiring for the 1941 Buick, it might tell me the wiring for my 1940 Buick is wired correctly. My next job is to check the wiring per Neil's drawings. I will let you know what I find.



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