vacuum starter switch - 1940 Century

[allcars]

The starter tries to engage on my '40 Century if I add any significant throttle when it is not fully warmed up. I believe the problem is in the vacuum switch that should cut out the starter once the car is started. The Shop Manual gives a procedure for checking the switch calibration (not a simple process), but that was for a new car. I'm wondering if the vacuum operation of the switch is the problem. I know a lot of the older "tap start" cars have been changed to push-button starter buttons over the years, but it doesn't seem to me like a real Buick Eight if it starts that way! Any suggestions on how to proceed welcomed.

[Guest imported_JPIndusi]

There are two ways engineered into the starting system on older Buicks with accelerator actuated starting to prevent the starter from actuating once the engine is running.

The first is to assure that the vacuum from a running engine is applied to the switch in the carburetor to move a steel ball to prevent the switch contacts from closing. The proper carburetor gaskets must be used to assure that there is a path for the vacuum to get to the steel ball. Another possiblity is that the switch is out of calibration.

The second method to prevent starter actuation with a running engine was to wire the coil on the starter switch on the starter to obtain it's ground from the generator armature terminal. Here is an explanation of how this works. There were variations in the actual implementation from year to year, but this is the general idea.

When the engine is not running and the ignition switch is placed to the on position by turning the key, and the accelerator pedal is depressed, +6 volts is appled to one end of a starter switch coil. Now if the other end of the coil is grounded, the coil will pull in and contacts will close causing the starter to actuate. A clever way to ground this side of the coil is to connect it to the armature of the generator or the ARM or GEN terminal on the voltage regulator. This provides a ground because when the generator is not spinning, the armature terminal is grounded through the brushes and armature windings. The armature windings introduce only minimal resistance since they are wound with very thick copper wire. However, once the engine starts, the armature is at +7 or so volts so the coil on the starter switch cannot pull in the contacts to actuate the starter.

For your car starter to actuate while it is running, both systems need attention.

I would check the starter system wiring against the shop manual. Also check that engine vacuum is reaching the starter switch on the carburetor.

Good luck.

Joe, BCA 33493

[Guest 49 Packard]

I'm having a similar problem with my '49 Packard. Have you come up with a good resolution?

[1938BuickLimited90]

I know its many years later but were you ever able to sort this issue out? I am having the same problem with my 1938 buick and am looking for a solution before I go and add a push button start.

Thanks!

[kgreen]

The 248 engine has a different set up than the 320 engine.  Both must have a good vacuum.  

 

For the 248: remove parts and remember the position of the cupped ball retainer.  The retainer goes in two ways but only works one way.  Clean the interior of the switch with a suitable solvent being sure airways are not plugged.  New gasket, reinstall.  If you have gasket leakage within the carburetor, your problem will persist.  Make sure your carb is in good working order.

 

For the 320: This switch also works on vacuum but cannot be disassembled and cleaned effectively.  At least I couldn't do it thoroughly enough for my satisfaction.  They get gummed up and dirty.  I replaced mine for a sure fire fix.  If you do take it apart clean it liberally with solvent and carefully clean the contact.  I am not sure if there is a critical gap for the contact but the contacts of course must be clean.  Here are some views of the vacuum switch for the 320.  As I recall, there is a thin rubber diaphram that becomes unusable once it is removed.  I am unaware of a rebuild kit.

Edited October 12, 2023 by kgreen
spelling, clarity (see edit history)

[Century Eight]

Kgreen.   Good explanation.  I didn’t know there was a diff with the 320 eng but a few years ago I had the same problem with my 1950 Super 263 c.i. After the carb was rebuilt i had all kinds of trouble with starting it and the starter continuing to spin after the engine was running.  The 1940 tech advisor Doug Seybold said most of the time it is a dirty carb switch.  Long story short, he was exactly right.  Cleaned it, and the problem was fixed.

 

Many beautiful ‘40 dashes were ruined when previous owners drilled holes and put a starter button right thru the engine turned dash.  At least they could have hidden it underneath.  Try all avenues and your car will start like it is supposed to, before you punch a hole and add a button.  

 

Besides the floor starter is a low tech theft deterrent!