this sounds familiar to me, but I cannot say for sure one way or the other;
I kept this window open thinking someone else might have chimed in by now.

cdnpont wrote:

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...do you lose current to the alternator field, and then have a dead alternator?

Or... do most GM's of our era have a secondary resistance wire run parallel to the idiot bulb for just that scenario?

Never would have given this a second thought until lately, and the wiring diagram does not describe it very clearly.

 

Just curious during the CS130D alternator conversion, which requires the resistance of that wee #57? bulb in series with the feed, or a resistor feeding the new "L" terminal of the CS.

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 I'm guessing that the state of the bulb shouldn't have an effect on the functionality of the charging system, since (A) the bulb circuit is only completed when there's a charging system problem, and (B) designing an electrical system such that it ceases to function when the warning light bulb (that is supposed to tell you that the charging system is not functioning properly) is burnt out seems kind of illogical. 

The reason that all the warning lights light up before you start the engine is so that you can check the function of the bulbs to determine if they are burnt out or not.

That said, I'm not familiar with GM electrical systems, so it's just a guess.

So you're saying that it's highly probable that GM decided to design the circuit such that the failure of the bulb which is supposed to indicate a failure, could itself cause a failure that will never be indicated to the driver.  I'll buy that.

Well, with no valuable experience or background knowledge with old GM electrical systems, the next tool is logic.  Does it pass the logic test?  Not really, from a functionality point of view, but perhaps from a 1969 corporate responsibility point of view.

Functionally, yes, if you turn on your key and happen to notice that your ALT light isn't on, you might conclude that there's a problem, and choose to not drive the car, call your local GM of Canada dealership and have your car towed in for service (replacing the bulb, after a number of tests and billable technician hours that would probably happen outside of warranty).

However, when do light bulbs burn out?  While they are on, is the answer.  So, you turn your key on, the light comes on briefly and burns out.  Unless you are extremely observant or technically minded, you might not perceive a problem.  The car starts, and everything works fine until 20 minutes down the road when it dies because the battery runs out of juice.  Not great for the customer, sidelined because of a burnt out light bulb, or perhaps the car cuts out just when you are attempting a pass or accelerating to beat traffic from a stop sign or left turn at an intersection.

In 2024, that's a safety recall which will ultimately cost the company a few million to correct, depending upon the fix and the number of affected units on the road.  In 1969, however, it's a problem that happens out of warranty, and there will be lawyers and accountants who determine whether the cost of potential lawsuits would exceed the cost of fixing the issue. 

That said, if there is a bypass wire or redundant circuit, then there's nothing to discuss.  I'm just having fun with the exercise, regardless, even though I'm admittedly not being very helpful. 

Cheers!

I'd be tempted to pull the bulb and try it.

I found these two diagrams in an old edition of Fix your Chevrolet. Apparently there is a resistance wire fed from the acc side of the ignition switch. So I think it would still charge with the bulb burnt out.