...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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65 Laurentian post, 67 Grande Parisienne 4 door HT.
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.
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1970 Formula 400 Carousel Red on black (std) interior "no drivetrain option" car (same base drivetrain as GTO) 1:411 1970 Firebird Formulas originally sold in Canada
...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.
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.
Research tells me that the bulb in the ignition on alternator feed serves 4 actual functions.
One, as a warning light of the alternators function. When you initially turn the key to on, the lamp lights, and at this point that circuit is providing the required resistive (lower) voltage into the alternator, briefly creating a magnetic field through a complete circuit. You are good to go.
Two, the bulb provides the needed resistance. On a CS series (or any internal regulated alt) hooking the field input to a full 12-14 volts will quickly burn out the rectifier circuit. It must be about 4 volts less than full.
Three, No lamp at key "on" indicates a burned out bulb or a break in the circuit from the key block to the alternator.
Engine starts, bulb goes out as the alternator now creates it own voltage and now back feeds a positive current back through that bulb circuit. No ground circuit, so no lamp lit. All good.
Four, Lamp lights from dim to bright during engine on indicating a failing alternator (dim and up), or a battery voltage too low to support the field (full bright). The circuit current is getting to a variable ground.
I was just curious if GM provided a resistive bypass wire of sorts around the lamp in the case of a dead bulb.
I actually don't think they did. No evidence in the elect schematic. So no lamp at "on" then no charge I guess.
It's just something that is never thought of. But I suppose the bulb will have a long life as it never stays lit for long. Cheers!
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65 Laurentian post, 67 Grande Parisienne 4 door HT.
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, if you turn the key to on and see no lamp lit, then you'll apparently not have any charging. So that in itself is indicating an issue. But who ever looks for the lamp at that point anyway...
But I do like how you word it Mark!
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65 Laurentian post, 67 Grande Parisienne 4 door HT.
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.
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.
Looking closer... there it is Paul. A thin 24 gauge Brown/white wire off the ignition switch. Small and likely long enough to provide the resistance required. Conspicuous in that it's the only 24 gauge wire in the harness, but no mention of it's resistive function. Converges with the Alt "Lamp" circuit at the bulkhead plug.
So the two wires combined still must give enough resistance to keep the voltage low enough at the Alternator, and yet if the bulb burns out, it still provides enough current to excite the alternator into life.
