Alternator energized while starting with HEI

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Logan

Logan

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I'm cleaning up under hood wiring and plan to install GM HEI on my '71 Dart 318. I searched FABO and see where the "IGN" and "resistor bypass" wires connect together and power the system. "IGN" is hot while running and "bypass" is hot during starting. This scheme has the alternator field powered during starting and I wonder if this is a good idea. After all, Chrysler seemed to go to great lengths designing an ignition switch to avoid this. Since the voltage drops during starting, it looks like the VR will ask for max alternator output while cranking. I was thinking of arranging two relays as shown to correct the situation. Comments?
 

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Lots of people have done this jumpering them together. I don't see the problem frankly. Field current is "about" 3-6 amps in general in the industry, depending on the alternator.

If your battery is so bad or so discharged that 3 or 4 amps is going to make the difference, it likely would not fire up, anyhow.
 
I build HEI kits for Mopars and I can tell you it doesn't bother a thing to leave that part stock.
Our installation instructions say to connect start and run wire together for the E coil power supply.
You might just cause yourself some charging system reliability issue's in the process of adding that extra wiring and relay.

But if you think it's better, then go for it
 
the only situation I could see it affecting would be if you had adapted a newr high amp alt and were using smallish alternator wires [like on mopes from the 60's] and add to that a weak batt and you then might start to think it could have some problems but i am not an electrical engineer at this time of the night {LOL}. Nothin wrong with extra saftey as long as you do it correctly.
 
In the standard ignition, while cranking, the ballast resistor still feeds the field in series. While the field current is a few amps, the alternator still places additional mechanical load on the starter, because it is being spun with some field current applied.

As others suggest, it still works fine.

Modern alternators have a delay circuit, it holds off generating for a few seconds after start, and also delays the energization of accessory loads Including the entertainment system.
 
The one issue that may occur is that the voltage to the charging system regulator will be low, and it will command full output from the alternator to try to compensate. This may present the possibility of damaging or burning a diode in an older alternator if you crank for a long time. One diode burned out would not be obvious to most drivers so this may be occurring without it being known.
 
The alternator is likely spinning too slow for that, an engine cranks at about 200 RPM. As I suggest, even with OEM wiring, field is applied via ballast resistance.

A bad diode often results in light flicker, due to interaction of regulator trying to compensate, and associated stability issue due to inductive lag in field control.
 
Modern alternators have a delay circuit, it holds off generating for a few seconds after start, and also delays the energization of accessory loads Including the entertainment system.


Did not know this. Explains my new Cummins Ram not charging when I first start it. Thanks:burnout: MT
 
nm9stheham said:
The one issue that may occur is that the voltage to the charging system regulator will be low, and it will command full output from the alternator to try to compensate. This may present the possibility of damaging or burning a diode in an older alternator if you crank for a long time. One diode burned out would not be obvious to most drivers so this may be occurring without it being known.
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LOL I don't see that happening. Pretty hard for an alternator at full field that's turning what, 200? (crank) RPM to generate much output?

Also, one blown diode often results in "tremendous" annoying radio noise.

"Back in the day" I was broke towards the "end of the month." This was the 69 383 RR, my firs Mopar, in San Diego. I was in the Navy. I think it had 2 good diodes left, after all that was !!45!! years ago. Anyhow, I drove it for maybe 2-3 weeks until I could afford to fix it.
 
I wondered the same thing, but figured as above that the ballast resistor still back-fed the field current, plus at low rpm there is little drag. Some late 70's cars (Aspen, Volare?) had some funky electronic circuit that might have disabled the alternator field while cranking (a few posts here).

Many GM cars (and my 80's M-B) use a Motorola-style alternator that is much different. It generates its own field current via a small 3-phase AC generator/diodes that is separate from the main power. It requires a "boot-up" current to get going, which it gets from several dash lamps. I wonder if many newer alternators are similar. My M-B manual says to immediately drive to the dealer if the "battery" lamp doesn't light when first turning the key to "ignition". M-B dealers probably charged a lot for that "emergency bulb replacement" job, and smiled wide as they offered free coffee.
 
BillGrissom said:
dealers probably charged a lot for that "emergency bulb replacement" job, and smiled wide as they offered free coffee.
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The few times I got screwed by some dealer, I got neither a smile, coffee, or vaseline!!!
 
Keep in mind that even with a low cranking speed, the alternator will be sourcing current into the very low impedance of the engaged starter, not just into the impedance of the battery. So a lot more current than normal can be sucked from the alternator through the starter, despite the lower cranking speed. Blowing diodes in alternators by jump starting another car with the jumping car's alternator running is a known problem......no reason to not think it would not happen even at very low speed.
 
nm9stheham said:
Keep in mind that even with a low cranking speed, the alternator will be sourcing current into the very low impedance of the engaged starter, not just into the impedance of the battery. So a lot more current than normal can be sucked from the alternator through the starter, despite the lower cranking speed. Blowing diodes in alternators by jump starting another car with the jumping car's alternator running is a known problem......no reason to not think it would not happen even at very low speed.
Click to expand...


I think you are completely off track, here. There is a point due to RPM that alternators simply don't put out much current, regardless of field current or load impedance. Lots of us have bypassed the ballast with no seeming ill effects

Jumpering a car with a RUNNING car is a completely different situation
 
Yeah, I am just trying to figure out why this was done in the first place. The voltage will drop at low RPM but I am not so sure about the alternator's current producing ability... I may have to read up more on that.... if I ever get time. I have never tested an alternator's current outputting ability at very low RPM to know...

It just may have been 'it works OK to have the ballast feed the VR' with limited current from the get go, it was just done that way. The parallel of IGN1 and IGN2 changes that. I also wonder if there are more dying alternators out there for such a reason than we ever know. And, I expect the early alternators with their older silicon, lower rated diodes would not be as robust as the newer one.
 
nm9stheham said:
Keep in mind that even with a low cranking speed, the alternator will be sourcing current into the very low impedance of the engaged starter, not just into the impedance of the battery. So a lot more current than normal can be sucked from the alternator through the starter, despite the lower cranking speed. Blowing diodes in alternators by jump starting another car with the jumping car's alternator running is a known problem......no reason to not think it would not happen even at very low speed.
Click to expand...

Yes there is a reason, 200 is not enough RPM, the battery is the dominant source. Give me a few days, I will use my clamp on DC ammeter and measure the alternator current. I will report back.
 
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