Internally Regulated Alternator Trivia - Bootstrap Current?

[Captain Obvious]

I did the internally regulated alternator upgrade a little while ago, and everything is fine with the exception of one ignorable low priority issue. Such a small issue that I wouldn't consider it a "problem", but more of a "curiosity" and it highlights some (what I think is) interesting alternator trivia. Goes like this...

 

Sometimes when I first start my car and the RPMs are (cold) low right after I start it, the alternator doesn't spin fast enough to snap into regulation.This is evidenced by the charge lamp being lit and the voltage indicated by the voltmeter. I can give it a little gas to pull the car out of the garage and just that little bit of RPM increase is enough to get it to snap into regulation and from that point forward everything is fine... Charge lamp goes out and the voltmeter shows that the alternator is outputting a healthy 14ish voltage.

 

The way the system is supposed to work is that the alternator pulls a small current (bootstrap) through the charge lamp and this current energizes the field coils until the alternator is spinning fast enough to produce field coil current by itself. Once the alternator is spinning fast enough to produce it's own internally generated field coil energy, it snaps into regulation and the bootstrap current flowing through the charge lamp stops and the lamp goes out. On my car, I sometimes need to increase the RPM's a tiny bit in order for this to happen. I'm not going to do anything about it, but I COULD also fix this by increasing the amount of bootstrap current.

 

So... If I'm not planning to do anything about it, then why am I bringing it up in the first place? Because I noticed an oddity on the 78 wiring diagram that would support my theories and potentially fix the issue. In the Engine Electrical section of the 78 FSM, they describe how the system works (page EE-13). And on that page (and the following page) there is a circuit diagram of the system that includes a resistor in parallel with the charge lamp. This resistor could provide additional bootstrap current to add to what is pulled through the charge lamp:

 

This mystery resistor is not shown on the complete car wiring diagram, nor on any of the circuit detailed descriptions in the Body Electrical section. Does anyone know if this resistor truly exists? If so, what's the value? Is it built INTO the voltmeter gauge, or is it buried in a wiring harness somewhere? Does it plug in like the tach resistor does? I simply can't sleep at night not knowing!!

[Zed Head]

Don't the windings in the alternator play a part also?  My 76 doesn't do that,it just goes directly to lamp off.  The alternator is an old 1981 junk yard unit that came with a motor I bought.  Best one I've had so far.  Maybe your alternator is not up to par.  Is it a reman?

 

Of course, that's not why you posted, it's the other mystery.  I have a 78 voltmeter in the garage.  I can measure resistance, for comparison, but someone else will have to do the other gauge.  My parts have been working well together and can't be disturbed.  I should see lower on the 78, assuming the lamp is the same, if the resistor is in the gauge.

[jfa.series1]

Bruce - I'm getting all torn up because you can't sleep at night over this.

 

Jim

[beermanpete]

I have seen this on other cars as well. I don't think it is a problem or defect. If it bothers you raise the idle a bit. Increasing the bootstrap current might help. You could try a higher wattage lamp for the charge light. The resistor is likely to ensure the alternator works if the lamp burns out.

[Captain Obvious]

I have a 78 voltmeter in the garage.  I can measure resistance, for comparison, but someone else will have to do the other gauge.

 

I don't know if my alternator is a reman or not. There's a lot of things that could affect this, and I'm right at the borderline.

 

Maybe your alternator is a tiny bit more efficient at lower RPM's. Maybe your charge lamp is slightly lower resistance than mine and passes a little more current. Or most likely, I suspect that your idle is simply higher than mine when you first start your car. I've got my AAR adjusted pretty low and because of that, I've got a slightly low cold idle. But in any event, you're right... It's more the mystery of the resistor on the 78 diagram than the way my alternator behaves.

 

You don't need anyone else to take any readings to check your 78 voltmeter... Just measure the resistance across the CHARGE bulb wires (green and blue) and then pull the bulb out of the socket and check it again. If there's a resistor built into the gauge (which I really doubt), then you'll still see some resistance between green and blue even with the bulb removed.

 

If the resistor exists at all, I suspect it's somewhere other than the gauge itself.

[Captain Obvious]

The resistor is likely to ensure the alternator works if the lamp burns out.

 

I thought about this as well and I agree. If it exists... 

[Captain Obvious]

Bruce - I'm getting all torn up because you can't sleep at night over this.

 

Haha!! Thanks Jim!  I'll be OK. No need for us both to be struggling!   

[Zed Head]

 

You don't need anyone else to take any readings to check your 78 voltmeter... Just measure the resistance across the CHARGE bulb wires (green and blue) and then pull the bulb out of the socket and check it again. If there's a resistor built into the gauge (which I really doubt), then you'll still see some resistance between green and blue even with the bulb removed.

 

If the resistor exists at all, I suspect it's somewhere other than the gauge itself.

Funny, I've never actually looked closely at the charge lamp before.  I had assumed that it was some special, high-durability bulb designed for its critical function of notifying the operator of a charging malfunction.  But, it's just a plain old bulb, glowing on the back of a piece of red transparent plastic.

 

I looked at my 78 gauge, and there's no need really to measure resistance since you can see the short sections of wire directly from the bulb socket to the connector.  But I did anyway to make sure one of them wasn't a magical piece of restrictive wire.  Both were plain old copper strands.  One was 0.1 ohm higher than the other but I think that was just poor contact from my pointy probe.

 

I'd guess that you could measure from the L connection at the alt back to the gauge bulb socket on an intact  harness, and from the gauge bulb to the power source, and learn something.  I didn't keep any body harness, just the EFI harness and odds and ends, like gauges and relays.  Maybe the brake check warning lamp is involved, I think that it's in parallel with the L wire.  It stays on after the swap,on a 76 anyway.  Has to be relocated to another power source.

 

Seems reasonable though, that there would be a parallel circuit.  Otherwise, a blown bulb leads to a dead battery.  Defeats the purpose of the bulb.

 

Back to you...

[Zed Head]

Now I'm worried, since I relocated my brake check relay power.  When my bulb blows I might be running on borrowed time.  Maybe, assuming.  Looking forward to the solution to this mystery.

 

 

Edit - The brake check relay doesn't seem to be involved, by the wiring diagram, but the other power sources at the VR do.   I'll have to pull my charge bulb some day and see if the alternator still charges.  I'm guessing it won't.  My battery is relying on one of those old corroded bulbs in the gauge.

 

Edit 2 - after a better look using the atlanticz site diagram it looks like the brake check warning lamp relay is involved, in 1978 anyway.  Seems like it may behave in a similar fashion to the charge light also, on when engine's off (alt not charging), off when the engine is running and the alt is charging.  Maybe the brake light comes on also for 1978 when the alternator has a problem.

Edited July 27, 2015 by Zed Head

[Zed Head]

Maybe that squiggly is just a generic marking representing all of the other power sources for L.  I looked at the 78 diagram and see the fuel pump control relay among several other possible "resistances" all branched with the voltmeter L wire.  Wish somebody would do the saridout procedure on 1978.  That diagram is hard to read.

 

Edit - forgot about the atlanticz version.  Much better - http://www.atlanticz.ca/zclub/techtips/wiringdiagrams/78_280Z_wiring.pdf

 

 

 

I think I need to add a circuit to my L wire.  Thanks for the warning CO.

 

Edit 2 - I've woven a web of my own comments around your thread CO.  See above.  Hope they make sense in the end.

Edited July 27, 2015 by Zed Head

[Captain Obvious]

OK, nothing special about the 78 gauge. I would have been surprised if there was. So if that resistor DOES exist, it's not in the gauge. Thanks for checking. If it exists, it's probably a plug-in like the tach resistor, but I don't have any 78 harness stuff to to poke around with either.

 

About the brake warning lamp stuff...

 

In 78, the brake warning lamp check relay is wired in parallel with the charge lamp and hence, it will supply additional current into the alternator rotor windings at bootstrap. So in 78 at least, it might not be that a blown bulb would render the charging system incapacitated. Depends on the resistance of the relay coil. I don't know the resistances of either the charge lamp filament or the brake check relay coil, but it would be interesting data.

 

In my case, I've got a 77 with a charging system rewired like a 78, including the brake check relay. However, prior to my alternator upgrade changes, my brake check relay had already been disconnected because it wasn't working properly. So in my case, there's a chance that if I were to wire in a 78 style brake check relay the additional current draw through the relay's coil would be enough to get me up over the bootstrap hump.

 

So yes, your charging system (and mine and probably everyone else who has done the internally regulated alternator mod) is relying on that single corroded bulb. But in theory at least, you would know that you had a problem because the charge lamp would not light up when you turned the key to ON before you cranked it. You're SUPPOSED to scan the entire dash for anomalies before your start the engine. I know nobody does this, but you're supposed to verify that the brake and charge warning lamps light up properly. It's a bulb check safety procedure. Just like the bulb check mode on a new vehicle. Every time you get into the vehicle. Make sure the bulbs work -  Check Engine, SRS, BRAKE, TEMP. Yeah, I don't do it either.

 

In 76, the brake check relay was driven by the alternator as well, but it was done a little differently.

 

In 76 and 77, they used a set of normally closed contacts and energize the coil when the alternator is working properly. So if the alternator stops working and the N connection goes dead, the brake relay coil de-energizes and lights the BRAKE warning bulb.

 

In contrast... In 78 they use a set of normally open contacts and de-energize the coil when the alternator is working properly. So if the alternator stops working and the L connection goes dead, the brake relay coil energizes and lights the BRAKE bulb.

 

That's why you needed to find a different place to connect your 76 brake relay coil. The sense of determining proper operation changed between 77 and 78. They both need a "high" to shut off, but the 78 won't "supply current", only "not draw current".

[Zed Head]

You wrote all of that, posted it, and didn't need to edit?  Maaaan.  I'm impressed.  I was thinking the same about checking the lights before starting the engine, and often do, but it would be that one busy week that would get me.

 

FastWoman has a 78.  Maybe she'll check her circuits for the sake of your sanity.

 

In the meantime, I might just add a second circuit to the L terminal, just for that busy week when I don't check the light.  This will also become a recommendation for all future alternator swappers.