[GMCnet] Alternator exciter wire power source

[Bruce Hislop via Gmclist]

So I have had a power resistor hanging around the coach forever that I planned to wire-in in place of the dreaded nichrome resistor wire. I checked
out the wiring diagram and for all years it shows the source of the exciter power as being from the ACCY power from the key.

I thought that's crazy because that means there is current flow in that resistor wire if someone leaves the key in the Accessory position for any
period of time. What's even more crazy is the GEN light is fed from the IGN contact. So that means there is current flow in the resistor wire
(alternator exciter wire), but the GEN light is not powered ON.

Is this the reason the nichrome wire has overheated in some coaches??? The key was left in the ACCY for long periods of time...? In which case the
APC cable would not prevent this current flow.

I could see these connections being reversed, then in the ACCY position the GEN light will be ON to remind you that you are discharging the battery,
but there would be no current flowing in the exciter wire.

So I planned on wiring the exciter current through my resistor from the IGN feed rather than the ACCY feed. The only time there should be current
flow in the exciter wire is when the Ignition key is ON and the engine is NOT running. In other words, the few seconds it takes to start the engine.
The only other time current will flow in the exciter wire is of course during an alternator failure.

So as part of my preparations to install my exciter power resistor, I proceeded to measure the voltage at Pin 1 of the alternator (exciter input) I
turned the key to ACCY and read 0 (zero) volts. So then I turned the key to IGN and read about 2 volts. So, my exciter wire only gets power with the
IGN ON (as it should be). I have tried to find the power source in the wiring harness, but that seems near impossible.

So are the wiring diagrams wrong and the exciter power is fed from the IGN feed only from the factory?

Or has my coach been modified to supply power from IGN as it should be?

Has anyone noted alternator exciter power in the ACCY position?

Can someone (preferably more than one person) test the exciter power at pin 1 of their alternator. This is on the 2 pin connector on the alternator.
Pin 1 is the exciter current and should show something between 2 and 4 volts with the engine not running. Try it on ACCY and IGN positions. The
second wire in this connector is the voltage feedback wire which should have battery voltage on it at all times.

Hopefully the wiring diagram is wrong and the exciter only gets power in the IGN position. If others have power in the ACCY position, well then I
don't know what the Engineers were thinking, but it could explain the overheated wiring harnesses.

Let me know what you find!

Thanks.


--
Bruce Hislop
ON Canada
77PB, 455 Dick P. rebuilt, DynamicEFI EBL EFI & ESC.1 ton front end
http://www.gmcmhphotos.com/photos/showphoto.php?photo=29001
My Staff says I never listen to them, or something like that

_______________________________________________
GMCnet mailing list
Unsubscribe or Change List Options:
http://list.gmcnet.org/mailman/listinfo/gmclist_list.gmcnet.org

[John R. Lebetski via Gmclist]

On accy I get about 4.5 VDC and on run (engine off) I get about 5.1VDC. Other terminal is always Batt voltage. Temp about 45F and was on shore power
with a combiner latched so about 14 VDC system wide.
--
John Lebetski
Woodstock, IL
77 Eleganza II

[Bruce Hislop via Gmclist]

John,
So from your readings, your alternator exciter wire is fed from the ACCY contact of the key-switch. I'm not sure why GM did that because the exciter
wire will be pulling about 1 amp in the ACCY position for no reason at all!

Now since the radio is powered from the BAT line from the factory, there is really no reason for ACCY position, but I could see a PO having the radio
rewired to the ACCY position after forgetting to turn the radio completely OFF and killing the chassis battery in short order. A stereo OEM radio
would consume several amps with its class A audio amplifier even with the volume turned down. It would make sense for PO's to have the radio changed
to the ACCY connection so you could be sure the radio was OFF!

But then turning the key to ACCY to listen to the radio for several hours would also put about an Amp of current in the exciter (nichrome wire)
dissipating about 10 watts into the wiring harness for hours on end. I could see this eventually toasting the insulation. However the APC will not
cure this issue.

Only moving the exciter power source to the IGN contact of the switch will fix this problem.

Thanks John for your feedback.

[John R. Lebetski via Gmclist]

Yes the only time I use Accy is to run the EL2 when in the garage so no fumes. Still confused why there is about 1/2 Volt difference between Accy and
run. Have not looked at schematic.

--
John Lebetski
Woodstock, IL
77 Eleganza II

[Bruce Hislop via Gmclist]

John,
The additional 0.5 volt comes from the GEN light being illuminated. It is powered from the switch IGN terminal only, so that gives a bit more exciter
current when in IGN position.

I can understand if they powered the GEN light on the ACCY terminal to remind you that in ACCY position you are draining the battery. The Nichrome
wire needs only to be powered in the IGN position. Maybe the Engineer intended to wire it this way, but it was a Monday morning when he drew that
circuit!

From the wiring diagrams the radio is constant powered from the chassis battery. I'm not sure why they did not power it from the House batteries
because the OEM radio consumed several amps. Likely because the OEM buzz box converter had no filtering or regulation and without a house battery
connected it would likely toast the radio.

So my Nichrome exciter wired has been reconnected to the IGN source where I believe it should be.

[John R. Lebetski via Gmclist]

Thanks for explaining the 1/2 Volt mystery. Good take aways for those reading this thread. This begs the question EOKO with points you have a crap
shoot as to whether points have landed open or closed. Not sure how warm the resistance wire and coil get with 100% dwell. With HEI I am making the
assumption that with no pickup pulses the module defaults to OPEN coil to ground path. Is that correct?

--
John Lebetski
Woodstock, IL
77 Eleganza II

[Bruce Hislop via Gmclist]

John,
I believe you are correct, that the HEI module open collector output to the coil would be OFF (no coil current) when the distributor is stationary (no
pulses from the reluctor). I would check that on mine, but it is EBL EFI with spark control so a different beast again.

Just a little tidbit of info for anyone. On electronic spark control, the reluctor pulse does not control the current spark event, it sets the event
timing for the next piston to fire. The reluctor pulse is set at a fixed 8 or 10 degrees BTC and this setting is used while cranking (under 400 RPM
or so). The spark for cranking is controlled by the distributor module itself.

Once the engine starts and gets over 400 RPM, the ECM sets an input to the distributor module high which switches the spark timing to an external
pulse from the ECM. This input wire has the connector you pull disconnect when you set the timing with ECM. This prevents the ECM from controlling the
timing so you can set the fixed reference timing of the distributor.

Under normal operation, the pulse triggers at say 10 degrees BTC, but that's too late for the current firing event, so it is used to time the next
piston firing event. The ECM delays its output pulse for the next piston to fire based on RPM and MAP spark table as well as other factors such as
knock sensor and lean cruise.

I've had this discussion with a couple mechanics who did not believe me!

[Matt Colie via Gmclist]

RF_Burns wrote on Wed, 17 February 2021 18:07

> John,
> I believe you are correct, that the HEI module open collector output to the coil would be OFF (no coil current) when the distributor is stationary
> (no pulses from the reluctor). I would check that on mine, but it is EBL EFI with spark control so a different beast again.

> <BIG snip>

> I've had this discussion with a couple mechanics who did not believe me!

Bruce,

When it comes to mechanics, your time is better spent arguing politics or religion. There is a reason why the code book for any ECU is big and thick.
Don't even bother trying to explain a ring counter.....

We had one in one of my labs one time and I was trying to explain that the system (this was port fuel with stereo EGOs) could actually adjust the
mixture on a per cylinder basis. He didn't even believe it while he watched and I took the control out of closed loop and then shortened the injector
pulse on one cylinder and he watched as the EGO spiked and the EGT for that cylinder dropped and the torque did as well. He still was marveling when
I set the control back to closed loop and it dialed the system right back in.

This was then and I am now about 20 years out of date on all this.

Matt
--
Matt & Mary Colie - Chaumière -'73 Glacier 23 - Members GMCMI, GMCGL, GMCES
Electronically Controlled Quiet Engine Cooling Fan with OE Rear Drum Brakes with Applied Control Arms
SE Michigan - Near DTW - Twixt A2 and Detroit

[Keith V via Gmclist]

Bruce,
Thats really interesting!
I always wondered how the ecu controlled the timing.
Yeah it's really hard to fire a spark 10 degrees ago, but not so hard to fire one in 35 degrees from now!
________________________________
From: Gmclist <gmclist...@list.gmcnet.org> on behalf of Matt Colie via Gmclist <gmc...@list.gmcnet.org>
Sent: Wednesday, February 17, 2021 7:02 PM
To: gmc...@list.gmcnet.org <gmc...@list.gmcnet.org>
Cc: Matt Colie <matt7...@gmail.com>
Subject: Re: [GMCnet] Alternator exciter wire power source