Alternator Anomaly

Stu Rasmussen

Stu Rasmussen

New member
Jan 29, 2019
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When I purchased the Birchaven the PO replaced the alternator as part of the deal.

I have a persistent 0.3 ampere drain (regardless of whether the ignition switch is on or off) on the engine battery which goes away when I disconnect
the 'sense' connector from the alternator.

The new alternator is an AutoZone DL7157 which according to the specifications is an internally regulated unit.

I suspect the new alternator is defective as that 'sense' terminal should only be a floating reference for the alternator's internal regulator.

Also, the alternator voltage is about 15.5 volts when the engine is running, even into a fully charged battery. I suspect the alternator is running
wide open and will eventually cook my batteries if I leave this go too long.

I'm looking for confirmation of my diagnosis before pulling the alternator and trying for a 'lifetime warranty' replacement.

Any other thoughts?

Thanks in advance

Stu

--
Stu Rasmussen W7QJ
Silverton, OR
'77 Birchaven
 
Stu have you tried removing one leg [test both of course] of the battery isolator and seeing what happens?
--
Rich Mondor,

Brockville, ON

77 Hughes 2600
 
Internally regulated and internal regulator are two different things. If you are using an isolator you need the sense wire.
Where are you metering the 15 plus Volts? If its at the alternator main terminal that's about right since battery Voltage will be .7 Volts less
after the isolator. Also Voltage should drop after warmup.
Your reman alt could have s bad diode trio that feeds the regulator causing the drain.
--
John Lebetski
Woodstock, IL
77 Eleganza II
 
Stu

15.5 volts is too high.

The sense lead is exactly as you describe.

AutoZone is usually good on warranty claims. This one will fail their wiz-bang microprocessor tester.

HTH

73’s

DE AD0LF

Dolph Santorine

Wheeling, West Virginia

1977 ex-Palm Beach TZE167V100820
Howell EFI/EBL , Reaction Arms, Manny Transmission

>
> When I purchased the Birchaven the PO replaced the alternator as part of the deal.
>
> I have a persistent 0.3 ampere drain (regardless of whether the ignition switch is on or off) on the engine battery which goes away when I disconnect
> the 'sense' connector from the alternator.
>
> The new alternator is an AutoZone DL7157 which according to the specifications is an internally regulated unit.
>
> I suspect the new alternator is defective as that 'sense' terminal should only be a floating reference for the alternator's internal regulator.
>
> Also, the alternator voltage is about 15.5 volts when the engine is running, even into a fully charged battery. I suspect the alternator is running
> wide open and will eventually cook my batteries if I leave this go too long.
>
> I'm looking for confirmation of my diagnosis before pulling the alternator and trying for a 'lifetime warranty' replacement.
>
> Any other thoughts?
>
> Thanks in advance
>
> Stu
>
>
> --
> Stu Rasmussen W7QJ
> Silverton, OR
> '77 Birchaven
>
> _______________________________________________
> GMCnet mailing list
> Unsubscribe or Change List Options:
> http://list.gmcnet.org/mailman/listinfo/gmclist_list.gmcnet.org
 
It sounds as if you >may< have the wrong alternator fitted. If you have the original wiring, You do NOT want an internally regulated alternator,
often referred to as a 'one wire' alternator. The proper alternator has the regulator inside the alternator housing, but uses the external sense wire
for regulation. 15 volts at the alternator output should produce ~~ 14.5 - 14.7 volts at the isolator battery terminals. A bit high unless the
batteries are discharged. As noted above, take it to AutoZone, have it tested, and confirm it's externally sensed.

--johnny
--
Foolish Carriage, 76 26' Eleganza(?) with beaucoup mods and add - ons.
Braselton, Ga.
I forgive them all, save those who hurt the dogs. They must answer to me in hell
 
Stu,

You have at least a problem in the coach wiring. That sense lead is supposed to be part of the ignition circuit. So there should be no power there
when ignition or ACC are not powered.

300mA is a little high for this, but it is not that far out of acceptable.

The 15.5 at the machine may be acceptable, because the output is supposed to be 14.3~.5 over the sense lead. So, check it again at the service
terminals of the isolator.

Matt
--
Matt & Mary Colie - '73 Glacier 23 - Members GMCMI, GMCGL, GMCES
Electronically Controlled Quiet Engine Cooling Fan
OE Rear Drum Brakes with Applied Control Arms
SE Michigan - Twixt A2 and Detroit
 
I'm pretty sure it is the correct alternator - it has the 2-wire loom to
connect the sense (battery) separately to the alternator as shown in the
wiring diagrams.

It also cross-references to AutoZone's alternator for a 1977 Oldsmobile
455 which is the installed engine.

And the issue is that when the sense wire _is_ connected it creates a
0.3 amp continuous drain on the engine battery which goes away when the
loom is disconnected. That should be a high-impedance comparator input
to the built-in regulator and drawing 0.3 amps 24/7 is a _lot_.

Before doing the R&R I will make sure that the PO didn't somehow reverse
the two wires in the loom whilst replacing the engine (which could put
battery to the alternator rotor continuously). That's a long shot but
stranger things have occurred.

Thanks for the input

Stu

> It sounds as if you >may< have the wrong alternator fitted. If you have the original wiring, You do NOT want an internally regulated alternator,
> often referred to as a 'one wire' alternator. The proper alternator has the regulator inside the alternator housing, but uses the external sense wire
> for regulation. 15 volts at the alternator output should produce ~~ 14.5 - 14.7 volts at the isolator battery terminals. A bit high unless the
> batteries are discharged. As noted above, take it to AutoZone, have it tested, and confirm it's externally sensed.
>
> --johnny
>
 
I'm pretty sure it is the correct alternator - it has the 2-wire loom to
connect the sense (battery) separately to the alternator as shown in the
wiring diagrams.

It also cross-references to AutoZone's alternator for a 1977 Oldsmobile
455 which is the installed engine.

And the issue is that when the sense wire _is_ connected it creates a
0.3 amp continuous drain on the engine battery which goes away when the
loom is disconnected. That should be a high-impedance comparator input
to the built-in regulator and drawing 0.3 amps 24/7 is a _lot_.

Before doing the R&R I will make sure that the PO didn't somehow reverse
the two wires in the loom whilst replacing the engine (which could put
battery to the alternator rotor continuously). That's a long shot but
stranger things have occurred.

Thanks for the input

Stu

> It sounds as if you >may< have the wrong alternator fitted. If you have the original wiring, You do NOT want an internally regulated alternator,
> often referred to as a 'one wire' alternator. The proper alternator has the regulator inside the alternator housing, but uses the external sense wire
> for regulation. 15 volts at the alternator output should produce ~~ 14.5 - 14.7 volts at the isolator battery terminals. A bit high unless the
> batteries are discharged. As noted above, take it to AutoZone, have it tested, and confirm it's externally sensed.
>
> --johnny
>
 
Yeah, that seems like how it _should_ be so I am digging a little deeper
before getting out the wrenches.

And it may or may not be related: the heater/AC fan comes on 'low' all
the time when the ACCessory lines are powered. Seems odd you can't
completely shut off the fan (and eliminate the battery drain therefrom)
if all you want to do is listen to the radio.

So I am probing further . . . . and further . . . .etc.

Stu

> Stu,
>
> You have at least a problem in the coach wiring. That sense lead is supposed to be part of the ignition circuit. So there should be no power there
> when ignition or ACC are not powered.
>
> 300mA is a little high for this, but it is not that far out of acceptable.
>
> The 15.5 at the machine may be acceptable, because the output is supposed to be 14.3~.5 over the sense lead. So, check it again at the service
> terminals of the isolator.
>
> Matt
>
 
If it fits the bracket and wiring harness fits ,it is correct.

On Mon, Dec 30, 2019 at 7:49 AM Stu Rasmussen via Gmclist <

> I'm pretty sure it is the correct alternator - it has the 2-wire loom to
> connect the sense (battery) separately to the alternator as shown in the
> wiring diagrams.
>
> It also cross-references to AutoZone's alternator for a 1977 Oldsmobile
> 455 which is the installed engine.
>
> And the issue is that when the sense wire _is_ connected it creates a
> 0.3 amp continuous drain on the engine battery which goes away when the
> loom is disconnected. That should be a high-impedance comparator input
> to the built-in regulator and drawing 0.3 amps 24/7 is a _lot_.
>
> Before doing the R&R I will make sure that the PO didn't somehow reverse
> the two wires in the loom whilst replacing the engine (which could put
> battery to the alternator rotor continuously). That's a long shot but
> stranger things have occurred.
>
> Thanks for the input
>
> Stu
>
>

> > It sounds as if you >may< have the wrong alternator fitted. If you have
> the original wiring, You do NOT want an internally regulated alternator,
> > often referred to as a 'one wire' alternator. The proper alternator has
> the regulator inside the alternator housing, but uses the external sense
> wire
> > for regulation. 15 volts at the alternator output should produce ~~
> 14.5 - 14.7 volts at the isolator battery terminals. A bit high unless the
> > batteries are discharged. As noted above, take it to AutoZone, have it
> tested, and confirm it's externally sensed.
> >
> > --johnny
> >
>
> _______________________________________________
> GMCnet mailing list
> Unsubscribe or Change List Options:
> http://list.gmcnet.org/mailman/listinfo/gmclist_list.gmcnet.org
>
--
Jim Kanomata
Applied/GMC, Newark,CA
jimk
http://www.appliedgmcrvparts.com
1-800-752-7502
 
Low speed blower on with the ignition is a stock situation. It was also done on GM cars of the period.

--johnny
--
Foolish Carriage, 76 26' Eleganza(?) with beaucoup mods and add - ons.
Braselton, Ga.
I forgive them all, save those who hurt the dogs. They must answer to me in hell
 
The Autozone DL7157 alternator is the correct one to use. It is a 100A
unit that is a common upgrade for our GMC's. I have use that alternator
for over 13 years in our first coach and had one failure in that time
frame. It replaces the factory stock unit without alteration which is
typically an63 or 80A rated units. It is advisable that you increase the
red wire between the alternator and the isolator/combiner depending which
you have at lease one size. The factory wire is a #10 gauge wire and it
would be good to go to at least a #8 gauge wire. I know a a number of
people including myself that have run the #10 gauge wire without issue, but
they and I had no excess 12VDC load needs that would damage (hot and melted
insulation) the wire. The biggest load is right after you start and the
charge tapers off fairly quickly. The only adder other than the wire
upgrade would be adding a double pulley and belts.

https://www.gmcrvparts.com/ProductDetails.asp?ProductCode=G112

As pointed out, without the sense wire the alternator does not alternator
correctly or at all. The alternator output should be around 14.7 +/- Volts
and maybe just a little less at the isolator/combiner terminal. I would
check the terminal ends of the sense wire and possibly change both out.
The cable ends on most all cables should be suspect if they are very old.
They may look good, but could be corroded internally, don't ask me how I
know!

From your comments on the HVAC system fan running on low,, that is a
feature added to the 77/78 Type II units as flow thru ventilation. You can
disable it by doing the following mod:

http://www.gmcmotorhome.com/tech/heatfan/index.html

You should take the resistor pack out and check for burnt out segments.

These comments are possible suggestion and mods to some of you issues,
yours may vary.

J.R. Wright
GMC Great Laker
GMCGL Tech Editor
GMC Eastern States
GMCMHI
TZE Zone Restorations
78 Buskirk Custom 29.5' Stretch
75 Avion (Under going Frame up Restoration)
On Location in Tucson

On Mon, Dec 30, 2019 at 8:56 AM Jim Kanomata via Gmclist <

> If it fits the bracket and wiring harness fits ,it is correct.
>
> On Mon, Dec 30, 2019 at 7:49 AM Stu Rasmussen via Gmclist <

>
> > I'm pretty sure it is the correct alternator - it has the 2-wire loom to
> > connect the sense (battery) separately to the alternator as shown in the
> > wiring diagrams.
> >
> > It also cross-references to AutoZone's alternator for a 1977 Oldsmobile
> > 455 which is the installed engine.
> >
> > And the issue is that when the sense wire _is_ connected it creates a
> > 0.3 amp continuous drain on the engine battery which goes away when the
> > loom is disconnected. That should be a high-impedance comparator input
> > to the built-in regulator and drawing 0.3 amps 24/7 is a _lot_.
> >
> > Before doing the R&R I will make sure that the PO didn't somehow reverse
> > the two wires in the loom whilst replacing the engine (which could put
> > battery to the alternator rotor continuously). That's a long shot but
> > stranger things have occurred.
> >
> > Thanks for the input
> >
> > Stu
> >
> >

> > > It sounds as if you >may< have the wrong alternator fitted. If you
> have
> > the original wiring, You do NOT want an internally regulated alternator,
> > > often referred to as a 'one wire' alternator. The proper alternator
> has
> > the regulator inside the alternator housing, but uses the external sense
> > wire
> > > for regulation. 15 volts at the alternator output should produce ~~
> > 14.5 - 14.7 volts at the isolator battery terminals. A bit high unless
> the
> > > batteries are discharged. As noted above, take it to AutoZone, have
> it
> > tested, and confirm it's externally sensed.
> > >
> > > --johnny
> > >
> >
> > _______________________________________________
> > GMCnet mailing list
> > Unsubscribe or Change List Options:
> > http://list.gmcnet.org/mailman/listinfo/gmclist_list.gmcnet.org
> >
> --
> Jim Kanomata
> Applied/GMC, Newark,CA
> jimk
> http://www.appliedgmcrvparts.com
> 1-800-752-7502
> _______________________________________________
> GMCnet mailing list
> Unsubscribe or Change List Options:
> http://list.gmcnet.org/mailman/listinfo/gmclist_list.gmcnet.org
>
 
The saga continues.

I pulled the alternator and found the nearest stock was about 40 miles away.

So I headed that way but stopped at an en-route AutoZone to have the first one tested since an 80-mil round trip with a good alternator would not
improve my mood.

Hooked it up to the magic tester (and three's another sad tale - the parts guy had no concept of how belts should work and it was sad to see the
current state of 'technicians' - but I digress) and it FAILED, so continue the journey to an in-stock alternator.

Exchanged it for a new re-manufactured unit - but we could not test the replacement at that store because their tester was smoked.

So, back to home base, installed the new alternator and . . . . and . . . . and the new one also draws 0.3 amps 24/7.

Either I got two duds or something else is happening. Tomorrow's project is pulling the new one and getting it tested (after doing some more probing
and checking).

Stay tuned.
--
Stu Rasmussen W7QJ
Silverton, OR
'77 Birchaven
 
I went through a similar thing with AZ in Georgia. Colonel Ken gave me and the alternator a ride to the AZ store. It tested bad but they did not
have one. We went to a second AZ store about 10 miles away that had one, but insisted on testing the original a second time. It also tested bad. I
got a replacement and it failed the same way when I installed it. This caused me to diagnose the problem farther and I found my problem was a bad
crimp connector on the APC and not either of alternators. I still had both of them because I had to sway my double pulley from one to the other.

I diagnosed and fixed the problem using the "new" one. Then I reinstalled my original which is still running fine many years later.

In return process of the "new" one I found that they do not know how to test a remote sense GM alternator. The needed to use a different cable which
neither store had. So if you want you want a new alternator from AZ, and maybe others, just take your's in to AZ and have it tested. It is
guaranteed to fail.

We had a similar problem on Dan Gregg's coach at a AZ store in Indiana. We went through 3 "bad ones" before we smartened up and found the problem to
be in the coach and not the alternators.

I have never seen a GM remote sense alternator test good at an AZ store.

If you get a replacement from them, do not just walk out of the store with it. Make them test it before they hand it to you. After enough "new" ones
fail their testing, they will figure out that they are testing it wrong or do not have the right tester set up.

Did you see the same .3 amp leak when installed on their tester?

I just remembered that you said their tester was broken. That makes it a little more difficult.

--
Ken Burton - N9KB
76 Palm Beach
Hebron, Indiana
 
Stu,
Have you metered the 2 pins to see which pin is wired to the battery Sense wire and to be sure the Sense wire is going to the correct alternator pin?

- Pin 1 is the Excitation power (OEM #16 Brown wire)(powered from IGN only) and should have maybe 6V with IGN ON and engine not running (voltage
dropped through the dreaded Nichrome wire)

- Pin 2 is Battery Voltage Sense. (OEM #12 white wire) Should be constant 12V from battery, its powered from the same wire as battery going to the
ignition switch. This should be a high resistance input to the regulator but I cannot find any current specs for it. I suspect no more than a couple
mA.

From the Alternator schematic, the Sense wire goes through a couple of resistors. These resistors have no values on the schematic I have, but I
suspect maybe 5k resistance in each circuit leg, so about 2.5K ohms input resistance which equals approx 12V/ 2.5K ohms = 4.8 mA. I can't see this
circuit "shorting out" causing a 300mA draw.

So that leads to the Exciter power pin 1. If you read some voltage there, then that is likely where your 300mA is going. There should be no voltage
on that pin with the IGN OFF. The PO may have done some mods, or maybe the Nichrome wire has melted in your wiring harness giving a phantom
voltage... however I would expect to see current flow to other circuits with the alternator plug pulled.

If you indeed see power on Pin 1, then I would remove the Brown wire from the plug and plug back in with only the White wire connected and see if your
300mA draw is gone. If so measure the voltage at the Brown (Excitation) wire and I would expect you to see 12V when it should be Zero volts. You
would then need to troubleshoot the source of that power. The Excitation wire should only have power with the IGN switch in the ON position. FYI,
300mA @ 12V is about a 40 ohm load. The PO may have bypassed the Nichrome with with a fixed resistor, but powered it from a constant source. However
300mA is enough to kill a good battery within a couple of days, so this would have presented an on-going chassis battery dead problem.

Here is an excellent Delco service manual on our alternators:
http://www.gmcmhphotos.com/photos/data/6522/Delco_Alternator.pdf

--
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
 
Glad you figured it out. But the 2 cavity connector should only go in one way and be held by a locking tab. I assume the locking tab/key is broken
off on yours. That said anything can be forced with enough ambition. Kind of how nothing is fire "proof". It all depends on how much heat is in the
equation.
--
John Lebetski
Woodstock, IL
77 Eleganza II
 
For what it's worth, my connector also goes in both ways -- and it does
have the locking tab in place. The APC cable also fits on the
alternator either way. It might be a flaw in how the rebuilt units are
made these days.

-Dave
1978 Transmode near Pittsburgh

-----Original Message-----
From: Gmclist [mailto:gmclist-bounces] On Behalf Of John
R. Lebetski via Gmclist
Sent: Thursday, January 02, 2020 5:06 PM
To: gmclist
Cc: John R. Lebetski
Subject: Re: [GMCnet] Alternator Anomaly

Glad you figured it out. But the 2 cavity connector should only go in
one way and be held by a locking tab. I assume the locking tab/key is
broken
off on yours. That said anything can be forced with enough ambition.
Kind of how nothing is fire "proof". It all depends on how much heat is
in the
equation.
--
John Lebetski
Woodstock, IL
77 Eleganza II

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