TheAMCForum.com Homepage
Forum Home Forum Home > The Garage > Electrical - non engine
  New Posts New Posts RSS Feed - Alt light is on/ ignition off
  FAQ FAQ  Forum Search   Events   Register Register  Login Login

Click for TheAMCForum Rules / Click for PDF version of Forum Rules
Your donations help keep this valuable resource free and growing. Thank you.

Alt light is on/ ignition off

 Post Reply Post Reply
Author
Message
jav70 View Drop Down
AMC Nut
AMC Nut
Avatar

Joined: Apr/20/2008
Location: New Jersey
Status: Offline
Points: 257
Post Options Post Options   Thanks (0) Thanks(0)   Quote jav70 Quote  Post ReplyReply Direct Link To This Post Topic: Alt light is on/ ignition off
    Posted: Sep/10/2014 at 4:31pm
Help..Just had my alternator rebuilt. When the ignition is in the off position the alt light is on. Turn it to on, and the light goes out. It is also out when the car is running. Voltage regulator?
AMC1970 javelin,1968 AMX 343, 1968 AMX 390
Back to Top
purple72Gremlin View Drop Down
AMC Addicted
AMC Addicted
Avatar
Charter Member

Joined: Jul/01/2007
Location: Illinois
Status: Offline
Points: 16591
Post Options Post Options   Thanks (0) Thanks(0)   Quote purple72Gremlin Quote  Post ReplyReply Direct Link To This Post Posted: Sep/10/2014 at 9:49pm
Think its the diodes in the alternator.
Back to Top
amcglass View Drop Down
AMC Addicted
AMC Addicted
Avatar

Joined: Dec/11/2011
Location: NW Ohio
Status: Offline
Points: 2588
Post Options Post Options   Thanks (0) Thanks(0)   Quote amcglass Quote  Post ReplyReply Direct Link To This Post Posted: Sep/11/2014 at 5:40am
Diodes I would think is a good place to start
Javmanpres

BBG Mark Donohue 401

74 Jav/AMX
71 Matador 2dr sedan delivery wagon
1902 Rambler Runabout   
70 T/A Javelin
Back to Top
rocklandrambler View Drop Down
AMC Addicted
AMC Addicted
Avatar

Joined: Feb/09/2013
Location: Nanuet, NY
Status: Offline
Points: 3953
Post Options Post Options   Thanks (0) Thanks(0)   Quote rocklandrambler Quote  Post ReplyReply Direct Link To This Post Posted: Sep/11/2014 at 7:16am
Just take it back to whoever rebuilt it and tell them the problem.
Past AMC's
1974 Hornet X (new)
1975 Gremlin X (new)
1964 Classic 660 Cross Country
1965 American 440-H
Back to Top
FuzzFace2 View Drop Down
AMC Addicted
AMC Addicted
Avatar

Joined: Jul/05/2007
Location: Angier, N.C.
Status: Offline
Points: 10343
Post Options Post Options   Thanks (0) Thanks(0)   Quote FuzzFace2 Quote  Post ReplyReply Direct Link To This Post Posted: Sep/11/2014 at 7:19am
I would also think the same as the others and to take it back but I would re-check to make sure you have the wiring right.
Dave ----
TSM = Technical Service Manual

75 Gremlin X v8 for sale
70 Javelin 360/auto drag car
70 Javelin 360/T5 Street car
Back to Top
billd View Drop Down
Moderator Group
Moderator Group

Forum Administrator

Joined: Jun/27/2007
Location: Iowa
Status: Offline
Points: 30894
Post Options Post Options   Thanks (0) Thanks(0)   Quote billd Quote  Post ReplyReply Direct Link To This Post Posted: Sep/15/2014 at 7:32am
Which alternator - 35 or 55 amp?
The 35 amp has a single isolation diode, the 55 amp a dual diode array. The 35's single diode is more simple in its mounting. The 55 is shifted to 180 degrees from the brushes and spans the negative diode mounting studs for its own mounting. If they messed up the insulators there it would likely burn out the internal diodes as it would short the output of the alternator. But in that case it would not charge.

The orange wire that comes from the dash ALT light connects to the positive diode array inside. The diodes normally block flow from the ignition switch through the light and to ground. With the switch off there's no power to the light so even a bad diode would simply ground the outside of the bulb, not power it. There would have to be power to the bulb somehow to have an internal diode cause this on 1970 and prior years.

HOWEVER, the isolation diode could do this - it's the only thing between that orange wire from the bulb and battery power. If the isolation diode was "bad" in a shorted way, then battery voltage via the output terminal could get to the orange wire, to the bulb where it would ground through one of the other items that's sharing the connection with the bulb at the ignition switch.

To test for that possibility, disconnect the battery wire from the alternator and see if the light goes out. If so then the isolation diode is shorted and not isolating. 

The bulb needs 2 things to light - power on one side, ground on the other. IF the key is off, there's no power on the key side. However, there MAY be ground on the key side through other bulbs or devices that share the spot on the ignition switch where that bulb normally gets power from.
OK, assuming that with the key off the inside can't power it, but the inside may ground it, that means there's power getting to it from the outside - the engine bay area. And the most likely spot for that is the isolation diode.

To test - car not running, key off, bulb lit, disconnect the voltage regulator connector at the regulator. Is the bulb still on? IF so, then it's not regulator. If not, then it's regulator.

Reconnect regulator, car not running, key off, CAREFULLY disconnect the battery wire from the alternator output stud. Did the light go out? If so, it's a bad isolation diode.
If not, then look for a mouse with insulation hanging out of its mouth.............. 
(Or compare your wiring to some of the pictures posted recently in this area.)
Back to Top
jav70 View Drop Down
AMC Nut
AMC Nut
Avatar

Joined: Apr/20/2008
Location: New Jersey
Status: Offline
Points: 257
Post Options Post Options   Thanks (0) Thanks(0)   Quote jav70 Quote  Post ReplyReply Direct Link To This Post Posted: Sep/15/2014 at 2:20pm
I decided to use it as a core and bought a reman at NAPA. The place that rebuilt it was close to where I work, which is an hour from home, and sort of out of the way. Installed the NAPA unit and everything is good! Thanks for the replies.
AMC1970 javelin,1968 AMX 343, 1968 AMX 390
Back to Top
jav70 View Drop Down
AMC Nut
AMC Nut
Avatar

Joined: Apr/20/2008
Location: New Jersey
Status: Offline
Points: 257
Post Options Post Options   Thanks (0) Thanks(0)   Quote jav70 Quote  Post ReplyReply Direct Link To This Post Posted: Sep/15/2014 at 2:22pm
To Bill D...Excellent reply. I will print that out and keep it in my Service Manual for the future. Thanks!
AMC1970 javelin,1968 AMX 343, 1968 AMX 390
Back to Top
davester304 View Drop Down
AMC Addicted
AMC Addicted
Avatar

Joined: Apr/05/2011
Location: Victoria BC
Status: Offline
Points: 621
Post Options Post Options   Thanks (0) Thanks(0)   Quote davester304 Quote  Post ReplyReply Direct Link To This Post Posted: Sep/16/2014 at 6:15pm
Just jumping in on this thread - sort of have the opposite. Got the Alternator checked - all good including the diode(s) - all worked fine for a few days, now, no Alt light with key turned on (not to start position) -until then the light came on and then started fine. Still started but now looks like no juice going back into the battery from the Alt - I am thinking it is the Voltage Regulator no longer doing its job? Got stranded at the gas station on my way to a show on the weekend too! Jumped it and got home no problem.
I have been having issues lately - that seemed to be solved for a few days -  if you have read my other postings in this category.
1970 Javelin SST
If you don't look back at it when you park it, you bought the wrong car
Back to Top
carnuck View Drop Down
AMC Addicted
AMC Addicted
Avatar

Joined: Mar/31/2010
Location: Seattle
Status: Offline
Points: 3942
Post Options Post Options   Thanks (0) Thanks(0)   Quote carnuck Quote  Post ReplyReply Direct Link To This Post Posted: Sep/16/2014 at 9:46pm
Could be the sensor wire at the connector broke.
Back to Top
 Post Reply Post Reply
  Share Topic   

Forum Jump Forum Permissions View Drop Down

Forum Software by Web Wiz Forums® version 12.03
Copyright ©2001-2019 Web Wiz Ltd.

This page was generated in 1.703 seconds.
All content of this site Copyright © 2018 TheAMCForum unless otherwise noted, all rights reserved.
PROBLEMS LOGGING IN or REGISTERING:
If you have problems logging in or registering, then please contact a Moderator or