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Alt light glowing at higher revs

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Blue68SST View Drop Down
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    Posted: Jun/19/2018 at 8:13pm
Driving home from a cruise night tonight, I turned my lights on for the first time in a long while. I then noticed my alt light glowing, brighter at higher revs, not at all at idle. Headlights off, just parking lights on produces the same result. Turning off the lights off altogether caused the light to fade, but it still glows faintly at higher revs. Putting on the turn signals at higher revs, with the lights off causes the alt light to glow brightly, again, not at all at idle. Bad ground? It's a 1968 AMX with I believe a Motorola 35 A alternator.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote 304-dude Quote  Post ReplyReply Direct Link To This Post Posted: Jun/19/2018 at 8:30pm
Check the condition of the fan belts. Any cracks or polished look in the V side may cause slipage. When more demand for current, such as lights, will cause the alternator to produce turning resistance. If the belt slips, the alternator won't produce enough power by drag.

Have had belts slip without squeal before. Possibly needs the belts adjusted, as in not tight enough.

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Post Options Post Options   Thanks (0) Thanks(0)   Quote Blue68SST Quote  Post ReplyReply Direct Link To This Post Posted: Jun/19/2018 at 8:45pm
The belt looks perfectly fine and has good tension. Worth changing?
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Post Options Post Options   Thanks (0) Thanks(0)   Quote 304-dude Quote  Post ReplyReply Direct Link To This Post Posted: Jun/20/2018 at 5:22am
Originally posted by Blue68SST Blue68SST wrote:

The belt looks perfectly fine and has good tension. Worth changing?


If belts are good and tight, then there may be a connection issue.

I would clean all the connections carefully, both at the alternator and regulator. One way to check if over charge condition is happening to cause light to function, is to hook up the voltmeter and test at high idle speed and gradually increase RPM. The voltage should be within 13.8v to 14.7v.
71 Javelin SST body
390 69 crank, 70 block & heads
NASCAR SB2 rods & pistons
78 Jeep TH400 w/ 2.76 Low
50/50 Ford-AMC Suspension
79 F150 rear & 8.8 axles
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Mopar_guy Quote  Post ReplyReply Direct Link To This Post Posted: Jun/20/2018 at 10:45am
Originally posted by Blue68SST Blue68SST wrote:

Driving home from a cruise night tonight, I turned my lights on for the first time in a long while. I then noticed my alt light glowing, brighter at higher revs, not at all at idle. Headlights off, just parking lights on produces the same result. Turning off the lights off altogether caused the light to fade, but it still glows faintly at higher revs. Putting on the turn signals at higher revs, with the lights off causes the alt light to glow brightly, again, not at all at idle. Bad ground? It's a 1968 AMX with I believe a Motorola 35 A alternator.



"Hemilina" My 1973, 5.7 Hemi swapped Javelin
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Post Options Post Options   Thanks (0) Thanks(0)   Quote 6768rogues Quote  Post ReplyReply Direct Link To This Post Posted: Jun/20/2018 at 6:27pm
Dealers used to paint the bulb black.
I had a 68 Jeep Wagoneer with the AMC 327 and the Jeep dealer routinely painted the bulbs black.


Edited by 6768rogues - Jun/21/2018 at 6:58pm
Content intended for mature audiences. If you experience nausea or diarrhea, stop reading and seek medical attention.

Located usually near Rochester, NY and sometimes central FL.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote 73Gremlin401 Quote  Post ReplyReply Direct Link To This Post Posted: Jun/20/2018 at 9:57pm
Originally posted by 6768rogues 6768rogues wrote:

Dealers used to paint the bulb black.


Ha!  I used a piece of electrical tape over the lens...

But seriously, everytime I've had an AMC that came down with a case of the dreaded motorola glowing ALT light, and the grounds all checked out OK and the belts were tight....it was a fried diode bridge on the back of the alternator that was the culprit.

In most of my cases, the alternator was still charging, just not at a rate that made the regulator or the dashboard light happy.

It is fixable, and you don't need to toss the whole alternator, or the regulator.  I'm going waaaay back in my memory banks on this, so I'm not going to offer any advice of exactly how to fix this, but surely someone else has come across this before here on the Forum that can help you get it sorted out.

Update - just read the very fine Billd write up on the test procedure - and yup, what he's written is what I remember, and it does often point to the diode bridge.  he points out a lot of the other things too, which is absolutely correct, and as old as these cars are getting now, means you really need to check them all.



Edited by 73Gremlin401 - Jun/20/2018 at 10:03pm
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81 Jeep J10 LWB 360/4-spd
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Post Options Post Options   Thanks (0) Thanks(0)   Quote FSJunkie Quote  Post ReplyReply Direct Link To This Post Posted: Jun/21/2018 at 2:04am
A bad connection causing high resistance somewhere in the circuit often causes this. A bad diode will do the same thing. 
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Talonfcc566 Quote  Post ReplyReply Direct Link To This Post Posted: Jun/21/2018 at 6:21am
Blue68SST,
I don't normally speak to any glow issue because Billd has put some much info out on this that following it will typically get you fixed.
Your problem sound somewhat like mine ... headlights on is what cause the Alt light on. Sometime at idle I saw it. I did everything anyone mentioned with no luck but learned a lot. I think FSJunkie might have it right. I fixed my issue by unplugging items. Taillight harness, running lights plugs, and then I got to my left headlight. When I unplugged it the alt light stopped coming on. I noticed something wasn't right with the connection and replaced the bulb even though it worked. Problem gone. I believe it was a voltage drop at that headlight causing the problem. Maybe this will help you? Best of luck either way.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote billd Quote  Post ReplyReply Direct Link To This Post Posted: Jun/21/2018 at 8:46pm
Originally posted by 6768rogues 6768rogues wrote:

Dealers used to paint the bulb black.
I had a 68 Jeep Wagoneer with the AMC 327 and the Jeep dealer routinely painted the bulbs black.

ANY dealership that did that was an idiot. AMC addressed the issue and it can be resolved with basic troubleshooting. Sorry, but that's a cheat way out of FIXING an issue - if it glows, there is a problem, PERIOD!
I'd fire any #$%#@ idiot that worked for me that painted a bulb instead of FIXING the problem. There is ALWAYS a reason. (and it's NOT normal no matter what the urban legend believers love to spread around as fact)

It is either a connection problem, ground problem, or in pre-71 cars, an isolation diode issue, and on 71 or later cars, a field diode issue, or as AMC admitted - a different bulb type is needed. 

If it only happens at higher RPM, you need to see "what's different" at higher RPM - more engine speed, engine movement, perhaps a BLOCK TO CHASSIS ground issue. Perhaps a GLAZED or slipping belt (both mentioned in both AMC and Motorola guides)
Can also be other connection issues - but basically on a 40+ year old car, assume all connections are suspect - all. Grounds included. 

Start simple - is the engine grounded properly to the chassis
Is the regulator grounded to the alternator (the black wire to the alternator.
(you can NOT look and tell - you have to remove the connection, look to see if it's shiny and if not clean it, and put it back together tight. AMC used three different connection methods on the regulator to alternator ground!)
Check all connections at the alternator. 
Depending on miles and age, and without any real solid information, and not having run any decent tests, if all else is GOOD, all connections, all fuses, grounds, etc. we don't know what condition the brushes or diodes are in. 

I'll try to find time to read the rest of the thread to glean some other information out, but them's the basics. 
The light SHOULD NOT GLOW. Only a fool would paint a bulb instead of fixing the bloody problem. AMC has a lot of information on the topic, TSBs, etc. and Motorola has as much if not more - and frankly, AMC cars were not that much different than others in many respects, it's just people FREAK OUT when they see motorola because of all the lies, false BS spread out there - and many repeating "urban legends" about them (and that's using a kind phrase compared to how I really feel about it)

When I fix an alternator (or car, for that matter), the light will NOT glow and no, it will not  be painted black. I've heard some whoppers, but if I found a car like that coming out of a dealership today, I'd have  a nice little call and letters to the district office of that car brand.
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