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ALT light on |
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White70JavelinSST
Supporter of TheAMCForum Joined: Aug/08/2012 Location: Minnesota Status: Offline Points: 4866 |
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Posted: Jun/03/2018 at 3:47pm |
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The ALT light comes on and glows brightly. when the engine is running. Using a voltmeter across the battery +/- terminals, and the engine running at 700 rpm the reading is 14.89 volts. ALT light is bright on. Engine off, key off, the voltage reading across the battery is 12.85. Any ideas? |
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70 Javelin SST, second owner, purchased 1972
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Samuelsc360
AMC Apprentice Joined: Aug/19/2017 Location: Chesnee SC Status: Offline Points: 145 |
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Replace the Diode Trio. |
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1971 SC360
1973 Ambassador Sedan 360 1973 Hornet X 360 1973 Javelin AMC 401 1975 Sportabout X 258 |
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billd
Moderator Group Forum Administrator Joined: Jun/27/2007 Location: Iowa Status: Offline Points: 30894 |
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No such thing on a 1970 Javelin........ that's later, in 71 and later. 1970 and prior didn't have a diode trio.
But it could be the isolation diode (or even regulator, fuse, or other things) Do a quick test - with the engine running check the voltage at the output terminal of the alternator. Now check the voltage at the terminal where the orange wire connects. You should have a difference of .8 volts (eight tenths of a volt) a maximum of one volt difference. If there is NO difference, or if the difference is over one (1) volt, you have issues with the isolation diode - the diode or diodes in that red heat sink plate on the alternator. Let us know if it's a 55 or a 35 amp, etc. and we can help from there (it's what I do - restore, rebuild, fix/repair, diagnose and more) 35 amp will have one diode in the plate, 55 amp is used on AC cars or heavy-duty charging system cars and has two diodes in the red plate. If you get .8v difference, then we move from there to diagnose, I have all of the original Motorola books and charts and more.
Check connections while you are at it - ensure the black wire is secure on the alternator, etc. PS - my bet is isolation diode. The reason is your voltage - it's about one volt too high. The regulator regulates at 14.8 on your car with an R2AM4 regulator but that's at the ORANGE wire - the output should be lower by .8 to 1.0 volts. Since your output isn't lower by about a volt, I suspect shorted isolation diode. |
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White70JavelinSST
Supporter of TheAMCForum Joined: Aug/08/2012 Location: Minnesota Status: Offline Points: 4866 |
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55 amp alternator Bill, purchased it rebuilt, installed when I added AC to Javelin. Still have the 35 amp Alt, original to car, still works, Still using original voltage regulator. Hoping to check things from your info tonight. I did read your posts at beginning of electric section about alternators too. Thanks Bill !
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70 Javelin SST, second owner, purchased 1972
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billd
Moderator Group Forum Administrator Joined: Jun/27/2007 Location: Iowa Status: Offline Points: 30894 |
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You're very welcome. Good luck - if I had TIME, I have a whole lot of stuff I'd either can and post, or write up.
Gotta keep these AMCs going. (and their owners but there's not a lot I can do about that part, LOL)
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White70JavelinSST
Supporter of TheAMCForum Joined: Aug/08/2012 Location: Minnesota Status: Offline Points: 4866 |
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O'Reilly can get them in rebuilt, 55 amp. guy said there's 50 of them available. Ten bucks to ship. $60.99 Limited Lifetime Warranty any opinions? |
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70 Javelin SST, second owner, purchased 1972
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White70JavelinSST
Supporter of TheAMCForum Joined: Aug/08/2012 Location: Minnesota Status: Offline Points: 4866 |
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Where do I find the diode trio part? Thanks
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70 Javelin SST, second owner, purchased 1972
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billd
Moderator Group Forum Administrator Joined: Jun/27/2007 Location: Iowa Status: Offline Points: 30894 |
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Yeah - good luck. There was recently someone who went through three of them before getting a good one that actually didn't cause the light to glow.
The picture looks "ok" but you don't know the pulley size, case type, and other things, that you actually get. Sure, the price is ok, but referring back to having to go through three to get a good one, and knowing they test them for charging only, and not much else, all I can say is "use at your own risk". I have a few reman on my shelves, using for cores, and have worked on several that were sent to me - that I discovered were reman, and the parts were wrong, questionable processes used (like painting over rust, weak diodes and so on) I can't compete with 60 bucks and won't try. The parts I use alone would be almost that much. A recent thread also showed someone with the alt light glowing - and after some back and forth, found it was the replacement alternator, he tried another and the light stopped glowing. That tells me the first ones had weak isolation diodes, not tested for the correct voltage drop under load. You will find people here who have gone with such things and have had good luck. That's how it goes. Some will say go for it - because they did and it was fine. I can't ever say "they are all bad", only comment based on experience, and what I have on my shelves (seeing first-hand what's inside and how they do it)
I also suspect "your mileage may vary" because a lot depends on the source of the part that the store sells. Some are better than others, for sure. |
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billd
Moderator Group Forum Administrator Joined: Jun/27/2007 Location: Iowa Status: Offline Points: 30894 |
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you can't because they were not used in 1970.
Motorola didn't use those until LATER. (there are also different causes anyway, don't just swap parts without a diagnosis) Further- Motorola never called 'em that, you'd need to ask for a FIELD DIODE if you went to a place that used Motorola terminology. |
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White70JavelinSST
Supporter of TheAMCForum Joined: Aug/08/2012 Location: Minnesota Status: Offline Points: 4866 |
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Billd, you are a wealth of electrical knowledge my friend. What I would enjoy knowing are a couple things, as I am beginning to think this is going to be a long drawn out process. Will it damage anything if I continue to drive the car with the ALT light on. The battery is an Optima Red Top. I haven't driven it more than ten miles with the light on so far. If that diode or diodes is failing/failed, does it tend to "cook" the battery? Other idea, until I can get the 55 amp Motorola repaired, if I put the 35 amp back in, will that kinda keep up with the charging when AC is on? Motorola's are finicky and few people rebuild them correctly. I almost gotta do it myself, and of course it's summer - more fun to drive em now than fix em... |
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70 Javelin SST, second owner, purchased 1972
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