Robbieboy Posted April 25, 2019 Share Posted April 25, 2019 My alternator isn’t charging. I have no ignition light when engine is stopped and ignition is on. The ignition switch functions properly as in the fan etc. Gets its feed when it should. I’ve taken the plug off the back of the alternator. One of the thin leads had a constant 12v supply - even when the ignition was off. Is that right?? Im doing some checking before putting a new alternator on and blowing that up too. The vehicle is new to me, and I’ve found some horrors so an incorrectly connected wire would not be a surprise. Thanks in advance for any guidance. 👍👍 Quote Link to comment Share on other sites More sharing options...
Snagger Posted April 26, 2019 Share Posted April 26, 2019 The thin wire (brown with yellow stripe, if I recall correctly) should only be live when the ignition is on. The bigger brown wires should be permanently live as they run straight to the battery. The small brown wire is what excites the coils in the alternator when the ignition is on, and will take some current drain if the ign is on but the alternator is not producing charge. The dash warning light is in that wire's circuit, so when the ign is "earthing" to the coils, the light illuminates. Once the alternator provides charge, it will produce a 14v charge on that wire trying to reverse the current, but as the batteries and ignition switch will also be at 14v through the battery charging wires, there is no potential difference across the bulb and no current in that wire. The large wires will be at 12v with the alternator giving no charge, but they will not have current in a healthy alternator as the circuit is controlled by the diode pack on the back of the alternator. Once the alternator is providing charge, the wires will be fed in the other direction through the diodes and will have 14v. If you remove the plug from the alternator and still have a light when the ign is on, then there is a short between the small wire between the bulb and alternator plug. If you got no light, then try removing the plug and shorting the small wire to a good earth - with the ign off, there should be no light, but it should come on immediately if you short it to the battery -ve or a good earth. Check the condition of the big wires to make sure they have no shorts and the terminals are clean. If that is all good, then there is no reason a new alternator should blow unless you have a bad battery. 1 Quote Link to comment Share on other sites More sharing options...
rtbarton Posted April 26, 2019 Share Posted April 26, 2019 If the ignition light blows the alternator won't charge as the light is part of the charging circuit. Also check the brushes. 1 1 Quote Link to comment Share on other sites More sharing options...
Robbieboy Posted April 26, 2019 Author Share Posted April 26, 2019 11 hours ago, Snagger said: Thanks for this! Really much appreciated. I dynamos are carp, but being able to see thecrelays move in a control regulator gives you loads more clues as to what’s going on... 😜 our with he multimeter tomorrow Cheers Quote Link to comment Share on other sites More sharing options...
Robbieboy Posted April 26, 2019 Author Share Posted April 26, 2019 Thanks both. Really appreciate your time in helping out. 🤞🤞🤞 for a solution tomorrow. I’ll kerp you posted. 👍 1 Quote Link to comment Share on other sites More sharing options...
Recommended Posts
Join the conversation
You can post now and register later. If you have an account, sign in now to post with your account.
Note: Your post will require moderator approval before it will be visible.