I think I have backfeed problem? - Page 1
Forum section: electronics / beginners

author: Vwfbk
author_rank: Newbie
timestamp: February 23, 2020, 03:07:34 am
content: Hello All! I’ve had an ongoing problem, when I turn off my ignition the engine doesn’t shut off. When I turn the key off the Alternator light stays lit, I lose power to my tach and I have 3 volts at my main relay at position 85 where ignition power comes in. I have 3 relays, one is my main relay, one for my fuel pump and one for my fans. When I checked the alternator to make sure it was charging correctly (it was) I pulled the lights on to put a load on it and the engine died. A friend suggested that I use an old school test light and touch it to position 85 on the main relay and his bet was that the engine would quit, and it did! Given that it did quit he suggested that the relay may be too big and that I should change it to a lower amperage to cure the problem I’m having. I wanted to post my finding to this forum to see if the smaller relay would do the trick? Thanks for the help!!

author: AVGresponding
author_rank: Super Contributor
timestamp: February 23, 2020, 09:30:33 am
content: The numbering convention for relays will tell you that 30 is common, and 87 NO, 87a NC. 85 and 86 are the coil.I'll confess that I've never heard of a relay being "too big" except in terms of unnecessary current capability.The question here is, why is the main relay coil getting enough current to keep it energised, when the ignition is turned off?The fact that you're measuring 3 volts there suggests that either a semiconductor is failing/has failed and is allowing current through when it shouldn't, or that there's a bad earth somewhere.Bad earths in cars are a very common cause of electrical problems, often showing up at this time of year (in the Northern hemisphere) when the weather has been wet, causing corrosion to accelerate.A can of WD40 or something similar and a bit of time making sure any connections to the chassis are good will probably fix it, but if it's a duff semi, that's a job for an experienced auto electrician.

author: Jwillis
author_rank: Super Contributor
timestamp: February 23, 2020, 03:23:05 pm
content: I think it has to do with where you've tied the ignition /micro power into the ignition switch . In your circuit the main relay is turned on by the ignition switch . When the ignition switch is turned off it's no longer turning the alternator field off because the relay is being fed by the still running alternator which keeps the relay on. This keeps the alternator field charged . Alternators are self exciting so they do not require a battery to continue producing current.The engine doesn't require a lot of current to keep running . So when you load the alternator with the head lights or a tester it's enough to deplete the field of the alternator and kill the engine.

author: AVGresponding
author_rank: Super Contributor
timestamp: February 23, 2020, 05:58:45 pm
content: It would be nice to have some more information.The field coil is a current sink, not a current source; it would require more than just that connected to 85 on the main relay to cause the engine to keep running. It sucks 10-20% of what the alternator kicks out in order to stay energised, but they aren't directly connected.The field coil is supplied through the main ignition relay, normally, so for it to be connected to the +ve coil connection on the relay, there's something wrong... it should be on 87, the NO.Question: Is the micro supplied from the ignition switch, or wired direct to the battery?

author: SeanB
author_rank: Super Contributor
timestamp: February 23, 2020, 06:27:09 pm
content: Vw and bad ignition switches, very common. Replace the switch assembly and likely the problem will be gone.

author: Jwillis
author_rank: Super Contributor
timestamp: February 24, 2020, 02:13:53 am
content: As soon as the stator voltage gets to about 0.7 volt the field diodes "open" and direct stator output to the rotor. This increases the rotor current flow and thus the strength of the spinning magnetic field, so the stator output rises.As the stator output rises it not only flows through the rotor, but also TRIES to flow backwards through the warning lamp bulb, reducing the current flowing through the bulb, so the bulb starts to dim.If the main relay is connected on the same line as the warning lamp ground which is connected to the ignition switch the relay becomes the new ground connection . The back flow from the stator will keep the coil on the relay charged. This keeps the charge light on because theirs no current coming from the battery to cancel out the back flow . The engine won't shut down.