Relay board receives 12v via Alternator

[fasterlaster]

I'm posting this in the board assembly forum since this question is more about hookups than tuning. Mods, please move if not appropriate.

I've been working on getting my MS2 working in my 1962 VW Bus. Things are progressing well, but there is one wiring hurdle I have left. Due to the wiring options when I cut off the ignition the engine continues to run. I believe I understand what is occurring, but at a loss for a good solution other than adding an additional relay. There is a single terminal from the key switch that provides switched power. This terminal feeds one side of the idiot lights in the dash including the Alt/Generator light. The other side of the Alt Idiot light is fed by the D+ terminal on the Alternator. The switch also feeds a fused terminal in the fusebox and more importantly the original coil + terminal at the back of the bus. This 12V coil signal is what I use to trigger the MegaSquirt Relay switched source. Ignition on, MS is on. Ignition off, MS is off.......unless the engine is running.

What I believe is happening is when the engine is running and the ignition key is turned off the Alternator continues putting out +14V and the MS Relay board relay continues to receive a 12V signal keeping that relay contact closed. With the engine on, but ignition switched off the Alternator idiot light glows, showing current is flowing.

I thought of a diode in the path from Light to switch, but I think that would cause the light to not function correctly. The only other thought I had was another relay on the Alt side of the ignition switch. Attached is a basic schematic for reference. If anyone has suggestions I'm all ears, since I can't get out and test drive the bus until I can turn off the engine without unplugging a wire.

2017-11-22_00-04-44.png (39.71 KiB) Viewed 482 times

[DaveEFI]

Is this a conversion from a dynamo? Normally, the alternator output is connected direct to the battery, as the internal diodes stop the battery discharging via it when the engine is stopped. Only the warning light circuit - which also supplies the initial volts to 'excite' the alternator - is switched by the ignition switch.

But this applies a modern all in one alternator. Early ones with an external regulator may well be different. If one of those is fitted, I'd be inclined to find out how it was done on the vehicle it came from.

An alternator disconnected from the battery may well continue to produce an output while turning. It might be possible to add a load to it via a relay when the ignition is switched off - enough to pull the volts down below what the MS relay needs to hold it on.

[fasterlaster]

Thanks for the response Dave. The vehicle was originally a generator with external voltage regulator (6V even), but you are correct about the charging circuit. I didn't include that portion in the schematic, and the wire going to the idiot light is only the warning light circuit. What is currently installed is a 12V alternator with internal regulator. The idea of a secondary load is an interesting one, I'll have to noodle over that.

[DaveEFI]

Again, modern alternators usually have a diode in the warning light circuit (internal to the regulator) to prevent back feeding. You could try adding one.

[terryly]

What i did to mine
https://c1.staticflickr.com/4/3026/2869 ... 2aaf_o.jpg

[fasterlaster]

It just seems too easy! Diode city here I come. Thanks again guys.

[fasterlaster]

Big thanks to Dave and Terry. Got home from work, and found a random diode sitting in my junk box. Problem solved. I also went back and found dozens of solutions to this on various forums, unfortunately I wasn't searching for run-on so I had no luck the first time around.