Alternator question - confusion (maybe Early-S-man??)
 

Today I was working on MikeZ's Frankenstein 356 6 cylinder conversion. The engine has an alternator with internal regulator. The D+ wire was not hooked up and consequently the battery was not charging. No surprise here. I guess I need to mention that the entire electrical system is far from anything standard but rather "unique" like the rest of the car? ;)

I located the alternator light in the instrument cluster with one side of the light hooked up to switched +12Volts (a.k.a. 15+). It is hot when the ignition switch is in the RUN position. I ran a wire from the other side of the bulb to the D+ terminal of the alternator. Everything worked fine. Switching the ignition to RUN and the alternator light comes on. Once the engine starts and revs a little the light goes out and the voltage comes up to about 14.2 Volts. The alternator is charging. When the engine idles very low the alternator light starts to flicker. Great, I thought I was done. Well.....

When I turned the ignition switch to OFF the engine kept running. The D+ feeds power through the alternator light into the ignition switch and keeps everything powered. The alternator light is lit and the engine keeps running. Once I pull the D+ wire from the alternator light the engine stops.

I checked the wiring diagrams for 83 911. From those it looks like the alternator light is simply hooked to +D and 15+. Nothing else. I am confused as to what prevents the D+ terminal from energizing +15 through the alternator light once the ignition switch is turned off. I am puzzled.

My only guess is that I have not enough load (instruments such as oil pressure, temp, etc are not used) on +15 other than the MSD ignition module. The voltage drop across the alternator light is not big enough to lover the voltage on 15+ to kill the ignition. What now? Could this be caused by a wrongly dimensioned bulb? Should I try and use a diode (anode to the alternator light, kathode to D+) in series with the alternator light or a ballast resistor between 15+ and ground.

Cheers,
Ingo

either put a diode in the D+ wire, or separate the fuel pump and ignition wiring from the D+ wire, so when the switch is shut off, there is nothing to feed back into. are they all on the same terminal now? the D+ wire becomes hot when the alt is charging. it's not a ground from the bulb that it used to be. that's what puts out the bulb, hot on both sides. so if the D+ wire is connected thru a switch terminal to the ignition and fuel pump power wires, there you have it.

John, the alternator light goes directly to +15. That is where the fuel pump and the ignition is powered from unless I miss something. I think D+ powers 15+ through the alternator light

(John, the alternator light goes directly to +15. That is where the fuel pump and the ignition is powered from)

there you go. diode or a separate 15 terminal.

Once again Dr. Walker has steered you in the right direction. A diode will prevent reverse flow through the warning lamp bulb into the ignition circuit. It does not matter which side of the bulb holder it's on, although it might be more conveninent to put it between the ignition switch and the bulb holder.

Thanks....

John,

I will try the diode. However, I am still confused why this does not happen on regular SC's

Probably because this has carbs and a fuel pump that is energized on the same circuit...it kept running with the ignition switch off and it died of fuel starvation....pump wasn't running when I tried it this morning when I turned the switch off....

We did the install....fixed it...thanks all....