When I turn the key off the engine continues to run until I step on the brake. Turning on the lights doesn’t make a difference but touch the brake and it’s off. Any ideas?

Ground issue?
Has it always been this way?

Just noticed this recently. Might have been there for a long time. I usually have my foot on the brake when I shut it off. Checked the bulbs and they are correct

Carbureted?

Absent the load from the D-Jet/L-Jet system, the alternator can back-feed the system through the GEN bulb.

Easy to test: pull the combo gauge, turn key on, ensure the GEN bulb is on. Start the car and verify GEN bulb goes off. Pull either wire from the GEN bulb (or remove the bulb itself) and turn off key; car should stop.

Your brake light may be creating a significant enough load so the alternator excite circuit can drop. So make sure you do this test without touching the brakes.

If that resolves it you need to install a diode in that circuit between the bulb and the alternator.

Poor electrical design.

This very thing is called out in the custom chassis harness I'm putting in my V8 build.

Need a diode in the alternator light circuit.

Yes 3.0 with carbs. I’ll try this tonight

I don’t know anything about diodes. What am I looking for and do I need to connect it in a specific direction?

I just did a few this a couple of weeks ago due to my Microsquirt setup. You need to insert it inline before your alternator warning light. It does need to be oriented properly as that’s what a diode does - allow current one direction but not the other. I will see if I can get a pic of it as installed behind my gauge. I bought a pack of 15 diodes and you’re more than welcome to have a couple if you want.

Hopefully this is the actual issue.

Here's a link on bird site about installing the diode for this purpose:

https://forums.pelicanparts.com/9871444-post14.html

Here's how I did it: I crimped or (poorly soldered) the diode to a couple of spade connectors. Then put it in line between the blue wire spade connector and the alternator light spade connector. You can't see the actual diode in the pics below because it's wrapped in electrical tape. I do recall that I initially put it in the wrong direction and the alternator charging light came on with car running. Switched it around, and the problem(s) were solved -- I had charging, and the car shutoff with the key.

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Billy,

Our electrical engineer Dad, would be laughing at you. Then giving both of us a lecture on diodes that would put both of us to sleep!

@beatnavy

Yes that is exactly my problem. I’ve got a trade for you. In exchange for a diode I’ll send you some of my handy dandy wire connectors. Got these from Mr Amazon. Put a wire in both sides and warm it with a heat gun. Solder will melt and the tube shrinks to seal everything up. The work real nice. Their cheep too.

Check your pm for my address and send me yours.

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I just bought a box / kit of those a couple of weeks ago off Amazon as well! They do work well.

It's all good, Billy. I'll get three of those out in the mail to you hopefully by tomorrow. I have another package I need to send to Jared that's been sitting in my kitchen.

I have a 3.0 with carbs and I don't have this issue?

Do you still have your relay board? I don't. I also have an exterior mount regulator. I doubt this has anything to do with you're issue, just putting it out there.

It seems to be related to MSD ignition systems that take power directly from the battery and sense the GEN light voltage. At the moment the ignition is turned off, the engine is still rotating and the alternator still producing voltage at the GEN light and somehow this keeps the MSD powered thinking the engine should still be powered.

It looks like the diode is in series with the GEN light but I'm not sure. Does anyone have a schematic drawing of where this diode goes?

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Thanks to all for your comments here. This solves the mystery of the place and purpose of the diode in the alternator wiring

@BeatNavy Hurry up and send that stuff to Jared so we can get some updates on his car... JK. You're a great part of this community with all your generosity!!! Thanks again for your contribution to my car!!

Heading over to the post office right now!

Thanks @JOEPROPER

Wow, if this is correct that has to be one of the worst design choices I have seen. By definition, shutting something off should result directly in that something shutting down.

The problem is that Porsche chose to use a simple incandescent light to "compare" system voltage (the S9 fuse) to alternator output (the D+ terminal). If the alternator was working properly then the voltages are the same and no light; if D+ is dead ground (bad alternator or VR) or way too high (bad VR not controlling the alternator) then the GEN light illuminates. Soup.

Problem comes when there's not enough load on the system to drop the voltage low enough to drop the alternator field circuit, so the alternator stays "excited" and producing current to the battery, which can then flow back through the GEN light to keep the alternator field excited, which continues to produce voltage to the battery...ad nausea.

Normally, the D-Jet/L-Jet system draws enough current to drop the system voltage low enough so this does not happen. But when you carb the car (or, in my case, install a very-low-amperage Microsquirt) that doesn't happen.

We run into similar problems with modern cars turned into racers: if you use a master cut-off switch to simply cut the battery wire, in most cases the alternator will continue to produce current to the system absent the battery and the car will continue to run. In that case you need to config the cut-off switch to also cut power to some other critical system to "break the chain", such as either power to the ECU or the field circuit wire. That's why many of the cut-off switches have additional normally-open/normally-closed circuits built in, in addition to the one for the main battery wire.

Yes, but the alternator light provides a calibrated current to excite the coils in the start-up phase. Thereafter, for those that can "read" the light, it provides troubleshooting information for when the system begins to fail.
I don't buy your carb argument; The same system was used by Porsche, and probably a lot of other manufacturers on cars with carbs. The 914/6 had carbs, the 914 /4 1.8 Euro had carbs.
When we learned how to drive, the first things we learned were: you turn the key this way to power up and start the engine; then you turn the key that way to shut it down. If it does not follow the simple rule, THEN YOU HAVE A PROBLEM.
The ignition switch provides switched power for a reason. I think the primary reason is to shut things down when the power is switched off.

I understand the issue and it seems there should be a better work around to this issue. Adding the diode in series with the GEN light does defeat one purpose of the GEN light and that is to alert the driver that the alternator diodes feeding the battery have died and the battery is not being charged. In this case the current through the GEN light flows from the VR to the battery.

It would seem a possibility to use a lower wattage GEN light so it doesn't power the car when the key is turned off and adding a diode across the GEN light to provide sufficient current for bootstrapping.

It's not "calibrated"; a simple touch of the D+ wire to the battery does it.

I know, as that's how I "excite" the alternator on my race car if I want to ensure the alternator is working. The car does not have the D+ wire (the GEN light) attached to anything, it's hanging in the breeze. Many times with RPM it'll come on by its own, but sometimes it does not. Touching it to battery voltage works.

So as long as there's enough current flow through the GEN bulb it'll work fine, no calibration needed.

My guess as to why it doesn't work with the LEDs? One of two possibilities: the LED just doesn't flow enough current through itself to excite the field circuit or...it's a Light Emitting DIODE... (likely the first).

GA

EDIT: As an aside, I do not recommend connecting the D+ directly to system/battery voltage if you're not using the GEN light. In my street car I have experienced - as others here have - a rare internal diode failure with the alternator where system voltage is still being controlled by the VR but D+ voltage is max, like 17+ volts. Symptoms are off or dim GEN light at idle that gets brighter with RPM (because there's a 5-ish volts differential between each side of the GEN bulb.) If you did not have the bulb in there you'd get 17+ volts current flow to fuse S9 and "I don't know what would happen with that."

It's really not an optimal design. But hey, it's roughly halfway through the whole history of the automobile so I cut it some slack.

Internally regulated 911 alternators need the diode, its a Porsche tech notice..

Installed the diode in the direction I thought it would work. Only had a 11.9 volt charge and it dropped lower when I turned the light on. The car would stop when i turned off the key. Turned the diode around and everything worked properly.

I had to do the same thing. I wonder if that diagram in that Pelican post was accurate.

Glad it's working out!

It's not. He posted:



You're not trying to keep power from the alternator getting to the bulb; you're stopping power from the S9 fuse getting to the alternator via the bulb.

Regardless, y'all fingered it out!

Ah, that makes sense. Thanks for the explanation, Greg!

And I thought everything I read on the Interweb was true...