Alternator diode for 83 SC

[shamrok]

Just getting an EFI/ITB project completed after a top end rebuild. Last issue (ha, yeah right!) I'm running into is that the car won't shut off. Reading the archives, it looks like 84 and later cars had a diode behind the charging light.

My SC has no such diode and I'm wondering how the normal SC charging logic worked with regard to the blue exciter wire? I eliminated a ton of the stock wiring, so it's possible I accidentally got rid of the stock diode somewhere, but I can't find a diode in the wiring diagrams anywhere and not sure where it should be.

Alternator appears to be charging fine (13.4-13.5v measured), so I believe the charging circuit is good, I just think there is some backfeed from the alternator while it's running.

[Trackrash]

Do you have an alternator with internal or external regulator? And is it the original alternator to the car?

[Superman]

'83SC alternators had internal diodes/voltage regulators.

[dannobee]

The alternator light is wired into the exciter circuit. When you shut the key off, the little bulb won't pass enough current to keep the ignition circuit powered up and the engine dies. If you bypassed the bulb and just substituted a wire, enough current back feeds from the alternator, back through the blue wire, and into the ignition, allowing the engine to stay running with the key off.

[shamrok]

So I’m still running the blue wire to the alternator. It’s one of the few wires I retained from the harness, and the alternator light in the gauge appears to be functioning normal. I don’t think anything is bypassed, but I’m guessing maybe the diode in the alternator might be bad, which shouldn’t (?) prevent the light to do its thing.

Any harm in just running a diode elsewhere along the blue wire, like behind the dash light in the later cars?

[Walt Fricke]

No diodes involved here, external to the alternator. About 6 of them inside it, but they can't cause this problem whether they are fine or burned out.

The internal regulator alternators required more exciting current - at least in order to start charging right away. This was done with a bulb which drew more current (lower resistance). The bulb Porsche chose for this purpose has a larger diameter base, which won't fit the bulb holder in the speedometer, so you can't just switch in the later bulb into the earlier holder.

So to use the old bulb and bulb holder, the resistance must be lowered. Porsche's tech bulletin for this shows using a 5W 91 ohm resistor (a fat sucker) across the bulb's two wires at the holder. One of these wires comes (via the ignition switch) from the battery, the other from the alternator.

With the engine not running, but key on, current passes through the bulb from the battery to a ground in the alternator,and the bulb lights. When the engine starts and the alternator starts putting out current, the voltage difference at the two sides of the bulb is so small that the bulb won't light - which is what you want.

When you shut the key off, the fuel pump stops being connected to a source of 12 volts (because there is a relay involved), and so does the ignition coil/CD box (because it gets its power via the ignition switch).

In racing, where an external cutoff is required to disconnect the battery, this issue comes up all the time - on a running engine, lifting either of the battery terminals won't stop the engine (if the key is on), since the alternator circuit provides the needed volts/current. These cutoff switches, to be efficient, have an additional switch on the cutoff, which is wired to shut off the fuel pump or the ignition system in a way that current from the alternator can't get to one or the other of those critical components. (there is a way of doing without this second switch, but on cars with the alternator in back and the battery in front you have to run an extra wire, which is always hot, from the battery back to the rear of the car - not the best way to be sure there can be no sparks in a crashed car)

So you have messed up the wiring somewhere, allowing power from the alternator to bypass the ignition switch to run the fuel pump.

The SC's CIS has a safety system - if no air is flowing through the intake, the fuel pump is shut off because a switch inside the CIS plumbing is closed to ground when the air sensor plate is in its engine off position. If you want the fuel pump to run without the engine running - like to check fuel pressure - you pull the connector to this switch (back behind the CIS stuff where it is hard, but not impossible, to reach. This lifts the ground, which in some almost magic fashion closes the fuel pump relay up front. Your aftermarket intake doesn't have this.

However, this trick to run the fuel pump with a stopped engine does require the ignition to be turned on. If wiring was done without understanding this, the most I'd expect would be a fuel pump which ran whenever the key was on. As long as you turn the key off, the pump should stop.

But somehow you've created some paths you didn't mean to create. Your aftermarket ignition system, for instance, should run through a relay (back in or near the engine compartment, and with its high amp power coming from the big starter cable so you have minimal voltage drop to the ignition). As long as you control this relay with a wire which is only hot with the key on, that should stop your engine just fine.

You need to get back to the wiring drawing board, and trace the relevant circuits.

[Superman]

Thanks, Walt. I did not understand a word you said but I am glad you said it.

For me, chasing down electrical problems is a SLOW process of, as you say, tracing and understanding circuits individually. One at a time. According to spec. I can read music and electrical diagrams at about the same rate. S - L - O - W

[Drbraunsr]

To get my '80 SC EFI project to shut off required a relay change from black to red (which has a diode). The relay I changed was in slot #2, counting from the firewall.

[shamrok]

Regarding my alternator wiring, it consists of just brown ground to engine case. Red to starter. Blue to blue in stock harness, that's it.

What you say about the fuel pump is something that concerns me and has me looking at possibly replacing with aftermarket, as it is running anytime that my ignition is in ON. I acknowledge that the stock setup had the intake air switch, which I obviously eliminated. But the stock pump is wired according to the below diagram.

As far as the fuel pump and ignition and everything, they're all on standard automotive relays according to the below diagram. The car will not shut off if the ignition is turned off, but the constant 12v is still hot. Conversely, if I turn off the constant 12v, but keep the key in the ignition ON position, the car will also not shut off. I must remove both sources of power for it to shut off.

If this diagram is not compatible with the stock Porsche setup, I think I may just wire the fuel pump relay to only get power via the ignition switch (to both pins 30 and 86 on the relay). If it's not in fact a diode that's required, I'm kind of stumped.

[dannobee]

Try this: With the engine running and the ignition switch off, pull the blue alternator exciter wire from wherever you wired it in. If the engine now dies, it's wired wrong.

The bulb must be in line between the ignition switch and the alternator, otherwise the blue wire will flow enough current to supply the ignition and not allow the engine to shut off. The bulb must be in series, not parallel with the blue wire.

[shamrok]

Okay, I'll give that a shot. I didn't modify any wiring from the ignition switch and just tapped into the blue wire from the engine harness before the 14 pin connector (which has been eliminated). Looking at the SC wiring diagrams, it's not very clear how I would otherwise attack this, as I don't understand from this how the blue wire interacts with either the bulb or the ignition switch, it just goes into pin 11 of the 14 pin connector, then....? I see a black goes back to the alternator, which I did not do, but I'm not sure of the logic there, maybe it's how the blue wire interacts with the internal diodes of the regulator? But then they both come and go from D+ and that's where I lose comprehension of what's going on, haha.

[al lkosmal]

I've run into this issues with installs ......here's an earlier post where i've described the fix for the run-on issue...scroll down to post #14

https://forums.pelicanparts.com/porsche-911-technical-forum/982002-msd-diode-placement.html



regards,
al

[shamrok]

Awesome, thanks Al. The more I search here, the more I see I'm not the only one who ran into this with EFI conversions.

[Walt Fricke]

Diodes - I now recall how a diode might be mentioned: the red stock fuel pump relay has one. You don't necessarily need a relay with a diode, because things work fine with, say, the horn relay, which is black and doesn't have an internal diode. The internal diode is there to prevent a sort of reverse voltage when things are suddenly disconnected, similar to how a spark plug works. But it seems this may be overkill, and isn't connected to the wiring problem you had.

[dannobee]

Dunno if it's actually a reverse voltage, per se, but an induced voltage because when you remove power from the coil, the iron core drops back down because of the spring tension and for an instant it becomes a generator and induces a voltage of up to 70 volts. That can harm some electronics. The diode eliminates that spike.

[shamrok]

Quick update, with the ignition off and disconnecting the blue wire, the engine won't shut off, so I don't believe this is the alternator backfeeding. Just to doubly make sure, I wired in a 1amp 50V diode to the blue wire and still no joy, she keeps on running.

I'm out of ideas, the only thing I can think of is to put the Microsquirt relay on switched 12v to both pins of the main relay in the above Microsquirt wiring diagram.

[timmy2]

Try installing diodes across 85 & 86 in your drawing for main and FP relays just like Porsche did. (Same orientation)

[dannobee]

That diode is built into modern relays because of the spike voltage that I mentioned earlier.

What about pulling the relay(s)? Does that make it die? Or pulling the fuses?

Are you sure that you wired the ignition switch correctly? The voltage at terminals 86 at the relays goes to zero with the key off and goes to 12v with the key on?

[shamrok]

Pulling main relay kills the car. The way it's wired is that the main relay powers the ECU, and the ECU is supposed to then ground the fuel pump relay, which gives power to the fuel pump, injectors, ignition, and wideband O2 controller.

I'm open to the possibility I screwed up the wiring somewhere, below is my wiring diagram of what is on the car now. I think something is not right as all the items that are wired together to the fuel pump relay pin 87 are always powered on with the key, even if the relay is pulled. As far as I understand, they should be off if the relay isn't there, it's as if they're directly controlled via switched 12v and not the relay.

As far as ignition switch wiring, I didn't change anything behind the dash, I just re-purposed the red wire that formerly used to send 12v switched to the CDI. Voltage to pin 86 (either relay) is interrupted when I turn off the key.

[shamrok]

Alright, I'm officially perplexed. This probably isn't the right place, but I'll mention it anyways. With the fuel pump relay pulled, the car starts and shuts off with the key. Everything that is wired to the fuel pump relay pin 87 is getting power from switched 12v, REGARDLESS of there being a relay there or not. Wat?!