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Posted by: worn Jul 1 2022, 07:32 PM

The diode between the alternator bootstrap circuit and the brake warning system has failed in my ‘72. To me, a diode is a tiny glassy thing you solder onto a PCB. I suspect though that an automotive diode might not best be ordered from Mouser. Can someone give me a path to replacement? How many Watts, etc? Thanks.

Posted by: Spoke Jul 1 2022, 07:55 PM

I would imagine any simple silicon diode would work with perhaps 100V standoff capability with 5amp current would be fine. Maybe someone has replaced this before and knows of an exact match. Personally I go to Digikey because their component selection guide is much better than Mouser.

Posted by: davep Jul 1 2022, 07:58 PM

Do you have a diagram showing the location in the circuit? Where is it physically located? If I can lay my hands on one it may be possible to identify. I have no idea why a diode is need there.

Posted by: jim_hoyland Jul 1 2022, 08:52 PM

Could be located behind the gauge cluster ?

Posted by: Spoke Jul 1 2022, 09:36 PM

If I'm reading this correctly, @http://www.914world.com/bbs2/index.php?showuser=13152 's '72 has a 911 engine in it. The diode in question is in series with the GEN light. This picture is from the '86 930 schematic showing the diode 911-617-108-00 in series with the GEN light. This diode failed on my 930 and stopped the alternator from starting up. If the diode is shorted or removed, the engine will not shut off since the GEN light powers a couple of critical circuits.

I don't recall which diode I used to replace this but I think any 100V, 5A silicon diode should work.

Here's a 100V, 6A silicon diode at Digikey that should work. It will cost you $0.74.

https://www.digikey.com/en/products/detail/vishay-general-semiconductor-diodes-division/GI751-E3-73/2140948


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Posted by: 914Sixer Jul 2 2022, 08:49 AM

You need one of these?


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Posted by: GregAmy Jul 2 2022, 09:15 AM

EXACTLY my problem when I Microsquirted my D-Jet 2L. There is no diode on the /4 GEN circuit but the D-Jet system draws enough load that it drops the connection and the car dies; not so with the modern MS system (simply amazing that an entire EFI system can be powered through that small incandescent bulb.) A diode fixed it.

A diode is a diode, as long as it can handle the amps in the desired direction. I installed a solar panel PV diode in the GEN circuit and have many left over if you want me to put one in an envelope for you:

https://www.amazon.com/gp/product/B01CXOQMJ8

Then again, at $7 shipped for 20 of them, it might just be better to order a pack for yourself and have them arrive tomorrow...

Posted by: worn Jul 2 2022, 09:51 AM

Bingo! The is what has failed.
For those wondering about the diode function i wrote a thread with a nifty diagram. The ‘72 was the most complicated. The engineers were trying to do more with less tying a single flasher and brake warning light into the seat belt and alternator circuits. The diodes prevent current backflow as the different components started or stopped.

Posted by: 914Sixer Jul 2 2022, 11:17 AM

Shoot me a PM with your shipping info.

Posted by: jim_hoyland Jul 2 2022, 11:22 AM

Did other years use the diode too ?
??

Posted by: Chris914n6 Jul 2 2022, 12:19 PM

It's used as a one-way circuit. My late 90s Nissan cluster has a couple for warning lites.
Sorry don't know the specs.

Posted by: worn Jul 4 2022, 12:23 PM

This is what is inside. I probably have one already

Posted by: Spoke Jul 4 2022, 04:08 PM

That diode connection looks aftermarket and not OEM. Was the left lead broken off when you opened up the insulation?

Posted by: worn Jul 4 2022, 05:10 PM

Not sure if it broke before or after. The outer package was identical to the one posted. At any rate I get megohm plus in either direction . I have one rated at one watt that i might try. Without it everything works except the test brake light with key on and ebrake off. In theory all three lights in my triple gauge should light up, but the red brake warning light doesn’t. I use the ebrake all the time to keep the cables limber, so the red light is going to be blinking on start up anyway.
Thanks Spoke and everyone else.