wiring coil relay

[supermanbidder]

im going about wiring my coil relay and my question is can i just get power from the = post on my alternator seeing its right there rather than running another wire to the battery and then back

thanks tom

[Glenn]

What's a coil relay?

The coil is powered from the ignition switch.
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Glenn
74 Beetle Specs | 74 Beetle Restoration | 2180cc Engine
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[supermanbidder]

http://www.thesamba.com/vw/forum/viewtopic.php?t=492821

[Glenn]

Never work on a car that needed one. The ignition just isn't that much of a draw compared to the headlights or rear defrost.
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Glenn
74 Beetle Specs | 74 Beetle Restoration | 2180cc Engine
"You may not get what you pay for, but you always pay for what you get"

Member #1009

#BlueSquare

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[ashman40]

If you are adding a relay in the engine compartment to power all or some of the ignition powered devices, you can use the alternator B+ terminal as a source. It is directly wired to the battery (+) post.

You can then use the black (#15) ignition wire that used to power the ignition coil as the source to energize the relay.

There is one problem.... you will likely need to add a 1A diode (1N4001) into the #61 Gen light circuit. This will partially disable how the circuit works but prevent engine run on.

Without the diode, the engine will likely continue to run after you turn off the ignition switch.
Most Bosch style relays only require 5.5v to remain energized. When you turn off the ignition switch you cut power from the ignition switch to the #15 circuit. But the Gen light is connected to both the #15 circuit at the fuse box and the D+ terminal on the alternator. While the alternator is spinning it is sending 12v from the D+ terminal towards the Gen light. The only reason the Gen light is not ON while the engine is running is because positive current from the fuse box is pushing back in the other direction, the current flows balance so no current flows. The Gen light remains OFF.

When the ignition is turned off the engine/alternator is still spinning. The current from the ignition switch stops and now the D+ current can flow through the #15 circuit (the Gen light will remain ON). This keeps the ignition relay activate and your relay continues to power the ignition coil with current from the alternator B+.
Adding the 1A diode into the #61 circuit with the "stripe" towards the alternator will allow positive current to flow towards the alternator allowing it build a field charge and in turn charge the battery. But this will prevent current from flowing back towards the fuse box. When your battery is weak and the alternator output is greater than the battery output normally the Gen light will come ON dimly. This will no longer happen as current will not be allowed to flow in this direction.

There is a good write up here:
http://www.mgtoronto.com/pdf/Tech/ignition-run-on.pdf
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AshMan40
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'67 Beetle #1 {project car that never made it to the road }
'75 Beetle 1200LS (RHD Japan model) {junked due to frame rot}
'67 Beetle #2 {2019 project car - Wish me luck!}