Ignition switch activated relay?

[Air_Cooled_Nut]

I need some guidance here from those that know automotive relays.

Okay, on my '76 fuse block the #7 fuse has NO WIRES going to or from it. This is good because I want to use it as a switched power source, so that it gets power when the ignition switch is in the ACC position.

Looking at my Bentley and verifying with my voltmeter, fuse #10 gets power through the ignition switch. I temporarily have a 10AWG jumper from the #10 power input to the #7 power input i.e. the bottom of the fuse holder. Everything works as expected.

However, what I'm afraid of is passing too much current through the ignition switch when both fuse locations are feeding things. My idea is to use a relay. The relay would be switched ON by getting power from the #10 fuse location. The thing is, when I run the power from the wire that comes off the ignition switch (Black w/Yellow stripe), go through the relay on terminal 85, then have a wire from terminal 86 to the #7 fuse box, the relay does not switch/engage. Note that there is electricity at the #7 fuse. It's acting like the relay isn't even there, as if it's just an odd looking wire.

So, from all the very simple diagrams I've seen, there's typically a wire that goes from the positive battery terminal, to a switch, to the relay, and then from the relay to ground. Testing the relays I have by connecting directly to power and ground, they do their job (click!). So, my question is: Can I run a "trigger" wire (say, 14 gauge? you tell me) from the #10 power feed to the relay and ground the other side of the relay directly to the chassis without having all of the electricity short circuit through the relay and thus burn something up? Or would #10 still receive its power, the relay would switch, and all would be good with the world?

If the above worked, I would have an independent power supply that goes through the switch in the relay -- terminal 30 has +12v and terminal 87 goes to the #7 fuse.

Am I simply over-thinking it?

Relay diagram for reference:

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Toby http://www.aircoolednut.com/
Did I mention that I'm an original Darksider?
'72 VW Squareback, 2007cc, GB 5-speed, rag top; '76 VW Riviera Penthouse Sundowner 2.0L; 2015 Audi S5 Cabby w/Stage II APR; '06 Ducati Sport Classic 1000; '14 Ducati Diavel Strada
The First Invasion

[bsairhead]

Yes. Pick up the ground from any good chassis point. The wire to #86 can be 14 or 16 or 18 as long as its fused correctly.

[telford dorr]

To expand on the above answer a bit, I'd do the following (assuming a '76 bus):

- connect a small (AWG 22, or whatever size is easy to work with) brown wire from relay terminal 85 to ground. A ground terminal on the back of the instrument cluster works well for this.

- connect a small black wire (same gauge as step above) from the output of fuse #10 (switched ignition power) to relay terminal 86. You may need a piggy-back type terminal for this. This provides fuse-protected power to activate the relay. As the relay coil pulls little current, the added load to this fuse is minimal.

- connect a red AWG 14 wire from the unswitched power side of the fuseblock (bottom of fuse 8 or 9) to the bottom of fuse #7. You may need a piggy-back type terminal for this. Connect another red AWG 14 wire from the top of fuse #7 to relay terminal 30.

- connect your new load(s) using suitably sized black wire to relay terminal 87. Install either a white (8 amp) or red (16 amp) fuse as needed for fuse 7.

When the ignition switch is activated, the relay will close, sending power to your new load(s).

Note: I limited the red wire size to #14, as this will allow a load suitable for a 16 amp fused circuit. If you are needing more current than that, I'd run a suitably sized wire from the big stud on the starter motor forward, and power fuse #7 with that (takes the new load away from the stock harness).
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'71 panel, now with FI
'Experience' is the ability to recognize a mistake when you're making it again - Franklin P. Jones
In theory, theory works in practice; in practice, it doesn't - William T. Harbaugh
When you're dead, you don't know you're dead. The pain is only felt by others.
Same thing happens when you're stupid. - Philippe Geluck
More VW electrical at http://telforddorr.com/ (available 9am to 9pm PST)

[Air_Cooled_Nut]

Telford, I'm so glad you provided your input! I was hoping you would

Okay, I'm good now, thanks for the help volks.
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Toby http://www.aircoolednut.com/
Did I mention that I'm an original Darksider?
'72 VW Squareback, 2007cc, GB 5-speed, rag top; '76 VW Riviera Penthouse Sundowner 2.0L; 2015 Audi S5 Cabby w/Stage II APR; '06 Ducati Sport Classic 1000; '14 Ducati Diavel Strada
The First Invasion

[NASkeet]

Keep in mind that there are two different ignition-controlled supplies; terminal-15 and terminal-X, the latter of which is only live when the starter-motor is NOT operating, thus taking load off the battery, when the starter motor needs every volt it can get!
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Regards.

Nigel A. Skeet

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[vwwestyman]

I will offer an alternative to how you access the power to turn the relay "on:"

Use the wire that powers the "you left the key in the ignition switch, dummy" buzzer!

That way, you can have that fuse energized any time that the key is in the ignition, whether it is turned on or not.

The handiest use of this function is if you want to have the radio on but not have the engine running. It can potentially damage things(e.g. burn out points) if you turn the key to ON but don't have the engine running for long enough.
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Dave Cook

President, Wild Westerner Club

1978 Champagne Edition Westy, repowered to '97 Jetta TDI
1973 Wild Westerner
My Thing

[telford dorr]

NASkeet - Good point: if he follows the wiring instructions as presented, his relay will be operating off of ignition switch terminal 'X'. I assume that the new loads may be significant, and are not likely to be needed during startup.
_________________
'71 panel, now with FI
'Experience' is the ability to recognize a mistake when you're making it again - Franklin P. Jones
In theory, theory works in practice; in practice, it doesn't - William T. Harbaugh
When you're dead, you don't know you're dead. The pain is only felt by others.
Same thing happens when you're stupid. - Philippe Geluck
More VW electrical at http://telforddorr.com/ (available 9am to 9pm PST)

[Air_Cooled_Nut]

Relay is triggered by fuse #10. Power is fed to fuse #7 from a power feed coming from a custom rear fuse box.

Relay location. Ground for the relay is also at the mounting point of the relay to the body. Yellow wire w/black stripe comes from the top of the #10 fuse so it is a fused/protected feed.

The wires at the relay. Snug but insulated and well-crimped (I have a pro ratcheting crimper with dedicated dies).

Rear fuse box:

Yes, my next task is to provide a fused feed wire to the rear fuse box.
_________________
Toby http://www.aircoolednut.com/
Did I mention that I'm an original Darksider?
'72 VW Squareback, 2007cc, GB 5-speed, rag top; '76 VW Riviera Penthouse Sundowner 2.0L; 2015 Audi S5 Cabby w/Stage II APR; '06 Ducati Sport Classic 1000; '14 Ducati Diavel Strada
The First Invasion