Wiring Question, Switched Hot @ Fuse Panel Options

[andys]

I need to run a some relays for my V8 conversion, and need a switched +12 volt source. I'd like to tap into a source off the fuse panel. My question is which fuses are 'on' when ignition is on (switched)? I'm pretty sure fuse #8 is switched, but which others are switched? I'd like to use #8 since it's for fog lights (which I don't have), but I believe it's constant +12 volts (since a relay switches them on). BTW, the wiring is incomplete so I can't simply power up the system and turn the ignition switch to read switched sources.
Thanks,
Andys

[Larry.Hubby]

Andys,

Yes, fuse 9 is switched, but it's a little more complicated than I may have indicated.

First, the two fuses, F8 and F9, are different capacities. F8 is 25A while F9 is 8A, so, depending upon what you want to hook up, you might need to take account of that.

Second, after looking at the schematic a little longer, the difference between the two fuses, and the reason they drive the voltage on F9 with a relay, is that the relay isn't switched by the ignition on signal(15), it's switched by the accessory signal (X). The X signal goes on when the key is turned, but goes back off when the key is turned all the way to the start position, so whatever is hooked to F9 will not run while the starter is engaged (radios are often hooked up this way).

Third, it depends upon what year car you have. All the info I've given so far is correct for '70 & '71 cars. From '72 on, they interchanged the roles of F8 and F9. I'm not sure why. It seems to have happened when they had to add the seat belt buzzer. They don't seem to have changed the fuse capacities when they made this change, except that F8 drops down to 16A for '74 and later cars.

So, bottom line is: if you have a '70 or '71 car, F8 & F9 are on with the ignition switch, but F9 goes off when the starter is engaged. If you have a '72 or later car, it's the same thing, but it's F8 instead of F9 that goes off when the starter is activated.

[andys]

Andys,

Yes, fuse 9 is switched, but it's a little more complicated than I may have indicated.

First, the two fuses, F8 and F9, are different capacities. F8 is 25A while F9 is 8A, so, depending upon what you want to hook up, you might need to take account of that.

Second, after looking at the schematic a little longer, the difference between the two fuses, and the reason they drive the voltage on F9 with a relay, is that the relay isn't switched by the ignition on signal(15), it's switched by the accessory signal (X). The X signal goes on when the key is turned, but goes back off when the key is turned all the way to the start position, so whatever is hooked to F9 will not run while the starter is engaged (radios are often hooked up this way).

Third, it depends upon what year car you have. All the info I've given so far is correct for '70 & '71 cars. From '72 on, they interchanged the roles of F8 and F9. I'm not sure why. It seems to have happened when they had to add the seat belt buzzer. They don't seem to have changed the fuse capacities when they made this change, except that F8 drops down to 16A for '74 and later cars.

So, bottom line is: if you have a '70 or '71 car, F8 & F9 are on with the ignition switch, but F9 goes off when the starter is engaged. If you have a '72 or later car, it's the same thing, but it's F8 instead of F9 that goes off when the starter is activated.

Larry,

Thanks for the info (confusion (IMG:style_emoticons/default/huh.gif) ). My car is a '75, so it apparently uses F9 as a switched hot, if I follow your explanation correctly. The relays I need to energize (on the V8 conversion) are for the radiator fans (the gronud is switched by the PCM, and I have run separate wires for power;+12volts). Since it's low current and the fans don't need to run durring start, I suppose I can run it from either fuse.

Thanks again Larry!

Andys