[O'Man]

I need a couple of extra switched 12 volt circuits for a project I am completing on a 72 Chevelle.

As I look at my fuse box I have two UNSWITCHED circuits on the upper left side of the fuse box. One is labeled "Power Accessories" the other is labeled "Ltr Clock". The Power Accessories has a 30 amp fuse and the Ltr Clock has a 20 amp fuse. Both have an unused blade clip next to them. I have no cigarette lighter in the car.

I am thinking what is to stop me from putting a relay off of one of those blade clips by connecting the power in of the relay to the blade clip. Then I can energize the relay with a 12 volt lead from a "switched" blade on the fuse box. Once the swithed lead is activated the relay is actuated then any accessories I connect to the relay power out are activated.

I am thinking of using the 20 amp "Clock Ltr" lug because I have no cigarette lighter in the car. The load from the accessories powered off the new relay will be very small just some electric instruments. Does this sound like an electrically logical way to get the extra switched connections I need?

Just another quick question ...does anyone know what runs off that Power Accessorie fuse? The car I am working on has very few power accessories, no A/C, no Power windows, no power seats, no power antenna. Is it possible that that Power accessories circuit is VERY lightly loaded in a low ption car like this one?

 

[Finally]

The relay is certainly an electrically 'sound' way of doing it. The cig lighter fuse will protect the load and it's wiring. The current to run the relay is nil.

Not sure what is on the Power Accessories fuse, pull it and see? Don't know what all circuits are available on a 72, no other switched circuits you could pick up?

 

[O'Man]

Believe it or not there are only two switched circuits that are equipped with the "open blade" at the fuse box. I could use those and might just do that but I am also gonna be adding a Vintage Air system and I expect that is gona need some switched power for its relays. I am hoping to avoid chopping into any wires and it just seems that the best deal is to rig up a relay off the Cig lighter circuit. That is a high juice circuit judging by the 20 Amp fuse. I am going t pull that 30 Amp fse and see what stops working....just wondered if anyone knew what was on that fused circuit.

thanks for your help

 

[John D]

I'll second Hank's comments! Great solution to not "hacking" up the wiring. The relay (and possibly a small terminal strip fed from the relay) could be buried up out of site, and only a pigtail/feeder visible off the "UNSWITCHED" lug on the fusebox.

 

[Elree Colby]

If there is nothing connected to the blade next to the accesories fuse then you have no load on that fuse.

 

[O'Man]

If there is nothing connected to the blade next to the accesories fuse then you have no load on that fuse.

Elree

Are you saying that there is likely nothing on that 30 AMP circuit right now? Obviously I need to pull that fuse and see what if anything quits when I pull the fuse but I HATE messing with factory wiring ...fires are just so FINAL. Not to mention that the car is in an under house garage. Burning the house down could be a really tough eventuality to explain to Mrs Oman.

I know that some of the factory A/C wiring harnesses plug a single "power" wire into the fuse box and that there are relays out there on the harness. What I am thinking , and I think I read you correctly, is that the fact that the car is lightly optioned means that that 30 AMP fuse is in all the cars reguardless of options ordered / not ordered. Connections are then made to the fuse box as required for whatever options the car is ordered with?

I am thinking I will take the 30 amp line (via jumper) and power the relay. Then I take a 12 volt switched jumper off one of the other switched baldes. I can then run up to 20/25 amps worth of accessories off the relay. Because the relay needs very low amperage to activate I won't overload whatever switched circuit I tap for the relay actuation power.

 

[no1dc]

oman I did basically the same thing to my sons car, but used the ignition terminal at the bottom of the fuse block to powers a supplemental ATC fuse box I located in the engine compartment on the firewall. I ran a 12 gauge wire form the ignition terminal to a 30amp in line breaker I mounted(dbl sided tape) to the side of the fuse box and continued to run the 12 gauge wire to the supplemental box in the engine compartment on the firewall.

Your idea of using a relay sounds good.

Not trying to hijack your post but would usung a relay be a better way to supply my supplemental electrical box than a 30amp inline breaker? I have used a relay AFTER the supplemental box 12V constant power for the supplemental vacuum pump. TIA Pete

 

[Elree Colby]

Oman

There are no wires attached to that fuse on the back of the fuse block, the only connection is the tab to the right of the fuse. It's there to supply power seat, power windows, etc.

 

[O'Man]

oman

Not trying to hijack your post but would usung a relay be a better way to supply my supplemental electrical box than a 30amp inline breaker?[/B] I have used a relay AFTER the supplemental box 12V constant power for the supplemental vacuum pump. TIA Pete

I think that after your supplimental box is OK as long as there is some sort of fuse back there before the relay. I asume that you tapped off the battery for the supplimental box? If you omitted a fuse in that line and it is not protected by a breaker you have a potential for a fire if the insulation were to wear thru however. If as I assume you took the juice off the battery I would put a fuse right at the battery to protect the line that feeds the secondary box.

All the relay does is allow you to put an "automatic / semi automatic" switch anywhere you want and have it electricaly activated. It can eliminate running big heavy wires all over the car ...depending of course on where you have located the accessory / accessories that are driven by the power that is switched thru by the relay.. If your vac pump is on the firewall and you are tapping direct off the battery you need a long line of fairly heavy wire to get to the pump no matter where the relay is placed.

Think of electric fans at the radiator. You put the relay up on the rad support someplace, run 12 volt power from the battery via a fairly hefty cable then activate your relay with a light guage wire from whatever switch mechanism triggers the fans to come on.

 

[no1dc]

I think that after your supplimental box is OK as long as there is some sort of fuse back there before the relay. I asume that you tapped off the battery for the supplimental box? Not for the iginition supplemental box. For that box I tapped off of the ignition spade on the fuse box in the car and ran a 12ga wire from that spade to an inline 30amp breaker then out from the 30amp inline breaker to the ignition supplemental fuse box on the firewall. The pump has two power sources, ignition and constant 12V. If you omitted a fuse in that line and it is not protected by a breaker you have a potential for a fire if the insulation were to wear thru however. The supplemental ignition box has an inline 30amp breaker in it.

If as I assume you took the juice off the battery I would put a fuse right at the battery to protect the line that feeds the secondary box. I do also have a second supplemental fuse box on the core support where the external voltage regulator used to be. This box is supplied by a jumper(6 ga) from the horn buss relay that receives it's power form the battery/alt. There are no breakers or fuses in these lines. The original fusible links from the battery to the junction block and at the horn relay buss bar on the 10ga factory wires are still there.

All the relay does is allow you to put an "automatic / semi automatic" switch anywhere you want and have it electricaly activated. It can eliminate running big heavy wires all over the car ...depending of course on where you have located the accessory / accessories that are driven by the power that is switched thru by the relay.. If your vac pump is on the firewall and you are tapping direct off the battery you need a long line of fairly heavy wire to get to the pump no matter where the relay is placed.

I guess my question is, would it be better to use a relay INSTEAD of an inline 30amp breaker to power my supplemental ignition fuse box? Also should I put a fuse or inline breaker in my power feed from the alt to the buss bab? This is a 6ga wire?

Think of electric fans at the radiator. You put the relay up on the rad support someplace, run 12 volt power from the battery via a fairly hefty cable then activate your relay with a light guage wire from whatever switch mechanism triggers the fans to come on.

TIA Pete

 

[O'Man]

A relay is not a substitute for a fuse. A relay is an electronically actuated switch. If there is no fuse in the circuit that the relay is switching on and off.... well then there is no fuse. If something happens to the line or to one of the accessories on that line then the wire can become the fuse. When the wire burns up and breaks the circuit is broken. It is during the time the wire is "burning up" that all the fun happens. Not a good situation.

If the actuation circuit for the relay has a fuse in it and something causes that fuse to go the relay is going to be deactivated and any accessories powered by the relay will shut down. This feature will not protect you from a failure on the other "side" of the relay.

For my money a fuse / fuseable link or an inline breaker should be somewhere in every circuit in the car. Factory systems are set up this way and any of the stuff we add to these cars should be protected by fuses / fuseable links or inline breakers.

I really don't like fussing around with electrical stuff...I do it only when I have to and I am hyper careful to double and triple check my connections. I try to use slightly heavier wire than might be needed, I try to fuse everything and I solder and shrink wrap all terminal ends / connectons I make. I have to much time and money in the car to risk a fire. Any circuit without a fuse for protection is just unacceptable to me so I guess my answer to you is make sure that you have every line you have added to the car fused unless it is protected by a factory fuse.

I had one car fire when the hedders on my GTO toasted one of the wires to the starter. Fortunately I was close to home. (50 yards actually). I could SEE the fire down by the starter inbetween the tubes but I could not do a darn thing to stop it. I roared into my driveway at about 50 MPH thinking I was gonna use the garden hose on the situation and threw open the hood. Fortunately the fire burned right thru the wire, the wire broke and that was the end of the event. I immediately junked the hedders and have never had a set on a car again. My point here.....don't take chances if you mess up a circuit or have some insulation burn off , or if an accessory fails and there is no fuse in the circuit you could lose the car OR WORSE.

 

[Dean]

For my money a fuse / fuseable link or an inline breaker should be somewhere in every circuit in the car.

That's for sure and as close to the power source as possible.

Granddaughter came over a while back driving her old TBird.
I looked and there was a piece of lamp cord connected to the positive battery cable, ran out the crack of the hood, draped over the mirror, through the driver's door and across the dash powering an aftermarket stereo.

She said her boyfriend had put the stereo in and I told her I think he is trying to burn you and your car up.