When I put in the relay to switch on the underhood 12v accessory fuse blocks, should I then unhook the ingnition switch blue and blrown wires that now feed it? It seems like two paths might be better than one, but might also make it harder to protect with fuses, and make it harder to trouble shoot things later on. Then again, there might be even bigger problems inherent in such a design that my neophyte mind is unaware of.

I have said many times that I am an electronic dunce. I mean that. But asking dumb questions has the potential to cure duncehood. Thanks for taking the time to think about this and voice opinions. I hope I add useful knowledge back to the community from time to time.

It seems as if the redundancy might be a useful backup in the even of a failure of the relay.

Sam

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I would. If those old wires are not on the same fuse, they could be fused higher, and could melt if the new wiring fuse pops. Or simply be confusing for debug.

Two paths can be better, but I'd rather run the two wires together, physically next to each other; or better yet, as just one large wire. After all, if one wire chafes and shorts to ground, then the whole circuit goes down--meaning, it's not true redunancy. [If both wire runs had fuses at either end (or were diode connected, so as to prevent backfeeding), then sure, it'd be redundant--but that's not how you're doing it.] If you're worried about the relays, just carry a spare. If you're making a block to hold the relays, put one or two spares onto it, not wired up; in the future they could be used in case you add something else. Or simply unplugged and swapped with the bad relay.

If you're worried about relay failure, I'd put small indicators on the outputs of the relays. Then you'd see if the relay was on or not. LED's would last a long time, with the proper resistor (or buy one with an integral resistor).

I'd pull the wires, tape them back, or cut them out. Then draw really good pictures of what you've done, for future reference. It's much easier to work with one page of schematics.

Lastly, the wires from the ignition switch: aren't you using them to just switch the relays? Meaning, you snipped the old wires that fed all the switched circuits so as to run the coils on the relays instead; and then used the relays to switch 12V through thicker gauge wire to all the circuits? Or are the old wires controlling the relays and feeding the old circuits too?

Sam
Your not NASA, get rid of the extras. It could really cause you some problems. Your ECU will not like EXTRA grounds . Ive work in the electrical field for 25 years ( and seen & made some bone head moves) but my worst was running a paraell electric system for a tractor.
Good luck,
Frank

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Scrapple: Because a mind is a terrible thing to waste.
73 Duster - Race Car
66 Dart Wagon - DD
178" FED
82 D150
All Slant powered

I deal with this kind of question very often in th context of headlamp circuit upgrades: Is it better to put each and every filament on its own fuse and relay? Left low, left high, right low, right high, each fused and relayed individually? Or is it better to put each circuit on its own fuse and relay (low beam circuit, high beam circuit)? Some people feel that the first option is better, because it mnimises the consequences of a failure: If a problem develops in the left low beam circuit, for instance, it won't affect the right low beam. They have a point, but doing it that way also increases the likelihood of a failure, for it doubles (or triples, or more) the number of potential failure points. More relays that could fail. More fuses and fuseholders and crimp/solder junctions and ring terminals and wires that could fail. So, which is better? Less chance of failure but with greater failure consequences? Or lesser failure consequences but a greater chance of failure? Unless there are extenuating circumstances, my preference is generally to keep things as simple as possible: The lowest possible component and wire count consistent with proper, safe, and reliable operation. This is especially true when the primary failure point (such as a relay) is an easy 20-second pop-out/pop-in replacement.

The same applies to wires: If you run two wires to feed a circuit instead of one, you might have some redundancy, but you also have twice the number of potential failure points. Net gain? Probably zero if not negative.

Of course, there's no one right answer that will apply to all the circuits in your car. One relay and one fuse for each and every individual electrical load is too far in one direction. All electrical loads fed by a single relay and fuse is too far in the other direction. The right point on the continuum depends on the circuits, loads, service conditions and reliability consequences involved.

To answer your question directly: No, you should have just one ignition feed circuit.

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一期一会
Too many people who were born on third base actually believe they've hit a triple.

Thanks all. The ignition switch will not be used to power the relay coil. A toggle will do that. The ignition switch will power all the original in car power requirements, just as it did stock. This would be radio, heater, and everything included in the stock chasis harness.

Sam

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Redundency is very good......in an airplane where the consequences of failure might be gravity reaching up and saying, "Come On Down!".

Otherwise, I also suggest you remove any unneeded or unused wiring/circuits. They'll only give you headaches when it comes to troubleshooting.

Roger

I like simplicity myself. Redundancy will just make troubleshooting and rebuilding harder, so I avoid it.

Lou

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Home of Slant6-powered fun machines since 1988

Just a thought to consider: if you feed various devices both from a relay and from the switch and don't plan it well, the system can stay self-energized and you'd have a tough time turning it off. Diodes can be used to prevent that.

I've seen this happen with an old mechanical voltage regulator - I was surprised to find the ALT light flash on and off around once/second only after I turned the car off. Minor little details will deterimine whether it never happens, or only sometimes, or always, so it's better to plan ahead to avoid it.

I figure that my Dart has been running flawlessly on non-redundant electrical systems (without failure, except for an alternator and a start relay) for 40 years now. Certainly the capacity of system could be improved... but redundancy? I should live so long to need the backup systems!



-Mac

(That said, I'll probably experience a total electrical failure tomorrow).