What Circuits For Relay?

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mopowers

mopowers

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For those who have rewired their cars, what all do you recommend running off relays? I realize that generally, they're meant for "high draw" circuits, but what constitutes "high draw?"

I'm doing some research before I tackle rewiring a car of mine and am trying to determine what to run off relays. So far, I'm thinking: headlights (high, low beam), fuel pump, fan (high, low speed), ignition box. For the starting circuit, I'll be using a Ford relay for the trunk-mounted battery and a CD relay for the charging circuit.

How about tail lights, brake lights, reverse lights, parking lamps, line lock, interior lights, alternator 12v reference/trigger, coil 12v, etc.?
 
mopowers said:
Thanks! I totally missed that thread. I'm still curious what constitutes a "high current" circuit.
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If item has a motor, requires a 20-amp or greater fuse... consider it high current. So much of our vehicles wasn't fused other than the fusible link outside bulkhead.
They pulled power for a convertible top pump straight off the amp gauge and ran large wires to a very robust manually operated switch with cycling circuit breaker mounted on it. Relays allow for much smaller gauge wiring to lesser switches. The larger high current wires go to relay and pump motor. it's the same principle as your starter relay. Could you imagine battery cables coming to a switch in your dash?
 
A relay is just an electrically controlled switch. It's like a light switch in your house but instead of you flipping the switch with your hand, you use electrical current to flip the switch.

It doesn't matter if what you're working with is high current or not. If you didn't have relays, you'd have like 30 switches on your dash that you'd need to remember to turn on and off all the time. They are used when you want something to turn on automatically when you turn something else on.

With the headlight relay kits, a relay is used because without a relay, you could potentially only have 10v getting to your headlights with all the losses in the bulkhead connector, splices, headlight switch etc. So instead of connecting that wire with only 10v to your headlights, you connect it to a relay on what I call the "trigger" side. Which is basically what flips the switch in the relay. It's no different than using your hand to flip a switch in your house.

The other side of the relay is wired to the battery so the relay is getting a full 12v from the battery, but it's not allowing that 12v to pass through the relay and on to the headlights until your 10v from your headlight switch gets applied to the relay. Once that 10v is applied, it flips the switch in the relay and allows the 12v to pass from the battery, through the relay and on to the headlights.

If you didn't have the headlight relay but still wanted a full 12v to your headlights, you'd have to walk to the front of the car and flip a switch under the hood or in the grille. That would be annoying, so a relay is used. In this case, the relay is used so you don't have a separate switch but it's also being used to give the headlights a full 12v. In most cases I'd say relays are used to avoid having to flip multiple switches. The voltage drop fix is more of a two birds with one stone kind of a deal.
 
I would lower that "current figure" a lot on these old girls if you are powering something off the key switch.

EXAMPLE.........it is well known that voltage drop in the "ignition run" circuit causes overcharging, because this drop lowers the voltage to the VR, and make it "think" the battery needs more. So in a FACTORY STOCK car running only factor loads in the engine bay, the "run" circuit in some cars (mine was one) can see a drop of 1 1/2V more or less. Guess what? You need a relay. That entire circuit is likely only about a 5A load.

Headlights. Factory stock headlights are somewhere around 60W. At 12V-14V that's somewhere around 5A ea, or 10A for two. It is also well known that factory headlight wiring is undersized. RELAYS One for high, one for low.
 
ESP47 said:
A relay is just an electrically controlled switch. It's like a light switch in your house but instead of you flipping the switch with your hand, you use electrical current to flip the switch.

It doesn't matter if what you're working with is high current or not. If you didn't have relays, you'd have like 30 switches on your dash that you'd need to remember to turn on and off all the time. They are used when you want something to turn on automatically when you turn something else on.

With the headlight relay kits, a relay is used because without a relay, you could potentially only have 10v getting to your headlights with all the losses in the bulkhead connector, splices, headlight switch etc. So instead of connecting that wire with only 10v to your headlights, you connect it to a relay on what I call the "trigger" side. Which is basically what flips the switch in the relay. It's no different than using your hand to flip a switch in your house.

The other side of the relay is wired to the battery so the relay is getting a full 12v from the battery, but it's not allowing that 12v to pass through the relay and on to the headlights until your 10v from your headlight switch gets applied to the relay. Once that 10v is applied, it flips the switch in the relay and allows the 12v to pass from the battery, through the relay and on to the headlights.

If you didn't have the headlight relay but still wanted a full 12v to your headlights, you'd have to walk to the front of the car and flip a switch under the hood or in the grille. That would be annoying, so a relay is used. In this case, the relay is used so you don't have a separate switch but it's also being used to give the headlights a full 12v. In most cases I'd say relays are used to avoid having to flip multiple switches. The voltage drop fix is more of a two birds with one stone kind of a deal.
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Thanks for the explanation, but I know what a relay does and how it functions. The question I've got is - when rewiring a car completely, what circuits warrant using relays on? I guess the question boils down to which circuits is a person willing to have the load go through the associated switch?

In addition to the circuits previously listed, I'm leaning toward using relays for tail/brake lights and maybe even reverse lights, since my battery will be in the trunk anyway.
 
I see no reason to use relays for tail, brake, or backup lamps The tail/ turn/ stop circuits are not known for failures, even turn signal/ brake switches are fairly low failure. I WOULD (if you rewire) use heavier wire in tail/ turn circuits especially if you ever tow a trailer.
 
67Dart273 said:
I see no reason to use relays for tail, brake, or backup lamps The tail/ turn/ stop circuits are not known for failures, even turn signal/ brake switches are fairly low failure. I WOULD (if you rewire) use heavier wire in tail/ turn circuits especially if you ever tow a trailer.
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That makes sense. As a heads up, I won't be using any factory wiring or switches (except for the turn signal switch). I'm starting form a clean slate. I do plan on up-sizing the wiring for the smaller circuits to help mitigate voltage drop.
 
mopowers said:
Thanks for the explanation, but I know what a relay does and how it functions. The question I've got is - when rewiring a car completely, what circuits warrant using relays on? I guess the question boils down to which circuits is a person willing to have the load go through the associated switch?

In addition to the circuits previously listed, I'm leaning toward using relays for tail/brake lights and maybe even reverse lights, since my battery will be in the trunk anyway.
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On top of what RedFish and 67Dart273 said, I would just look at the schematic and use relays in the same places that the factory used them in. There are only a few used unless you had the fancy add ons like power seats, windows etc.

Most of us only use them to compensate for voltage loss or for aftermarket add ons that would require running large gauge wires all over the place, like RedFish mentioned. Sounds like you've got that figured out with your trunk mounted battery.

I'd take into account that relays add another failure point in the circuit so I wouldn't add them unless absolutely necessary. Wires and quality splices don't normally fail randomly but I've seen a lot of relays fail. The standard headlight switch connectors don't normally have issues under stock application. They potentially can if you start adding fancy lighting that draws more current than the switch/wiring was designed for. If you think you're adding more than what the factory did on those circuits, then I would consider using relays to keep some of the load off the switch.
 
ESP47 said:
On top of what RedFish and 67Dart273 said, I would just look at the schematic and use relays in the same places that the factory used them in. There are only a few used unless you had the fancy add ons like power seats, windows etc.

Most of us only use them to compensate for voltage loss or for aftermarket add ons that would require running large gauge wires all over the place, like RedFish mentioned. Sounds like you've got that figured out with your trunk mounted battery.

I'd take into account that relays add another failure point in the circuit so I wouldn't add them unless absolutely necessary. Wires and quality splices don't normally fail randomly but I've seen a lot of relays fail. The standard headlight switch connectors don't normally have issues under stock application. They potentially can if you start adding fancy lighting that draws more current than the switch/wiring was designed for. If you think you're adding more than what the factory did on those circuits, then I would consider using relays to keep some of the load off the switch.
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All good info - I appreciate it. Just noticed you're in Sacramento. Howdy neighbor, from across the river.
 
mopowers said:
All good info - I appreciate it. Just noticed you're in Sacramento. Howdy neighbor, from across the river.
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Howdy, I used to work over there at the CHP academy where they outfit their vehicles. Keep us updated on your build!
 
My 2 cents

Don't ever draw power from the battery, for anything that is normally running with the engine on, on most OEM wired muscle era mopars.

Don't use cheap chinese relays. I've been doing relay kits for 10 years. Had one failure which was related to corrosion from OSPHO being applied nearby. I use only tyco/bosch relays. I'll never use a chinese relay, unless Tyco moves to china, failure rates much higher with those units.

I know the OP is doing a major rewire.
 
ESP47 said:
Howdy, I used to work over there at the CHP academy where they outfit their vehicles. Keep us updated on your build!
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Thanks man, I appreciate it. Here's a build thread of the project if you're interested. It's still got a ways to go, but I'm slowly chipping away.

Mopowers' 66 Dart Build

crackedback said:
My 2 cents

Don't ever draw power from the battery, for anything that is normally running with the engine on, on most OEM wired muscle era mopars.

Don't use cheap chinese relays. I've been doing relay kits for 10 years. Had one failure which was related to corrosion from OSPHO being applied nearby. I use only tyco/bosch relays. I'll never use a chinese relay, unless Tyco moves to china, failure rates much higher with those units.

I know the OP is doing a major rewire.
Click to expand...

Thanks Rob. I'll definitely be using Tyco relays. Interestingly, the gentleman at Haywire said MAS relays are good too. He said he's gone through them by the thousands and has never heard of a failure.

Probably a stupid question, but why not pull power directly from the battery for anything running when the engine's running? Is it just preferred to pull from the alternator output instead for those items?

You're right about the "major rewire." There ain't a damn wire in this thing. Haha!!

upload_2022-1-20_19-37-15.png
 
If you add a lot of high current draw devices [ elec fan, air con etc ] without using relays, then you would have to upgrade wire thickness to carry the increased current, otherwise you will get large voltage drops which may affect the operation of the devices. It is not practical, in fact dangerous/risky [ like a fire ] trying to pull large amounts of current through the terminals of bulkhead head connectors & ign switches that were NEVER designed to carry those large currents.
Use relays. The typical Bosch 30 amp relay has a coil resistance of about 70 ohms. So it draws less than 0.2 amp & will not overload existing circuits or wiring. Use the relay contacts to do 'the heavy lifting'.
 
crackedback said:
My 2 cents

Don't ever draw power from the battery, for anything that is normally running with the engine on, on most OEM wired muscle era mopars.

Don't use cheap chinese relays. I've been doing relay kits for 10 years. Had one failure which was related to corrosion from OSPHO being applied nearby. I use only tyco/bosch relays. I'll never use a chinese relay, unless Tyco moves to china, failure rates much higher with those units.

I know the OP is doing a major rewire.
Click to expand...
.................................................
Who sells Tyco ?
I used relays on my LED light bars and used a $5 relay from Auto Zone
 
67Dart273 said:
I see no reason to use relays for tail, brake, or backup lamps The tail/ turn/ stop circuits are not known for failures, even turn signal/ brake switches are fairly low failure. I WOULD (if you rewire) use heavier wire in tail/ turn circuits especially if you ever tow a trailer.
Click to expand...
I think we would sell agree that it is gard to imagine that MA Mopar used 18ga wire for years tail/brake lite circuits. 18ga!! One way to help is to use LED's for the marker lights and even tail and backup lights. You can do it for the brake/turn signals but the relay has to be changed or a resistor has to be added to up the current so the turns will blink. I like the LED's to reduce the load on these old wires. (And yes I know LED's may not be legal for brake lamps...not sure why as they are certainly just as bright!!)
 
Dfr360cuda said:
5 bucks each on Amazon

View attachment 1715857195
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Bingo!
Notice new cars have fuse block strapped to + terminal on battery. Under hood and close to destination.
My thought was to use original wiring as “activation” for relay that is close and directly wired to battery - to assure full 12v is on the circuit.
Hide it all under the battery tray for original look.
 
Dfr360cuda said:
.................................................
Who sells Tyco ?
I used relays on my LED light bars and used a $5 relay from Auto Zone
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Waytek, terminal supply any decent electrical house should have tyco/TE/Bosch relays.

That amazon ad has a tyco/bosch relay pictured. If the number stamped on top starts with a V or 033 and made in Portugal it's a real deal piece. If something else shows up, nope.
 
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6pak said:
Bingo!
Notice new cars have fuse block strapped to + terminal on battery. Under hood and close to destination.
My thought was to use original wiring as “activation” for relay that is close and directly wired to battery - to assure full 12v is on the circuit.
Hide it all under the battery tray for original look.
Click to expand...

DO NOT do that on a OEM wired muscle era mopar...

If you change the charge path, that's OK... not with the bulkhead and ammeter in place.
 
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