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Discussion Starter #1
I am building a very basic Ford Model-A rod (see Sluggo the '31 Ford in Builds). It is basically power-nothing. I plan relays for headlights (hi and low), starter (of course), my electric cooling fan has two speeds and each needs one. I'm running triple Stromberg 97's. The Stromberg folks recommend 2.5 lbs fuel pressure so I plan 2 Facet fuel pumps (one for the primary, one for the 2 secondaries). Anything else require a relay ? The car will soon return from the paint shop and wiring seems like a likely next project.
Buzz (MoonBat)
 

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True Hotrodder
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If there is nothing else in your rod pulling a heavy amperage then that should take care of it. Have fun!
 

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Guess I would do it a little different. I would not use a relay for the headlights, just a dimmer switch. Quality wire and large enough will overcome the amp draw. Why a relay on the starter? The solenoid is in fact a large relay. Just run a #16 gauge wire from the key start position, thru the neutral safety switch then to the solenoid. Where is your brake light switch? If you are using a pressure type then I recommend a relay for it because they don't handle the increase draw well. Are you using a lock up converter?
 

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Old(s) Fart
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Why do people want to complicate wiring and add potential failure modes with a bunch of unnecessary relays? GM built tens of millions of cars that didn't have relays in any of these locations, and they worked fine. You use a relay in situations where the control switch is remote from the load and power source, or where the control switch voltage is different from the voltage being switched. The latter case is common on newer cars, where just about every function runs through the computer (yeah, even the lights and starter). Since solid state devices tend to run on 5V systems, the relays are needed to switch 12V to the lights or starter. Also, circuit boards and solid state devices used in the ECU aren't sized for high current. On a basic car with normal ignition, headlight, and dimmer switches, these switches are sized to carry the current for the anticipated loads (unless you're using HID lights) and don't need relays. And if you've got a GM engine and starter, the "relay" is already built into the starter solenoid. Just use wire gauges sized to carry the current. Why make it more complex than it needs to be?
 

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Relays for the headlights depends upon the bulbs used. Regular ol sealed beams work just fine through the head light switch and the dimmer. Put in hi end halogens and the headlight switch will be the first to give up. Repaired a friends car, he had halogens, he complained the headlight switch got too hot to touch. When I went to check it out I found he was only a short ways away from a fire. The switch had gotten so hot the terminals were all loose. So, relays depend on type of light.

Two speed cooling fan, no big issue here, just less of a load if only running on low. I feel ( and have a single speed fan in my street rod ) a single speed should be more then adequate for any condition.

Two fuel pumps? Just one more area for trouble to develop. Run a single pump, it will more then keep up with the 97s, matter of fact you'll most likely need an in line pressure regulator to keep it down to the 2.5 PSI.
 

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Discussion Starter #7
Relays for the headlights depends upon the bulbs used. Regular ol sealed beams work just fine through the head light switch and the dimmer. Put in hi end halogens and the headlight switch will be the first to give up. Repaired a friends car, he had halogens, he complained the headlight switch got too hot to touch. When I went to check it out I found he was only a short ways away from a fire. The switch had gotten so hot the terminals were all loose. So, relays depend on type of light.

Two speed cooling fan, no big issue here, just less of a load if only running on low. I feel ( and have a single speed fan in my street rod ) a single speed should be more then adequate for any condition.

Two fuel pumps? Just one more area for trouble to develop. Run a single pump, it will more then keep up with the 97s, matter of fact you'll most likely need an in line pressure regulator to keep it down to the 2.5 PSI.
Thanks for youf comment. My headlights are sealed beam so I guess I'm OK there. My headlight switch has two separate positions for hi and low. I'll try it without relays.
The triple deuce linkage uses the middle for starting and low speeds. It opens the outer two at greater speed. I thought I would keep the pressure down (second pump off) in the outers to reduce the chance of leaking (97's are prone to leak). I'll try it with a single pump first
Thanks again. Triple 97's look SO cool.
Buzz (MoonBat)
 

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Old(s) Fart
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Relays for the headlights depends upon the bulbs used. Regular ol sealed beams work just fine through the head light switch and the dimmer. Put in hi end halogens and the headlight switch will be the first to give up. Repaired a friends car, he had halogens, he complained the headlight switch got too hot to touch. When I went to check it out I found he was only a short ways away from a fire. The switch had gotten so hot the terminals were all loose. So, relays depend on type of light.
Incandescent sealed beam headlights are 35 watt, or about 2.75 amps each. Sealed beam halogen replacements are 50 watt, or about 3.9 amps each. Even if you double that for a four lamp headlight configuration, that's 16 amps. The GM style headlight switch has a 20 amp circuit breaker built in. Yes, if you are using H4 halogens at 100 amps each, you'll exceed the capability of the breaker in the switch.
 

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Discussion Starter #9
Incandescent sealed beam headlights are 35 watt, or about 2.75 amps each. Sealed beam halogen replacements are 50 watt, or about 3.9 amps each. Even if you double that for a four lamp headlight configuration, that's 16 amps. The GM style headlight switch has a 20 amp circuit breaker built in. Yes, if you are using H4 halogens at 100 amps each, you'll exceed the capability of the breaker in the switch.
Thanks again.
Buzz
 

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Only use one fuel pump BUT Install a return pressure regulator. I doubt you can find a fuel pump to put out 2.5 psi.
On the cooling fan use a cooling fan thermostat to control when the fan comes on and this will act as a relay.
 

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My adjustable fan controller triggers a 40 amp relay to drive the fan . That way the ( expensive) adjustable controller doesn't have to carry the fan load . A low pressure fuel regulator will take care of managing an electric fuel pump.
 

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You run relays to keep power at power suckers to lessen strain on wiring and factory switches.

All a relay does is switch power. They are very tough little boxes. I have submerged, frozen, melt, caked them in mud, and installed them in spots where they were vibrated constantly. When ran within the voltage they provide and connected tight they will outlast most other electrical components.

Now with a fuel system you want a return system. This will allow the fuel to circulate keeping it cool up to the regulatior. I have ran pumps capible of 80psi and several hundreds of gpm on tbi and carburated engines. That regulatior is only going to let the preset psi into the rail.
I have never ran below 4.5 though. Going close to 2psi you may need to run a regulatior that does have a max psi it can hold back.

But you only need one pump and a regulatior to keep a constant psi of cool fuel feeding the 97's. The rest can go back to the tank.

Relay wise you place one next to the pump, one next to the headlights, one next to the starter, one next to the wiper motor, and one next to the heater. I like to keep the relays within a foot of these items often having all but the headlights inside the cabin with shrouded wiring going outside.

The power wires feeding these relays go to a positive and negative post on the firewall.

The alternatiors sensing wire goes to the post so when the relays kick on and that startup draw happens the alternatior can react instantly.
 

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One of the beauties of relays is that you can use them to turn off/on either or both. Also that you can control it with a ground or power. You can also make it a latch on, so that once momentarily engaged, it stays that way until it is reset.
I like to use ground control where ever I can, so that if the control wire somehow grounds out, the worst that happens is something comes on instead of blowing a fuse. Then there is the small current to larger current thing.
 

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MGK, tell me more about how to make a relay latch. Only way I know is to use a latching relay like the ones found in a VW used for high/low beams.
 

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525415


Common pin = keyed power
NO contact = to load
one coil pin is grounded, the other is control(triggered by whatever event)
If you jumper from the NO term to the control, once the event triggers the relay, it will feed the load and the relay, causing it to latch on, until keyed power is removed from the common terminal. which could be accomplished by turning the key off, or by a momentary break switch in series with the feed to common or in the jumper.
 
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