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Electric Relay Placement

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Lin

Lin

Jedi Knight
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Hi folks,
As a general rule, do you physically place electric relays closer to the controlling switch (dashboard) that operates a device (for example, fog lights or electric radiator fan) or closer to the device itself (front of car or radiator). Just curious if there is a general rule.

Thanks,
Lin
 
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Re: Electrical question

Hi Lin--

There are probably several factors to take into account--space, appearance, etc--but since one of the advantages of using a relay is that it allows you to use lighter wire between it and the power source/switch (versus "hard wiring" the appliance) it would make sense to mount the relay as close as possible to what it is controlling.

Seeya in Crystal Springs.
 
Re: Electrical question

I agree with Michael. In my mind there are two reasons for the relay to be close to the device. 1. is that you do not want to have the high current or the larger wires coming into a small switch. the 2nd reason is that you want an efficient circuit so you should pull the larger wires the shortest and most direct route to the load device. Therefore the relay which is going to open and close those larger wires should be out there not pulled into an area such as the dashboard. An example of such a circuit is the starter solenoid. The large wires that have to run between the battery and the starter would be difficult to pull them into the dash area and also to attach them to the ignition switch. Therefore the location of the starter solenoid is near the starter itself and in a very direct path from the battery to the starter. The solenoid is simply a relay. The smaller wires are then pulled between the solenoid's coil and the ignition switch or a separate starter button.
 
Re: Electrical question

I would counter that the production cars I've owned had the relays in a central location with the fuses, often in the passenger compartment.

It would be interesting to see an authoritative source on this question.
 
You have a good point Steve. Many do have them centrally located. I usually do see them in an enclosure under the hood. My two Nissans have it that way. But of course thatā€™s Nilsson. ��. Too me I think I would consider the size of the cables and the easiest routing.
 
vette said:
Cool. And they could still be grouped with the mounting tab shown.
Click to expand...

When I added relays for my headlights, I originally placed them out of the engine compartment inside the fenderwell near the headlights. Didn't take too long before they got wet and failed. These solved the problem.
 
In my BJ8 there is a open area (like a box) on the inboard side of the firewall that was empty having no purpose. It was large enough to hide a second Lucas dual fuse box and 3 relays mounted on a metal plate.
 
Many rally cars have them centrally located also, more fir the reason of easy / quick replacement than where they are best in the flow of electricity.

i am sure this will be similar for car manufacturers

plus most are inside the car so more protected from the elements.
 
Hi All,

Steve, yes, many manufacturers have centralized protection centers for fuses, relays, and diagnostic computer plugins and, due to their electronic complexity, were designed that way. Our Healey harnesses were not and operating amperage for all devices are carried and controlled with elements directly wired into each circuit (i.e. Headlight Circuit, etc.).

Although this was not as much of an issue with few attached devices of low draw, many of us have added many more devices that have dramatically increased the amperage draw through circuit wiring. With aging, corrosion, and hardening, additional resistance results in increases in even the original equipment amperage draw with harnesses required to carry more power flow. Adding upgrades and add-ons through the same harnesses have caused many to exacerbate the situation with the switch from their original 23-amp generators for a 65+ alternators in order to meet the power demand.

So, what are you trying to protect? If you are installing relays to reduce the amperage flow through your harness from operational draw to that of switching only, I would suggest you place the relays closer to the device and create a harness to the nearest source of high power (i.e. generator/alternator). As with fuses, if you want to protect the circuit, place the fuse at the power source of the circuit but if your intention is to protect the device, place the fuse close to the device. Keep in mind that, in most cases the Healey's circuits and devices are not fuse protected and running additional wire to a central point only extends the circuit for questionable access convenience. Also, with a fuse block, you are limited what it provides. However, you can centralize critical components and compliment with the use of in-line fuse protection on all other circuits and components you wish to also protect. Although this combined approach will eliminate any limitations, it does require the creation of additional documentation that references the circuit and its location of fuse protection to ease future location.

Since all my relay and fuse connections are lubricated with dielectric grease and weather tight to eliminate moisture penetration over the substantial extended time of forgotten attendance, I trust the small benefit derived from a central fuse box and/or relay box is far less then the accepted limits.

Just my thoughts,
Ray(64BJ8P1)
 
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