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Aftermarket relay location?

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Old 07-17-2014, 12:13 PM   #1
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Question Aftermarket relay location?

So i plan on adding some aftermarket relays for reverse lights and possibly a front light bar/cubes. i want to keep as few wires as possible from entering the cabin (but i will if need be) so i considered mounting the relays in my engine compartment. where does everyone mount them and why? im not sure the connectors would stand the test of time if i mounted them in the engine compartment. my switches are located in the change bin in the front of my center console.
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Old 09-23-2014, 06:00 AM   #2
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I am going to screw mine somewhere on the outside of the battery box. I have a SRM backup light kit coming in the mail. When I do it ill take a picture.
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Old 09-23-2014, 09:08 AM   #3
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The relays that I've come across (including the one in my SRM kit from Rigid) recommended them being inside the cabin...not the engine bay. I ran my SRM relay through the large firewall grommet on the driver's side (required a vertical slice of the grommet) and ziptied the relay as close to the firewall as possible. That protected the relay and still gave me enough wiring length to get to the switch (mounted next to the rocker/dimmer) and get the leads to the back of the truck (following the factory loom down the framerail).
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Old 09-23-2014, 10:47 AM   #4
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I have all of mine (3 total) mounted in the engine bay, on top of the front rail on either side of the radiator. They're easily accessible if anything goes wrong or you want to change things around and it's easy to route wiring neatly around the edge of the bay to keep things neat. This location also keeps them above most of the heat being generated and trapped down inside the engine bay and since they're in the front they get a lot of air movement to help keep them cool. I packed all of the connections with dielectric grease and have had no issues in the 2 years I've had them installed there. We definitely experience lots of road grime during the winter and I work for an electric utility, so the truck has been through plenty of unpleasant conditions and this location has still worked just fine for me.
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Old 09-23-2014, 11:36 AM   #5
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Default Random Thoughts...

A couple of ideas gathered from doing this stuff over the Decades...

Nothing succeeds like success, so the experience of benchwarmer4203 sounds good to me. It's what I've done. Automotive Grade Relays can take the proverbial heat. I've used them for Trailer Brake Duty, etc..

1. If you're concerned about Environmental conditions under-Hood, consider putting the Relays in the smallest, sealed 'Tupperware'-type Container you can find. Or, something similar... All you're trying to do here is copy how Ford reliably mounts Relays on top of the Radiator.

2. Run the req'd +12 VDC to the Relay Coil, and use the Dash/Cubby Switch[es] to complete the Ground side of the Relay Coil Circuit to Chassis Ground. This means the Wire run through the Firewall is carrying only Chassis/Circuit Ground; not +12 VDC. This makes possible electrical shorts a non-issue. This is the way Dome Lights were wired through the Door Switch in the Stone Age. All the Switch did was complete the Circuit to adjacent Ground.

3. Trailer Wire comes as 4 Wire, Color-coded 'Ribbon' Cable. It snakes through tight spots pretty well. Using it means you can ID the different Switched Circuits by Wire color, and have a few 'spares' for any future Electrical Control Projects.

4. Of course, always use a Fuse/Circuit Breaker for the Supply +12 VDC as close as possible to your new Relay 'Bank'. This protects against Mister Murphy and his Law.

I hate re-doing Guano, so I don't. 'Overkill' it the first time, and you're done.

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Last edited by Engineer Guy; 09-23-2014 at 11:40 AM.
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Old 09-23-2014, 12:00 PM   #6
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I mounted mine on the rear side of the battery box. There is enough room on the top lip to drill a hole and bolt the relay.

I use this type of relay with metal bracket. I bend the tab backwards so it can sit on top of the battery box lip.

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Old 09-23-2014, 05:49 PM   #7
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My Rigid D2s came with a relay (regular Duallys do not) - I put on an existing bolt just above where the firewall grommet is. Not sure what item is there already but it's got a flange and a bolt already, so the relay using the same bolt/location fits just fine.
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Old 09-23-2014, 07:02 PM   #8
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I just don't see how putting the relay in the engine compartment would be a bad idea, I know its not recommended, but there is already a bunch of electrical stuff in there. I see relays put outside of the "cabin" at work all the time.

Manufacturers also usually recommend soldering all connections, but lots of people don't.
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