Relay for Trailers on LT? - BMW Luxury Touring Community
 
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post #1 of 5 Old May 27th, 2009, 7:18 am Thread Starter
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Relay for Trailers on LT?

Do you really need to use a relay on the LT when hooking up a trailer? Why can't you just splice into the harness? What are the potential problems one might have if you run your trailer lights without using a Relay?

Thanks

Tom
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post #2 of 5 Old May 27th, 2009, 9:06 am
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Re: Relay for Trailers on LT?

Quote:
Originally Posted by Plasterman
Do you really need to use a relay on the LT when hooking up a trailer? Why can't you just splice into the harness? What are the potential problems one might have if you run your trailer lights without using a Relay?

Thanks

Tom
YES you need a relay !!!! If not you can FRY the wiring harness CLICK HERE for good info. call them for the one for the BMW LT

Stevie Shreeve
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post #3 of 5 Old May 27th, 2009, 9:12 am
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Re: Relay for Trailers on LT?

There are a couple of issues but it really boils down to personal preference. There are certainly folks who have done it without using relays.

The first issue is the wire gauge for the tail lights and turn signals are sized to supply the rated voltage and current to the light bulbs. Adding additional light bulb loads will increase the current in the wires as well as the voltage drop across the wires. As the current increases the voltage will drop and the the relative brightness of the all the light bulbs will decrease. How serious this effect becomes is dependent on the wire gauge and how many light bulbs are paralleled together. The other issue to consider is that the trailer wiring harness will add additional wire length and therefore additional voltage drop to the trailer lights. Most modern automotive systems can handle doubling the load fairly well without significant dimming of the OEM lights or creating a heat problem in the wiring. Whether these effects are acceptable is again a matter of personal choice.

The second and probably more important issue is that if something happens to the lighting on the trailer, say a short to ground, a fused and relay isolated system will not take down your bikes lighting system and anything else that may be on that fused circuit. Only the fuse in the trailer circuit would be blown. Again it is a personal choice if this is a serious enough concern to warrant isolating the trailer circuits.

As far as frying a wire harness is concerned, the tail and brake light circuits are fused on the K1200LT and I would guess on just about any automotive product worth a hoot. Wiring burns up when un-fused or high amperage rated circuits get cross wired or shorted to the chassis or to ground return wires of lower power rated circuits. Someone would have to have seriously screwed the pooch when they were splicing wiring for anything like this to happen. Mis-wiring an isolation relay setup is more likely to create this scenario. In a properly wired system the worst thing that should be able to happen is blowing the lighting circuit fuse. The whole purpose for fuses is to protect the wiring from being damaged because of short circuits or unexpected high current loads.

I usually invest money and time trying to improve my visibility on the road for any motorcycle I own. Either of these are reasons enough for me to isolate the trailer circuit from the bikes circuits. YMMV


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Last edited by jwd98056; May 27th, 2009 at 11:35 am.
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post #4 of 5 Old May 27th, 2009, 11:30 pm
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Re: Relay for Trailers on LT?

Quote:
Originally Posted by Plasterman
Do you really need to use a relay on the LT when hooking up a trailer? Why can't you just splice into the harness? What are the potential problems one might have if you run your trailer lights without using a Relay?

Thanks

Tom
Tom, no need to spice just use duplex spades. Use the relay for insurance.
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post #5 of 5 Old May 28th, 2009, 12:15 pm
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Re: Relay for Trailers on LT?

Also the size of the wire at the tail light assy. is 1.0mm where back further in the harness it is only 0.5mm. So don't be fooled by what you see at the tail light assy. I highly recommend an isolator.

John
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But lack DE, MA, RI and CT with the 2005 LT

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