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Trailer wiring causing limp mode?
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Old 29-03-2017, 03:14 PM
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Default Trailer wiring causing limp mode?

Hi guys, when I'm towing my boat trailer with the lights on, my D40 (2014 series 6 manual) seems to lose all power (cannot rev, no turbo etc). I assume this is limp mode?

As soon as I turn the lights off it returns to normal.

This only seems to occur with my boat trailer, other trailers are ok so it would point to my boat trailer wiring.

However, what would cause this to occur? The lights on the trailer (LEDs) all work fine.

Would appreciate your thoughts!

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Old 29-03-2017, 08:05 PM
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That's an interesting fault. It shouldn't happen! I suspect something's wrong with the second pin from the left of the trailer socket on the trailer. That should be the clearance light pin if it's using standard wiring - I always refer to this page for my trailer sockets.

It's curious that it should cause limp mode. I also wonder if there's a cross connection between that and the pin beside it (3rd from the left on the trailer) which may be fed by the car's reversing signal? That could bring the reversing light circuit on, which might indicate to the ECU that your power should be restricted because you're in reverse - it's a bit of a leap, but a possibility.

Try disconnecting those two wires from the trailer socket (you don't need either during the day) and see what it does.
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Old 29-03-2017, 09:54 PM
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does sound like there is a bad connection or something to do with the Lights Wiring to trailer. i would use a multimeter and check for good 12 volt power at the plug harness and at the tail lights, i know you can,t check the LED as there sealed but probe the wire at the tail lights to get the voltage. has to be something very simple.
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Old 30-03-2017, 08:30 AM
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Quote:
Originally Posted by Old.Tony View Post
That's an interesting fault. It shouldn't happen! I suspect something's wrong with the second pin from the left of the trailer socket on the trailer. That should be the clearance light pin if it's using standard wiring - I always refer to this page for my trailer sockets.

It's curious that it should cause limp mode. I also wonder if there's a cross connection between that and the pin beside it (3rd from the left on the trailer) which may be fed by the car's reversing signal? That could bring the reversing light circuit on, which might indicate to the ECU that your power should be restricted because you're in reverse - it's a bit of a leap, but a possibility.

Try disconnecting those two wires from the trailer socket (you don't need either during the day) and see what it does.
I seem to recall d40s having something where if the brake lights come on, it reduces power. So the only thing I can think of is that when the headlights are on, power is being fed back into the brake light circuit on the vehicle. It could be a stray wire in the trailer plug or a bridged wire somewhere else in the wiring loom, but will take a bit of diagnosis with a multimeter to find the short...
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Old 02-04-2017, 04:54 AM
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Some interesting comments there guys and it does help. I added a rollbar with the inbuild LED brake light and a friend wired it into the cars trailer plug. Not sure why only my boat trailer does it but the cars trailer plug has been 'messed' with.

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Old 02-04-2017, 04:56 AM
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Actually, come to think of it when I turn my park lights on when the boat trailer is connected it does turn the rollbar brake light on! This is when limp mode or power loss starts. When I turn park lights off its fine again (including turn signals and brake lights on the trailer).

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Old 02-04-2017, 04:58 AM
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Quote:
Originally Posted by bods View Post
I seem to recall d40s having something where if the brake lights come on, it reduces power. So the only thing I can think of is that when the headlights are on, power is being fed back into the brake light circuit on the vehicle. It could be a stray wire in the trailer plug or a bridged wire somewhere else in the wiring loom, but will take a bit of diagnosis with a multimeter to find the short...
This sounds exactly the problem. As I said below, when park lights are on power is being fed back into the brake circuit as it causes my rollbar brake light to come on (as its wired into the trailer socket)...

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Old 02-04-2017, 06:11 AM
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Sounds like it may be something in the trailer wiring. Can you try the same trailer on a different vehicle and see if the same issue happens?

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Old 03-04-2017, 02:09 PM
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Quote:
Originally Posted by bods View Post
Sounds like it may be something in the trailer wiring. Can you try the same trailer on a different vehicle and see if the same issue happens?

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I dont have access to another car with towball unfortunately but I think (hope) its on the trailer side.

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Old 03-04-2017, 09:21 PM
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Fair enough. Best bet would be to googe a picture of your trailer plug (unless you know what the pin diagram is) and test for continuity between the tail and brake light circuits. Obviously there is somewhere, it just depends whether it is in the trailer plug itself or one of the connections at the lights on the trailer...

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