Strange electrical issue after changing rear light wiring harness

[Sevoris]

Hello everyone! I have a tiny issue and I'm hoping someone here might point me in the right direction.

I bought a Roadster a couple of days ago - I absolutely love it - but it came with a small problem. The rear left indicator and light weren't working (brake light was working). Long story short, I discovered a corroded and partially broken off connector for the rear light and replaced the whole left wiring harness with an OEM part to avoid merely splicing the wires to change the failed connector. The lights, apart from reversing light, were now working fine.

Today I went for a drive and things got interesting.

First, when braking, the front position lights came on and the illuminated speedo display went dark. Then it went to normal whenever I got off the brake. It still does this.

Second, when I turned on the lights as it got dark, the gearbox error signal came on the dash along with the ABS light and a generic warning light (picture below). The ABS and gearbox continued to work normally despite this, but then the gearbox suddenly jumped out of gear at motorway speed and reverted to Automatic mode despite me being in top gear and cruising steadily. It then worked perfectly fine in that mode, but the symbol remained. It did this twice, after I once again put it in Manual.

Lastly, the front left light suddenly went out and, as I stopped to check, I noticed that the brake lights - all three of them - were constantly on. I then did a small experiment, where my friend confirmed that the brake lights stayed on even as I drove away but worked perfectly fine if I turned off the lights - as if driving by day. So if the lights are off, the brake lights work as intended but if I turn the lights on the brake lights are also on.

Since none of this was happening before I changed the wiring harness I assume that's what caused it. The question is, where did I go wrong and how do I fix it? Disconnect the battery to reset the ECU? Check the fuses? Summon Cthulhu?

PS: the only light actually not working at the moment is the reversing light as I didn't realise the bottom of the bulb there was corroded as well, but I'm not sure if one malfunctioning bulb could be the cause.

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[tolsen]

Bulbs incorrectly fitted.

[10:45PM]

Check when sidelights are on that they're both illuminating at 5watts, check with sides on that when brakes are pressed, both lights come up to 26watts total, and finally check when sides are off and brakes are on that both bulbs illuminate at 21watts.

When you've discovered that one of these tests fail, take out and check all of the rear bulbs , for correct placement and correct fitting within its receptacle.

[Sevoris]

Check when sidelights are on that they're both illuminating at 5watts,

Noob question but how would I check that?

Also, bulbs did this? Damn. I think I'm going to start by replacing the last non-functioning bulb (reverse) and see if that makes a difference in case it's causing a problem. Then stock up on bulbs and see what's what.

[10:45PM]

If they are fitted badly (jammed in willy nilly for instance) they can short between the two side and brake circuits , and the car doesn't like that.
Also make sure any replacement bulbs are EXACT type replacement (same numbers)

noob question but how would I check that?

Check that both tail lights light up at 5 watt ish brightness (i.e. they are equal and not 21watt brake light brightness), then check when operating the brake pedal that both lights go brighter (21watts+5watts side brighter) ........ and slightly dimmer with brake pedal pressed, but sides are off cos you have to deduct the 5watts side brightness cos you've turned them off)

Geddit?

[2minits]

The only bulb that needs checking is going to be the stop tail light in the side you've messed with, it might be worth changing the bulb as it could just be a dodgy one, the symptoms you've got all point to either the wrong bulb or a badly fitted one in the stop tail socket

[EmilysDad]

You get some weird & wonderful effects on a Roadster will iffy bulb connections. Just putting mine back in the garage (a recent luxury for it) I noticed that as I pressed the brake .... both bulbs worked, though the passenger was very slightly slow to fully illuminate), the back lights on the pods briefly flickered. New bulb, and it's sorted.

[bob-in-dav]

Ok - had similar issues with two Fortwo's in a week - panel lights and side lights coming on with brakes etc.

In both cases all that had been done was a rear bulb change but it was the method used which caused the problem.

I usually remove the multi plug from the rear cluster and open the unit up on my lap.

It is when you reconnect the multi plug to the cluster that you need to ensure that the little blade connectors of the cluster are actually going in and making good contact within the loom plug and not just being pushed back into the light.

[Sevoris]

Issue solved.

Thank you all for your help! I would have panicked from the sheer amount of electrical buggery if I hadn't tried what you suggested first. Speaking of which, I found the culprit!

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Yes, it seems that when I changed the rear light I replaced it with the wrong bulb. As soon as I got the new one today and pulled the old one out I saw the problem. The single connector on the old one was touching the two separate connectors for brake and normal lights, causing problems. As it is, every single problem seems to have been alleviated by a simple bulb change!

[137699]

Nice easy fix. Next job for you - log onto www.theroadster.net and hook up with the rest of the roadster owners!

[Sevoris]

Nice easy fix. Next job for you - log onto www.theroadster.net and hook up with the rest of the roadster owners!

Done&done!

On a side note, I'm surprised how many Roadster there are in my area. Even a coupe.