[968 GUY]

saw something in here about brake lights affecting the cruise control (automatic speed control). I noticed mine was not working the other day and was preparing for the worst until I saw the thread which included info that a burnt out brake light bulb could shut cruise off. Brake lights were working, but I had replaced bulbs w/LED's. Since I had also heard that LED's may trigger a warning that a bul;b was burnt out,i changed them. Have not driven the car yet, will later today and find out if that was the prob...hope that is all it was.
Ken

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[968 GUY]

Yep, LED's in the brake lights will not allow the cruise conrol to work. Glad that was all it was as I am about to do a 3500 mi. trip. Put less than 2000 mi on the car since I got it Oct '08. Hate to put all those miles on in one shot.

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[flash]

interesting - i wonder what exactly about them causes the failure

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[968 GUY]

The only thing I can think of is that since LED's pull less power, it may act like a burnt out bulb. I brlieve that I have seen somewhere that a burnt out bulb will not allow the cruise to engage. Might be worth a test session.

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[flash]

hmmm - i am thinking that the ground potential is maybe the issue, but indeed a test is in order

lol - and you are just the guy to do it - thanks for volunteering - have fun - let us know

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[968 GUY]

I am satisfied with what I have found out, and believe that I am correct in what I stated about bad bulbs and also the LED's. Hopefully this would be helpful to someone with cruise probs.

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[flash]

well, it is helpful if somebody decides to put leds in place of their bulbs, but why don't you do everybody a favor and check it out? seems easy enough - we know that using the wrong bulb does cause the cruise control to fail, but i'm not sure about a burnt one

i'd do it, but i'm in the middle of working out the drive belt routing for the supercharger and making the mounting plate today

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[968 GUY]

works with 3rd brake light unplugged, don't have a spare burnt out bulb to try right now.

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[flash]

yeah - that supports my theory that it isn't a burned bulb issue - it likely is more related to the led itself - i couldn't imagine a burned bulb disabling the system (though with porsche you just never know) - but, something with the wrong resistance, or hitting the wrong pin would do it

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[gryphon]

I had a similar problem with a 1984 gray market 928. It seemed that the additional lighting that was required to make the car US legal affected the electronics (perhaps resistance?) in some manner and disabled the cruise control system. I don't remember the details but do know that itI took an automotive electrical specialist to sort it out.

Tom

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