Tail light bulb and cruise control '81 300SD

[tyl604]

1981 300SD. My cruise control has not worked for a while. At first started being a bit wonky and then finally just quit.

I seem to remember that the tail light circuit is somehow involved with the cruise and using the improper tail light bulb will stop the cruise from working.

Can this be true?

[Sugar Bear]

Yes, it is true. Use quality bulbs such as Osram or MBZ genuine of the correct wattage. IIRC they are 5W/25W. Do not use US 1156 or 1157 bulbs.

[tyl604]

Ouch. I actually need to check. Might have a 1157 installed.

Thx. Good info for all.

[Diseasel300]

So long as you have incandescent bulbs installed and they are working when you press the brake pedal, the brake circuit is unlikely to be the problem. The cruise control amps are notorious for taking a dump, often randomly disengaging by themselves or going into a runaway acceleration event before cutting out randomly. Sometimes they just die for no reason with no warning.

Assuming your brake lights work, next stop is the cruise amp. Kris Rose used to repair them, not sure if he still does. He has a profile here with the username JamesDean

He has a website here:
https://www.cruisecontrolrepair.com/cruise-control

[Sugar Bear]

1157's can also heat damage the housings.

[tyl604]

SugarBear - Ok; confused. Tried to find Osram but still find Sylvania-Osram 1157. Can this be the original bulb @ $1.89 when I see others on Ebay advertising the bulb at $12. And still says 1157.

Osram-Sylvania Tail Light Bulb; 12V - 21W/5W, Clear | 1157
Audi, BMW, Dodge, Land Rover, Mercedes, Mini, Porsche, Saab, Volvo
$1.89
Reg$1.97
List$1.46

Wonder what the dealership wants for this bulb? Tried the Osram website but can find no auto bulb at all. Just looked at RBM dealership-Atlanta; they still list a 1156 at $4. No mention of Osram.

[cmac2012]

Originally Posted by Sugar Bear

Yes, it is true. Use quality bulbs such as Osram or MBZ genuine of the correct wattage. IIRC they are 5W/25W. Do not use US 1156 or 1157 bulbs.

Wow. I had not heard about this. I check the lights at boneyard and pocket bulbs that look not burnt - I do the same thing with spade fuses. I have a lifetime supply of spade fuses.

I figure what the hell, it's the reimbursement program for the three dollar entrance fee at Pick n' Pull these days.

But from what you say, I may have to use those bulbs for my other non- MB rigs.

The cruise control on my SDL has never worked, but miraculously it works on the 300D. But it's been sketchy lately. Sometimes it surges, and then backs off, at first this was only on downhill, it's doing it more and more on the flat. Also on long trips, about once every hour or two it will suddenly quit and I have to quickly get my foot on the gas pedal and reset it.

Now if you'll excuse me, I'm going to go check my bulbs.

[Sugar Bear]

Check Osram 7528.

[tyl604]

Yes - remember James Dean. Actually I go all the way back with this problem with putting a cigarette filter in the black round box on the wheel well - which did not work.

Have resisted asking JD for a rework and just wondered about the bulbs. Pretty sure after 42 years I have replaced these bulbs and bet I have 1157s. Need to go down to the garage and check.

Appreciate the input. Would rather start out with $8 in bulbs as a first try. Just confused about the exact 1156-1157-Osram/Sylvania bulb and have little trust in the dealership;.

[tyl604]

SB - will check Osram 7528.

Thx.

Just ordered 2. $14 from Prime.

Hoping; much appreciated. Maybe James Dean will be next if this does not work.

[Diseasel300]

Have run 1157's in all of my MB's and never experienced a burned/melted tail lamp assembly. If the lights are on when you hit the brakes, it isn't the problem. The cruise control amplifer is passing current through the brake light bulbs when the pedal is released, so if the bulbs are intact, cruise works. If you burn out a brake light bulb, cruise is disabled. It's a safety feature so the amp can tell when the brake switch is activated.

Presently have 1157's in my SL and can confirm that not only is the tail lamp housing NOT melted (nor has it in 28 years of running), but the cruise works fine.

[Sugar Bear]

Had the inner silver housing melt on a W115. I'm glad yours is ok.

[tyl604]

Cmac - your symptoms are exactly what I experienced before mine quit running period.

Interesting that no one seems to remember the cigarette filter solution; I must be getting old.

Searching the dark recesses of my mind, I seem to remember that there was actually something about the metal that was used in the original Mercedes bulbs, that the new 1157s did not use that metal and that is what caused the problem. No one else remembers this???

[Sugar Bear]

I don't specifically remember the metal being the issue, it could be. I definitely remember bulbs causing issues though.

[tyl604]

Osrams will be here Fri. We shall see.