Author Topic: Transmission safety switch  (Read 246 times)

jtwoods4

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Transmission safety switch
« on: 21 April 2019, 09:57 PM »
Here's a picture of what Mercedes calls the transmission safety switch. it's underneath the kick panel on the passenger side of the 1980 300sd. does anybody know what the heck the switch does? Part number

001-545-62-32
1980 300SD

Squiggle Dog

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Re: Transmission safety switch
« Reply #1 on: 21 April 2019, 11:23 PM »
This switch causes the transmission to go into second gear when the engine is idling at a stop with your foot on the brake, to help prevent the vehicle from creeping forward. Then upon acceleration from a stop, it puts the transmission briefly into first gear and then back into second gear to aid with acceleration.

Its second function is to prevent the transmission from being overloaded from too much torque from the engine. If it detects a situation (through wires connecting to the tachometer, overload protection solenoid, and ALDA system) which might cause the engine to put too much force on the transmission, it keeps it from shifting into the next highest gear.

At least this is how I understand it from memory.
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1967 W110 Universal Wagon, Euro, Turbo Diesel, Tail Fins, 4 Speed Manual Column Shift, A/C
1980 W116 300SD Turbo Diesel, DB479 Walnut Brown, Sunroof, Heated Seats, 347,000+

floyd111

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Re: Transmission safety switch
« Reply #2 on: 21 April 2019, 11:57 PM »
is this only for diesels? Can't seem to find a link for a gasoline MB

jtwoods4

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Re: Transmission safety switch
« Reply #3 on: 22 April 2019, 06:17 AM »
This switch causes the transmission to go into second gear when the engine is idling at a stop with your foot on the brake, to help prevent the vehicle from creeping forward. Then upon acceleration from a stop, it puts the transmission briefly into first gear and then back into second gear to aid with acceleration.

Its second function is to prevent the transmission from being overloaded from too much torque from the engine. If it detects a situation (through wires connecting to the tachometer, overload protection solenoid, and ALDA system) which might cause the engine to put too much force on the transmission, it keeps it from shifting into the next highest gear.

At least this is how I understand it from memory.

Wow thanks Squiggle dog. I talked to the local Mercedes mechanics up the street who have been doing this for 30 years and the old-timer there said he has only ever replaced one of these in his entire career. Strange that this broke somehow.on my car. Anyway thanks for the explanation.
1980 300SD

Squiggle Dog

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Re: Transmission safety switch
« Reply #4 on: 22 April 2019, 10:01 AM »
is this only for diesels? Can't seem to find a link for a gasoline MB

Correct. The diesel engines put out a lot of torque, and since the 300SD was an experimental vehicle which Mercedes was forced to put out on the market due to needing to keep the overall fuel economy of their S-class fleet at a certain level in order to be able to sell them in the USA, they made sure to cover all bases as they couldn't afford to have any glitches in the 300SD. It was not only the first S-class to have a diesel engine, but it was the first turbocharged diesel engine they put into a production passenger car, and in a very short time frame.

The engineers went overboard on making sure the new 300SD wasn't a failure. The oil system was designed so that oil which was cooled by a comically large (6" wide?) oil cooler would be routed to the piston skirts to cool them. The fuel system was set up with an ALDA and overboost protection system which would restrict the fuel supply if more than a certain boost pressure from the turbo was reached, and the transmission was designed to be kept from shifting into the next highest gear during this situation.

However, many people (including myself) simply disable the ALDA and overboost system, and it allows more performance out of the engine without restriction, and isn't prone to decreased performance over time due to the vacuum tubing getting clogged with soot. It's been found that in real world driving, a lot of the safety devices regarding the 300SD's engine were overkill, but the engineers couldn't take the risk.
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1967 W110 Universal Wagon, Euro, Turbo Diesel, Tail Fins, 4 Speed Manual Column Shift, A/C
1980 W116 300SD Turbo Diesel, DB479 Walnut Brown, Sunroof, Heated Seats, 347,000+

Squiggle Dog

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Re: Transmission safety switch
« Reply #5 on: 22 April 2019, 10:02 AM »
Wow thanks Squiggle dog. I talked to the local Mercedes mechanics up the street who have been doing this for 30 years and the old-timer there said he has only ever replaced one of these in his entire career. Strange that this broke somehow.on my car. Anyway thanks for the explanation.

Eventually things inside these units can fry, but like most electronics, they can be opened up and repaired.
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1967 W110 Universal Wagon, Euro, Turbo Diesel, Tail Fins, 4 Speed Manual Column Shift, A/C
1980 W116 300SD Turbo Diesel, DB479 Walnut Brown, Sunroof, Heated Seats, 347,000+

jtwoods4

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Re: Transmission safety switch
« Reply #6 on: 29 April 2019, 06:29 PM »
Well guys I replaced the transmission safety switch and the fuse is no longer blowing. However I noticed my transmission does not kickdown when starting from a complete stop. if I press my accelerator pedal all the way down to engage the kickdown switch it blows fuse 4 again. is it possible that the kickdown solenoid at the transmission is causing the fuse to blow?
1980 300SD

Squiggle Dog

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Re: Transmission safety switch
« Reply #7 on: 29 April 2019, 07:15 PM »
Yes, very likely that the solenoid on the transmission is bad. I ran into this when I replaced the transmission in my 300SD. The switch on the used replacement transmission was bad. So installed the one from the old transmission and all was well.
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1967 W110 Universal Wagon, Euro, Turbo Diesel, Tail Fins, 4 Speed Manual Column Shift, A/C
1980 W116 300SD Turbo Diesel, DB479 Walnut Brown, Sunroof, Heated Seats, 347,000+

jtwoods4

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Re: Transmission safety switch
« Reply #8 on: 29 April 2019, 07:17 PM »
excellent I have an old transmission out in the garage and I pulled that solenoid earlier tonight and hooked up 12 volts to it and it does click so hopefully that's the problem and I can just put it in the new car car. Thank you squiggle dog  :)
1980 300SD

jtwoods4

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Re: Transmission safety switch
« Reply #9 on: 30 April 2019, 06:28 PM »
Well that was absolutely the problem. Here's a photo of the kickdown solenoid from the transmission you can see the back of it has started to bust open. not sure how the hell that happened thus Illinois it only has 54000 miles on it (original miles on car). Anyway the spare that I had in my parts car makes the kickdown work perfectly.
1980 300SD