1968/1969 USA Emissions Component - What is it?

[JPMOSE]

On my 1968 250SL; USA version; Automatic Transmission,  there is a Bosch electrical component (metallic finish - about 2 1/4" square) behind the battery.  It has a connector on top; it's bolted to a metal plate and has a Bosch relay on the backside of the metal plate.

WHAT IS THIS? I'm guessing that it's a speed sensor used for the Fuel Shutoff solenoid.

The BBB doesn't have a write up on this that I could find (looking at the Service CD for Mercedes with early and late BBB shop manuals).  The Haynes book has a better write-up on this.  From what I can see, this particular component is only on 1968 250SLs with Automatic and 1968/1969 280SLs with Automatic (through May 1969 with introduction of the new automatic transmission); USA versions only.  I believe that past May 1969, USA 280SLs with Automatic have a black plastic speed sensor that sits back to back with a similar looking relay box???

From the Haynes diagram,  the speed sensor appears to be similar to a relay.  On the pre May 1969 cars, it appears to be activated by a pressure switch on the transmission and the ignition coil.  I haven't studied the later configurations.

So....am I right...is this is a Speed Sensor?  If so,  how does it work?  What is the failure rate of this component?  Can it be rebuilt?

I am looking for an education from the experts out there....I really enjoy learning about the functionality of everything under the hood....thanks for your help!

Best Regards,

J. P. Mose
1968 250SL

[waltklatt]

Does your car have transistorized ignition? Sounds like the likely place to install it, but only on the 1970-1971 280SL's I have seen this.
Can you post a picture of the mystery box?
Walter Klatt
1967 220SL-diesel
1963 230SL-gas

[TheEngineer]

It is a speed sensor. Has a relay in it that will close when the engine RPM is over 1100RPM. The function is to provide a signal to your fuel cutoff solenoid on the injection pump. The circuit is described in the BBB under 00-74/2: When you have the foot off the pedal because you are going downhill, the microswitch on the fire wall closes, the speed switch is closed and the solenoid cuts fuel to zero delivery. When you approach a stop, the engine RPM drops below 1100, the speed switch opens and your engine gets fuel again. You can test system operation by connecting a test light between ground and fuel cutoff solenoid terminal. Mine works intermittently: Sometimes it works just fine, sometimes not. I have had everything apart and it's the Speed Switch. But I've been unable to fix it. It's most frustrating. Good thing is, it doesn't make any difference.

[RBurg]

Just disconnect it and tape the wire to something.

Ron - Minnesota
71 280SL Tobacco "O GIGI"
97 E420 Silver Mist

[JPMOSE]

Thanks for the replys so far!

Peter....I assume the speed sensor detects RPM from the coil?

Ron...I know I can disconnect it, but I am interested in all things electrical and mechanical....I like to know how things work.  I sold a Linhof to a Ron in the St. Paul area three-four years ago.  It wouldn't be you by chance (I noticed that you are a photographer)?

Best Regards,

J. P. Mose
1968 250SL

[TheEngineer]

J.P: I don't know. I don't have a wiring diagram for the speed relay or repair instructions. I have opened it by removing the one screw in the center. There is a choke coil and a printed circuit board. If you find out how it works, please let me know. I'd love to fix mine.

[JPMOSE]

Peter,

I took my speed sensor apart on Sunday.  Moisture was trapped in it, which corroded many components.  I attemted to clean it up though.  I disassembled the relay next to the speed sensor and it was like new.  I hooked a circuit tester to #1 on the speed sensor -- the light stays on continuously (even at 850 RPM).  Hence, I am either testing it incorrectly or the speed sensor is stuck in closed position.  As far as it's functionality, I would assume it needs, at minimum, the output from the ignition coil at 1100 RPM before it closes the relay within the speed sensor.  Doesn't the coil provide output intermittently (faster as the points open faster with increased RPM)?

The '68/'69 Speed Sensor is a discontinued part.  But, Mercedes Germany has 17 in stock for a mere price of $750 each!  It's a good thing fuel shutoff isn't necessary for operation of the car.

This is the only problematic component of the fuel shutoff system.  I believe the oil pressure switch, micro-switch, fuel shutoff solenoid, neutral safety switch, relay and wiring are all fairly reliable.

Although I enjoy getting items to work correctly, I think the speed sensor may be a lost cause.  It is only on USA versions and I doubt there is much interest by many to get it to work.  Hence, I will probably give up on it.

Best Regards,

J. P. Mose
1968 250SL

[TheEngineer]

J.P.: Great minds think alike

'69 280SL,Signal Red,113044-12-007537,tired engineer, West-Seattle,WA

[knirk]

Question about the fuel shut off solenoid: If stuck closed will the engine run in idle, or stop completely.

I have had a engine problem on my 69 us-model the last few days. After down hill driving, or deceleration before a turn I get no responce when pressing the gas pedal. It will not stop, but run in idle. And after a few seconds it runs normal again.



Per G. Birkeland
69 280 sl aut 834
Norway

[JPMOSE]

Per...when you state "closed" I assume you mean the same as engaged.
If the fuel shutoff solenoid is engaged, the engine will shut off immediately at idle.

Best Regards,

J. P. Mose
1968 250SL

[knirk]

Thanks J.P.

I removed the cable to the fuel shut off relay and took the car for a 35 mile drive. No stops or problems at all. Easiest fix ever!

Per G. Birkeland
69 280 sl aut 834
Norway

[ja17]

Hello JP,

Here is some information an wiring diagrams on the system which is not found in the BBB but is from a factory bulletin. The system is different for the automatics and standards.

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Joe Alexander
Blacklick, Ohio

[Benz Dr.]

quote:Originally posted by theengineer

J.P.: Great minds think alike

'69 280SL,Signal Red,113044-12-007537,tired engineer, West-Seattle,WA

 You left out the best part of that saying.

  '' And fools seldom differ. ''

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