Speedrelay

[TheEngineer]

My 1968 280SL with automatic transmission has an engine RPM sensing relay. It closes (a contact) when the engine speed is over 1200 RPM. It's part of the emission control system, but it also saves gas. When the car is coasting, such as going downhill, and the engine speed is over 1200 RPM it cuts off the fuel supply to the injection pump. You can tell if it doesn't work when the car backfires when coasting. When it works, the car also slows downhill much better in a lower gear. The relay is completely solid-state and I'd like to find a replacement or substitute.  Bosch Part Number is 0 332 010 001

Shown cover removed and clip attached to monitor function

[Richard Madison]

Peter:

My '69 Manual sometimes backfires going downhill.

Does a Manual car (1969) have this same relay? Where is it is located? Can a home mechanic test it and repair it?

Richard M, NYC

[TheEngineer]

Yes Richard, I think so. The system is described under "USA Abgasreinigung" page 00-78 and I have to apologize, because I only have the German edition of the shop manual. The English version was out-of-print. On my car I can test the operation easily because I have installed a switch that cuts the fuel supply. Originally that switch was to prevent unauthorized persons from starting the car.

[114015]

Hello Richard,

Yours shouldn't have these relays because your car is a European version. As to my knowledge, only US versions (code 49/1) of the later 280ies had the "USA Abgasreinigung" (US emission control). These are located (somehow) behind the battery - large & black.

Best,
Achim

[TheEngineer]

Very interesting: I took the speed-relay out, inspected it and put it back in. It worked just fine – once. I rigged a test lamp per the book and it came on when I coasted. It only comes on when the transmission has shifted into 3rd or 4th. And then it quit working. It’s the speed-relay; all other switches work just fine. I can tell by connecting a test lamp directly to the speed-relay and running the engine: Over 1200 RPM it comes on, under 1200 RPM it goes out. Just like the book says it should. I took it out again and inspected it again. Found nothing. Put it back in and it worked fine. Went for a half hour test drive and it worked the whole time. It even comes on when I coast to a stop sign. There is no popping in the exhaust at all when it works. Car runs very nice. There has to be an improvement in gas milage when it works. But I can’t re-create the failure and I don’t know why it works now. It probably will quit again. Help! Also, I must correct my earlier post: The control board (circuit-board) is solid state but it controls a mechanical relay, all inside the housing. And yes, I have inspected the relay for operation and inspected the points. The unit gets it's signal from the "1" terminal of the coil. It's a chopped signal and when it becomes faster, the relay closes. If someone has a wiring diagram of the circuit board, it would help a lot. As of now I suspect heating to cause the failure, maybe a cold solder joint? On my second trial, I ran with the cover removed.

[wwheeler]

Peter,

I also have this large square relay (0 332 010 001) and have been trying to figure how it works. Mine is on a '68 280 SE automatic and has a phillips screw in the middle that holds the cover on. Everything you say is true about it's operation. The only thing I have done to mine is remove it and clean it up. I need to test it and will do so like you.

The relay is available from the Classic center. I don't have the number right now but can get it for you tomorrow. You'd better sit down though for the price... $960.00. They probably don't sell many except for people who have to have it.

Good luck

[wwheeler]

New MB part number is #000-545-01-32

[TheEngineer]

Wallace: Where did you find that part number?
I did something to the relay and it has been working just fine. I have driven the car about 50 miles and it works every time I take my foot off the accelerator. I don't know what I did! I removed the cover and maybe this made it run cooler. I have re-installed the cover and we'll see what happens. There is one 3300 ohm resistor that shows evidence of overheating. It still checks out resistance- wise. It's a fascinating project - I'm glad I'm doing it on my own car and not for a customer! I'm thankful to be retired. I can't afford to spend $960 for a new relay. It's not something that makes the car not driveable and besides, the 280SE sedan has the same set-up. I should be able to find a relay at the junk yard. When I was young, I built myself an entire car from parts from junk yards. I'm terrible cheap! I even buy used tires.

[wwheeler]

I got the part number from the Classic Center when I sent in the Bosch number. I agree, since the item is not critcal to the function of the engine, the price of a new relay is hard to justify. I took mine off and sprayed Eastwood's clear zinc spray on it to make it look better. Not a substitute for zinc plating, but a whole lot easier.

[TheEngineer]

After years of trying to find the problem I finally traced it down to the 4-pin connector. You can see it in the photograph at the top of the relay. Note, that I have the bakelite shell removed to have access to the connector pins. With the shell removed it worked just fine for more that a hundred miles. When I re-installed the connector shell, it worked once and then quit. When I pulled the connector-with the shell installed-up by about 1/16 of an inch, it worked again. When I push the connector down as far as it will go, it works once and then quits working. I inspected the solder joints on all the connector pins with an ohm meter and found them to be good. I'm now running with the connector pulled up and it has worked every time so far. I think there is the ghost of a disgruntled Mercedes mechanic hiding somewhere!