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Overload Protection Device Switching Unit 1979 W116 300SD
I've been trying to figure out exactly how these cars are supposed to work before I start guessing.
After studying the electrical schematics for my '79 300SD W116 I'm a bit confused. There is a "Transistorized" box on the schematic that's called "Overload Protection Device Switching Unit" I believe this is the little black box located in the front passenger kick well on the right. This device looks like it works in conjunction with several switches and senders: Intake Manifold Pressure Switch(Over boost sender on top of Intake Manifold) Transmission Pressure Switch (???) Switch Over Valve (AKA Changeover valve on firewall connected to Over Boost sender and ALDA) Kick Down Switch (Behind Accelerator pedal) Stop Lamp Switch (Brake light switch) Automatic Transmission Solenoid valve (12V solenoid located on passenger side of transmission) TDC Sensor (Not sure where this is located) Here is a link to some good information on how this works: Operation engine-transmission-overload protection I heard there is a pendulum switch inside the overload protection device switching unit (not sure if this is true). This pendulum is supposed to sense when the vehicle has come to a complete stop. When this happens it will energize the 12V solenoid on the transmission to kick it down to first gear. If this box is not working properly your car will never start off in 1st gear, only second gear. My question is: Where is the Transmission Pressure switch located? I have quite a few projects going on with my car(s) and house. I haven't been able to crawl under it for a while to locate the transmission pressure switch. I was wondering if anyone knows where it might be. Phil |
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I can only find the kick down switch and the I believe, the pressure control rod. I imagine you mean that, a rod instead of an electrical switch. Follow that rod to the transmission.
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83 SD 84 CD |
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Thanks, but I'm aware of the pressure control rod (bowden cable on later models). Please see the photo I attached to this message. I circled the transmission pressure switch in red. I got these schematics from:
Mercedes-Benz Model 116 I was hoping maybe it this switch was only on a different model, but it says MOD 300SD ONLY on the drawing. Phil |
#4
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As I understand it that control unit does 2 things:
Kick the transmission down to first from 2nd when you remove your foot from the brake pedal (the car rests in 2nd). Limit boost in first gear to protect the transmission. -J
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1991 350SDL. 230,000 miles (new motor @ 150,000). Blown head gasket Tesla Model 3. 205,000 miles. Been to 48 states! Past: A fleet of VW TDIs.... including a V10,a Dieselgate Passat, and 2 ECOdiesels. 2014 Cadillac ELR 2013 Fiat 500E. |
#5
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3 Things from link: Operation engine-transmission-overload protection
Engine-transmission overload protection To prevent damage to engine and transmission in the event of faulty operation or during extreme situations the engine-transmission overload protection performs 3 functions: 1. Engine overload protection. 2. Transmission overload protection. 3. Shifting-down automatic transmission into 1st gear when moving off. |
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I see.
I can't really tell where, physically where that switch is located tho.
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83 SD 84 CD |
#7
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Quote:
Your wiring diagram is not easy to read, but it does have the wire color code for the switch. Looks like "Grb"? Is that the German abbreviation for green? If you can decipher the wire color, look for a connection on the transmission with only that color wire, single wire.
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Respectfully, /s/ M. Dillon '87 124.193 (300TD) "White Whale", ~392k miles, 3.5l IP fitted '95 124.131 (E300) "Sapphire", 380k miles '73 Balboa 20 "Sanctification" Charleston SC |
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Google translate says "gelb" is yellow, "grün" is green.
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Respectfully, /s/ M. Dillon '87 124.193 (300TD) "White Whale", ~392k miles, 3.5l IP fitted '95 124.131 (E300) "Sapphire", 380k miles '73 Balboa 20 "Sanctification" Charleston SC |
#9
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It looks like it says BR/GN for Brown & Green.
I will have to crawl under there today to take a look. I'll take a picture if I find it and post it here. Thanks, Phil |
#10
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I got under the car today. I found the pressure switch. It was easy to find. Right next to the vacuum modulator. I removed it but couldn't make out the part #. It is a VDO part. I tested continuity on it. Shows open which is what it should be with the car not moving.
While I was under there I removed the kickdown solenoid and checked it by applying 12V to it. It clicked and I could see a little pin moving inside of it. I would still like to find the part # for this switch in case it does not work in the future. Phil |
#11
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I found the part # on EPC. It was right under my nose. P/N: A0045429917
It's $130.25 on Pelicanparts I attached a screenshot from EPC to this message. Phil |
#12
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Quote:
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2022 Nissan Frontier Pro4x 1990 Mercedes 420SEL 1950 Packard 8 Previous German cars- 2001 Mercedes SL500 1983 Mercedes 300SD 2011 BMW 328i (manual) 2008 BMW 535xi (manual) 2006 BMW M3 (manual) 1980 Mercedes 300TD 2006 BMW 750i 1996 Mercedes E300D 1994 BMW 740i 2006 BMW 330xi (manual) 1999 E300DT Smoke silver/black 253k 2012 BMW 535i Xdrive M-Sport 1984 300TD-T 304k 1988 Mercedes 560SEL (parts car) 2010 BMW 550i xdrive 2017 Audi A4 Presitge 1979 300SD 313k 2003 E500 189k Last edited by turbozeke418; 01-27-2019 at 01:33 AM. |
#13
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That valve clicks with the key on, engine off? That valve has a constant 12V with the key on and closes with a ground signal from a pressure switch on the intake manifold.
I don’t know why that valve would affect shifting. Why is it getting a ground signal with the engine off? Something might be wrong with the pressure switch. Are you sure it isn’t the heater valve clicking? Sixto 98 E320s sedan and wagon 02 C320 wagon |
#14
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When I heard the noise I put my hand on it and could actually feel it click
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2022 Nissan Frontier Pro4x 1990 Mercedes 420SEL 1950 Packard 8 Previous German cars- 2001 Mercedes SL500 1983 Mercedes 300SD 2011 BMW 328i (manual) 2008 BMW 535xi (manual) 2006 BMW M3 (manual) 1980 Mercedes 300TD 2006 BMW 750i 1996 Mercedes E300D 1994 BMW 740i 2006 BMW 330xi (manual) 1999 E300DT Smoke silver/black 253k 2012 BMW 535i Xdrive M-Sport 1984 300TD-T 304k 1988 Mercedes 560SEL (parts car) 2010 BMW 550i xdrive 2017 Audi A4 Presitge 1979 300SD 313k 2003 E500 189k |
#15
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If my eyes don’t deceive me as to which valve that is...
Here’s information on the pressure switch that triggers the valve - https://www.pelicanparts.com/techarticles/Mercedes-W123/22-ELEC-Boost_Sensor_Replacement/22-ELEC-Boost_Sensor_Replacement.htm Confirm the valve by following one plastic line to a banjo bolt at the tail end of the intake manifold, and the other line to a banjo bolt on the ALDA atop the IP. Then confirm the signal by checking continuity between the switch and one pin in the valve connector. The other pin should get constant 12V with the key on, engine running or not. Back to the switch, check for continuity between the single electrical post and ground. It should be an open circuit until the setpoint of 0.85 bar or whatever spec is. If it’s a continuously open circuit longer than the typical valve clicking frequency, maybe the wire between the pressure switch and valve is grounding somewhere. If the switch is sending a periodic ground signal with the engine off, spray some brake parts cleaner into the open end of the pressure switch then see if the periodic ground signal persists. Again, I don’t see how this valve can affect the transmission. It’s not part of the vacuum system. Sixto 98 E320s sedan and wagon 02 C320 wagon |
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