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

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.

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

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

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.

I see.

I can't really tell where, physically where that switch is located tho.

My guess is on the right side of the transmission. I have a copy of the ATSG repair manual for 722-3 and 722-4 transmissions (starting with 1980 models). There is a port on the right side for measuring "working pressure". That may be where this switch is located.

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.

Google translate says "gelb" is yellow, "grün" is green.

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

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

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

I know this is an old thread but came upon and have a similar situation. I also have a 79 300SD. The other day when I was working on the car I had the key in the on position but the car was off. Occasionally I would hear a clicking noise periodically. It sounded like something trying to switch and then immediately switching back. I was able to narrow it down to the the over pressure switch. My car does not start off in first gear ever. If I try to do it manually the trans shudders and free revs between 1st and 2nd. Did replacing this fix your transmission shifting issue? I would assume something is bad in it if it is clicking periodically??

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

When I heard the noise I put my hand on it and could actually feel it click

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