Overload Protection Device Switching Unit 1979 W116 300SD - Page 2 - PeachParts Mercedes-Benz Forum

tyl604 · #16 01-27-2019, 07:47 PM

Guys, that looks an awful lot like a plain old switchover valve. One vac line (think the bottom) goes to a nipple on the back of the valve cover and the other goes someplace else. And there is (on mine) a third nipple on the top which is left open to the air. It may be hidden under the plastic cover on the top.

It prevents overboost on the ALDA.

At least that looks like a version of the switchover valve on my '81 300SD.

turbozeke418 · #17 01-27-2019, 11:15 PM

Quote:

Originally Posted by tyl604

Guys, that looks an awful lot like a plain old switchover valve. One vac line (think the bottom) goes to a nipple on the back of the valve cover and the other goes someplace else. And there is (on mine) a third nipple on the top which is left open to the air. It may be hidden under the plastic cover on the top.

It prevents overboost on the ALDA.

At least that looks like a version of the switchover valve on my '81 300SD.

It only has 2 hoses. The one on the left goes to the back of the intake on the engine and the bottom goes to the ALDA. Then there is an electrical connection to it. I disconnected and bypassed it today and I don't notice any difference.

Alec300SD · #18 01-27-2019, 11:47 PM

Turbozeke418's photo is of the bulkhead switchover valve used to prevent overboost.
When the switch is grounded, the solenoid valve vents the boost to the atmosphere.
The ALDA no longer sees boost pressure and the fuel is limited to the IP and the engine is is essentially normally aspirated at that point.

The overboost switchover valve used on 1980 and later MB turbodiesels has only one path to ground.

Evidentlly MB was concerned that a boost heavy launch from a dead stop could cause catastrophic transmission failure on the 1978 and 1979 turbodiesels.

The engineers came up with the "Overload Protection Device Switching Unit". (The link is to the image in post # 3 of this thread)
When this unit is working properly, pin number one on this unit provides an additional ground signal path to the overboost switchover valve.
Pin 7 of the switching unit gives the signal to the "Automatic Transmission Solenoid Valve" to allow the electrically controlled downshift from second gear to first gear when taking off from a dead stop.
Thusly, you can't have a boost-assisted takeoff from a dead stop.

Because you activate these circuits on every takeoff, the components in the switching unit do wear out (after 40 years...really? go figure!).
The componets in the unit that are most likey to have failed are the internal switch (between pin 1 and pin 3) and the internal solenoid (between pin 3 and pin 7).

The switching unit is located in PS footwell.
It is a black box with 8 pins and is covered by a carpeted piece of trim.
The trim piece has plastic grill insert for heat dissipation.
Note: There is a hidden Phillips head screw in the grill insert that secures the trim piece in place.

The fact that the turbozeke418 reports that the bulkhead mounted overboost switchover valve is activating with the engine not running, more likely than not means something is wrong with the switching unit (or its inputs).

Because turbozeke418 has reported erractic transmission behavior with a mechanically induced first gear start sequence,
I also suspect something is amiss with the transmission control inputs (bowden rod length, bowden rod bushing on PS of transmission, sticky throttle linkages, etc.),
or the transmission itself (low fluid pressure, incorrect fluid level, clogged valvebody, etc.).

First order of business, the abilty to successfuly have a mechanically induced first gear start should be tested.

Unplug the 8 pin connector to the switching unit and the 2 pin connector to the kickdown switch (located beneath the accelerator pedal).
Alternatively, unplug both the green and white/green wires from the Automatic Transmission Solenoid Valve.
Then, with the engine running, brake applied, select L and then select D.
When you then takeoff you should be able to acheive a smooth first gear start.

If not, IMHO, any mechanical issues need to be addressed before anything electrical is addressed.

turbozeke418 · #19 01-28-2019, 12:18 AM

Alec300SD Thank you so much for the information. Now I have something to work with and trouble shoot from!!!

turbozeke418 · #20 01-28-2019, 07:31 AM

Quote:

Originally Posted by Alec300SD

Unplug the 8 pin connector to the switching unit and the 2 pin connector to the kickdown switch (located beneath the accelerator pedal).
Alternatively, unplug both the green and white/green wires from the Automatic Transmission Solenoid Valve.
Then, with the engine running, brake applied, select L and then select D.
When you then takeoff you should be able to acheive a smooth first gear start.

If not, IMHO, any mechanical issues need to be addressed before anything electrical is addressed.

I just unplugged both and no effect or change at all. The car starts off in 2nd gear. If I manually put it in L and floor it, the car bucks, chirps and free revs between 1st and 2nd gear (almost like when you're teaching someone to drive stick that can barely grasp the concept of an automatic). I recently changed the fluid and the filter and the fluid was nice and red and didn't smell burnt at all.

sixto · #21 01-28-2019, 11:37 AM

How about if you hold it with the brakes, put it in L, then gently get on the accelerator. Do you get the same indecision or does it feel more like you’re fighting first or second gear exclusively? How long does it take to engage gear when you shift from P or N to D first thing in the morning?

Sixto
98 E320s sedan and wagon
02 C320 wagon

Alec300SD · #22 01-28-2019, 02:29 PM

Quote:

Originally Posted by turbozeke418

I just unplugged both and no effect or change at all. The car starts off in 2nd gear. If I manually put it in L and floor it, the car bucks, chirps and free revs between 1st and 2nd gear (almost like when you're teaching someone to drive stick that can barely grasp the concept of an automatic). I recently changed the fluid and the filter and the fluid was nice and red and didn't smell burnt at all.

Sorry I wasn't clear on the expected test performance results.
You want to first ascertain if your transmission is mechanically sound.
If your car performs as expected, then mechanically the transmission is working well and you can then proceed to check the electrical side of things.

Mechanically forcing a first gear start (with the kick down switch and the switchover unit disconnected) after shifting from Drive to Low and back to Drive before launching from a dead stop, will make the transmission hold each gear to the upper limit of the shift rpm setting (about 4200 rpm).

You should have 3 distinct shift points, 1-2 upshift at about 30 mph, 2-3 upshift at about 50 mph, and the final 3-4 upshift at about 78 mph.

'Desert Rose' performs this way, starting at the 2:49 time mark on the attached video from Doktor Bert.
Doktor Bert is the PO of 'Desert Rose'.
https://www.youtube.com/watch?v=PFhaRefTRUM

With the test you just performed as you described, you only tested the 1-2 upshift...if you had enough roadway available to reach 4200 rpm and about 30 mph.
Repeating the test as you did it is sufficient to check for 1-2 upshift capability.
If you can't get a 1-2 upshift when you repeat your test (starting from a dead stop in Low and flooring
it to 4200 rpm and reaching about 30 mph), then something mechanical is amiss.

turbozeke418 · #23 01-31-2019, 03:25 AM

I tried it all and it's still the same. I does NOT want to go in 1st and fights and jerks every time I try to. Tires chirping etc. I went to check the vacuum line on the trans and the modulator is on the passenger side not the drivers side. It also appears to be a white modulator also and seems little to no info online about them.

tyl604 · #24 01-31-2019, 04:51 PM

The modulator just adjusts how crisply the tranny shifts. It is connected forward by a plastic vac line to the VCV (vacuum control valve). The vac just ahead of the VCV should be around 12-15 ihg and it drops pretty quickly to zero when you hit the accelerator. You can check this with a Mityvac.

As I remember you turn the modulator clockwise for a firmer shift and CCW for a softer shift.

But the modulator does not sound like your problem.

Alec300SD · #25 01-31-2019, 08:35 PM

Quote:

Originally Posted by turbozeke418

I tried it all and it's still the same. I does NOT want to go in 1st and fights and jerks every time I try to. Tires chirping etc. I went to check the vacuum line on the trans and the modulator is on the passenger side not the drivers side. It also appears to be a white modulator also and seems little to no info online about them.

Okay, so it sounds like the transmission won't go into first gear with mechanically forced request for a first gear takeoff.
Hopefully, it is nothing major.

It could be due to low fluid pressure (low fluid volume, leaks, weak pumps, worn clutch packs, dirty valve body, etc.), or binding shift rods, worn shift rod bushings, etc.
The torque converters are known to drain down and it can take a bit of time for the fluid pressure to build on startup while the torque converter gets refilled.

I'd try a test of the Automatic Transmission Solenoid to see if it functions properly.
Unplug the switchover unit, and unplug the kickdown switch.
Attach a remote starter switch (or equivalent) the two male connectors of the kickdown switch harness.
This allows you to remotely active the Automatic Transmission Solenoid by hand at different engine revs (not just at WOT).

Take the car for a spin, get the engine and the transmission both up to operating temperature.
When driving in fourth gear, in the warmed up car, activate the handheld remote kickdown switch.
You should be able to downshift (4th down to 3rd) via the switch if the Automatic Transmission solenoid is working properly.

If the solenoid works, pullover and place the car in Park.
For giggles try and see if you can get a first gear start with your remote kickdown switch.
Engine and transmission at operating temperature, brake applied, shift from Park into Drive.
Slowly accelerate, and immediately after starting to move, activate the remote handheld kickdown switch.
If it downshifts, congrats, you had a 2-1 downshift (which should have been followed by a 1-2 upshift as you continued to slowly accelerate).
If not, you really may have a mechanical issue with the transmission that does not allow it to get into 1st gear.

Note: this last test was emulating what the switchover unit does when it works properly.

Let us know how you fare.
Again, I hope it is nothing major.

Edit: Another thought, the chirp you have reported may actually be a very harsh 1-2 upshift that occurs very quickly.
To test this theory, plug in the switchover unit and the kickdown switch.
With the engine and transmission at operating temperature, brakes applied, shift from Park to Drive.
Remove foot from brake pedal, do not accelerate at all...let the car creep slowly forward and slowly build up speed.
You may get a 1-2 upshift that is only detectable on a slower than normal takeoff speed.

turbozeke418 · #26 03-17-2019, 06:39 PM

sorry to keep commenting on this thread, but it has had the most response for my issues. I went under the car today and determined that there are no wires going to my kickdown solenoid. I applied positive juice to it and it clicks so I know it works. Now to figure out where the wiring went for it? I attached to pics. My kickdown solenoid is on the passenger side front of the trans. There are also to wires that come through the floor and one is connected to the back of the trans and there is another one just hanging there in the same area. Is there any chance that should have been connected to the solenoid? Maybe someone worked on it in the past and forgot or intentionally left it disconnected?

Alec300SD · #27 03-17-2019, 10:06 PM

Yes, that wire with the boot goes to the kickdown solenoid.

If you pull back the rubber boot, there should be two wires on a ring terminal.
The screw may still be present in the ring terminal.

The green wire connects at its other end to the kickdown switch.
The green/white wire connects at its other end to pin 4 on the switching unit in the PS footwell.

If you reconnect the wire to the kickdown solenoid you should regain first gear starts,
and also regain downshifts when activating the kickdown switch under the accelerator pedal.
(So long as the other parts of the circuit work properly.)

turbozeke418 · #28 03-19-2019, 07:43 AM

I connected it back....and it was disconnected for a reason. Everything seems to work exactly like it should (electrically). The transmission is the problem. Bucks and jerks like crazy when 1st gear is initialized. For now I'll leave it disconnected until I find a replacement tranny.

Alec300SD · #29 03-19-2019, 10:21 AM

If it is tme for a replacement tranny, consider Sun Valley Transmissions.
They have a stellar reputation...from what I have read.

turbozeke418 · #30 03-19-2019, 10:32 PM

Quote:

Originally Posted by Alec300SD

If it is tme for a replacement tranny, consider Sun Valley Transmissions.
They have a stellar reputation...from what I have read.

I'm in NJ and not looking to pay a ton for the trans. If I aquire one, my mechanic (autobahn) quoted $900 to remove and install the new one. They did that for me in my 1984 300TD with a used trans and it drove perfectly out the door.

I've been wondering and not sure if you know or not, but why only a 722.120 for this car when it is the same OM617 turbo that was in the cars until 1985? Why can't I just throw in a 722.3xx trans (they're everywhere) with a mechanical speedo? Just seems odd. only 1 trans from a 3 year span can be used. FRUSTRATING!