W124 Overboost solenoid in dump mode
 

I am pretty sure my solenoid is broken/defective and is dumping the pressure signal to the ALDA. Warm weather begets tinkering for me, so after checking and rechecking, I just bypassed the overboost (OB) solenoid on the 300TD and ran the intake pressure direct to the ALDA. I've been hesitant to do this for safety reasons in the past. Yay and yikes the wagon behaves more like my E300!

Does the OB switch trigger the OB protection solenoid? How do I test the OB switch either on or off the engine? What is effected, soot/fuel/other, if my OB protection circuit(switch/solenoid) has been in 'dump' mode for quite awhile(years)

The 300TD has been a dog since I got it (2/2014) regardless of some of the things I have checked/done after researching here. The car runs very well otherwise and I thought it was turbo related and beyond my abilities for now. Over the past year I have R&R'd-Cleaned IM, replaced return lines, diesel purges, cracked Inj. nuts to bleed air, checked boost(11-12 psi w/T in line to/before solenoid), cleaned fittings/lines on the OB circuit, replaced air/fuel filters...etc.

Therefore, I'm thinking the solenoid has been dumping pressure when energized.


PS:Its my kids car so it has been OK to me for it run like an NA...ya know nice n slow.

Personally I would just remove the Alda and see how it shifts, if it's okay just leave it off. That's what I did a year or two ago, shifts are okay a little hard but I probably don't have every adjustment quite right etc. You might have better economy w/ the Alda. Regardless I think this could be compensated for if you're just smart about how you drive.

I remember when my OB got stuck on, (however it works). Car was seriously slow, like 50% less power.

These days I find it hard to need anything beyond half pedal travel.

You could figure out a "kids" mode on/off :D

Yep, very noticeable difference with the pressure line going direct to ALDA. Did you leave it bypassed or fix/replace switch/solenoid. Soleniod is NLA apparently and switch is $80 or so where I could find it. I'd like to fix this then move onto the ALDA if it get to that.

There is a pressure switch on the driver's side of the intake manifold, has a single wire attached. That switch tells the OB circuit when to dump the signal to the ALDA (1.1 bar +/- 0.15 bar). It is a normally open switch (infinite resistance unless the overboost state exists).

The switchover valve has two electrical connections, one is power (black/red wire) and the other should be to the pressure switch (brown/black wire). When the pressure switch closes and provided a path to ground, then the switchover valve should dump the pressure signal to the ALDA. Briefly energize this switchover valve, should be an audible "click" when it dumps the pressure. If you have a mity-vac or other tester, you can probably also test the various openings to see how pressure or air is routed in the two different states.

I think the switchover valve is susceptible to getting clogged with soot, and sometimes spraying carb cleaner or brake cleaner through the openings can restore function.

Hmm, well my soleniod (switchover) has just one plug. Now Im thinking the pressure switch is funky and sending the signal to dump. I have cleaned both the switch and solenoid with brake cleaner in the past. Perhaps I need to dig a bit more in the switch orifice to get any crud out.

I'll check the solenoid with a blow method with the car on and see where the air is coming out.

Failing that, I will buy new/used parts.

Mine is gone, wouldn't work without the alda anyway haha. I'm not worried about it w/ the pressure actuated wastegate.

Switchover valve / solenoid should have one plug with two pins/connectors.

If you disconnect the plug to the pressure switch, and the car has normal power, then the pressure switch is your problem.

Great thank you for the info. I will do that. Also went to junkyard and found quite a few w124s to pick at, so now have spare pressure switch and solenoid(unknown if they work as yet)

BS. Its a $3 solenoid, found on every benz in the junkyard built in the last 30 years.

The switch does not work that way, if it failed then you would potentially overboost. It is very unlikely you will ever overboost if the waste gate is working and even then very unlikely you would actually damage anything.

Spray some Wd40 in the valve and blow it out until its clear.

My bad $83.25 for the new sender unit

Pelican Parts - European Automotive Parts and Accessories - Porsche • BMW • Mercedes • Volkswagen • Audi • Saab • Volvo • MINI

and that solenoid is def. not $3 either

Pelican Parts - European Automotive Parts and Accessories - Porsche • BMW • Mercedes • Volkswagen • Audi • Saab • Volvo • MINI
Hope I get free shipping on the $1010.50 one :O
Anyway, I got some possible replacement from a JY. Diesel or gas MBs, w124s anyway, are not common in a big city like Philly so PnP opportunities are slim. Agreed on the WD40 shower, BTDT a few times. I wish it were that simple.

Thanks for the input.

I would just bypass and install a gauge at those rates.

Should I be concerned about soot getting into the ALDA using a direct run from the IM to it? I'm thinking yes and if so is there some filter mod out there?

I have R&R'd the IM, got it cleaned out and have cauterized the EGR vac line(didnt have a BB and wanted it to 'look' stock for emissions JIC), so it should be disabled. I did notice a wee bit of oily residue on the original pressure switch when I took it off; i.e. it wasnt sparkly clean. I've been checking boost and seem to max out at 11 psi, havent tried on the highway yet.

The JY pressure switch did not seem to do much to send signal to solenoid or made no difference. Of course, I buggered the JY solenoid and broke 1 nipple off, so thats out for the time being.

Thanks for any thoughts.

My pressure switch was also dead, ditched it. Soot gets into the aldo with directly or indirect connection. Theres nothing you can do about that other than block off the EGR. Install a boost gauge and never worry about it again.
https://i.imgur.com/MHdETCah.jpg