'91 350SDL overboost protection switch delima
 

I just replaced the crossover solenoid (over boost protection switch MB part # 001 540 86 97) on the left fender wall and still have no boost. Apparently the old and new switch still think that there is too much intake manifold pressure (boost) from the turbo and they shut down fuel at the ALDA. Without hooking the car up to a computer and pulling codes, how can I diagnose the pressure switch sender on the manifold? If I disconnect the single wire from the pressure switch sender on the manifold the crossover solenoid still cuts off fuel. Additionally the crossover solenoid gets very hot when hooked up.

Any suggestions?

Driving without the turbo gets old fast.

Mav

I got rid of the overboost protection solenoid (ops) on my 300D about 9 months ago. After getting rid of it I tested the boost pressure and it only goes up to around 9 psi when I have it loaded up. That works fine for me and I have not had any issues.

If you want to keep it in working order you should check out the pressure switch. You should be able to find it somewhere on the intake manifold - it has a single wire attached. Try disconnecting it and see if it allows the ops to drop out. If it does then you have a switch issue.

Jim,

Thanks for the quick reply!

A couple other things to note:

1) If I disconnect the single wire from the sending unit on the manifold, the OPS is still inoperative, both the old one and the brand new one from MB.

2) I learned on diesel discussion forum that once the pressure sending unit on the manifold detects about 1-1.1 bars it creates a ground circuit and switches the OPS. What has me puzzled is that disconnecting the sender should cut that ground and the OPS should work fine…but it doesn’t.

Any thoughts?

Mav

hmmm, yea, you would think that. Have you tried disconnecting the ops and running the line straight to the ALDA. I am assuming that you have cleaned out the banjo fitting at the intake.

Jim,

Everything is real clean; in fact I removed, with the help of a machine shop, about 1.5lbs of crud from the intake. I further cleaned all the lines and fittings.

All I need to do to make the car run like new again is disconnect the two wire plug from the OPS and I have boost. I’m a bit concerned though, of bypassing the OPS based on the premature failure reputation of the 603.970.

Mav

Maverick,

Was the 1.5 pounds of crud in intake area loaded up around the 1 or 2 intakes? I keep hearing some say there is a likelihood that the crud is being ingested, causing damage to the cylinder walls or rings and starting the downhill process ....

My 350 SDL has about 130K on it and I love it, but wish there was something I could do that would at least make me feel better that there is a chance it won't do whatever it does ...

George,

Ya, the crud was all around the crossover and the plenums for cyl. 1 & 2. After I disconnected the EGR and re-routed the blow-by to under the car, I don't have near that much buildup.

As for all the negative stuff you'll read on this forum about our motor, try not to let it get to you. I've asked every MB and private garage mechanic I come across about the failure rate of this motor and have yet to have someone substantiate it. I've come to the conclusion that the problem is WAY overblown! Every MB lead mechanic I've met says that in the cases that they have heard of, the driver failed to maintain the motor with oil proper changes.

Change your oil HOT every 3,000-5,000 miles and drive the snot out of her!

Keep in mind that those on this forum who poo poo the "S" class diesels likely drive 190's and 240's and 300D's and have what I call " 'S " envy!

See ya...

Mav :D

My ops is leaking oil.
 

What does this indicate? Should I get a new one?? Thanks.

Update on my 350 SDL ...

R&R intake manifold and removed at least 1.5 pounds of crud ... coked oil, from the looks of it. Machine shops dipped and cleaned it for about two weeks ... then I chipped and swabbed it until generally clean (no chunks to ingest). Both the vacuum fitting and the pressure sensor in the manifold were very nearly covered ... likely was still getting some function ...

Question: what does the vacuum fitting provide to the control system?

I replaced the glow plugs (just easier with manifold off) and it does start and idle smoother. Process of R&R was pretty straightforward ... biggest issue is remembering where all the little fuel line clips go ... replaced all of them as they were very brittle and most broke on removal.