Do not ignore switchover valve - PeachParts Mercedes-Benz Forum

tobybul · #1 04-06-2007, 08:28 PM

I just want to share what I did to restore power on my 85D... I searched and read recent threads on cleaning of switchover valve but there didn’t seem much mention on this specific topic.

We always talk about how the switchover valve works and that it gets dirty and so on but seem to not emphasize the need to clean it. If one has lack of power problems on these 20+ year old diesels, I now believe that one of the 1st things to do is clean the boost line that comes from the manifold and goes to the alda. This includes not only the 2 banjos and the line but also the SWITCHOVER (SOV) valve. It is a simple and inexpensive job that for me worked.

On mine, the banjos and the lines where really not plugged up. They looked a bit sooty right at the banjo and cleaning them did not make any difference. But only recently did I go in to cleaning the SOV (after reading Steve Brotherton's article on transmission tuning). Upon cleaning the valve, it was like - WOW!!!

I feel that all the adjustments and doctoring I did such as alda adjustment, purging, fuel additives, changing fuel filters, timing, etc, etc to get the car out of the sluggish mode was like barking at the wrong tree imho

. In my case none of them worked. Knowing what I know now, I would not have messed with the alda. I think, it gets the engine out of specs plus it creates a domino effect with other components. Also, I ended breaking the alda seal that I now feel I did not need to. Call me old fashioned but I prefer keeping these cars at factory specs when possible. Don’t get me wrong, changing maintenance and wear items such as filters, etc are important and should be done.

IMHO, the boost line from the manifold to the alda is the lifeline to good power for the engine. Because of the exhaust gases from the manifold, the SOV becomes susceptible to particulate accumulation. The diameter or orifice of the SOV is much smaller than the diameter of the boost line and about the same as the banjo orifices. So in a sense it is a choking point for the boost gases. Heck, this thing was probably plugged up 10 years ago already.

Of course, if cleaning the SOV and boost line does not do it then I would say that would be the time to look at other things. But I definitely recommend cleaning ALL the boost line components first when experiencing low power problems before going in to anything else.

SGT John · #2 04-06-2007, 08:43 PM

Is this thing at the top of the picture the switchover valve?

I cleaned my banjo bolts earlier today, and broke the vac hose that goes to this blue device.

SGT John · #3 04-06-2007, 08:45 PM

krap, forgot to include this

tobybul · #4 04-06-2007, 08:56 PM

The switchover valve is behind the brake booster fastened to the firewall with 2 screws. Trace the boost line that comes from the banjo bolts and it will lead you to the sov. I remove the sov and the 2 boost lines and electical connection attached to it, sprayed the openings with brake cleaner. Most effective if sprayed thru bottom opening rather than side. Lots of black stuff came out. Dried thoroughly and reconnected.

The blue device is the vacuum amplifier which the 85's have.

SGT John · #5 04-06-2007, 09:09 PM

I removed the whole line from banjo to banjo...there is nothing else... Does that mean that the PO removed it or do some cars not have it?

Let me confirm that this should be on the hose running from the back of the crank case (erm throttle body maybe) to the ALDA unit...

tobybul · #6 04-06-2007, 09:23 PM

Quote:

Originally Posted by SGT John

I removed the whole line from banjo to banjo...there is nothing else... Does that mean that the PO removed it or do some cars not have it?

Let me confirm that this should be on the hose running from the back of the crank case (erm throttle body maybe) to the ALDA unit...

One banjo is screwed to the manifold just below the air cleaner towards the back (close to the firewall). The other banjo goes to the alda. The lines go from each of them to the SOV behind the brake booster.

JimmyL · #7 04-06-2007, 09:40 PM

I thought that was an overboost protection solenoid. Do I have that confused with something else, or are these just different names for the same thing????

tobybul · #8 04-06-2007, 09:58 PM

Quote:

Originally Posted by JimmyL

I thought that was an overboost protection solenoid. Do I have that confused with something else, or are these just different names for the same thing????

I believe I have heard it called that too.....

SGT John · #9 04-06-2007, 10:39 PM

I have nothing. it's a long straight hose with a banjo on each end.

tobybul · #10 04-07-2007, 01:38 AM

Quote:

Originally Posted by SGT John

I have nothing. it's a long straight hose with a banjo on each end.

Its possible then that it was removed.... a way to check is there should be a loose electrical connector meant for this valve close to where the SOV should have been at the firewall.

I guess that means that the alda is constantly getting pressure without any relief... not sure what the effect of that would be... I'll defer to others on this one.

300SD81 · #11 04-07-2007, 01:42 AM

Quote:

Originally Posted by tobybul

Its possible then that it was removed.... a way to check is there should be a loose electrical connector meant for this valve close to where the SOV should have been at the firewall.

I guess that means that the alda is constantly getting pressure without any relief... not sure what the effect of that would be... I'll defer to others on this one.

The ALDA is always supposed to get pressure. The only time the valve activates is when there is overboost, only way I can think of overboost happening on a stock setup is if the wastegate fails..