What The Heck Is This?!?! - PeachParts Mercedes-Benz Forum

DanDizzo · #1 07-13-2008, 08:38 PM

Girlfreind has a 1980 300sd. About 125000 on a stuck odometer. Very Clean and rust free. Well, I'm relatively new the whole vacuum scene, but I've done enough reading and diagnosis to understand most of the discussions here. My question is, What is this switch? Why isnt it hooked up? what should it be hooked up to, if anything?

This is on the driverside firewall left of the brake booster.

I appreaciate any help you can offer.

tyl604 · #2 07-13-2008, 08:57 PM

Not the same style as on my 81 300SD but I believe this is your vacuum switchover valve which gives you the boost from the turbo. Look through the threads for this item and I believe you will find all the info you need.

Good luck.

duxthe1 · #3 07-14-2008, 12:15 AM

That is a switchover valve that is supposed to keep boost from reaching the aneroid (boost fuel enrichment) until the engine warms up a bit and on some models during an over-rev (I think). The 617 engine has proven to be a very tough unit and many (understatement) have gone hundreds of thousands of miles with it bypassed. Unless you commonly drive off on sub-freezing cold starts then you will be fine as is.

tyl604 · #4 07-14-2008, 08:39 AM

Replace it. Mine went out at 252K miles (at least per my indy) and I replaced it. Put the hoses on the wrong way at first and the car had absolutely zero power. Changed the hoses to the proper nipples and bingo - back came the power. So I think you need it to be operational.

pawoSD · #5 07-14-2008, 09:10 AM

Its there to protect the engine from over-boost, I'd get it hooked up properly!

tangofox007 · #6 07-14-2008, 10:31 AM

Quote:

Originally Posted by DanDizzo

Well, I'm relatively new the whole vacuum scene, but I've done enough reading and diagnosis to understand most of the discussions here.

Anything that you may have read that suggects that the switchover valve has anythng to do with vacuum is erroneous.

The switchvalve is normally open so that manifold pressure is "transmitted" to the ALDA so that the ALDA can deliver fuel in proportion to manifold pressure. Manifold pressure is normally regulated (at the top end) by the turbo wastegate. In the event that the wastegate fails to properly regulate boost pressure and an overboost condition occurs, a switch in the intake manifold closes to provide a ground path to the switchover valve, causing it to actuate. When actuated, the switchover valve vents pressure from the ALDA and blocks pressure from the manifold so that fuel delivery will be reduced as a secondary method of preventing an overboost condition.

tyl604 · #7 07-14-2008, 01:10 PM

TF7 - interesting answer. I thought it helped the turbo to provide boost. When I had the hoses on the wrong nipples, I had no boost. On the expressway it was a real dog. Then when I finally figured out which hose went where, I got all my power back. Sounds like you are saying that the power should be there anyway but the switchover valve just prevents overextending (maybe blowing out) the turbo charger. Is this a correct interpretation? If so, what happened to my power on the expressway before I properly routed the hoses?

vstech · #8 07-14-2008, 01:24 PM

it is possible you had the hoses oriented so the pressure was connected to your "vent" line, this would have plugged up the alda, and prevented flow...
more likely was you had a plugged up banjo bolt, and the mechanic cleaned it for you...

tangofox007 · #9 07-14-2008, 02:15 PM

Quote:

Originally Posted by tyl604

TF7 - interesting answer. I thought it helped the turbo to provide boost.

In the "normal" position, the switchover valve "allows" the ALDA to function normally; the ALDA increases fuel delivery as boost increases. When the valve is actuated, it blocks manifold pressure to the ALDA. That prevents the ALDA from functioning normally, meaning that fuel delivery will not be increased in proportion to boost level.

Quote:

Originally Posted by tyl604

When I had the hoses on the wrong nipples, I had no boost. On the expressway it was a real dog. Then when I finally figured out which hose went where, I got all my power back.

That's is precisely what you should have expected. If you have the hoses connected to the wrong ports on the switchover valve, you will replicate the "actuated" condition of the switchover valve. (Pressure can't get to the ALDA.)

Quote:

Originally Posted by tyl604

Sounds like you are saying that the power should be there anyway...

Of course it "should" be there. (Not that I said anything to that affect.) But it's not going to be there if the switchover valve is actuated, if it's blocked or if it's not connected in a way that prevents pressure from reaching the ALDA.

DanDizzo · #10 07-14-2008, 09:38 PM

Quote:

Originally Posted by tangofox007

Anything that you may have read that suggects that the switchover valve has anythng to do with vacuum is erroneous.

The switchvalve is normally open so that manifold pressure is "transmitted" to the ALDA so that the ALDA can deliver fuel in proportion to manifold pressure. Manifold pressure is normally regulated (at the top end) by the turbo wastegate. In the event that the wastegate fails to properly regulate boost pressure and an overboost condition occurs, a switch in the intake manifold closes to provide a ground path to the switchover valve, causing it to actuate. When actuated, the switchover valve vents pressure from the ALDA and blocks pressure from the manifold so that fuel delivery will be reduced as a secondary method of preventing an overboost condition.

I wasnt implying i had read anything that said that switch was vacuum related, i was just trying to say i wasnt sure what this switch did, and it appeared to be a "vacuum" related switch.

I did run the lines back to it and the car runs the same. Now, I'm wondering, have these switches failed in a way where they wouldnt prevent over-boost. In other words, I'm wondering why the previous owner would have by-passed it if it wasnt running poor with it connected.

vstech · #11 07-14-2008, 09:44 PM

it's unlikely you would have an overboost condition. so it's normal that with it connected the car would function the same.

DanDizzo · #12 07-14-2008, 09:52 PM

Quote:

Originally Posted by vstech

it's unlikely you would have an overboost condition. so it's normal that with it connected the car would function the same.

I was expection it to run the same, i was just confused why its been unhooked. is there some secret we all dont know about, that is if we unhook this switch we will get whiplash type acceleration?!?!?!

Maybe then, I can take the ol' girl to the track......

tyl604 · #13 07-14-2008, 10:32 PM

If the car runs the same with the hoses re-connected, I have two theories: 1) the old switchover valve does not work and should be replaced or 2) the hoses are not on the correct nipples. The car should have more pep with the valve working than without the valve.

Am I missing something?

tangofox007 · #14 07-14-2008, 11:00 PM

Quote:

Originally Posted by tyl604

The car should have more pep with the valve working than without the valve.

Am I missing something?

The valve is normally open, which is the same as having the valve bypassed. The valve cannot do anything to help performance; it can only limit performance when actuated.

tyl604 · #15 07-15-2008, 08:44 AM

Got it; I had it backwards.

Thx