Actually after more reading it should say how critical is the changeover valve? I have no idea how to adjust boost and since lots of folks talk about but it doesn't seem taht anyone has explained it... Unless I missed something.
Anyway here is my original message.

I have read and read but there seem to be conflicting opinions as to how critical the overboost protection might be in these older turbos.
Some appear to be saying that with worn wastegate spring and other factors that the turbo probably can't make enough to hurt the motor but on the other hand I have read where these turbos can make up to 40 psi in manifold pressure which is not good.

The reason I ask is that when I first got this car it screamed in the 0-70 range. After doing some work, valve adjust (all were so tight you couldn't insert a feeler gauge), timing, and fuse replacement the car started running like a pig. I mean in the top end it felt like a naturally aspirated 300D.
This lack of power has been driving me mad for the last couple of days. Can you imagine? You adjust things the way they should be and it runs worse...
Anyway...
I went out for a road test this morning and just out of curiousity pulled the plug on the overboost switch on the firewall (actually the changeover valve) and low and behold the car took off!!!

Now I am not one to want to blow this motor as it only has 159K on the clock but then again I don't want to drive a pig either.

The PO should not own a car. This thing was destined for the scrap heap but I am trying to resurrect it.

__________________
'99 S420 - Mommies
'72 280SE 4.5 - looking to breathe life into it
'84 300SD Grey - Sold
'85 300SD Silver - Sold
'78 Ski Nautique

From where does the signal to ground the changeover valve come from so that it knows to let the alda cut fuel?
Is this a function of a throttle setting sensor or some switch on the turbo or what? Or is this simply a function of the changeover switch itself that it senses the pressure increase (full boost, no throttle) on the manifold and then it manages the changeover?

I think I just answered my own question.

But could somebody confirm that then what happens is that when the boost drops i.e. engine rpms come down and the exhaust gases stop turning the turbo so hard (hence less pressure) then the switch turns back on and controls the pressure to the alda? Wouldn't this mean that in a heavy hill climb the fuel supply would be alternately richened then with overboost, leaned, then richened, then leaned... you see my confusion?

Obviously I'm trying to get a handle on this in my weak old mind.

__________________
'99 S420 - Mommies
'72 280SE 4.5 - looking to breathe life into it
'84 300SD Grey - Sold
'85 300SD Silver - Sold
'78 Ski Nautique

Been still doing more reading and I think I have a better handle on things. Would like a little input though if possible.

Can someone confirm if the pressure switch (on top of the manifold) is a negative ground switch? In other words does it ground to the engine making a complete circuit when it senses over pressure?
If I'm getting this then it would complete the circuit for the changeover valve allowing the alda to sense outside air, not boost air thereby reducing fuel and hence power.

Therefore to test this switch you could disconnect it and see if you have power and then reconnect it and see if the power is cut back. If cutback at normal operating ranges then the assumption would be that this switch is bad (or mabe just clogged from inside the manifold).

__________________
'99 S420 - Mommies
'72 280SE 4.5 - looking to breathe life into it
'84 300SD Grey - Sold
'85 300SD Silver - Sold
'78 Ski Nautique

I went through this about a month ago, and was told that the sensor in the intake manifold would complete a ground. There's constantly power at the changeover valve when the key's on, and if the sensor senses an overboost condition, it completes the ground, making the air that was going to the ALDA instead vent to the atmosphere.

I would try this: With the key on but the car not running, put the leads from an electrical tester into the two connections at the plug. If the meter shows 12 volts, the sensor's definitely bad. If it tests okay, start the car and with it at idle, see if you get 12 volts. If not, I'd rev the engine until you do get 12 volts...assuming everything else is happy, you shouldn't be able ot get 15 psi of boost with no load on the engine, so if you get 12 volts at all in this test, the switch's bad.

If everything still checks out okay (I doubt this; I'll bet that that sensor's bad), you can try what I did. When I found out that mine was bad, after driving like an old granny for a month I took a 1156 bulb and used electricla tape to attach a positive and a ground wire to it. I then ran these two wires into the engine compartment and connected them to the plug for the valve. With this setup, the light will come on whenever the sensor closes.

This is a temporary measure until I get a boost gauge.

Hope this helps you some...good luck!

__________________
2001 VW Jetta TDI, 5 speed, daily driver
1991 Ford F-350, work in progress
1984 Ford F-250 4x4, 6.9l turbo diesel, 5 speed manual
Previous oilburners: 1980 IH Scout, 1984 E-350, 1985 M-B 300D, 1979 M-B 300SD, 1983 M-B 300D
Spark-free since 1999

Lights on and you know you can beat that Toyota. Light out and you better not be racing...

__________________
'99 S420 - Mommies
'72 280SE 4.5 - looking to breathe life into it
'84 300SD Grey - Sold
'85 300SD Silver - Sold
'78 Ski Nautique

Dsiconnected the changeover valve because as Warden pointed out my pressure switch on the manifold is indeed the culprit.

Here is what happened. While doing the valves and timing etc I changed out a bunch of fuses, some of which were blown.

Well guess what? One of them powered the changeover valve because after doing all this work the car ran like a slug...

Anyway, now it chirps th tires in the 1/2 shift, sweeeeeeet...

I think the banjo bolt on my 84 is crusted since the valve is working on that car and it drives like a slug too... Hopefully, just maybe it can be diagnosed and fixed just as easily.

BTW- for those of you wondering I am going to install a new pressure switch in the next day or two and I will stay off the throttle until then.

__________________
'99 S420 - Mommies
'72 280SE 4.5 - looking to breathe life into it
'84 300SD Grey - Sold
'85 300SD Silver - Sold
'78 Ski Nautique

Umm...actually, at least with how mine's set up, it's the other way around. With the light off, it's safe to lay on the throttle, but the moment the light comes on, I have to take my foot out of it.

I was originally going to leave the valve off and put a boost gauge in its place, but after reading a section on Jeff's DIY page, the jury's out...either way, a boost gauge's going in (along with a pyro when I can afford it), but I may get a new valve in addition...

The guy who Jeff linked to said that the valve engages whenever you let off the throttle at high RPM's, and that leaving the valve off can lead to over-fueling while decelerating. I'm not sure on that, though; I've actually never had my light come on in the time I've had it installed... *shrug*

I believe this is only the setup for early cars, my 83 does not have this feature I finally determined after finding no info about it here. It's tied to some relay/module I also don't have, which I think tied to the trans somehow. It's all in the FSM, in so-so english..

Wonder why they got rid of the off throttle venting of the ALDA?

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One more Radar Lover gone...
1982 VW Caddy diesel 406K 1.9L AAZ
1994 E320 195K

In all honesty, I questioned that part from the beginning. I would assume that the ALDA adds fuel to the fuel being injected based on boost readings, but that the lever connected to the pedal would still have the final say in how much fuel goes to the injectors. Therefore, with no pressure on the pedal, you'd be getting idle fueling regardless of what signal the ALDA was getting. This would negate the valve venting boost when the pedal doesn't have any pressure on it

I could be wrong there...anyone know for sure?