What do you guys make of this-

Like many among us, I suspect my ALDA is malfunctioning.

I put a T in the line between the overboost and the ALDA and it pegs at 10psi on the highway.

Then, I disconnected the line from the overboost to the ALDA to test the ALDA itself. It doesn't hold any pressure at all, nada.

Tried running the car with the alda disconnected at the overboost, plugging both disconnected orifices. Runs like a dog.

So... If my ALDA doesn't hold any pressure, it's shot? But if it's shot, why does my car run different when it's disconnected?

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1984 300CD

How did you test the pressure to the ALDA?

You can adjust the ALDA, or remove it.

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83 SD

84 CD

I tested the pressure to the ALDA by putting a T in the line between the overboost and the ALDA. It pegs at 10psi. If I T the line before the overboost, it reads around 12psi.

I tested the ALDA itself by disconnecting the line from the overboost to the ALDA, and hooking up a MityVac where the overboost would have been. Tried pumping it up, builds no pressure.

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1984 300CD

The first paragraph is a proper test with a good result.

The second, is not a proper test. What you did was pull vacuum FROM the ALDA, thereby reducing pressure and expanding the capsules. You need to send pressure TO it, via a controlled compressor. Hopefully, the aneroid capsules weren't ruptured.

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83 SD

84 CD

I don't understand, I had the mityvac set to pressure, I was pumping PSI not pulling vacuum...?

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1984 300CD

Ah, you may have a different mity-vac, got a pic?

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83 SD

84 CD

It just has a switch for pressure or vacuum. Think I figured it out... bad washers! Replaced the inside washer with a copper one and it pumps over 10psi, but leaks pressure quickly. Is this normal?

Noticeably more responsive after this quick fix, i'm adjusting the ALDA again for higher pressure.

Does it make sense to get different Boost psi readings before and after the overboost sensor (towards ALDA)? What is the best test for the overboost itself?

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1984 300CD

Sounds normal all the way around.

The turbo is supposed to send 10+ psi, up to 11.6, or 0.7 to 0.8 bar.

The switchover valve is a bleeding device so the ALDA won't go crazy. The over boost preventer is working, you've checked it by the above test, less coming out of the switchover valve than going to it.

I'm sure the ALDA won't hold indefinite pressure, it has to allow the pressure signal to escape. Otherwise it would blow up.

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83 SD

84 CD

Leaking switchover valves, interesting thought that.I think I will replace the one in my 91 on principal. BTW the switchover valves are a dime a dozen on the W124 gassers and they have clean air running through them...

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OBK #42

Pressure escapes back to the manifold as the engine revs down. The switchover valve blocks the boost signal and vents the ALDA line when energized so I don't see why the ALDA needs its own vent. But it could have one, I suppose.

Sixto
87 300D

The ALDA does not have a vent, and the switchover valve does not change pressure.

The ALDA is a sealed device. The line through the switchover valve is either open from the center to middle fitting, or closed where the top fitting vents the middle fitting, and the lower fitting is blocked (overboost condition vents air to the ALDA so that there is no boost enrichment which reduces power and boost pressure). When the switchover valve is open, pressures at all points in the line from the intake plenum, through the switchover valve, and in the ALDA are all the same.

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Gone to the dark side

- Jeff

The engine supplies sufficient volume to maintain 10 psi at the ALDA. This yields about 85% of the full performance capability of the engine.

The Mityvac supplies negligible volume and, with any leaks within the ALDA, no pressure is observed.

To obtain the factory performance (about 15% greater than your current performance), you'd need to replace the ALDA.

Alternately, you could just remove the ALDA completely if you can drive the vehicle judiciously and don't mind the lack of overboost protection.

Brian,

So, the ALDA should hold pressure when tested? Replacing the washers allows pressure to build up, but it won't hold.

I'm reading 11-12psi max with the MityVac T'ed in between ALDA and overboost. Is this closer to factory performance?

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1984 300CD

I would expect a brand new ALDA to hold pressure when tested. I'm sure that none of the 20 year old units can achieve that anymore.

If you read 12 psi with the Mityvac, you're done. The engine will provide more than sufficient volume to get the unit to perform adequately.