should an ALDA hold vacuum? - PeachParts Mercedes-Benz Forum

sixto · #1 10-13-2003, 02:09 PM

The 300SDL ALDA bleeds vacuum quickly to about 5 inHg then slows progressive taking more than a minute to bleed to 0. Does this mean anything?

Is there a MightyVac attachment that will let me test with pressure?

Thanks,
Sixto
95 S420
91 300SE
87 300SDL
83 300SD

Rick Miley · #2 10-13-2003, 02:26 PM

Well, you could take the cap off the top port on the MityVac and connect the hose there. But the guage only reads vacuum, not pressure. And pressurising something that way is clunky at best. Maybe try a bicycle pump with a universal rubber attachment like you'd use to blow up a raft.

I don't have much ALDA experiance, but believe it should hold pressure up to about 13 psi - that's the output of the turbo, right? Don't know about holding vacuum. If the ALDA is off the IP, then you should be able to see the rod moving when you apply air pressure.

Beagle · #3 10-13-2003, 09:13 PM

The ALDA is an aneroid (i.e. a sealed capsule) calibrated to sense very small pressure changes, typically between sea level and 10 000ft or manifold boost pressure if turbo. Applying vacuum or pressure will certainly wreck the capsule either bursting or straining it.

A vacuum simulates taking it into space – way outside its range!

erubin · #4 11-24-2003, 02:48 AM

I decided to "test' my ALDA with vacuum this evening before reading this post. Uh-oh did i damage the ALDA by pulling a vacuum on it? i noticed something odd which at the time i ignored. the first time I pumped the Mityvac i noticed it held a vacuum. then i pumped it down some more and now it wont hold any vacuum. Did i bust it? Will test drive it tomorrow.

I read an article (ALDA REPAIR ) that suggested testing it with a mityvac, which is why i did this.

I only have one complaint about my car's performance since i bought it a year ago. The pickup is sluggish at low RPMs when cold particularly up a hill but when warmed up it's good. I've already cleaned (didn't need it) the banjo bolt, both fuel filters are new, valve adjusted OK, overboost protection was bypassed by previous owner (one day I'll restore this).

For some reason i can't post a picture (the attach file and browse box don't appear) in this message. Instead you can see my ALDA by going to this other post of mine. Someone in that post commented that i had a "virgin ALDA" .photo of ALDA

engatwork · #5 11-24-2003, 07:38 AM

I called the local Bosch distributor and was quoted approximately $340 for a new ALDA. You can get new ones - they are just expensive as all get out

tkelley · #6 11-24-2003, 08:53 AM

Out of necessity I have gotten much deeper into the ALDA than I wanted to. The unit has two sealed brass barometric cartridge's which sense baro pressure and push on each other to set the initial position of the enrichment rod. The enrichment rod, the one that goes into the IP is sealed by a cup seal on the shaft.( this is the one that usually leaks) The upper cartridge is attached to the case and the bottom one is free to move. When pressure from the intake (turbo boost) is introduced to the case the bottom cartridge is pushed out causing the IP to flow more fuel. The easy way to test a ALDA is to hook the line going into the alda to a mity -vac (use the pressure output ) and see if the rod extends as pressure is applied. The unit is not designed to hold vacuum, only pressure. Also the port does not apply this pressure to the inside of the baro cartridge and therefore will not damage the unit. ( dont apply more than 10 psi)

Beagle · #7 11-24-2003, 02:42 PM

When pressure from the intake (turbo boost) is introduced to the case the bottom cartridge is pushed out causing the IP to flow more fuel.

When manifold pressure is applied to the ALDA chamber the aneroid is compressed drawing the control rod IN not out. This in turn allows the rack to move further back at full throttle increasing fuel delivery. Conversely, as altitude, increases the control rod moves out reducing max. delivery. In a nutshell the ALDA is a variable stop on the rack ensuring that at full throttle the fuel/air mixture is correct. At all throttle settings below full throttle the ALDA has no effect whatever on performance or power.

A Diesel engine always runs lean, that is why they are so efficient.

The easy way to test a ALDA is to hook the line going into the alda to a mity -vac (use the pressure output ) and see if the rod extends as pressure is applied.

The pressure in the ALDA chamber surrounding the aneroid will remain the same as manifold even if there is a slight leak and on an NA engine is left open to atmos. anyway. I come across very few problems with ALDA’s and most of them when the customer has messed with it! Applying pressure or vacuum proves nothing and is a sure-fire way to ruin your ALDA and as Jim points out they cost $$$$......why do you think I’m in this business?