Cliff’s notes: How do I disable/bypass the ALDA?

Description: Please read on –

I have had a new symptom some up on my ’83 300SD. When cruising above about 2,000rpm, I notice that once I pass about 20% throttle position, I am not getting any more power out of the engine. To put it another way – it’s like once you push the pedal in about 20% of the way, it doesn’t matter if you push it any farther, you are still getting about 20% throttle.

This started just a few days ago, and I made no changes to the car at the time this issue came up. I am going down my to-do list in order of easiest to check to hardest. I have replaced both fuel filters, made sure I had a full tank of fuel, and looked at all the vacuum lines. I also triple checked the lines for any air.

At this point, I am pretty sure it has to do with fuel pressure/delivery. Before attaching pressure gauges to check fuel pressure/vacuum (before and after the Lift Pump, respectively), I would like to disable/bypass/shoot to hell the ALDA. As far as I understand, this unit reads the vacuum/boost signal, and “feathers” the fuel delivery, as not to deliver too much fuel when you floor it before the turbo spools up. I would think that a faulty ALDA that always thinks you are under vacuum (read: turbo is never spooling up) even when you are under boost – would present the exact same conditions that I am experiencing. And yes, I know that the turbo is building boost (as I can hear a huge “whoosh” when I let off the gas and the EGR valve opens).

So I ask, how do you disable/bypass the ALDA? I remember some people talking about it disabling it, and just having to make sure and not give it too much throttle before boost (or else you’ll just bellow out fuel smoke).

On a side note, does anyone know an acceptable vacuum for the input of the Lift Pump (how much “sucking” can the Lift Pump do?), and also – what is a “normal” pressure right after the Lift Pump. I’m hoping that I guessed right with the ALDA, and won’t have to get to checking fuel pressure/vacuum…though I might anyways out of curiousity.

Thanks in advance for the help!

__________________
1983 300SD ~250k

The ALDA you mean? You disable it by removing it. 24 or 27mm wrench I think. You'll lose the overboost protection too though.

Before you do that, it's easier to check if the boost line going to it from the intake is plugged up, by blowing through it. If no air goes through then there's you problem, clean it out.

ALDA!! WOW I'm an idiot....yes, I'm talking about the ALDA. No wonder i wasn't finding any search results!! Good call on the vacuum/boost line. I'll definitely check that first. I'm not in front of the car right now, but I think I'm remembering the correct line...is it the white-ish colored one? Or the other one on the side of it?

__________________
1983 300SD ~250k

Oh hey...I found a really nice pictoral on cleaning the banjo bolt to switchover valve to ALDA. Check it out:

http://www.dieselgiant.com/mercedesaldaboostsystemservi.htm

__________________
1983 300SD ~250k

Also, check the turbo switchover valve. I had the old style (round) and it was frozen shut. It is only supposed to close if boost pressure gets too high.

__________________
RRGrassi


70's Southern Pacific #5608 Fairmont A-4 MOW car

13 VW JSW 2.0 TDI 193K, Tuned with DPF and EGR Delete.

91 W124 300D Turbo replaced, Pressure W/G actuator installed. 210K

90 Dodge D250 5.9 Cummins/5 speed. 400K

Yeah, I'm thinking of throwing mine out (I also have the old round one) and replacing it with a boost gauge...I already drive the car like a jet (monitoring EVERYTHING as a drive)...so one more parameter to always watch wouldn't bother me at all. I just want my little bit of power back.

__________________
1983 300SD ~250k