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Mercedes ALDA boost system service


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All parts used in these pictorials are available at the diesel parts for sale page


Mercedes ALDA  system service


Sometimes when these old turbo diesels get some accumulated miles on them, they just don't have that fresh feeling anymore.  One common area of performance degradation is the ALDA circuit.  In a nut shell the ALDA unit sits on top of the fuel injection pump and contains 2 aneroid capsules and some internal linkage, that enriches the fuel deliver in response to altitude and intake manifold pressure.  When the turbo charger starts making boost the small banjo bolt  and accompanying plastic line, transmits this pressure signal to the switchover valve on the firewall.  If over boost or a few other parameters are NOT present, the outlet of the switchover valve will let the ALDA receive the manifold pressure signal.  

Since the ALDA responds to changes in pressure, it will allow more fuel to pass into the engine if the pressure is increased.  This in turn gives you more power etc. etc.  So if the ALDA, switchover valve or line are clogged or not working correctly, then the car will feel very sluggish.  Common complaint are, the turbo does not feel like its doing anything.  The most common and 1st place to look, and easy to fix solution to a non functioning system is, cleaning the banjo bolt and plastic line.  Due to the nature of the layout of the system, the line from the switchover valve to the ALDA does not usually need attention as far as loading up with soot.

So follow along as we get the system soot free!

Safety and security tips:

Please remember to recycle all your used fluids at an appropriate recycling center.  Be mindful to not spill or splash fluids on yourself, others or the ground.  Also as a safety tip please remember anytime you are working on, around or under your car, to wear safety glasses and secure the car with wheel stops and approved jack stands! 




This is the ALDA component on top of the injection pump.  Notice the small black cap in the center.  There is a small screw below that, that  some people will "turn up" to try and get more power.  Don't do it unless you know exactly what you are doing.  You will probably break the screw messing with it, and then your ALDA will not work at all.


This is the switchover valve on the firewall right next to the vacuum brake booster.  One plastic line goes in from the banjo bolt and the other line goes out to the ALDA.  You will notice that the line that goes from the valve to the ALDA, will not usually have any soot inside.  Don't bypass this valve!  It is a system protection device to prevent over fueling from over boost.  If you bypass it to trick the ALDA, you may end up buying a new engine.


This is the plastic line that goes from the banjo bolt to the switchover valve.


This is the plastic line that goes from the switchover valve to the ALDA.


Banjo bolt on the back of the intake manifold with the plastic line attached.  Be careful when taking off the bolt so that you don't break the line.


Bolt coming out.  Notice the soot on it.  Yuk!  There is a washer on each side of the banjo fitting so don't drop or loose them.


Bolt removed and look at that soot.  Most turbo cars will be as bad or worse than this, if  its never been cleaned.



There is a hole inside the banjo fitting that mates with the plastic line and its usually clogged with soot also.


Look at that restriction.  I have seen them 100% clogged up on all the holes before.


Poor shot of the end, but there is a hole there and its about 90% clogged.  Clean the bolt with some brake cleaner and a small wire brush.  The brake cleaner will remove 99% of the gunk.


Ahh a nice clean bolt with the holes open 100%


The end of the bolt is also nice and clean now.  Since this bolt was so clogged, there WILL be a noticeable increase in power, since the circuit will now sense the rise in intake pressure and respond with more fuel.



I removed the plastic line at the switchover valve and sprayed brake cleaner through it.  Only a small amount of soot will come out.  The banjo bolt always clog's up first, and actually prevents further sooting up of the line.  That little spring clamp is a real bear to get off and back on.   Be patient.


Nice clean fitting and line ready to be reinstalled.


Another angle of the same fitting.


Banjo bolt and washer back in the intake manifold.  This job only takes about 20 minutes from start to finish and is really worth the effort.

Now that was an easy task right?


DVD of this repair is coming soon!