N75 test

Engine running at idle.

Select the ENGINE Controller

Choose BASIC SETTINGS -04,

Choose GROUP 011 (or turbo controls from the pull-down menu)

Choose GRAPH if you want, or just watch the numbers.

The N75 duty cycle will cycle from OFF to ON every 5 seconds or so. (about 10% duty cycle to about 90% duty cycle.)

The MAP actual pressure should cycle at least 30 mbar with a stock turbo if the test is done at idle - as the ALH engines do. (less change will probably occur with a larger than stock turbo.) If the engine accelerates to 1400 rpm (as the PD engines do that I am familiar with) then the MAP should change by at least 80 mbar. I would say that most PD engines with stock turbos change about 150 mbar from min boost to max boost at 1400 rpm.

The turbo vane actuator should cycle up and down about 11-14 mm (depends upon turbo - others may have different travel) and it should pull up firmly onto the full travel stop set screw.

You can hear a definite sound change in the exhaust - very noticeable.

If you hook up a vacuum gauge to the N75 output to the vane actuator it should change from about 2" hg vacuum (or less) to about 20" hg vacuum (or more).

So with this test you can check whatever you need on this control loop for basic troubleshooting.

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When you're in basic setting 011 and you press the "On/Off/Next" button, the engine RPMs come up to about 1400 and the N75 will cycle between full vacuum and no vacuum every 5-10 seconds or so.

When it's doing this, either use a mirror to look behind the engine or get underneath the car and watch the turbo actuator. Verify if it's getting the VNT arm pulled all the way down to the stop screw or not when the computer tells the N75 to switch full vacuum to the turbo.

If it does NOT pull all the way down to the stop, we need to figure out why. You will need a vacuum pump with gauge to further diagnose. First things, put the vacuum pump to the turbo actuator directly and measure how much vacuum it takes to pull the actuator to the stop. It should be about 18" mg. If it moves very little and leaks back down quickly, you have a bad actuator.

If it does move all the way to the stop at about 18" of mg and it doesn't leak vacuum, the actuator is fine and correctly adjusted. Move on to checking vacuum.

Run basic setting 11 again, this time measure how much vacuum the hose you just removed from the turbo actuator is getting. It should cycle from at least 25" mg to zero every few seconds. If it can't get more than a max of 20" of mg, there's a leak somewhere and it is absolutely affecting boost control.

I generally start by removing the small hose that goes to the large vacuum pipe going to the brake booster. That small hose feeds the vacuum to the n75 and the various other vacuum changeover valves the BEW has (one for intake manifold runner flaps, another for EGR cooler flap...). Put your hand vacuum pump with gauge onto the nipple that you just removed the hose from. You're now measuring how much vacuum is coming out of the vacuum pump/tandem pump directly.

On the BEW, I've had that black plastic pipe crack on the underside of one of the 90 degree bends it has and create a big vacuum leak. I've also seen a few tandem pumps where that large nipple that the vacuum pipe connects to is worn or loose and is creating a vacuum leak...

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Maf test

Yes the MAF test is BASIC SETTINGS-04, then choose group 003. It will raise the idle (on most PDs) to 1400 rpm and vary the EGR. You should see the MAF reading go from about 470 to about 230 as it shifts every 5 seconds or so.