Boost Pressure

To check turbocharger boost pressure, connect Pressure Gauge (617 589 02 21 00) to intake manifold after removing plug.
Using a dynamometer, drive vehicle in driving range "S" on 300 Series and "2" on 190D at full load and 4000 RPM. Boost pressure should be 10.1-11.6 psi (.71-.82 kg/cm² ).
If boost pressure is too low, check air filter and air intake duct for obstructions. Check turbocharger for leaks between manifold and turbine housing, compressor housing discharge and intake manifold, and between intake or exhaust manifold and cylinder head.
Check pressure line between intake manifold and aneroid compensator and overload switch-over valve. To check valve, turn ignition switch to position "2". Disconnect plug on valve and check for battery voltage at Black/Red wire.
If not present, check fuse No. 4 or wiring. Check for ground condition of Brown/Black wire. There should be no ground connection below boost pressure of 16 psi (1.13 kg/cm² ). If ground exists, check pressure switch in intake manifold or its wiring.
Other possible causes of low boost pressure would be a defective wastegate, requiring turbocharger replacement, or problems with the fuel injection pump, requiring removal, testing, and repair.
If wastegate does not open, causing boost pressure at full load operation to exceed 16 psi (1.13 kg/cm² ), check hose between compressor housing and wastegate. If hose is leaking or is kinked, replace hose. If not, replace turbocharger.
When dynamometer test is complete, road test vehicle with tester inside vehicle. Drive vehicle in "L" or "S" range at 4000 RPM. Fully depress accelerator pedal. Hold engine speed at 4000 RPM. Fully depress accelerator pedal. Hold engine speed at 4000 RPM with brakes (short test duration only). Boost pressure should be 10.1-11.6 psi (.71-.82 kg/cm² ). If not, repeat steps 3) through 6).