2001 HELP!! no turbo pressure.

Hi, i have a 2001 sprinter 311cdi 85,000 miles and i'm getting no boost pressure..the turbo is spinning up fine, but there isn't enough vacum getting through to operate the waste gate. I changed the pressure coverter thing under the air box behind the headlight..bit of an improvement. Also changed the other sensor switch on the other side..checked pressure and vacuums from all pipes feeding..all is good, Waste gate is not stuck..it is just not being told to close..and all boost pressure is being dumped..
It's driving me mad! Any ideas please!??
Could it be so many fault codes, that they need clearing??

Adam

check the harness from the transducer under the air box which runs under the radiator to the other headlight.
ps
also check for 9volts i think at the plug which goes into the the transducer.

Had diagnostics done today...loads of faults. cleared most but these came back instantly..

P1188 Element switch/Hydraulic pump.

P1470 Charge Pressure Control open circuit.

I guess that looks like the loom as you said..If it could be anything else. Could you put it down,
Thanks again