Low turbo boost - '86 5KCSQ

[quk@isham-research.demon.co.uk (Phil Payne)]

In message <34468F1C.B30B02C9@sjm.infi.net> George Simmons writes:

>  I'm also interested in the actual operation of the frequency valve; two
> vacuum(pressure?) hoses in, a vacuum(pressure?) hose out to the
> wastegate, plus a two terminal electrical connection from the engine
> control unit.....   Anyone know the theory of operation?

There are two input lines - 'vacuum' from the turbo inlet and 'pressure' from 
the inlet manifold.  The ECU modulates the frequency valve to control the air 
pressure in the top chamber of the wastegate, and thus control boost.

One of the most important factors is the wastegate diaphragm - it has to be 
airtight for this to work ...
 
As in all cases of sudden low boost - pull the turbo --> intercooler hose at 
the intercooler end and check for debris.  Not a likely thing to find, but an 
easy test to carry out - with potential for preventing real engine damage. 
Hopefully (and probably) you'll find nothing more that oil and you can carry on 
debugging.

-- 
 Phil Payne
 Committee Member, UK Audi [ur-]quattro Owners Club