problems with cruise control and diff lock

[Kelvin]

Not long after I bought an '85 4KQ in '85 the cruise control failed. My 
Audi dealer fixed it under warranty (and billed Audi for several hundred 
dollars worth of labor) but a year later it failed again. Even though the 
car was still under warranty, I wasn't happy with something failing twice 
in such a short period of time. So I stuck my head under the hood and 
began nosing about. As Ero Rademar noted, both cruise control and 
differential lock need vacuum to work. Well, I very quickly found a 
neoprene vacuum hose was badly cracking where it was pushed up and over a 
nipple. So I bought about a one-foot length of rubber hose with nylon 
reinforcing and a couple of stainless steel hose clamps at my local auto 
parts store and replaced that original hose. Presto the cruise control 
worked and continued to work without fail for the next 11 years and 
130,000 miles I owned the car. For less 10 minutes of labor and less than 
$1.00 worth of parts, I fixed it. So check your vacuum hoses and check 
valves. The cure is probably as simple and as inexpensive as mine was.

Kelvin