Cruise control on Volvo 760 Turbo

The cruise control on my Volvo 760 Turbo stopped working, first
gradually over a few days, then completely.

This happened before, and a mechanic fixed it saying it was a problem
with "hoses", charging $70. From what he said I concluded that a hose
was just loose, but I really don't know anything about this topic.

Would somebody please guide me to information where I can educate
myself on the subject?
Ideally, some Web link with pictures and description, but any info will
be better than what I know.

Or, if someone knows and can describe what's likely going on with a
flaky cruise control, that would help, too.

Thanks for your help and advice,
Wolfgang

 

The cruise control is very simple. It consists of the following
components:
The turnsignal and cruise control multifunction switch
The brake pedal safety switch
The cruise control servo
The cruise control vacuum pump
The electronic cruise control unit.

In order for the cruise control to work the following conditions must be
met:

There must be power to the unit, the switch and the vacuum pump, both of
the latter are supplied by the control unit.

The vacuum hose that runs from the servo (on the throttle housing
bracket under the hood) to a T fitting must be intact. [One leg of the
hose on the T fitting goes to the vacuum pump suction port (small ID)on
the left (driver's side) inner fender, the other goes through the
firewall to the cruise control safety switch(es). [One switch if
automatic, two if manual transmission] All hose ends must be free of
splits and fit snugly on their fittings. A quick check would be to
remove the hose from the servo and suck on it. Is should hold vacuum. If
it doesn't and the hose ends are all intact then the brake pedal safety
switch is leaking vacuum. Reconnect the hose. Remove the hose at the T
fitting and see if the leg going to the servo holds vacuum. If the hose
is ok and there is a leak the servo is defective]

There must be continuity to ground through the brake light bulbs. [The
safety switch is both a vacuum switch and an electrical switch. A ground
from the control unit conects to the switch which in turn passes the
connection to the ground side of the brake light switch which completes
the circuit to ground via the filament in the brake light bulbs. If the
brake lights are burned out the cruise won't work. If you step on the
brake pedal the ground circuit is broken and the cruise is disengaged]

The speed signal must be in excess of 30 mph.

80% of the time there is a vacuum leak. 15% of the time the safety
switch is defective, either leaking vacuum or not making contact. The
other five percent of the time either the vacuum pump has quit, the
switch makes poor contact between set speed and resume positions on the
switch or some junction in the control unit got spiked or hit with a
stray neutron.

That's about it.

Bob