The 30 year old vacuum hoses were showing some significant signs of deterioration so I decided to pull them and replace with new. Trog uses a vacuum based system for locking the differentials, engaging front wheel drive and the power take off (PTO). It’s a bit of a mess with hoses having to run up into the front cabin to be switched via manual valves. Volvo was a bit sloppy with this system. Where a nice neat manifold would minimize mess, they went with a series of T’s.
The front wheel drive actuator is controlled by a 12v solenoid.
FWD is engaged when any of the following are true:
- FWD switch enabled in the main cabin
- Gearbox put in low
- Hard braking (there is a pressure sensor on the brake line)
Most of the old hoses. They were replaced by Gates fuel line. The fuel line is rated for vacuum service and won’t degrade quickly with oils or gasoline vapors that will creep into the vacuum system. The vacuum system is “powered” by a port on the intake manifold of the engine. After then engine is turned off, residual fuel in the carburators evaporates and makes it’s way into the vacuum system (not much, but over time, enough). That gas can also degrade the membranes of the vacuum actuators (and brake servos), so I intend to put a charcoal filter inline with the intake manifold hose.
It took almost 25′ of 3/8″ Gates 27004 hose and 25′ of 1/4″ Gates 27002 hose to do the system.
I still need to redo the hosing for the vents on the various gearboxes. To prevent stuff from getting into the gearbox vents, Volvo has run a network of 1/8″ish hose connecting them. The output uncermoniously dumps into an interior panel stiffener box tube.
In the future, I want to replace most of this tangle of hoses, at least the ones that lead to the manual switches in the main cabin, with solenoid activated valves
I believe Skinner 3000 series valves should work, specifically 3133BJA7LNC4 RR T1J1 C1 12 VDC
The common manifold eliminates a lot of redundant hose. It is a little unclear whether the valve seating on this are appropriate for vacuum on the common side. Also, I’m not sure if the Cv of the valves are sufficient to move enough air to engage the actuators in a short enough amount of time.