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The question was, where to put the solenoid........ only ONE poster with an answer in response - "after the factory solenoid". Any other thoughts, warnings, does anyone really know??? I'm inclined to agree with @5Picker, but a reason why would be appreciated......
Since the truck doesn't monitor the vacuum it makes no difference where you put it from that perspective. The truck simply monitors the vacuum solenoid coil - it has no idea if there is actual vacuum or not.
I don't know how different the vacuum routing is on the newer trucks but on mine it would be difficult to intercept the vacuum line feeding the factory solenoid. The line is only a few inches long and it runs right up against the coolant reservoir. There would not be a lot of options for mounting an additional solenoid. As far as the concern for modifying the truck's wiring harness, if you do it correctly the results work flawlessly, will last the lifetime of the truck and are virtually undetectable by the average human. But, to each their own...
I guess it's probably pretty limited use but why does this not come from the factory? My first 4WD truck (1979 Chevy K10) had three levers on the floor - any one of them could be used in any combination with the others - F/R travel gears, Hi/Lo range and 2WD/4WD (plus it had manual locking hubs...).
because people are getting dumber and/or lazier. Simple as that. you think people who need tailgates automated can handle operating a manual 4wd lever never mind two or three? We are lucky Ford still offers the SD with column shifters, unlike dials or whatever else other brands use.
I guess it's probably pretty limited use but why does this not come from the factory? My first 4WD truck (1979 Chevy K10) had three levers on the floor - any one of them could be used in any combination with the others - F/R travel gears, Hi/Lo range and 2WD/4WD (plus it had manual locking hubs...).
Probably because driving a single set of wheels, with lots of torque multiplication from the transfer case, is not good for the rear end. With the 6.7 putting out over 1,000 ft.-lbs. of torque there is a very real possibility of drivetrain damage. Your 1979 K10 probably didn't make enough torque to break the track bar mounting bolt loose on a modern Super Duty.
Probably because driving a single set of wheels, with lots of torque multiplication from the transfer case, is not good for the rear end. With the 6.7 putting out over 1,000 ft.-lbs. of torque there is a very real possibility of drivetrain damage. Your 1979 K10 probably didn't make enough torque to break the track bar mounting bolt loose on a modern Super Duty.
OP for example has the 7.3 Gas motor. Twice the torque would still place it well under the HO's peak torque, any power management notwithstanding.
I was going to put a solenoid downstream of the factory solenoid too. I wound up just capping the solenoid and using the hubs as manual lockers.
The problem I ran into was a normal 2-port solenoid had the potential of trapping the vacuum signal to the hubs if it was closed after the factory solenoid was energized. This would leave the hubs in an unwanted locked position. I’ve since found a 3-port solenoid that would have worked the way I wanted with the common port tied to the hubs, normal on tied to factory solenoid, and normal off tied open to atmosphere. When energized, the 3-port would block the factory signal and vent the hubs to atmosphere. I just never had enough interest to buy and install one as the manual system is working just fine for me as I rarely need 4x4 anyway. An added bonus is I can exercise the transfer case and front diff occasionally by just switching to 4H in the cab.
When power to the solenoid is removed, it vents the downstream portion. There is no way to 'lock in vacuum' or stick the hubs in a locked condition based on vacuum. Unless you put a new vacuum switch downstream that does not vent when de-energized. Then, you just shot yourself in the foot when you should have just done the relay mod I wrote up four years ago.
This is an important behavior everyone needs to understand. If your hubs are locked via vacuum (not manually turned) and you turn off the truck, the vacuum vents and the hubs unlock. If you go to park on an icy hill for example and put it in park (while in 4WD) and the truck is stationary, it may be stationary because only the front wheels have traction. If you turn off the truck, the front hubs will unlock and the truck is back in 2WD and sliding down the hill or driveway. If there is a lot of tension on the vacuum locked hubs, they won't actually unlock until the tension is removed. But, if hub tension is light, or the truck rocks a little (wind, getting bumped, or a heavy dad-bod like me jumps out of the truck), they will spring unlock until the truck is started again.
There appears to be a "vent" in the factory vaccum system downstream of the factory "2 port" vacuum switch.
This weekend, I will insert another 2 port vacuum switch between the factory switch and what appears to be a vent in the vacuum line to the auto locking hubs. It would seen that even if vacuum is applied to the hubs, then switched off by the add on vacuum switch, the vacuum in the hubs should bleed off through the factory device.
