I have looked at the vacuum lines that run to the auto locking hubs on my 2024 F-250. (I replaced the fixed pass side hub with an auto locking hub) From another post - 2 wheel Lo mod - I am considering adding another vacuum switch to control the vacuum source to the auto locking hubs. The vacuum source seems to be from a line that originates from near the throtle body.

Vacuum line to the vacuum solenoid


Vacuum line to vacuum solenoid

If I insert another on/off solenoid - in the vacuum line, to keep the hubs from locking, should it be inserted before (upstream) the factory solenoid, or after the factory solenoid (downstream) - between the factory solenoid and the auto locking hubs. My concern is: will the location of the added, on/off solenoid, affect whatever electronic signal that is sent to the factory solenoid that controls the vacuum to the hubs? I appreciate any insight anyone has to manage the vacuum flow to the factory vacuum switch.
The 2 port vacuum switch that I have can be turned on or off with an upfitter. My thought is; turn off the vacuum to the factory vacuum soloenoid using added vacuum switch and the upfitter, then switch to 4 Lo, then reverse the process.

 

141 views - no response, so I changed the title -

 

why??
I see 0 point in doing something like this.

 

So you can use low range in 2WD. (without it locking in the hubs)

Put the solenoid inline after the factory one.

 

Or do the typical mod that just disables the factory vacuum switch via an upfitter switch for 2WD Low range.

 

🤔 that makes 0 sense
so you put the truck in 4 low, but don't lock or have the hubs auto lock. so basically the t-case is in 4 low spinning the front driveshaft to the axle, the axle shafts are spinning, but the stub shafts are not locked at the hubs....
what is the point??

 

I did that mod on my 2018 Supr Duty PSD - worked fine. A spare vacuum swith from another project, prompted the question, on / off with an upfitter switch, no wire cutting, no relay wiring, etc, etc

 

Many landscapers do this mod so they can use Lo range when backing a loaded dump trailer into tight spaces.
My friend uses this method because he has a longer, steep, paved driveway. He has to back his R/V up, to get it in its parking space. Lo range give him plenty of power but allowing the hubs to lock in binds up the drivetrain something awful.

 

i guess i don't understand. if the hubs aren't locked, your not getting 4wd

 

Correct. The purpose is to have the 4L gearing without having 4 wheel drive engaged.

 

For paved surfaces when you need the low end grunt but you can't use 4wd.

 

Is there any function advantage of using the extra vacuum switch compared to the other way with adding a relay?

 

All of the electrical options require cutting into the factory wiring harness, no?

To me that advantage of this approach is that you can tap power from any hot source and common ground, and if you want to put the truck back to stock you just replace a short length of vacuum tube.

 

No function advantage but the OP's way "may" be less obtrusive if you ever want to go back.
Just depends on who/how the mod was made.

 

I like this idea as I have been hesitant to cut the factory wiring harness

Do you have a pic of the vacuum switch or a part number?