Superduty “4×4 not engaging” problems? This is a very common issue, here is how to fix it. It might be a quick patch/replacement of a vacuum line, or a solenoid replacement. I made a quick video that will only take a couple minutes for you to figure out what the problem is, and depending on the problem, the actual fix shouldn’t take you more than 15 minutes. When I had this problem I couldn’t find a good resource to troubleshoot, so hopefully this helps.
First, is your vacuum pump working? You will hear it when you turn the truck to ON and engage 4wd, you disconnect a vacuum hose (pump senses drop in pressure and tries to re-pressurize), or if there is a vacuum leak the pump will just engage for a few minutes as soon as you turn truck to ON. Assuming you’re able to get the pump to engage, let’s move on (otherwise replace the vacuum pump).
Next, is your solenoid working? Simply disconnect the vacuum line that comes directly from the pump and goes INTO your solenoid, and plug the vacuum pump line directly into the upstream vacuum line that’s headed to your hubs. Check my video for how to do this in detail. You will completely bypass the electronics of the 4wd system and apply direct vacuum to your hubs. If you have no leaks in the vacuum lines that go out to your hubs from the solenoid, then the vacuum pump should shut off in under 45 seconds (once it fully pressurizes).
Now if the vacuum hub continues to run for over a minute, you have a vacuum leak in your lines, but test whether your hubs are engaged by jacking up the front tires and spinning one tire with 4hi engaged. Your passenger tire and driver side tires should both spin together now (but in opposite directions if you have an open diff), but if your hubs are NOT locked, then each tire will spin completely independently of one another. This means you have a vacuum leak or your vacuum pump is not functioning (again this should be very obvious if it’s not, test the vacuum with your finger over an active line, it should suck air). To force your hubs to engage just spin the manual locking hubs to “LOCKED”.
If you have any questions, please comment below.