Under the Hood: Troubleshooting shift to 4WD in F-150

McClatchy-Tribune Information Services

September 23, 2019

Q: I have a 2013 F-150 4x4 (bought new) with less than 19,000 miles. When I shift into 4-wheel drive under load, such as going up a steep incline, there is a loud bang under the truck. Is this a concern? If I anticipate the shift and make it on the flat preceding a hill, no problem. The nearby dealerís mechanic who checked it out couldnít get it to react similarly (no surprise, as itís all level paved roads nearby). But if I forget to anticipate when pulling my 22-foot travel trailer up an incline on a gravel road and shift into 4-wheel for extra traction, the resultant noise is concerning. I hope to read your comments in my local newspaper, the (Victoria, B.C.) Times Colonist.

ó John

A: John, your truck is equipped with an ďelectronic shift on the flyĒ system that does allow shifting from 2-wheel drive (2WD) to 4-wheel drive (4WD) high range at any speed and 2WD to 4WD low range below 3 mph with the transmission in neutral. The transfer case contains an electronically controlled clutch which is used to synchronize the speed of the front driveline with the rear driveline during 2WD (2H) to 4WD high (4H) shifts.

The second half of the shift process employs the IWE (integrated wheel end) system, consisting of spring-loaded vacuum hubs, which engage or disengage each front wheel hub to the outer axle. Engine vacuum is delivered or not to each hub via a solenoid thatís controlled by the transfer case control module. This is perhaps the more sluggish of the two systems. A check of the vacuum reservoir and hoses for leakage might be considered.

I believe the transfer case clutch takes care of business just fine whether you are under load or not, but smooth shifting of the hubs is likely a differing matter. Assuming the transfer case shift completes before the hubs, there would be considerable force exerted on the sliding/engaging hub mechanism when the drivetrain is under load. Perhaps one hub engages first, and the bang is the front axle internals slamming hard as the second engages? This is not a good thing and is something to be avoided!

It can be difficult to always predict when 4WD operation may be required for the road or trail ahead and itís not prudent to engage 4WD when on dry pavement. Perhaps when surprised by the need for a 2WD to 4WD shift on a hill, you can back off the throttle momentarily to allow the shift to complete before again bearing down, or better yet come to a stop?

This brings back memories of my 1951 Chevy pickup with engine vacuum operated windshield wipers. When climbing a long grade in the rain, I needed to occasionally back off the throttle momentarily to get a couple of wiper sweeps!