1999 Ford Ranger (107,000 mi.): 4X4 worked great a few months ago, with a slight squeak in the front right axle (when engaged). Now that it’s 10 degrees outside, the 4X4 won’t engage. The light comes on, I believe I can hear the engagement motor turning, but the front axles just won’t turn. The 4X4 Low also engages normally in Neutral, but only spins the rears. It warmed up for a few days last week, and presto – it worked again. Seems to be a cold weather symptom. Any suggestions? It would be nice if I could use 4X4 in the snow.

The Concept:

Ranger Pulse Vacuum Hublock (or PVH) 4×4 system. This part-time 4×4 system with wheel-end disconnect offers optimized fuel economy in a robust design that requires no regularly scheduled maintenance under normal driving conditions. The system allows silent 4WD shift on the fly at any speed or temperature and does not require reversing the vehicle to disengage the hublocks.

The Weaknesses:

The system depends on vacuum. Vacuum is supplied through tubing to an electrically controlled valve to actuators in the hubs that engage and disengage the front wheels. If vacuum is lost due to a leak, the system cannot operate. You could also have an electrical failure of the 4×4 shift solenoid valve that controls the application of vacuum to the hublocks.

The electrical solenoid valve is mounted in the right front wheelwell, between the bumper and inner fender liner. The front part of the wheel well splash guard liner can be removed for access.

The PVH solenoid is controlled by a computer called the GEM (Generic Electronic Module). The GEM sees the 4×4 switch as an input, along with the vehicle speed. When vehicle speed is right and 4HI is commanded, GEM pulses the solenoid. The solenoid applies and releases vacuum to the hubs. The GEM inputs and outputs can be monitored with the Ford NGS scantool. But you probably have an issue with something besides the GEM (since it is also controlling the transfer case, meaning it is getting inputs and commanding outputs appropriately), and the NGS is not an absolute necessity once you understand the system.

Here is a vacuum schematic:

Here is a picture showing a vacuum pump connected at the wheel end (also called actuator, hublock above). The vacuum pump is pumped up to 20 in Hg, then monitored to see if it holds vacuum. Try rolling the vehicle a little in neutral to see if vacuum holds while rotating.

If vacuum cannot be held, the hublock (wheel end) must be replaced. Or it could be modified in a few different ways as described in the pages below. One way is to install manual hubs. The other involves modifying the existing parts slightly for manual operation, by removing a screen to provide a finger hole so the hublock can be operated in a push on/push off fashion. If you are wanting to modify it, make sure it is actually bad first. The hublocks could be fine and the PVH solenoid valve could be the fault. PVH solenoid can be tested to see if it is capable of switching vacuum on and off when the 4wd switch is switched between 4hi and 2wd.


Yes, extreme cold can affect the hardness of the seals, making them less pliable. Extreme cold can also cause electrical problems if there are cracked solder joints or loose connections anywhere, the metal contracting when cold can cause an open circuit. This has the same effect as a broken wire. This type of phenomena is possible where connections are made too. If a female terminal of a connection becomes spread apart too far, the male terminal can lose contact if it is a borderline type situation. All the terminals of the relays, module, and shift motor have to be good. In general, a fault like this only strikes one or two terminals, not all of them at once. The terminal fit can be checked by seeing how tightly each individual terminal fits its mate. It should require significant force to pull the connection apart. The catch is when you have a multi-pin connector (as almost all car connectors are made). If one terminal is loose, the others still make it hard to pull apart a connection. So, the surest check is to visually inspect them all, and either remove suspect terminals from the main plastic hardshell and fit them into their mates to check individual fit. If you have a sample of the proper size mating terminal, you can use it as a tester by seeing how well each female grips the “tester” male sample. Make sense?

You really need to verify what is happening when the fault is present. Get under the truck and feel on the shift motor, trying to determine if it sounds like it is going through the full sweep. There are some faults that can occur with the shift motor, so just hearing it is not enough, it could be running, but shutting off too soon. Read more about the shift motor and how to rebuild it for some common failures here:

Rebuilding A Transfer Case Shift Motor

For the front hublock system, you need to connect a vacuum gauge to see if anything is happening when you have the problem. If it is inconvenient, what I suggest is to get a length of rubber tubing of the right size to attach to the nipples at the hublocks. Get some 3-port vacuum tees. Cut a 2 inch piece of tubing and attach one end to the tee and the other to the stock vacuum line. Run the middle leg of the tee to the cab and attach to a vacuum gauge. This is a temporary setup that will allow monitoring of the vacuum in the PVH lines. It will allow the hubs to continue working while waiting for the problem to happen. When you notice 4×4 not engaging, watch the vacuum gauge and see what happens when you switch the 4×4 switch. If no vacuum pulse is seen, then you need to do the hublock leak test, test the vacuum lines for leaks by plugging one end and pumping vacuum on the other end–seeing if it holds. Track through the vacuum system, connect the vacuum gauge directly to the solenoid and the vacuum source, and see if you can catch any vacuum problems.

At the solenoid, you should be able to hear it click on and off when the hubs engage. Get a listen to this while the system is working and then listen for it when the system is not working. If there is no solenoid click, then no vacuum pulse can be sent to the hublocks. This could be a problem of the solenoid getting water into it and freezing, since it is mounted ahead of the right front wheel area. It could also be water in the vacuum lines freezing, a simple fix of disconnecting, blowing them dry with compressed air. Drying out the solenoid might help it too.

Here are the wiring diagrams to give you better understanding of the system.

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