Here's the thing: in a traditional lever actuated system there is one thing that can break - the lever and linkage. And even if the linkage breaks, you can always crawl under the truck with a hammer and push the shifter into place to engage 4-wheel drive. In a vacuum operated system there are six - yes, six major components that can fail. Further, the system defaults into 2-wheel drive, so if any of the components fail, the truck slips out of 4-wheel drive. So, without further ado, here are the six reasons:

1. A broken fan belt. Yes, that's right, if the belt that drives your alternator is broken, or even slips (as it might well do during a deep water crossing), the alternator will stop turning. This will cause the vacuum pump, run off the back of the alternator shaft, to stop pumping. This will stop the vacuum, and the truck can slip out of 4 wheel drive. Imagine this occurring in a fast flowing river.
2. A broken vacuum pump. On a diesel, the engine produces very little vacuum. Instead, vacuum is produced via an auxiliary pump. Should any part of the pump break, vacuum will stop, and again, the truck can slip out of 4 wheel drive.
3. The solenoids that open/close the vacuum lines could fail. Should these solenoids fail, or any of the electronics that operate them (fuse, dash switch, wiring in-between), the truck will not go-into or out-of 4 wheel drive.
4. The steel lines could rust. Toyota runs most of the vacuum lines with steel to prevent them from collapsing. Because there is only air running inside of them, they are prone to moisture on the inside as well as the outside (as opposed to steel brake lines that have brake fluid inside of them). Therefore, corrosion occurs from the inside-out, which is non-detectable until there is a failure.
5. The rubber lines could crack. Short sections of the system use rubber lines to allow for vibration and alignment. Old rubber can crack, and even a tiny pinhole will break the vacuum.
6. And last, but not least, there's the whole diaphragm assembly. This is the unit placed in front of the transfer case that actually engages 4x4. It has a bonnet lined with a rubber diaphragm, a vertical shaft connected to the rubber, and a cam that translates the vertical shaft into a horizontal shaft. It is encased in a 2-piece cast body, with a gasket sealing it together. Any part of this piece can fail, and again will render 4-wheel drive useless.

OK, so what if you've got this system? Get rid of it. We are looking for a conversion - if you've done it or know of a way, leave us a comment below. Otherwise, we'll post the fix when we know!

The solution is to use a charge equalizer (battery equalizer). This is a unit that constantly works to ensure that both batteries are equal. It works on a voltage differential - if it senses a voltage difference between the two batteries, then current is directed from the higher battery to the lower battery. As the differential approaches zero, so does the current.

A charge equalizer works especially well if you are tapping 12 volts to run accessories such as a radio, fridge, or laptop. With the vehicle turned off, the accessories can run, draining the 12 volt battery. Meanwhile, the charge equalizer supplements the drain by taking charge from the high side battery to the low side one.

We bought our equalizer from G&S Cruiser Parts. This unit is made by Solar Converters, who offer excellent support. We paid $220 CAD for the 20 amp model (20 amp @ 12 volts).

Installation

There are 4 wires coming out of the box: black (grnd), white (12 volt), and red (24 volt), and an LED light that indicates the internal fuse health. The red one gets connected to the 24 volt terminal of the high side battery (where the starter lead is connected). The white one gets connected to the 12 volt terminal of the low side battery, and the ground is connected to the negative terminal of the low side battery. Connect the wires directly to the battery terminals; do not connect the ground to the frame. The unit needs to see the actual voltages coming directly from the batteries.

There are two internal fuses in the unit. One blows if the amperage going through the equalizer is too high. The other one blows if the ground wire is disconnected from the battery while either of the two hot wires are still connected. Any time I do any work on the truck, and have to disconnect a battery, the hot wires (white and red) must be unplugged from the charge equalizer.

The unit should be installed in the engine compartment at a convenient location. Inevitably the fuses will blow at some point, and you need to be able to remove the top of it to change them. I put mine on my drivers fender apron. I cut the LED wire and lengthened it to run back to the dash. I drilled a small hole to press-fit the light in.

A Note on the LED Light: There are two fuses, but the LED only indicates one of the fuse's health. This fuse is the one that blows if the ground wire is disconnected while the 12v or 24v lead are still connected to the batteries. When the LED light is off, then this fuse is blown. If the other fuse blows due to an overload of amperage there is no indication. If you suspect that it has blown, you can confirm it by turning the unit off, putting a voltmeter across each battery and measuring the voltage. Turn the unit back on, and re-check the voltage. You will likely see a small change with the unit on, and if so then the fuse is ok.

Setting the Voltage

There is a potentiometer inside the equalizer. You need to change the balance of the equalizer by setting the potentiometer. To do this, both batteries must be fully charged.

1. Connect the 12v lead to the battery and measure the voltage of the 24v lead to ground. It should be around 24-26 volts. Set the potentiometer to around 25.
2. Disconnect the 12v lead, and connect the 24v lead. Measure the 12v lead voltage. It should be half that of the 24v lead. If not, set the potentiometer so that it is.
3. You need to iterate this a few times until the 12v lead is half the voltage of the 24v lead.