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2017-04-15
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Hand Throttle Installation for Cummins 4BT on a Jeep Wrangler

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In 2012, my Jeep Wrangler's venerable 4.0L died and I went through a complete engine swap for a Cummins 4BT. One, of many, side effects of this swap for an entirely mechanical engine was losing my Jeep's cruise control. After a long drive from New Jersey to South Carolina, my leg was stiff and screaming at me to figure out a solution. I opted for a hand throttle.

Q&A

What is a hand throttle?

  • Very simply, it's a device that allows you to control the vehicle's throttle with your hand instead of with your feet.

Why would anyone want a hand throttle?

  • These are pretty common devices for the handicapped, allowing folks without the full use of their legs to drive.
  • In off-road situations, imagine the 4x4 is precariously balanced and both feet are engaged on the clutch and the brake. No matter how fast the driver moves, by the time the brake is released and the throttle engaged, the vehicle has rolled backward and has to tackle the obstacle again. With a hand throttle, the driver gets a "third foot" and can feather the throttle simultaneously to releasing the brake and engaging the clutch.
  • You're NOT supposed to use a hand throttle as a cruise control ... but you can.

Why should you NOT use a hand throttle as a cruise control?

  • A regular cruise control disengages automatically as soon as the brake is depressed, or the clutch is released, or the vehicle crashes. A hand throttle will stay engaged until the driver releases it which means in an emergency, if the driver panics and forgets - the vehicle will continue plowing ahead.

How does a hand throttle work?

  • In a gasoline powered engine like the stock 4.0L, the "gas" pedal actually only controls air flow into the engine. When the pedal is depressed, it pulls a physical cable to the throttle body linkage in order to open a butterfly valve inside the air intake. Technically, it should be called an "air" pedal.
  • In a diesel powered engine like the Cummins 4BT, the gas pedal pulls a physical cable but this time it is linked to a fuel pump like the Bosch VE Pump or the Bosch P-Pump and actually controls the flow of diesel.
  • In a modern "fly-by-wire" computer controlled engine ... well, you're shit out of luck.
  • The hand throttle simply runs a second mechanical cable to the throttle body or fuel pump back into the cabin where it's pull is controlled by a lever. In reality, the lever is nothing more than a gear shifter from an old school mountain bike (before the days of index shifting).

Installation

First, you have to get a hand throttle. TeraFlex and Rubicon Express both offered kits but now it seems only the TeraFlex option is still around. But you can literally replicate the kit using nothing but a friction shifter from a bicycle and save 75% of the cost. Despite sounding like a bargain to just buy the shifter, you'll still need cable housing, rubber bushings, and cable fittings in the end such that for the cheap price, it's easier just to buy the kit.

Remove the shifter knob from your Jeep. If you still have the stock shifter, you need to pop the gear pattern indicator out. Underneath that indicator is a nut holding the shifter grip onto the stalk. If you have an aftermarket shifter knob, you already know how to do this.

A bicycle shifter is meant to attach on a "fatter" handlebar, so it needs a rubber grommet to make up the diameter difference. The kit includes one. At first, it doesn't look like the grommet's tiny opening will fit over the shifter stalk but with enough thumb force, it will. Make sure to get the grommet down at least halfway. If you don't, the shifter will slam into the dash console when you're in 1st, 3rd, and 5th gears.

Now slip the shifter down over the grommet and begin tightening it down. The rubber will compress quite a bit so make sure the shifter is sufficiently tight and doesn't move around during shifting.

At this point, thread the shifter cable into the shifter. You can see how pulling back on the shifter will pull the cable. In time, you'll get a physical feel for how much shifter throw is necessary to replicate pedal depression. You will want to route the cable housing down the shifter shaft and towards the firewall. If you have to choose a "side", choose the passenger side to avoid getting cables near the driver's feet. Zip ties are handy for securing the cable housing but remember to allow enough slack for the shifter itself to physically move through it full range of motion.

You can reattach your shift knob at this point. I've always had a preference for the Rocknob stone designs. In this configuration, I can control the manual transmission and the hand throttle simultaneously with my fingertips. At this point, your work inside the Jeep is basically complete.

There is already a grommet in the middle of the firewall. You can punch a small hole through this grommet and then force the cable housing through it into the engine bay. This location is ideal because it will not bind up the cable and allow it to move easily.

The highlighted area indicates the grommet's location.

There is a metal plate on the Cummins 4BT that extends just aft of the fuel pump. This plate is where the existing cable housing from the gas pedal terminates and the bare cable extends out to the pump. Drill a small hole into this plate to attach the cabling housing termination lug. It simply fits through the hole and bolts up snug. Thread the cable through this lug and fit the cable housing into the fitting. It will be loose at first, but once the cable is secured to the pump, everything will be held in place.

Now the cable for the gas pedal and the cable for the hand throttle will extend in parallel and attach to the fuel pump. The simplest way to attach the cable is to use an electrical ring terminal. Thread the cable through the terminal's crimping end, loop it around the ring, and then thread it back through the crimping end. Do NOT crimp it down yet!

Loosen the nuts holding the existing cable to the fuel pump's control lever. Slip the ring terminal onto the bolt and re-attach the nut. With the cables in place, pull out the slack through the ring terminal's crimp end. When you are satisfied the tension roughly matches that of the gas pedal (at idle), go ahead and crimp down the ring terminal. The use of some blue locktite is recommended to ensure the nuts on the control lever do not loosen under the 4BT's vibration. NOTE: Make sure the shifter is actually in its "idle" position when making this adjustment.

At this point, the installation is complete. It is recommended to crimp a small 1.6mm bicycle cable end cap onto the cable. This will keep the metal cabling from slowly unwinding and fraying.

Conclusion

Installing a hand throttle onto a Jeep Wrangler is a remarkably cheap and easy task. This is also a very handy feature to have on the all-mechanical Cummins 4BT diesel engines. The hand throttle makes certain off-roading tasks easier allowing for more precise transitions from full-brake to forward travel by eliminating the brake-to-gas foot transition. Of course, they're also quite handy as a poor man's cruise control especially on long highway drives.

Bear in mind, the devices are NOT intended to be used as a cruise control but in practice, most people without a factory cruise control probably use them that way. If used this way, it is very important to train yourself to be cognizant that your vehicle WILL NOT STOP if an unexpected traffic event comes up ahead. You must be prepared to both stop and transition your hand back to the shifter to disengage the hand throttle. Some folks might mitigate that situation by resting their hand on the shifter which is why it is important to remember that puts undue stress onto the transmission's synchronizers causing premature wear.


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