Braking News

| April 11, 2005

It takes more than flipping an on/off switch to get the most from your engine retarder.

Without an engine brake, diesels don’t produce much braking compared with car engines because they lack a throttle – the biggest source of engine braking in a car. Truck diesels are also small for the weight of the vehicle, which is one reason trucks sometimes run away on steep downgrades.

Clessie Cummins, who invented the Cummins diesel, also invented the Jacobs Engine Brake, commonly known as the “Jake Brake.” The Jake Brake and its competitors use hydraulics powered by the engine’s existing valvetrain parts to convert your truck diesel into a retarder that stops almost as hard as the engine pulls under power.

The engine brake adds little to engine wear. Its biggest advantage is that it works by compressing and releasing air, eliminating the friction between dry parts of a service brake system, so it will never overheat. Having an engine brake not only will help you descend steep grades safely, it will significantly cut your maintenance costs because it will save your service brakes.

Proper operating
An on/off switch allows you to turn the engine brake off altogether or allow it to operate when the throttle and clutch pedals are fully released (your feet are off).

The brake is also controlled by a “Low/High” or Low/Medium/ High”dashboard switch with two or three positions, so you can match the power of the brake to road conditions. With a two-position switch, you get braking action on either all six cylinders or three of them, giving either full power or half power. With three positions, the switch settings engage six, four or two of the engine’s cylinders, giving full braking power, two-thirds or one-third power.

There is also an accelerator pedal switch. If the main power switch is on, the switch turns the brake on when the driver releases the throttle but cuts it off as soon as he applies power.

A few vehicles also have a brake pedal switch, so the engine brake can never be used alone – only after the service brakes are applied.

Follow this procedure to operate your engine brake:

  1. The engine brake depends on a free flow of oil, so always wait until the engine reaches normal operating temperature before switching it on. Always turn the engine brake switch off when shutting the engine down so it will not operate with the engine cold.

  2. The most critical factor in operating an engine brake safely is understanding the effect of road and load conditions. Unless your tractor or truck has a front drive axle that is engaged, the engine brake retards only the drive wheels, creating potential for a jackknife if the road is wet or icy. This problem is worse with a single-axle tractor, so never operate an engine brake when running bobtail, or with an empty trailer on a slippery road.
  3. When driving loaded under conditions that are just slightly slippery (for example, in the rain), first observe whether or not the truck remains stable when backing out of the throttle. If it does, then turn the brake switch to the lowest setting. Make sure there is plenty of room on both sides and in front in case you need to recover. Next, activate the brake cautiously, ready to turn it right off or declutch if the tractor’s rear wheels begin to turn out.

    Remember to turn the engine brake back off or declutch right away if the tractor starts to skid, fishtail or even jackknife. Advance to a higher setting only if the rig remains quite stable; do this with the same caution used initially. If the rig starts to jackknife, recover as above.

  4. Under dry road conditions, use the maximum power setting for abrupt stops, as in traffic. When descending long, gradual grades, start at the lowest power setting and increase it, as necessary, to hold your speed where you want it. You will often be able to alternate between two power settings to keep the truck at the right speed, using the more powerful of the two settings to drop a few miles per hour, then switching to the other to bring the truck back up to your desired speed of descent.
  5. Descending a long, steep grade always requires using a lower gear and descending slowly. Always activate the engine brake before reaching the downgrade to check that it is working. With an engine brake, you can use a higher gear and still get enough engine braking to minimize use of the service brakes. The lower the gear you use, the greater the retarding ability of the engine brake and the cooler your service brakes will remain. The Jacobs manual estimates that, depending on load, you can descend a 6 percent grade at 25 mph using your Jake brake. The ideal is to find the “control speed,” which is the speed and gear that cause the brake to maintain the constant speed you want with little or no use of the service brakes. Remember that using too high a gear can cause overheating of your service brakes even with a good engine brake.
  6. The engine brake’s power also increases rapidly with an increase in engine RPM. Any time you need maximum braking power, you should downshift until the engine is between 1,800 RPM and the maximum speed allowed for engine brake operation (usually 2,100-2,300 RPM). Look in your owner’s manual to find out what that allowable RPM is. Then observe it very carefully, as bent valve-train parts will typically be the result of over-speeding the engine. This is abuse, not covered under the engine warranty.
  7. The truck gains speed during the shift on a steep downgrade, which means you need to downshift at a lower RPM. When downshifting on a downgrade, always make sure you have plenty of RPM to make the shift. If your governed speed is 1,800 RPM, you should not downshift much above 1,200-1,300 RPM. If you can’t make the shift, go right back into the next higher gear.
  8. When shifting with the clutch, the engine brake is inactivated. But many drivers shift without the clutch and leave the switch energized to speed the shift (sometimes called “Jake shifting”). This is risky because the powerful engine brake slows the engine crankshaft very fast. This can subject the drivetrain to extreme torque.
  9. On trucks with an electronic engine, the brake is de-activated at about 1,000 RPM. Some engines with traditional, hydro-mechanical injection have switches to cut off the brake at low RPM, too. But if your vehicle doesn’t, always remember to switch off the brake when running down low because the retarding force becomes alarmingly potent. This will also prevent stalling and rough operation.

Making adjustments
An engine tune-up involves not only adjusting the engine’s overheads – the valves and injectors – but the engine brake itself. In fact, you need to remove the engine brake housings in order to access the valves and injectors for adjustment.

We received the help and advice of Jacobs regional Manager Bob Hillen and technician Bill Lattemann at Penn Detroit Diesel in Fleetwood, Pa.

“Doing this is not something anyone should be afraid of,” Hillen says. “But, since you are dealing with overheads, it’s essential that all adjustments be to specification and that all torque values are observed.”

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