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Saturday, February 2, 2002
Trailer brakes should be inspected and serviced at yearly intervals or more often as use and performance dictate. Before beginning any procedure outlined here, make sure the trailer is properly positioned on a level surface, raised and blocked safely with the tires completely off the ground. Always use properly sized jack stands and never leave weight on the jack itself. Most do-it-yourselfers usually perform these procedures one side at a time.
Remove the drum by first removing the dust cover, cotter pin and spindle nut and washer. Carefully remove the outer wheel bearing and cover it with a shop towel so dust does not settle on it during the time it is removed. Pull the drum away from the brake assembly and over the end of the spindle. Take care not to ding the spindle threads! (it may be easier in some instances to first remove the tire and wheel).
Inspect the magnet arm for any loose or worn parts. Check the shoe return springs, mounting springs and adjuster spring for deformation. Manually move the actuating arm or lever and check for any binding or rubbing. Replace the magnets if they show signs of uneven wear. Some magnets may have recessed screws on the side. If the wear is down to the screws or rivets, they should be replaced. In some instances it will be necessary to have the armature plate resurfaced. Call your local service facility if an armature has developed a pattern of grooves due to worn or damaged magnets.
It is wise to replace all the same items of the same axle even if only one side necessitates replacing. If, for instance the right side magnet is worn, but the left side is okay, replace them both. Keep all like components on the same axle, the same age.
Two areas of the brake drum are subject to wear and require periodic inspection: the drum surface where the brake shoes make contact during stopping and the aforementioned armature surface where the magnet contacts. The drum surface should be inspected for excessive wear or heavy scoring. If the wear marks are worn more than 0.020 inches, or the drum has worn out of round by more than 0.015 inches, then the drum surface should be turned. If scoring or other wear is greater than 0.090 inches, the drum must be replaced. Newer brake drums have an incorporated armature—all in one piece. In which case, if either the drum or the armature exhibits signs of grooving or scoring beyond 0.090 inches, the complete assembly will have to be replaced. To ensure proper contact between the armature face and the magnet face, the magnets should be replaced whenever the armature face is resurfaced.
Clean each brake assembly carefully with a soft, damp rag or brush. Never use compressed air to blow out the brake assembly. Some brake shoes may contain asbestos materials that are harmful. Make sure the brush or rag that is used is indeed damp. This will eliminate the spread of dust particles that can be inhaled. It is further recommended that a face mask filter and eye protection be worn while working on electric brake assemblies. Before reassembling any parts that may have been taken apart during the inspection, apply a small dab of white lithium grease to the areas behind the shoe where the shoe rubs on the backing plate.
Unlike most automotive brakes that are self-adjusting, electric brakes require periodic adjustment to keep the shoes and drums properly spaced. The main symptoms of brakes in need of adjustment are brakes that get hot while driving down the road (the shoes are dragging against the drum) or brakes that simply will not hold. Adjustment is simple but important.
Expand the shoes until you cannot rotate the tire or drum any further. This centers or “seats” each shoe evenly against the inside surface of the drum. Then back the adjustment off in the opposite direction until the wheel turns freely but has a very slight drag during a single rotation. Replace the plug to keep dirt and moisture out. Repeat this process on all brakes.
(Note: Electric trailer brake adjustment procedures are visually detailed in the RV Doctor’s Do-It-Yourself RV Care DVD.
Having the initial adjustment at each electric brake on the trailer is just the starting point. For a properly set-up towing configuration, next, the type of brake controller and its correct calibration must be considered.
Also called actuator-type controllers, this type relies on pre-programmed internal electronics to determine how much voltage is sent to the trailer brakes. The longer the brakes on the tow vehicle are depressed, the more voltage is supplied. One drawback to this type is that if the brake pedal is released too soon, not enough braking voltage will be delivered. If the brake pedal is pressed again, it takes more time for the voltage to ramp up. It is crucial that timer-based controllers be adjusted carefully for the exact trailer load and the speed it will be traveling.
The majority of inertia electronic controllers regulate the amount of voltage delivered to the trailer brakes by employing an internal pendulum or by bending a fiber beam, depending on the manufacturer. This type of controller will apply a proportional amount of voltage to the trailer brakes as is needed and mandated by the actual force of the tow vehicle coming to a stop. As the tow vehicle slows, more voltage is delivered to the trailer brakes. All electronic controllers have a sensitivity adjustment for varying loads in the trailer. Obviously, the heavier the trailer and the stowed gear, the more braking power is needed.
Properly installed, today’s smart controllers can be fine-tuned to these varying loads. Likewise, a gain adjustment is sometimes included that will help prevent the trailer brakes from locking up under normal towing circumstances. The electronic controllers of today automatically compensate for this.
Additionally, when braking on hills, most controllers will allow a little more power to the trailer brakes while going downhill and slightly less when going uphill. Another useful feature includes integral diagnostics which assures the driver that the trailer and tow vehicle are indeed electrically connected.
Some units contain a fully modulated manual control should the operator want just a touch of trailer brakes to compensate for a change in road condition or another corrective measure. Manual operation of the controller does not activate the stop lamps as in those controllers that monitor the stop-lamp switch.
Concerning inertia brake controllers only, synchronizing the controller to the electric brakes on the trailer will ensure the proper amount of voltage is being delivered to the brakes at any given time.
Because of fluctuations in tow vehicle battery and alternator output levels, and the differences in trailer cargo weights, etc., it’s important to synchronize the brakes often. The first step is to make certain the brakes on the trailer are properly adjusted as explained earlier in this article.
Next, be sure to read the operating instructions for your specific inertia controller. There are accelerometer and decelerometer styles of inertia-type brake controllers and slight differences in the synchronizing procedures may be necessary.
The trailer and tow vehicle must also be loaded as they would for normal traveling. Full water tank, full propane containers and empty holding tanks, plus all the canned goods and adult beverages in the cupboards, etc. Both vehicles should be at or near their regular traveling weight.
Begin by setting the gain level on the controller to the mid-point or slightly higher. While driving on a fairly level surface at 25 mph, apply the tow vehicle brakes. The goal is to get the gain set to a position just before the trailer brakes lock up. If the brakes lock up at 25 mph at your initial setting, reduce the power setting until the trailer tires do not skid at that speed. It’s probably wise to actually get the brakes to lock up first and then keep reducing the gain a little at a time until they no longer lock up or skid.
Optimum braking and the least amount of stopping distance are both achieved just before the trailer brakes lock-up. It may take a few attempts to obtain the perfect synchronization, but take your time. Proper setup will result when there is no sense of the trailer pulling or pushing the tow vehicle during the braking process.
Some controllers also have a modulating or sensitivity adjustment that also must be addressed. The purpose of this adjustment is to vary the threshold voltage to the brakes depending on the amount of pressure on the tow vehicle brake pedal and how fast the towing configuration actually comes to a stop. It’s the amount of voltage first applied once the brake pedal is depressed. Think in terms of a panic stop vs. a slow gradual stop. If the brakes are grabby or “harsh” during braking, it may be necessary to reduce the threshold voltage or lower the sensitivity. Again, brake lock-up should be avoided. Refer to your specific brake controller literature for the correct method of adjusting the threshold voltage.
Most braking problems that cannot be corrected by either adjusting the trailer brakes and the brake controller can generally be traced to electrical system failures. Obviously, mechanical issues with the electric brakes will be evident during your periodic inspections. Worn magnets, bent arms, gouged drums, worn linings, etc. are pretty conspicuous. In order to troubleshoot the brake system electrically, you will need a voltmeter and an ammeter.
Brake system voltage is best measured at the magnets in order to check the entire system. All voltage starts at the tow vehicle battery, passes through the controller, through the electrical connection between trailer and tow vehicle, and ultimately is applied at the brake magnets. Electrical integrity is crucial for safe stopping. The engine of the tow vehicle should be running when checking the voltage, so that a low battery will not adversely affect the measurements.
Attach the voltmeter in parallel with any of the brake magnets. The voltage should be zero volts at the beginning of the test. As the controller is activated, or the brake pedal depressed, the voltage should gradually increase to about 12-volts DC. This slow climb in voltage is termed modulation. No modulation means that when the controller begins to apply voltage to the brakes, the controller immediately applies a high voltage, which causes the brakes to apply instantaneous maximum power.
The threshold voltage of a controller, remember, is the voltage applied to the brakes when the controller is first becoming activated. The lower the threshold voltage, the smoother the brakes will operate. Too high a threshold voltage causes grabby or harsh brakes.
System amperage is the current being drawn by all brakes on the trailer. The engine of the tow vehicle should again be running. One location to measure the current is at the brake output wire right at the controller. Sometimes, however, this is easier to accomplish at the electrical connector plug at the rear of the tow vehicle. The brake wire must be disconnected and the ammeter put in-line or “in series” with the brake wire. Make sure the ammeter has sufficient capacity and note the polarity to prevent damaging the ammeter (a range of 0 to 15 amps is sufficient for single or tandem axle trailers; 0 to 25 amps is needed for triple axle trailers with six brakes).
Individual magnet current draw can be measured by inserting the ammeter in the line at the specific magnet you want to check. Disconnect one of the magnet wires (or simply slip an inductive-type ammeter over one of the wires) and attach the ammeter. By far, the most common electrical problem is low or no voltage at the brake magnets. Common causes for this condition can usually be attributed to one or more of the following:
• Poor connections
• Open circuit
• Insufficient wire size
• Broken wires hanging below the axle
• Improperly set up controller
Another common electrical problem is shorted or partially shorted circuits usually indicated by extremely high amperage readings. Possible causes are:
• Internally shorted magnets
• Defective controller
• Brake wire shorted to ground somewhere in the system
• Corrosion between the ground wire and the brake wire in the electrical connector plug
All electrical troubleshooting procedures should start at the controller. Try to eliminate the brake controller as the problem first. Make sure the controller is set up according to the manufacturer’s recommendations. If the voltage and amperage are not satisfactory, proceed to the connector and then to the individual magnets to isolate the problem source. Twelve volts output at the controller on the brake wire should equate to a minimum 10.5 volts at each magnet. The voltage will be higher if the tests are made with the engine running as suggested. Nominal system amperage at a basic 12.0 volts with cold magnets and the controller gain adjustment at maximum, the current at each magnet should be as follows (approximately):
Brake Size Amps/Magnet
7 x 1-1/4 inches 2.5
10 x 1-1/2 inches 2.5
10 x 2-1/4 inches 3.0
12 x 2 inches 3.0
Some modern electronic brake controllers are equipped with a built-in continuity tester, voltmeter or ammeter. When the trailer is properly connected to the tow vehicle, an indicator lamp is lit on the controller. This informs the operator that there is a good connection and current and voltage can flow from the controller, through the electrical connector to the brake magnets and to ground.
Properly adjusted electric brakes, a sound electrical path and a correctly installed and synchronized brake controller all contribute to not only your safety, but to an overall positive towing experience as well. And remember, RVing is more than a hobby, it's a lifestyle!
More RV Doctor Resources
DVD: Do It Yourself RV Care The RV Doctor, Gary Bunzer, shows you how to maintain and care for your RV. This 63-minute DVD was produced in cooperation with the Recreation Vehicle Industry Association. The small price you pay for this will come back to you in all the money you save on your RV maintenance and repairs. Learn more & buy here.
The RV Owner's Handbook Learn how to perform preventative maintenance and repairs on your RV from RV technical guru Gary Bunzer. Learn about towing with a motorhome, winterizing your RV, holding tank systems, heating systems, water pumps, trailer towing, spring shakedown, water systems and much more. Easy to understand, even for mechanical novices. Learn more & buy here.