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Rack and pinion drive: electrical diagnosis

A brake-motor rack-and-pinion room has to release its own brake and then drag the room through two pinions, so a weak battery, a bad ground or a control set too low all look like a dead motor. Measure at the motor with it trying to run, and know what the control and the brake are doing before you condemn anything.

The short version

How a brake-motor rack and pinion moves the room

The gearmotor runs off the coach battery and turns a square drive shaft (the torque tube) that ties the drive rail to the idler rail. A spur gear on each outer rail meshes with the gear rack inside its inner rail, so both ends of the room move together off one motor.

When you let go of the switch, the motor stops and its brake holds the room where it is. Lippert's version of the Power Gear manual says it plainly: releasing the switch locks the room. Power Gear's controls have load sensing, so when the room hits full in or full out the control shuts the motor off. Lippert's center drive through-frame is the same idea mechanically, with gear packs on the racks and a cross shaft between them.

Switch, relay control and current limiter

On Power Gear relay controls, the wall switch doesn't carry motor current. It sends a 12 V signal, and the control switches the motor. When you replace an old 14-1098 control with a 14-1130, Power Gear has you move the switch's center wire from ground to a 12 V source through a 3 A fast-acting fuse. Miss that and the new control never sees a command.

Power Gear also built current limiter (510116), sensing (140-1154) and two-stage (140-1144) controls. If the motor won't turn at all, Power Gear's chart lists a current limiter pot set too low, and the two-stage control has its own amperage check (82-E0264-T). E-Sync and M2-Sync controls are programmable and have their own encoder tests.

What the motor should draw

Put a clamp meter on one motor lead and run the room both ways. Power Gear published run and stall figures for its gearmotors, and they're your yardstick. Running near the run figure with the room moving is healthy. Climbing toward stall as the room slows means the load is going up: binding, room height, debris in the racks.

Lippert lists its Klauber D-300 through-frame motor at 30 A and 12 RPM. A motor that hums and pulls toward stall without turning is being held by something: a bind, a brake that isn't letting go, or a jammed gearbox. Find which before you order a motor.

Power Gear gearmotorRated outputRunStall
2.5 in, light duty150 in-lb at 15 RPM15 A30 A minimum
2.5 in, medium duty300 in-lb at 15 RPM15 A40 A minimum
2.5 in, 296:1400 in-lb at 10 RPM8 A40 A
3 in, square gearbox350 in-lb at 20 RPM16 A35 A minimum
3 in, heavy duty400 in-lb at 15 RPM15 A55 A minimum

The motor brake

The brake is on the back of the gearmotor under a rubber boot, with a lever that releases it by hand. Lippert's copy of the Power Gear manual says motors built from 2009 on have a spring-lock brake lever and earlier ones don't. Leave the boot on: Lippert says removing it voids the warranty.

If someone left the lever released after a manual override, the room can't lock and won't seal, and Power Gear's chart lists exactly that under a room that won't seal. Check the lever on every call where somebody cranked the room before you got there.

Some motors run the brake on its own lead. Lippert's above-floor manual labels a brown motor brake wire, and Power Gear's motor list includes a 3-wire unit. On those, check the brake lead's connection along with the two motor leads.

Voltage under load and the breaker at the battery

Power Gear's test is simple: there should be 12 V DC at the motor while it's running. If there isn't, charge or replace the battery, then go after the wiring. Measure at the motor leads with the switch held, then across the battery posts. The difference is what the wiring, breaker, control and grounds are eating.

Power Gear protected these circuits with 15 A and 20 A breakers. The relay kits came as 20 A for main rooms and 15 A for bedroom, wardrobe and single-rail rooms. Lippert's line on grounds applies here too: over 90 percent of electrical problems are bad grounds, and a star washer at the chassis ground is recommended.

  1. Charge the batteries or plug the coach in. Lippert wants a full 12 V DC charge before testing.
  2. With the switch held, check for 12 V at the switch. On a Power Gear relay system, also check the control's output to the motor.
  3. At the motor, check the incoming leads for 12 V, splitting at splices if you have to. If it's there and the motor does nothing, the motor is bad; Lippert says it isn't field serviceable.
  4. No 12 V at the motor: inspect wiring and connections between switch, control and motor.
  5. No 12 V at the switch: inspect battery-to-switch connections, breakers, relays and fuses, then start over.

On the job

  1. Get the batteries charged or the coach on shore power, and turn on the slide system switch or key.
  2. Look under the room first: transit bar, obstruction, debris in the racks, brake lever position.
  3. Clamp a motor lead and meter across the motor leads, then run the room both ways.
  4. No voltage at the motor: work back through the control output, switch signal and its fuse, breaker, battery and grounds.
  5. Good voltage and no draw: open motor or connector. Good voltage, near-stall draw and no rotation: brake, bind or gearbox.
  6. Motor turns but the room doesn't: stop here and go to the mechanical checks.
  7. Slow, with good voltage and high draw: check room height and binding before you blame the motor.

Numbers

Shop tip

Hook up the clamp meter before the first run. The first stroke after a room has sat tells you the most about binding, and you only get that stroke once.

Where this comes from

Try it on a service call

Put the lesson to work on a hands-on troubleshooting case.

Keep going on slides and leveling

Learn it

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Codes and specs

On the job