Pothole Protection and Tilt Lockout: Why It Won't Drive

Most of the calls we get about a lift that will not drive are not faults. The scissor lift is doing exactly what it was built to do, and the operator is standing on it wondering why it has stopped.

Three interlocks can stop an electric scissor lift, and they trigger in a predictable order. Knowing that order turns a two-hour service visit into a three-minute walkaround, and it also tells you when the machine really does need a technician.

The pothole protection bar and its limit switch seen from below.

The three interlocks, in the order they bite

Platform gate. On every scissor lift we build, the gate has an interlock that inhibits lift and drive when it is not fully closed. The check has to be done with the platform off the ground, because a gate that is wired correctly but adjusted loosely can read closed at the bottom and open at height. If the machine lifts but will not drive, start here.

Pothole protection. The bars or pads under the chassis deploy as the platform rises and must reach their limit switches before the machine is allowed to drive at height. This is the interlock that surprises people, because it engages on ground that looks flat. A 40 mm recess at a loading dock, a drain grate, a trench cover, an expansion joint with a missing filler: all of them are potholes as far as the machine is concerned.

Tilt. The scissor lift has two angle limits, not one. On our standard models the allowance is 1.5 degrees for lifting and 3 degrees for driving, and the higher models are set slightly differently. Exceed the lift limit and the platform stops where it is. Exceed the drive limit and the drive stops. Both are recoverable: lower the platform, move to level ground, and the machine resets.

How pothole protection is wired, and what fails

The bar hangs under the chassis on a linkage and drops as the platform rises. A limit switch confirms that it has dropped. If the bar meets an obstruction on the way down, or the switch does not close, the controller inhibits drive. That is the design intent, and it is the reason a machine parked over a drain will refuse to move even though it drove onto the drain ten seconds earlier.

Four failures account for nearly all the real faults we see:

• A bent bar. The machine was driven over a kerb while stowed, the bar took the hit, and it now hangs at an angle that keeps the switch open. Visible from standing height if you look along the chassis line.
• Water in the connector. The switch sits in the dirtiest part of the machine. A corroded pin gives an intermittent signal, so the fault appears in rain and disappears in the workshop.
• Harness chafe at the scissor pivot. The cable follows the stack, and a machine with 3,000 hours on it will have worn through a tie-wrap somewhere. Look at the bundle where it crosses the pivot, not at the switch.
• A misadjusted switch after a tyre change. New tyres raise the chassis a few millimetres and change the geometry. If a machine developed this fault immediately after tyre work, the switch is the suspect, not the controller.

The three-minute field test

Do this before calling anyone, and write down what happens:

1. Lower the platform fully. Switch off. Push each pothole bar up by hand and let it drop. Both should fall freely and click.
2. Switch on, stand clear, and lift the platform one metre. Watch the bars deploy and listen for the switches.
3. With the platform at one metre, open the gate. The machine should refuse to lift and refuse to drive. Close it and confirm both come back.
4. Drive onto a known slope or a piece of timber 30 mm thick under one wheel, and see whether the drive cuts out. If it does, the tilt sensor is working and the site is the problem.

If all four behave, the electric scissor lift is fine and the site is not. If any of them does not, you have narrowed it to one interlock before the technician arrives, which is usually the difference between one visit and three.

The one thing not to do

Do not bypass a pothole switch to get the machine moving. It is the most common contributing factor in scissor lift tip-overs, because the protection exists for the one moment when a wheel drops into something the operator did not see. A machine with a bridged switch will drive happily until the day it does not, and the failure is not gradual.

We have seen it done with a cable tie holding the lever up, which is at least honest, and with the switch removed and the plug shorted, which is not. Both void the warranty, and both would fail the first inspection under any regime.

When it is genuinely the controller

If all three interlocks test clean and the machine still refuses to drive, the fault is in the controller or its inputs, and that is a diagnostic-code job rather than a roadside one. On our machines the controller logs the last inhibit, which is why the first question our service desk asks is whether the display showed a code rather than what the operator was doing at the time.

Errors get resolved faster when the machine is described by what it refuses to do rather than by what it stopped doing. "Lifts, will not drive, bars deploy" reaches a diagnosis in one exchange. "It broke" takes a week. Our service desk takes both, but prefers the first. The wiring is the same across the self-propelled scissor lift range, so the test above works on a 6 m machine and a 16 m machine alike.