Disabling HPLO on Early 4QD Models

This applies to control boards issue 15 or earlier. With issue 16 a bridge point was added that may be linked out to disable HPLO.

High pedal lockout is a safety device to make sure the rider cannot switch on the controller with their foot on the throttle: to do so would cause the vehicle to start off at the ‘throttle’ speed. On some vehicles this is considered unsafe. HPLO ‘locks out’ the controller until the throttle is returned to zero when the controller resets and starts operating normally.

By definition – removing a safety feature could then be construed as making the machine unsafe!

For most uses, the HPLO can be over-ridden by using the inhibit input. Connecting the inhibit input (pin 1) to ground (pin 6) forces the internal demand speed to zero, irrespective of the throttle position. So if the controller is turned on with inhibit engaged, the HPLO will not engage and, as son as the inhibit is released, the controller will ramp up to selected throttle speed.

However there are applications where HPLO is a nuisance, so this page describes how it is embodied on the 4QD series controllers and how it can be permanently disabled..

The circuit above shows the power-up reset, inhibit and pot fault detect circuitry. At power-on the internal 12v supply powers up and the 10ยต capacitor charges up via the 100K to about 8v (2/3 of 12v supply). However the NPN transistor is biased on only when ‘Demand Zero’ is true (high). If there is any demand speed (from the throttle, this line is low and the transistor is held off, so pin 13 is held at +12v and IC4 does not change state – the Inhibit line is held low, inhibiting controller action until DZ does go high (the throttle is returned to zero).

If the throttle is low at power up, DZ is high, pin 13 of the IC is held at 4v and the controller is enabled automatically after the capacitor has charged. When pin 14 goes high, inhibit is removed and the transistor is held on via the 100K from pin 14 to the transistor base.

It follows then that to inhibit the HPLO function, the transistor must be biased on permanently. A 100K resistor from transistor base to +12v will accomplish this. Or you could simply short out the transistor (which is what the link on issue 16 control boards does)!


The drawing shows the centre of the top board of the controller, the end furthest from the 6 way connector. The view is from the component side. The two arrows indicate the base of the transistor and the internal +12v line.