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Pro-120 Fault Finding Tips

Faults on the Pro-120 series are not common, but there are a few which occur, mainly because the ‘bare board’ style of the controller makes it susceptible to certain abuses. These fault finding tips may help you. When fault finding, it is useful to take notes of the results: these will help greatly if you need to contact 4QD technical support for help.

Battery condition flashes when ignition switched on, but then goes out

A direction relay will probably click on and off while the ignition is on.

This is a normal symptom of the High Pedal Lockout operating properly because of an external fault. So it usually indicates a mechanical fault causing the throttle pot not to return to zero.

See Ignition relay won’t stay pulled in

Bad pot earthing.

This sort of fault is only likely to happen on model locos, especially where double heading.

Theoretically, if the wiring was bad such that the pot reference (pin F of the 6 way connector) was connected to chassis, an earth current could flow through pin F. This will probably blow the fuse track to this pin. If this fault happened, the pot would still find its reference through the faulty earth – but the current in this earth is putting a voltage on the bottom of the pot (which is what caused the fuse track to blow).

Under these circumstances, the High Pedal Lockout could quite possibly detect this spurious earth signal as the pot not being zeroed. More information on this on the page Earth fuses: why do they blow?

See Ignition relay won’t stay pulled in


When MOSFETs fail, they can usually be seen to be visibly damaged. A service page is available for the NCC series explaining what else may fail and how to test. Pro-120 circuitry is essentially the same as the NCC.Be warned that replacing blown MOSFETs in a Pro-120 is not an easy task and requires skill! It is easy to damage the board, by removing the through hole plating and by stripping top-side gate tracks.

  • Switching Waveforms and Servicing

Controller trips or cuts out

The controller will switch itself off in the event of certain fault conditions. These are explained fully in the page Pro-120, VTX, Scoota and NCC series controllers. Internal power supply and protection circuitry.

  • Parking brake is drawing excessive current. This is somewhat speed dependant so if this is the problem the controller may work at low speeds but could trip erratically as speed is increased.Unplug the parking brake and see if the fault stops. The parking bake should be able to supply 1 amp. More than this may cause tripping.
  • Internal 9v1 line is too low. This will happen if:
    • battery voltage falls too low. Check the battery voltage as seen by the controller (so you also check power connections): i.e. between pins A and F of the input connector.
    • pot draws excess current. Could be a wrong value pot – or you may have wired it wrong.
  • Intermittent ignition line. Check the gain control is correctly adjusted as explained in the instruction manuals – all instruction manuals are available on-site.If the gain is correctly adjusted then when the ignition is switched off at full pot speed the controller will properly ramp down to zero speed and switch itself off. If the gain is set incorrectly, the controller will switch off immediately (but only at full speed). If this is incorrect an intermittent connection in the ignition wiring will cause tripping – but only at full speed

Controller is dead or ignition relay won’t hold on

Smell the controller: failed controllers usually have a noticeable burnt smell where MOSFETs have blown. Otherwise problem is usually a dud pot or pot wiring, or the HPLO is detecting a fault and operating!

If the trickle resistor (see PRO-120 series controllers – Key components) getting hot when the speed pot is at zero speed? This indicates the capacitors cannot charge. Unplug the parking brake and 6 way connector: if it still gets hot there is an internal fault.

  1. Read Pro-120, Scoota and NCC series controllers internal power supply and protection circuitry.
  2. Is the ignition switch fitted and working?
  3. With the ignition switch off, check there is full battery voltage (measured with meter negative on controller’s B- terminal) on the yellow wire to the ignition switch – connector pin A.
    • If there is no voltage here, is the on-board fuse track to connector pin A blown?
    • If there is some voltage on the yellow wire, but it doesn’t rise to full battery, what else is connected to the yellow wire? If anything draws current from the yellow wire, the main capacitors won’t fully charge and the ignition circuit may not come on.
    • If there is (nearly) full battery voltage here then operate the ignition switch. The power relay should click as is closes and this voltage should stay at (or rise to) exactly full battery voltage. If the relay clicks but the voltage drops, see Ignition relay won’t stay pulled in.
  4. Check the voltage between pins D and F on the 6 way connector:
    • Mark 1 controllers: about 8.3v when the ignition is turned on.
    • Mark 2 controllers: about 7.7v when the ignition is turned on.

    Exact voltage is unimportant, but it should be steady and not vary much (10% maybe) as the pot is turned.

    • If there is no (or very little) voltage here, check the fuse track to pin F of the connector: if this blows then the pot fault detector operates and the controller will be dead.
    • If the fuse is intact, and there is no pot voltage, then the zener diode may have failed short circuit. If any accident occurs to the board, it is designed to fail safe and in particular this zener is chosen to fail short-circuit. If this happens, the power relay driver also may be damaged. Measure its base-emitter voltage (should be around 0.7v)
    • If the voltage varies greatly as the pot is turned the pot is wired wrongly, so that it is is progressively loading or shorting out the voltage as it is turned. As the pot draws more current, this voltage is reduced and if it drops significantly the power relay will click off as the controller’s internal fault detection system operates.This turning off of the power relay will cause a significant drop in the voltage on pin A and the pot voltage will fall only slowly as a consequence of this drop.It is just possible that some other fault is turning off the power relay as the pot is advanced, but this is less likely and other symptoms will usually be present. Disconnect the parking brake and motor to make this test more positive.
    • On 12v controllers only it is possible for the hiside current limit to get confused. If this happens, the controller will not switch off from the ignition. It can happen, for instance, if the ignition is switched on with no motor connected. Disconnect the battery and wait a few minutes for the main capacitors to discharge (or short M+ to B- to discharge them). Make sure the motors are connected before reconnecting the battery.

    See Pro-120 series controllers – Key components for identification. The zener is item 1 and the relay driver is item 24 (Tr 8)

  5. Is there anything other than a pot (minimum value 5K) drawing current from the 9v rail? The controller does not have spare power for outside devices and if these cause the 9v rail to fall, the ignition relay will disengage.
  6. Is the High pedal lockout engaging? Its operation is to immediately de-activate the ignition relay if the controller is turned on with the pot not at zero or faulty or wiring is faulty.
  7. Is the parking brake faulty? Unplug it and manually disengage the brake.

See also Ignition relay won’t stay pulled in

Connections overheat

We use and recommend these crimp connectors because they are reliable. The F-type crimps we supply require a special (expensive) tool and it is nearly impossible to operate this so as to make a bad crimp. However the commonly-available, yellow-insulated crimps are of variable manufacture and many different tools are available, most of which can give a bad crimp.

Solder is a high resistance material and is must be used very carefully if it is carrying high current. Not suitable for connections of this type.

Screw connections are subject to variable tightening and mechanical irregularity. Bolting to a circuit board is therefore problematical.

A properly crimped connection is ‘gas-tight’ – the strands of wires are deformed and squeezed into hexagonal cross section. All the air between the strands is excluded and the metal of the stands forms a pressure weld, making an extremely good and reliable joint.

Bad Crimps

Properly crimped connections are very reliable and the push-on connections are very good if properly made so overheating here always indicates the connections are bad.

Badly made crimps let air at the strands of the wires, which oxidise over time and the joint resistance increases. As they degrade, they start to get hot – which accelerates the degradation. In extremes, this heat can be transferred to the circuit board terminal and we have seen about two cases where the circuit board was actually burned as a result.

Mechanically stressed connections

Every time a push-on connector is removed and re-made, the springiness of the connector relaxes a little. So is a connector is removed too many times, the contact resistance increases and this can cause heating.

The effect is heavily exacerbated if there is any sideways pressure on the connector: maybe the wire is too short and is pulling continuously at the connector.

Controller will only go in one direction.

As you advance the controller from zero speed, one relay should click in forward direction and the other relay should click if reverse is selected.Note: the controller may not reverse properly if there is no motor connected (i.e. the output is open circuit): it requires some sort of load (resistor or motor) or the highside current limit engages and prevents correct operation.

  • Controller fails to react to reverse switch and goes forward regardless
  • Controller fails to react to reverse switch and goes in reverse regardless
  • Same relay clicks, both directions
  • One relay clicks, other does not
  • Relays click properly

Controller fails to react to reverse switch and goes forward regardless

This indicates the trouble is in the switch of wiring, and not in the controller. The reversing signal is not getting to the controller.

Check the voltage on pin C of the 6 way connector (black wire) – it should change from zero (forward) to full battery (reverse). It it’s not doing this then the switch of wiring is faulty.

Controller fails to react to reverse switch and goes in reverse regardless

There is a signal on the reverse line which the reverse switch is not removing.

  • Is the the switch is faulty?
  • Is there water in the system? Water can form a partial short and if water gets into the connector it could cause a partial short between, perhaps. ignition and reverse.

Measure the voltage on pin C of the 6 way connector (black wire). When forward is selected this should be zero volts. If not zero, than a voltage above approximately 6.5v will engage reverse.

Relays click properly

Usually this means the controller is working but the gain is set too low. On a standard controller, reverse is at half of forward speed. If the gain is set low, forward speed is low and half of this low forward speed may not be enough tom get the motor moving.

Another theoretical possibility is that there the relay contacts are dirty. However these relays used do have high contact pressure and tend to be self cleaning as a bad contact will, initially, get hot – which clears the problem before it shows up.

One relay clicks, other does not

A relay could be stuck in the de-energised state. Or it could be stuck in the energised state.

Replacing relays is not a simple task: it needs a lot of heat and there is a strong possibility of damaging the circuit board. Such damage may not be repairable.

If one relay is permanently energised, there will be no relay click as the speed is advanced from zero in that direction but the motor will operate. There will be a relay click in the other direction, but no motor operation.
If one relay is permanently de-energised, there will be a relay click in that direction, but no motor operation. In the other direction there will be no clock but the motor will operate.
  • The most likely fault is a blown relay driver transistor, but these are not so prone to damage by bad handling in the VTX. The old NCC was not surface mounted and was more prone to mechanical damage. The drivers could fail short-circuit, permanently energising the relay, or open, de-energising the relay.
  • Another possibility is that the deceleration ramp has been set too fast. This can cause the motor to be braked by the relay contacts rather than by the MOSFETs. If the relays change state while there is significant motor back emf (i.e. while the motor is still running too fast) there is an arc at the relay contacts and this can weld the relay contacts together. Usually causes the relay to stick in the energised state.Usually such welding is minor (it is called a micro-weld) and a sharp tap with a screwdriver handle, on the cover of the affected relay is sufficient to release the weld.
  • Overloading if sustained and at a high level can overheat either the internals of the relay, or the soldered connections to the board. Eventually the relay will fail.
  • Another possibility, rare, but we have seen it, is that there is a small particle of solder lodged in the armature gap. These relays are hand soldered during manufacture and on rare occasion solder particles can spatter and stick. If they get dislodged, sod’s law says they are bound to get stuck in the worst possible place (i.e. between relay’s armature and pole-piece). If so, the relay will never energise.

Same relay clicks, both directions

This would imply that there is a fault in the reversing switch or the reversing logic on the board.


Half speed reverse will not disengage

Sorry about this – you’ve spotted a manufacturing defect. We don’t make many, but we are only human! The half speed reverse must disengage if the link is properly broken. It follows that removing the header is not breaking the link. So look for a solder bridge across the appropriate pins.

Controller switches itself off as soon as throttle is engaged

With issue 9 (see Issue number history) the parking brake driver is protected against faults in the parking brake wiring.

This protection will cause the controller to switch itself off as soon as the parking brake is released (as the throttle is engaged) if there is a fault.

So unplug the parking brake, disengage it mechanically and, if the machine now functions, the parking brake or its wiring is faulty and the controller is simply reacting to this.


Controller will operate with wheels off the ground but gives no power

Controllers never fade away. When they die, they go usually with a bang and fail completely. Lack of power is always a battery or wiring problem! You need to measure the battery to properly see what is happening. A batter condition meter, if fitted, measures the battery – but is switched on and off by the Pro-120’s power relay so only what it measures before this happens is useful.

The Pro-120 does not have battery discharge protection: this can however be engaged, see the instruction manual. However if the battery voltage falls too low, the Pro-120 will turn power down to avoid the relays disengaging. So with a normal dead or discharged battery the voltage should fall to about 15v but no lower and the controller’s relays should not disengage.

Not all batteries fail gracefully, Sometimes a battery fails (usually a single cell) such that, above a certain current, the battery voltage suddenly falls very low – in effect the cell suddenly switches open-circuit. This is too sudden for the Pro-s undervoltage to work and the power relay will disengage.

Either of these effects could also be caused by bad connections or wiring to the batteries.



If all the wiring is good and the controller is pushed to the limit, it will overheat. Although there is an overheat sensor fitted, there is actually no single point that gets predictably hottest first, so measuring only one ‘hot-spot- is not perfect, but it would not be possible to monitor all possible points that can get hot.

It is quite possible for the controller to get so hot that the solder melts. This can happen well before any electrical failure, but once the solder melts it will run and cause a problem.

If the controller is overheating, it indicates that it is in fact working properly and that the system is simply asking too much of the controller.

Legitimate overheating may show up in several ways:

  • Thermal sensor engages: this reduces the controller current limit to an effectively zero level. In most applications, this will happen first but it depends on the way the controller is mounted, the wiring and the nature of the overloading.
  • Solder around MOSFET leads melts. These MOSFETs are rated to carry 60-75 amps, but the leads are not! The copper on the Pro-120 is double sided and starts off at 2 oz. It is then plated up. All this means a lot of heatsinking abound the MOSFET legs.
  • Solder melted around relay contacts. This may also mean that the relay contacts themselves are heated but the soldering melting does not necessarily indicate this.
  • Main capacitor overheating. The plastic housing may soften and split – this happens first and does not indicate the capacitor has failed. Then the capacitor bulges at the top and or bottom and eventually ruptures. The function of the main capacitor is explained on the 4QD-TEC site and heating here is associated with long battery leads.


Ignition relay won’t stay pulled in

See above under ‘Controller is dead’.As explained in Pro, Scoota and NCC series controllers internal power supply and protection circuitry, the 9v rail’s presence is used to power up the ignition relay.

The ignition relay will fail to power up properly under the following fault conditions:

  • Main capacitors not adequately charged before the ignition switch is closed.
    If you are running anything at all from the normally live connection of the ignition switch (i.e. pin A of the controller), the current this draws can stop the main capacitors charging adequately.If you do not allow enough time between connecting the battery and switching on the ignition – the capacitors will not have charged up. This is particularly so on the 48v controller.
  • 9v rail does not establish.
    In this case, the main capacitors do indeed charge fully and pin A of the 6 pin connector does rise to near the full battery voltage but the voltage falls again as soon as the ignition is closed.This indicates either an internal fault – or are you trying to power something other than a pot (min value 5K) from the pot supply? Pinch more than a couple of milliamps from the 9v line and the controller will detect a fault. The internal current consumption varies with the speed, the battery voltage and other factors such as temperature, so this fault could show up erratically.
  • High Pot Lockout engages.
    The Pro-120 senses whether the pot is at zero resistance when it switches on and the ignition will not latch on if there is significant resistance here. There will however be a click as the relay operates and then releases again immediately.A direction relay will probably click on and off. If a BCM-524 is connected, this will flash on when the ignition is switched on, but will quickly extinguish.Make sure that the throttle control is returning properly to zero. If HPLO is inappropriate to you, then the manual explains how to deactivate it.


Relays drop out

If the battery voltage falls low enough, any relay will drop out. The Pro-120 series┬áhave under-voltage cutback which prevents this. As supplied, the level on the 24v controllers is around 15 volts, enough that the relays will hold. However – cutback is disabled on the 12v controllers.

However – if the voltage dip is causes by something other than the controller, then there is nothing the controller can do to prevent the dip and, if it dips too low, the relays will indeed drop out.

Even if the controller catches itself – such a dip does mean that the batteries and/or wiring are inadequate to give the current required. There’s nothing the controller can do about that!


System lacks power

This probably is not a controller fault. Controllers either work properly – or they fail. For a controller to partially fail is almost unknown.

If you push a Pro-120 hard, one of two things happens:

  • Undervoltage cut back operates.
    If the battery voltage drops too far, the undervoltage system reduces the current available to the motor.

    • Undervoltage cutback is explained in our guided tour of controller features.
    • Undervoltage cutback level was reduced from around 21v to around 15v with issue 9 controllers. See Pro-120 series controllers – Issue number history.
    • A description of how undervoltage circuit operates is available in Pro-120 series controllers – protection circuitry on site.
    • The resistor that controls this cutback level is indicated in Pro-120 series controllers – Key components (item 25).
  • Controller overheats. See section on Overheating

Has the system just been setup or is it a system that worked properly once, and has failed?

  • New system – Lacks Power
    Generally this implies that you are overloading the controller. Either the vehicle’s weight is too high, the reduction gearing too low (so the machine’s top speed is too high) or you are simply expecting too much of it.Best advice is to use our on-site current calculator and work out what you are asking of the machine.If you are really push a controller too hard, it overheats. There is a separate section on overheating.
  • Existing system, has failedAlmost invariably, this indicates trouble in the batteries or the battery wiring.Is there any sign of anything getting hot? See overheating.

    If there is a battery condition meter fitted, does it show that the battery voltage is dropping? The condition meter, to be useful, must show the voltage inside the controller, i.e. it must be fitted to plug into the 6 way connector, as described in the instruction manual.

    If you are measuring with a separate meter, compare the voltage in the hand controls with that across the battery. Any difference between he two must be dropped in the wiring or the connectors.

Trickle resistor overheats

See Pro-120 series controllers – Key components to identify the trickle resistorThis resistor pre-charges the main capacitors as soon as the battery is connected and before the ignition switch is operated. When the power relay closes, this resistor is shorted out, connecting full power to the controller.

If it overheats will ignition off one of the following faults is present:

  • High pot lock out (HPLO) has engaged because the speed pot was not at zero the ignition was switched on. There may still be enough power flowing for the motor to slightly rotate, but the resistor will get hot. The resistor will not get hot unless the speed pot is advanced from its minimum.
  • Motor or its wiring is shorting to battery -ve. Disconnect motor leads to check.
  • Short in control wiring, white wire (pin A of 6 way connector) to battery -ve. Unplug 6 way connector to check.
  • Board has failed: in particular one or other MOSFETs have died.

If the resistor only gets even warm with ignition on it is likely that the ignition has been turned on with with pot not at zero and hplo is engaging.

Other pages relevant to this

  • Pro-120 series controllers – Key components
  • Pro-120 series controllers – Issue number history
  • Switching Waveforms and Servicing
  • VTX and NCC Service Manual The Pro-120 is essentially an enhanced version of this controller.
  • Internal power supply and protection circuitry
  • Instruction manuals are available on-site
  • Earth fuses: why do they blow?