Figure 1. The area of interest in the speed controller.
Figure 1 shows that the speed control is a 5 kΩ (5000 Ω, not 5000 kΩ) and that it's output will increase from 0 to 12 V as it is adjusted from L (low?) to H (high?) speed.
... if I move the pot about a quarter of the way around; the motor is already at the maximum speed I want.
It sound as though your drive is at full speed at 3 V out (25%).
To increase the useful span of the pot we need to have 3 V out at full rotation.
simulate this circuit – Schematic created using CircuitLab
Figure 2. Modified speed controller. Options A and B.
It may be as simple as adding 15 kΩ in series with the top of the pot as shown in Figure 2, Option A. This will result in 3 V at the top of the pot.
Alternatively put in a fixed 10 or 12k resistor and a 5 or 10k preset to allow you to tweak the max more closely to suit as shown in Option B.
You might see some non-linearity as the pot is loaded by R21, etc. If that's a problem we may need to scale all the resistance values down.
Just a caution: it might be worth checking what would happen should the pot go open circuit. Here's how to do it safely:
- Disconnect the motor.
- Measure the voltage on the W wire with the pot at mid-point.
- Unsolder the wire from the pot wiper and measure the voltage on the wire (not on the pot).
- If the voltage falls to zero then the motor should stop if the wire falls off.
- If the voltage rises past 3 V (your maximum) then you need to consider if you need to put a pull-down resistor on W. You could try 100k or more and see if that's enough to bring it back down to a safe value.
- If that doesn't work you may need to monitor the motor itself and put in independent speed monitoring and safety cut-out.