Is a screened (shielded) cable mandatory on the potentiometer wiring, and where is the shield connected?
It is not mandatory on short and clean routings, but if the cable is long or runs alongside supply, ignition and relay harnesses, a screened cable is recommended. On the GS740-3P the shield is connected to the green lead, that is to the potentiometer GND. The shield must be connected at one end only; it must not be earthed at both ends, creating a ground loop.
The potentiometer signal is a high-impedance analogue voltage; noise picked up on it is reflected directly in the throttle position and leads to jitter in the speed.
When to use a screened cable:
- If the distance between the cab and the engine is long.
- If the cable runs in the same harness as the supply, starter, alternator or ignition cables.
- If there are switching loads nearby such as relays, contactors or inverters.
Shield connection: the screen is connected to the green lead, that is to the potentiometer GND. This is the connection method stated explicitly in the product data. Earthing the screen at the far end as well creates a ground loop and most of the time makes matters worse.
Additional measures: run the signal cable separately from the power cables, cross them at right angles if they have to cross, and solder and insulate the ends securely. The chassis itself matters too: oxidised or loose chassis points lead to reference drift and random speed changes.
If the jitter in the speed persists despite these measures, first check the potentiometer and the wiper contact; a worn potentiometer behaves like a source of noise.
Full article: Electronic speed and throttle control on a diesel engine