What would you normally have to do for hot runner maintenance? You’d look for dead shorts caused by pinched wires, open circuits, and you would want to measure the resistance of the heaters to ensure they are in a healthy range.
Let’s start by measuring the resistance of the heaters. Put your meter in ohms, make sure the test leads are good and then go to each one of the heater circuits (you have to know the wiring diagram), touching each pin for each circuit.
Measuring resistance will not tell you if you have a direct short. The only way to do that is to put one lead on the body of the hot half and then touch each of the pins with the other lead to ensure no continuity. Tip: You may see a reading on one of the zones but pay close attention to the acronyms on your meter. If there is an M, it’s displaying mega ohms. If that is the case, it is only reading the mineral inside the heater.
Now, let’s measure the resistance of the thermocouples (t/c) to verify we do not have an open. Again, knowing the wiring diagram, you would put the leads into the corresponding circuits to ensure you have a contact. Note, most hot runner systems use grounded thermocouples, so we won’t look for shorts (they are a short by definition).
So, why would you want to measure the resistance of the heaters? You’re looking for resistance values 20% more than the norm or higher. This is an indication of corrosion and fatigue which means the heater will likely fail soon. You would want to change out that heater now while it’s not in production.