Temp Controller Shows --- or Jumping? 3 Tests to Tell Thermocouple from SSR

Temp Display Shows --- or Jumps?

3 Tests to Tell a Bad Thermocouple from a Burned SSR

Cover: the longitudinal and transverse temp controllers on the panel

You heat up the machine and the display suddenly shows --- - or the numbers climb on their own and never stop. The first instinct is to replace the controller - but a new one often still does not heat. Money spent, problem unsolved.

01  Know the Panel: Two Controllers, Two Circuits

“The panel is on the middle of the right-side cabinet, with one longitudinal and one transverse temperature controller; the controller is continuously adjustable from 0 to 399C.”
— VFFS manual - control panel

First tell which controller is faulty: the longitudinal one runs the back seal, the transverse one runs the end seal. Troubleshoot the faulty circuit - do not mix the two.

Fig.1: the 3-test flow - read the symptom first, then measure with power off

02  Test One: Read the Symptom to Pick a Direction

•  Display shows ---: usually the thermocouple loop is open or loose - the controller gets no signal;

•  Temp runs away and keeps climbing: usually the solid state relay (SSR) is burned through, contacts welded, heating never stops;

✔  The symptom points the way: no signal -> thermocouple; won't stop -> SSR. Do not swap the controller first.

03  Test Two: Watch the SSR Indicator Light

The SSR sits in the cabinet with its own indicator. During heating the light should follow the controller. If the target temp is reached but the light stays on, the relay contacts are welded shut - the direct cause of runaway heating.

•  Light blinks on/off with heating: the control signal is fine; suspect the heater or thermocouple;

•  Light stuck on, or never lights: suspect a failed SSR - replace it per the manual.

04  Test Three: Measure the Heater Resistance (Power Off)

⚠  Cut the main power before measuring. Electric work on control circuits must never be live, and must be done by a qualified electrician.

•  Set the multimeter to ohms and measure the heater: a reading means the element is OK;

•  Infinite (open circuit): the heater burned out - replace it;

•  Near zero: the heater is shorted - replace it too.

Fig.2: the fault row, causes to fixes, one-to-one

05  Two Small Causes People Miss

•  Loose connector terminal: a loose plug gives an intermittent signal and makes numbers jump - re-tighten the terminal;

•  Over or volatile voltage: the machine runs on 220V; abnormal supply can kill the controller - check the supply before swapping it.

Final Thoughts

The order is read symptom, watch the light, then measure with power off. --- means thermocouple; runaway means SSR; only then measure the heater or swap the controller. This way you do not replace every part one by one.

Stuck with a mysterious temp fault?  Send us the symptom (--- vs runaway) and a photo of the SSR light, and we will guide the diagnosis. Reply "manual" to download the VFFS Instruction Manual (PDF).