The WP20 and WP25 don’t use numbered fault codes. They report in words, and almost everything they complain about sits outside the box: a load cell signal, a speed signal that isn’t arriving, the link to the electronics at the frame, or a belt and frame that have changed since the last calibration. Copy the message word for word, fix the cause, then clear it.
This guide covers the messages we see most, what a calibration involves and why the zero moves. For what a service visit covers, see Tecweigh belt weigher and WP-series servicing. If you’re weighing up a WP50, see what changes on a WP50. We’re an independent service company, not a Tecweigh dealer or agent.
What are the WP20 and WP25, and where does the WY15 fit?
They’re Tecweigh’s weigh processors: they read the load from the weigh frame, count pulses from the speed sensor and turn the pair into a rate in t/h and a running total. The WP50 replaced both, but plenty of WP20s and WP25s are still working on Australian mine and quarry conveyors.
On the WP20, the load and speed signals are conditioned by a card out at the scale and sent back over a serial link, so a processor complaining about the scale may really be complaining about that link. The WP25 can be wired either way. Underneath is usually a WY15, a single idler belt scale that carries its own calibration weight. Older WY frames measure by displacement rather than with strain gauge load cells, so check the alignment before you replace anything.
What do the common WP20 and WP25 fault messages mean?
Write the message down exactly as it appears, with what happened just before it. The words are the whole diagnosis.
| Message | What we check first | When to call us |
|---|---|---|
| Auto zero error or auto span error | Weight in the wrong position, material on the belt or under the weigh idler, skirting rubbing, or the belt stopped part way. Clear it, run a fresh zero, then the span. See why a Tecweigh won’t finish its zero or span. | If it errors on a clean, empty belt with the weight correctly placed. |
| Load error | Water in the junction box, cable damage, loose terminals. Then watch the load reading rise as the calibration weight comes down. See load error. | If the reading doesn’t rise with the weight, or the error survives drying out the junction box. |
| Zero belt speed | Is the belt really stopped? If not, check the wheel turns freely, then cable, terminals and supply, then that the board switches suit the sensor. See zero belt speed. | If the sensor turns and the wiring checks out. With no speed the scale totalises nothing. |
| Interlock open | Nothing is broken. An input set as an interlock has opened, so the rate, the totalising and every output are disabled. Find which input and why. | If the plant signal is there and it won’t clear. Never bypass the interlock. |
| Low power | The supply reaching the electronics at the frame: a long or damaged cable run, or a poor connection. | If it survives a wiring check. |
| Scale data error | Comms to the electronics at the frame: a damaged cable, a reversed pair or a termination link in the wrong place. See scale data error. | If it survives a wiring check. With low power, it’s what a lightning strike looks like. |
| Self test at power up | A pass means the settings survived. If a WP20 reports it rebuilt them from its backup or from factory defaults, your calibration may be gone. See calibration lost after a restart. | Straight away, before anyone recalibrates over it. |
Resetting a message over and over hides the evidence without fixing anything. If the same one keeps coming back, the condition never went away.
What does a calibration on a WP20 or WP25 involve?
Work on or near a conveyor, or inside an electrical enclosure, needs isolation under your site’s procedures and trained, authorised people. If that’s not you, stop and call us on 1300 101 666.
Check the stored settings first: a calibration on top of a wrong weigh span or belt length is a tidy way of being wrong. Then:
- Record the as-found figures before anyone touches anything.
- Warm up. Run the belt empty until the belt and frame settle. The WP25 documentation asks for at least half an hour, the WP20’s for about an hour. Give it the hour on a cold morning.
- Belt speed. Match the display to a timed run over a marked length. More than about 2% out needs investigating. After a new belt or splice, re-measure speed and length and re-enter them.
- Zero on a clean, empty belt over whole belt revolutions.
- Span against a known load: the frame’s calibration weight, a chain, or on the WP25 an electronic span from the load cell ratings and conveyor incline (which method suits which scale). The WP25 shows the change and asks you to accept or reject it. The WP20 doesn’t ask.
- Material test: a known tonnage at the normal rate, weighed on a weighbridge. It’s the only check that covers the whole system.
- Save it (WP20). Writing the settings to permanent memory is a separate step after any change, a fresh zero included. Skip it and the backup is still the old one.
The numbers we work to: repeat span runs within about 0.1%, and a routine span that has moved more than about 0.25% needs attention. We also record the running rate with the calibration weight applied, so the next visit can confirm in minutes that the calibration has held.
Who adjusts what. On our service visits we adjust the zero and span when the checks show it’s needed. Between visits, site should run a zero about every two weeks and after a big change in the weather, and check the span but record it rather than accepting it. We recommend a service calibration every quarter so the scale keeps up with the seasons. See what happens during a calibration visit.
Why does the zero or span move on a Tecweigh scale?
Because the belt and frame changed, not because the processor drifted. Zero moves with build-up, material lodged under a weigh idler, belt tracking, tension, a seized roller or wind. After a proper clean, a one-off change of around 1% is normal. A zero that keeps walking needs investigating, not re-zeroing. A new belt beds in over a week or two, so check the zero after the belt work and again once it has settled. See why a belt weigher zero drifts.
Two settings hide that movement without raising a message. Automatic zero tracking re-zeroes the scale when the rate sits below a threshold, so build-up gets absorbed and the scale under-reads. The dead band stops anything totalising below a set rate, which loses light feeds and the tail of every run. Both leave the factory switched off, so if either is set, someone set it. See why a Tecweigh under-reads at low rate.
Span moves for bigger changes: a frame knocked by a loader, a replaced load cell, or a new belt of a different thickness. See what zero, span and repeatability mean.
What speed sensor and load cell points matter?
Speed. Both processors carry hardware switches that must match the sensor fitted, and the electronics will run happily on the wrong signal. The usual sensor is a wheel on a freely swinging arm, close to but not directly over a return idler. A wheel that won’t spin, or an arm that’s seized or tied back, reads low or not at all. A simulated speed setting left on after fault-finding counts on a stopped belt. Where the old sensor is worn out, our USR speed sensor mounts on the tail drum or a trailing arm, and we’ll confirm it suits the input first. See our speed sensors guide.
Load cells. The cells run at a low excitation voltage, so a typical cell at capacity gives only around 10 mV. A resistance check across the bridge condemns a cell more than about 10% off its expected figure, and a dynamic check confirms the signal rises as the calibration weight comes down. See our load cells guide.
Alignment. The weigh idler and the three idlers either side are aligned as one system, typically to within about 0.3 mm; the exact target depends on the scale. Disturb one without realigning and the scale is out. Never weld on or near a scale.
How does a WP20 or WP25 talk to the plant?
The rate leaves as a 4–20 mA loop that isn’t isolated on either generation, so a loop feeding more than one device needs an external isolator. An output that works into a test meter but fails in the plant is nearly always the loop load, and the WP25 drives a lower maximum loop load than the WP20. Relays give a totaliser pulse and high and low rate and speed alarms.
The WP25 has a single RS485 port that talks either to the plant or to the frame electronics, never both, which catches out anyone adding a PLC link later. An optional card adds Ethernet or Modbus. A framing error means the two ends of the link are set up differently. Outputs are disabled during a calibration cycle and while the interlock is open, so plant logic will alarm during a service. See getting belt weigher data into your PLC or SCADA.
Need a manual for your equipment?
Power the unit up and note the version it reports, then photograph the processor label and the WY frame plate. The WP20, WP25 and WP50 each have their own manual, and WP50 material doesn’t describe the older units.
Talk to us about manuals for your equipment. Send the make, model, serial number and software version (a photo of the label is fine) and we’ll help you find the right document.
When should you call us rather than keep resetting it?
Call when the same message comes back after the obvious cause has been dealt with, or when clearing it would overwrite a calibration you can’t get back, such as a unit back from a power event on default settings. Totals that no longer agree with the weighbridge belong on the list even with a clean screen.
We keep the settings of every scale we look after in AIMM, our asset management system, so a unit that loses its configuration is a reload, not a recommission. We’ll tell you straight whether to repair it or replace it. Call 1300 101 666 or email admin@accurateindustries.com.au. See booking a belt weigher service for what to send.
Frequently asked questions
Can a Tecweigh belt scale be calibrated without test weights?
Yes. Both accept a calibration chain, and the WP25 can work out a span from the load cell ratings and the conveyor incline. Only a physical load puts real force through the frame, and only a material test checks the whole system.
We recalibrated a WP20 and it came back with the old numbers. Why?
The new settings weren’t saved to permanent memory. The scale runs on them, but the backup is the old copy, and if the memory is disturbed by a flat battery or an electrical spike, the unit restores that copy.
Do I have to replace my WP20 or WP25?
Not because of its age. If it calibrates, holds its zero and agrees with a material check, it’s doing its job. See what changes on a WP50 before anyone commits to a change-out.