CVT Common Fault Signs: Describing Symptoms Accurately
Symptoms in plain words, so a buyer can describe them accurately to a technician or supplier. This guide does not diagnose.
Design diagram only, not a product photograph.
Why a symptom guide must not diagnose
This guide describes the common fault signs associated with a CVT, in the vocabulary of the company materials, and it refuses, deliberately, to turn any sign into a diagnosis. The reason is not caution for its own sake. It is that a symptom and a diagnosis are different kinds of statement, and confusing them harms the reader. A symptom is what is experienced: slipping, a change in ratio behaviour, a leak, a noise. A diagnosis is what is wrong: which part has failed, and why. A guide that has never seen the vehicle cannot make the second statement, and a guide that pretends to can send its reader to order the wrong part with confidence.
The guide's purpose is narrower and more useful: to give the reader the words to describe a symptom accurately, so the technician who does see the vehicle, and the supplier who will be asked for a part, receive accurate information instead of a guessed diagnosis. Accurate description is not a lesser service than diagnosis. It is the precondition for diagnosis, and a reader who describes well will be diagnosed well, while a reader who arrives with a confident wrong diagnosis will be harder to help.
The symptom vocabulary in the materials
The company materials provide a fault and quality vocabulary, and the CVT-relevant signs in it are: slipping, for clutch and pulley slipping; internal leakage and pressure loss, for the pulley cylinder in process step 6; oil leakage, in the general sense of sealing failure, which the process documents address through sealing-surface preparation and sealing checks; and, in the finished-unit testing description, abnormal noise and hydraulic condition as things to notice. Those are the signs this guide uses, because they are the signs the materials support. No sign is added from general knowledge, and no sign is given a vehicle-specific form, because the materials give none.
Each sign is described below in plain words, with the candidate rows it can point to, and with the description questions that make it useful. Candidate rows are not diagnoses. They are the list a technician should check, in turn, before a row is ordered.
Slipping
Slipping is the loss of clean grip between belt and pulleys, experienced as a loss of drive, a flare in engine speed without matching road speed, or a hesitation in ratio change. The slipping guide on this site explains it in full, and this guide summarises it: the candidates are the belt, the pulleys, the cylinder pressure and the control side, step motor, valve body and solenoid. The description questions are: when does it occur, at low speed, high load, cold, hot, during ratio change or always; is it constant or intermittent; has it worsened gradually or appeared suddenly? A slipping report that answers those questions is a useful report. A report that says only slipping, without them, leaves the technician to guess where to start.
Pressure loss and internal leakage
Pressure loss is discussed in this guide alongside ratio behaviour: a transmission that does not hold the ratio it should, or does not change ratio as it should, is a behaviour to describe to a technician, who can check whether pressure is involved. Internal leakage is the cylinder-side cause that the process documents name: pressure escaping inside the cylinder, so the pressure that should move or hold a pulley is lost without any visible external leak. The two are related as cause and result, and a symptom report should keep them separate: report the behaviour, ratio not held or not changed correctly, and let the pressure test determine whether internal leakage is the cause. The pressure-test guide explains that test, and a buyer who has read it will ask for the test by name rather than accepting a general testing claim.
Pressure loss can also come from the control side, where pressure is not directed correctly to the cylinder, and from the supply side, which this site does not describe as a CVT row. A symptom report that says pressure loss therefore points to a chain of candidates, cylinder, control and supply, and the investigation must check them in turn. The report's job is to describe the behaviour accurately, not to pick the link.
Oil leakage
Oil leakage is visible, unlike internal leakage, and it is therefore tempting to treat it as self-diagnosing. It is not. Visible oil tells the reader that sealing has failed somewhere, and the process documents address sealing through housing refurbishment, sealing-surface preparation and sealing checks in bench testing and finished-unit testing. But visible oil does not say where the seal has failed, whether the leak is the cause of a drive symptom or a separate finding, or which row should be ordered. A leak report should say where the oil is seen, how much, whether it is fresh or old, and whether any drive symptom accompanies it. Those details let a technician find the seal, rather than replacing a row on the basis of oil alone.
Noise, vibration and hydraulic condition
Abnormal noise and vibration are listed in the finished-unit testing description as things to notice, and hydraulic condition is listed alongside them. As symptoms, they are the least specific signs in this guide, and the guide says so. A noise can come from the belt, the pulleys, the bearings or elsewhere in the vehicle, and a guide cannot attribute it from a description alone. A vibration can come from the transmission or from outside it. Hydraulic condition, as a symptom, is usually reported indirectly, through the drive symptoms above, rather than observed directly by a driver.
The useful description questions for noise and vibration are: when does it occur, under what load and speed, does it change with ratio change, and has it appeared with any other symptom? A noise report that answers those questions gives a technician a starting point. A noise report that only says noisy does not, and a guide that attributed a noise to a specific CVT row from such a report would be guessing.
How to write a symptom report
Write the report as a list of observations, not as a conclusion. State each symptom in plain words, when it occurs, how it has changed, and what accompanies it. State what has already been checked, if anything, and what has not. State the part rows already fitted or replaced, if any, because a symptom that persists after a replacement points back to the other candidates. Send that report to the technician first, and to the supplier only with the technician's findings, because a supplier asked to supply from a symptom alone must guess the row, and a guess is how wrong parts are delivered.
The repair-shop page on this site shows that sequence as a procedure: symptom to questions to checking to row to written inquiry. The selection guide shows the row-first decision that follows checking. The inquiry checklist shows every question the final inquiry should contain. This guide is the first step in that sequence, and it is complete when the reader can describe a symptom without also claiming to know its cause.
One last caution. A symptom guide is sometimes read as a list of things to worry about, and a reader who has just noticed a noise may leave this guide convinced a major fault is present. That is not the guide's intent. Symptoms vary in severity, only a technician with the vehicle can judge severity. The guide's intent is the opposite of worry: it is to replace worry, and guessed diagnosis, with accurate description, which is the fastest route to a correct answer.