Mercedes P0718 – Input Speed Sensor
Mercedes P0718 – Input Speed Sensor Intermittent Explained. Mercedes P0718 – What Does It Mean?
If your Mercedes-Benz has stored fault code P0718 – Input/Turbine Speed Sensor “A” Circuit Intermittent, it does not necessarily mean the transmission has failed or that the input speed sensor has completely stopped working.
Unlike P0717, where the Transmission Control Module (TCM) can no longer detect a valid input speed signal, P0718 tells us something different.
The signal is present—but only some of the time.
At one moment the TCM receives a normal input speed signal. A fraction of a second later, that signal may disappear, become erratic or return an unrealistic value before recovering again.
From the driver’s perspective, the transmission may appear to fix itself.
From the TCM’s perspective, however, the input speed signal cannot be trusted.
At Brisbane Tuning & Turbo, we regularly diagnose Mercedes 7G-Tronic (722.9) transmissions displaying intermittent speed sensor faults. The challenge is not simply identifying the fault code. The challenge is determining why the signal becomes intermittent.
Why the Input Speed Sensor Is So Important
The input speed sensor measures the rotational speed entering the transmission after power passes through the torque converter.
The TCM continuously compares this value with:
- Engine speed
- Intermediate transmission speed
- Output shaft speed
- Selected gear
- Expected gear ratio
Using this information, the transmission can determine:
- Whether a shift has completed correctly
- Whether a clutch is slipping
- Whether the torque converter is operating normally
- Whether the gear ratio matches the selected gear
The input speed sensor therefore forms part of the transmission’s continuous self-checking system.
Without reliable speed information, the TCM can no longer accurately control shift timing or verify transmission operation.
What Does “Intermittent” Actually Mean?
The word intermittent is the key to understanding P0718.
Unlike P0717, where the signal is completely missing, P0718 usually indicates that the signal appears and disappears unpredictably.
For example, the transmission may operate perfectly during one journey.
The following morning it may enter limp mode.
After restarting the engine, everything appears normal again.
Twenty minutes later the fault returns.
These changing symptoms are often frustrating because the transmission appears unreliable rather than completely failed.
From the workshop’s perspective, this behaviour usually indicates that the signal is being interrupted rather than permanently lost.
Why Mercedes Stores P0718
P0718 is stored when the TCM detects interruptions in the input speed signal.
The interruption may last only milliseconds.
However, because the TCM depends on continuous speed information to control clutch application and monitor gear ratios, even brief signal interruptions can trigger a fault.
The TCM does not know whether the interruption is being caused by:
- A failing conductor plate
- Damaged wiring
- A poor electrical connection
- Low system voltage
- Electronic failure
- Connector contamination
Its job is simply to recognise that the input speed signal is no longer stable.
Determining the cause is the purpose of proper diagnosis.
The Most Common Cause – Mercedes 722.9 Conductor Plate Failure
On the Mercedes 722.9 transmission, the input speed sensor is integrated into the conductor plate.
As the conductor plate ages, repeated heating and cooling cycles, vibration and long-term exposure to transmission fluid gradually stress the internal electronics.
Unlike a complete electronic failure, ageing components often fail intermittently.
Initially the vehicle may drive perfectly for days.
The driver may experience one unexpected harsh shift before everything returns to normal.
Weeks later the transmission enters limp mode.
Another restart restores normal operation.
This progression is typical of many electronic failures.
Eventually the intermittent fault often develops into a permanent fault, at which point P0717 or additional speed sensor fault codes may also appear.
For this reason, P0718 is often an early warning that the conductor plate is beginning to deteriorate.
Other Possible Causes of Mercedes P0718
Although conductor plate failure is common, experienced transmission specialists avoid assuming it is always responsible.
Several other faults can interrupt the input speed signal.
Damaged wiring between the transmission and the vehicle may momentarily interrupt communication as the engine and transmission move during acceleration.
Poor electrical connections inside the transmission connector may also create intermittent signal loss.
Corrosion, fluid contamination or connector damage can increase electrical resistance and produce unpredictable faults.
Low battery voltage or charging system problems may affect transmission electronics, particularly during engine starting or heavy electrical load.
Previous transmission repairs should also be considered.
A wiring harness that has not been correctly secured or a connector that is not fully seated may only lose contact under vibration.
Because intermittent faults can have many different causes, replacing components based solely on the fault code may not resolve the problem.
Common Symptoms of Mercedes P0718
Vehicles affected by P0718 often display inconsistent behaviour.
Common symptoms include:
- Intermittent limp mode
- Harsh shifting that later disappears
- Delayed gear engagement
- Unexpected shift quality changes
- Check Engine Light
- Transmission warning message
- Vehicle operating normally after restarting the engine
- Fault occurring only after the transmission reaches operating temperature
- Fault appearing only under certain driving conditions
One of the most useful pieces of information a customer can provide is when the fault occurs.
For example:
Does it happen only when cold?
Only after driving for thirty minutes?
Only while towing?
Only during motorway driving?
Those details often help narrow the diagnosis considerably.
How We Diagnose Mercedes P0718
Intermittent faults require a different diagnostic approach from permanent faults.
If the vehicle arrives at the workshop operating normally, simply reading stored fault codes rarely identifies the underlying cause.
At Brisbane Tuning & Turbo, we begin by performing a complete transmission scan and reviewing all stored and historical fault codes.
We then analyse live transmission data.
Instead of asking whether the input speed sensor works, we ask whether the signal remains stable throughout the entire drive.
We monitor:
- Engine speed
- Transmission input speed
- Intermediate speed
- Output speed
- Selected gear
- Actual gear ratio
- Transmission temperature
If the input speed suddenly drops to zero while the transmission continues operating normally, our attention immediately turns towards the conductor plate, wiring and electrical integrity.
If the interruption only occurs when the transmission reaches operating temperature, heat-related electronic failure becomes much more likely.
We also inspect battery voltage, charging performance, transmission wiring and electrical connectors.
Only after analysing all of the available evidence do we determine whether the fault is electronic, electrical or mechanical.
A fault code provides a direction.
It does not identify the failed component.
Mercedes P0718 Repair Options
Once the underlying cause has been identified, the repair becomes considerably more straightforward.
Damaged wiring or poor electrical connections may require repair or replacement.
If diagnosis confirms internal conductor plate failure, replacing the conductor plate together with the appropriate programming and adaptation procedures is generally the correct repair.
Where additional valve body or transmission faults are identified, those issues should also be addressed before returning the vehicle to service.
Replacing the wrong component may temporarily appear to solve the problem, only for the intermittent fault to return weeks later.
Proper diagnosis avoids unnecessary parts replacement.
Can You Continue Driving with Mercedes P0718?
Many vehicles with P0718 remain driveable.
However, intermittent faults tend to become more frequent rather than disappearing permanently.
Because the TCM cannot rely on an unstable input speed signal, shift quality may deteriorate and limp mode may become increasingly common.
Early diagnosis usually reduces both repair time and repair cost by identifying the problem before additional transmission faults develop.
Frequently Asked Questions
Does P0718 always mean the conductor plate has failed?
No.
Although conductor plate failure is one of the most common causes on the Mercedes 722.9 transmission, wiring faults, poor electrical connections and voltage problems may also interrupt the input speed signal.
What is the difference between P0717 and P0718?
P0717 indicates that the TCM has lost the input speed signal.
P0718 indicates that the signal is intermittent—it appears and disappears or becomes unstable during operation.
Can heat cause P0718?
Yes.
Many intermittent electronic faults become more common as transmission temperature increases.
Heat-related conductor plate failures are well recognised on higher-mileage transmissions.
Will clearing the fault code repair the transmission?
No.
Clearing the code only removes the stored fault information.
If the intermittent fault remains, the code will return once the TCM detects another interruption.
Does P0718 mean the transmission requires rebuilding?
Not necessarily.
Many vehicles require conductor plate replacement or electrical repairs rather than a complete transmission rebuild.
Professional diagnosis determines which repair is actually required.
Mercedes P0718 Diagnosis and Repair
Mercedes fault code P0718 does not automatically indicate a failed transmission.
It tells us that the Transmission Control Module has detected an input speed signal that is no longer stable enough to rely upon.
The interruption may be caused by conductor plate failure, damaged wiring, poor electrical connections, voltage supply issues or other transmission-related faults.
Because intermittent faults can be difficult to reproduce, replacing parts based on assumptions often becomes expensive.
A structured diagnosis identifies exactly why the input speed signal is becoming unstable before repairs begin.
At Brisbane Tuning & Turbo, we don’t replace transmissions based on assumptions. We diagnose the cause first, explain our findings, and then recommend the most appropriate repair.
If your Mercedes has stored fault code P0718, intermittently enters limp mode or is displaying unpredictable transmission behaviour, contact Brisbane Tuning & Turbo to book a professional transmission diagnostic. We’ll identify the underlying cause and provide clear, evidence-based repair recommendations.
