Error Code P0704 is defined as Clutch Switch Input Circuit Malfunction. This is a generic trouble code, meaning it applies to all vehicles equipped with the OBD-II system, especially those made since 1996 up to present. This includes vehicle models from but not limited to Ford, Honda, Mazda, Mercedes Benz, Volkswagen, etc. Specifications on the definition, troubleshooting, and repairs, of course, vary from one make and model, and powertrain configuration.
When Error Code P0704 is stored, this means the PCM (powertrain control module, also known as ECM or engine control module in other vehicle makes) has determined a malfunction in the clutch switch input circuit. This code is only applicable for vehicles with manual transmission.
The PCM electronically monitors certain functions of a manual vehicle. Among these functions are the shifter position and clutch pedal position. For some models, this includes the turbine input and output speed, which determines the degree of clutch slippage.
The clutch is a mechanical coupling that links the engine to the transmission. In most cases, it’s operated using a rod (with food pedal on the end) which pushes the plunger of a firewall mounted hydraulic clutch master cylinder. When depressed, the clutch master cylinder will cause the hydraulic fluid to push into the slave cylinder (which is monitored by the transmission). The slave cylinder then actuates the clutch pressure plate, allowing the engine to engage and disengage from the transmission as necessary. Some models use a cable operated clutch, but this has become a lesser common system. Depressing the pedal with the left foot will disengage the transmission from the engine. On the other hand, releasing the pedal will allow the clutch to engage the engine flywheel, propelling the vehicle to the desired direction.
A clutch’s primary function is to act as a safety feature so the engine will not start with the transmission unintentionally engaged. The clutch switch is designed to interrupt the starter signal (from the ignition switch) so that the starter will not be activated unless the clutch pedal is depressed. The PCM and its other controllers use input from the clutch switch for various calculations of engine control, automated braking functions, and hill-hold and stop-start implementation.
Error Code P0704 refers to input circuit of the clutch switch. Thus, it’s important to consult with the vehicle service manual (or similar) for the exact location of the component, and other specific information about the specific circuit as it relates to the vehicle.
This code can cause multiple interruptions that affect the drivability of the vehicle, safety, and traction functions. Some of the most common symptoms include:
- No start condition or intermittent engine
- Excessive engine RPM at idle
- Disabled traction control system
- Increase in fuel consumption
- Some vehicle models may experience safety functions
There are many potential causes for this code, such as:
- Faulty clutch switch
- Worn clutch pedal lever
- Bushing clutch lever
- Blown fuse
- Burnt fusible link
- Shorted or open wiring and/or connectors in the clutch switch circuit
- Defective or programming error in PCM
How to Check
As with most codes, checking with a TSB (technical service bulletin) is a good starting point to diagnose this code. Other things you need include the basics, such as a scanner, DVOM (digital volt/ohmmeter), transmission gauge, and vehicle service manual (or equivalent).
Start the diagnosis by visually inspecting the clutch switch circuit wiring. Test all system fuses and replace any blown fuse. Next, load test the battery, then check the battery cables. Check the alternator too.
Find the diagnostic connector, connect your scanner and retrieve all stored codes, including their freeze frame data. Take note of this information. Then, clear the codes and then take the vehicle for a test drive to see if the code comes back immediately.
If the code comes back, then check the battery voltage at the input circuit of the clutch switch using the DVOM. Some vehicles come with multiple clutch switch to accommodate multiple functions. Thus, make sure to consult with the service manual to determine how the clutch switch is laid out.
If the input circuit has battery voltage, then depress the clutch pedal and check for battery voltage on the output circuit. If the output circuit doesn’t show any voltage, then you can suspect either a misadjusted or defective clutch switch. For this, make sure the clutch pivot lever and pedal arm are in good condition. Also, check for play in the clutch pedal bushing.
If both sides of the clutch switch has voltage (when the pedal is depressed), then check the clutch switch input circuit at the PCM. This can be a battery voltage signal or reference voltage signal, consult with the manufacturer’s specifications for the vehicle. If there is an input signal at the PCM, then you can suspect a defective or programming error in the PCM.
If there’s no clutch switch input signal at the PCM connector, then you can disconnect all related controllers and then use the DVOM to test the resistance for all system circuits. Repair or replace shorted or open circuits (between the clutch switch and PCM) as necessary.
How to Fix
Depending on the diagnosis, common repairs for this code include:
- Replacement of damaged clutch switch
- Properly re-adjusting the clutch switch
- Repair of an open or shorted circuit, ensure a properly fitted clutch switch
Since this code will cause the vehicle to experience hard or not starting, the most common misdiagnosis when dealing with this code is in the starter. Many people jump on replacing the starter, which will not solve the problem.
If the vehicle is hard to start, then this can be extremely dangerous, especially if the vehicle stalls in traffic.
Many times, worn clutch pivot arm or clutch pedal bushing may be misdiagnosed as a defective clutch switch.
When checking the system fuse, make sure the pedal is depressed. Some fuses may seem normal at first test, then fail when the circuit is placed under load.