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PCM Relay – The Power Center of The Car: Here’s What You Need To Know

PCM Relay - The Power Center of The Car

The PCM relay, also known as the powertrain control module relay, comes with a power relay that provides battery voltage to the right PCM circuits. This type of contact relay is activated using a signal wire or the ignition switch. This relay applies to all vehicles that use the OBD-II system, especially those vehicles that are newer than 1996 to present.

Auto Repairs Are EXPENSIVE


Let’s dive a little deeper about what the PCM relay is, the signs and symptoms of a failing PCM relay, what the OBD-II system is, the error code P0690, and the possible fixes of the diagnostic codes that usually show up in relation to the PCM relay and electronic control module. 

What is the PCM Relay?


The functions within a car is controlled by a computer, which even controls the engine itself. This is the case for the engine and the related functions, which are controlled by the engine control module, or the ECM. the ECM monitors and changes the operation and power distribution to both the fuel and injection systems, engine timing, exhaust gas circulation, emissions, ignition, and other systems within your car. In order to put power to each of these systems, the engine  control module needs to be powered as well.


The power distribution center is located under the hood and contains almost every single one of the vehicle's fuses and relays. The ECM power relay, or the PCM relay, acts as a switch that provides power to the ECM. when the ignition key is turned off, the switches within the PCM relay no longer receive battery. If the PCM relay stops switching and working like it should, the vehicle will not be able to function properly. 


In addition, if the PCM relay is stuck open, no power can be driven to the engine control module, preventing the vehicle from turning on and starting. Iof the relay is stuck closed ,then too much power will be given to the electronic control module via the PCM relay and the battery will drain out. 

Symptoms of a failing PCM relay


The electronic control module, which we know is the computer responsible for controlling all of the electronic components within the vehicle, controls almost all electronic functions, like emissions, ignitions, and fuel. It is a very important part, if not the most important piece, of the entire management system. 


The ECM power relay is the part in charge of sending power to the ECM. If the PCM relay fails, then the vehicle will have all sorts of problems and be unable to run, or have dangerous problems with the driver and passengers. Usually, the driver will notice some symptoms that will clue them into issues with the PCM relay – let’s check it out. 



  • No power when you turn the key



One of the first signs and one of the more obvious symptoms of an issue within the PCM power relay system is no production of power when the key is inserted into the ignition, and the relay fails to provide power to the entire vehicle. The failed PCM relay will turn off the dashboard lights and other key safety warning lights that are normally illuminated when you turn the key in the ignition. Furthermore, and more importantly, the vehicle will usually be unable to start and it might be impossible to turn the key. 


  • Engine will not start



Going off of the first symptom of the PCM relay failure, there is a chance that the engine will not be able to start or turn over when you insert the key, and it might get stuck in the ignition. The fuel and the ignition system, along with other diagnostic materials, are generally controlled by the electronic control module, which is powered by the PCM relay.


If the PCM relay fails and cannot power the ECM, the entire engine system that powers the vehicle will not have power and will only be able to crank, but might not even be able to start. 


  • Dead Battery or Battery Draining



Lastly, a failed PCM relay can cause the battery to drain over time due to the switch constantly being turned on, or a dead battery. If they relay short circuits, it can drain the computer battery, even  when the vehicle is turned off. This will drain the battery over time and it will go dead, since power is being used by the computer. 

Causes of PCM Relay malfunction


There are certain causes of the PCM relay malfunctioning, which can be seen by the aforementioned signs and symptoms listed above. There are several possible causes for the code, the error code P0960: ECM/PCM power relay sense circuit high. 


The first cause is that there is a damaged or defective PCM power relay. The second cause is that there is a blown fuse or fusible link. There are certain signs that your car has a blown fuse – this can help you diagnose and pinpoint the cause of the PCM relay. 


Usually, a blown fuse just causes a minor electrical problem, like your blinkers not working or some of your air conditioning not working well, but in some cases, a blown fuse can result in your car not starting. A blown fuse can be due to the fact that a fuse has heated up, melted, and impeded the electricity flow. Sometimes, it can even be due to the driver overloading the system by turning on all of the related accessories. 


The last cause of the PCM relay malfunction is that there is an open circuit between the PCM and power relay. This will result in the code P0685, which is the sign of a defective PCM power supply relay, a low system voltage, damaged or disconnected connectors, including battery terminals, a defective ignition switch, and a failed PCM or control module. 


The best way to fix this code which relates to the open circuit of the PCM relay is to troubleshoot the code P06805. You can do this by recording the fault cdes to diagnose the issue later on. Then, locate the PCM power relay and determine which wire leads to what part of the terminal and the function of each wire. If you do not do this step, it can lead to a misdiagnosis and short circuiting. 


After you have determined the circuits of the PCM relay, perform an inspection of hte wiring, keeping an eye out for disconnected and burnt wiring and connectors. If you do not find any visible damaged or disconnected wires and there are no blown fuses, then you can start to test for voltage in the PCM relay. 


If you do not find any low voltage at both the primary and secondary terminals on the PCM relay, then the cause of the issues is probably a defective ignition switch in the relay. If the ground circuit is open, then retighten the grounding points or determine if the circuit is shorted to battery positive. 


If all voltages and ground circuits are correct, then remove the PCM relay adn test it by paying attention to internal resistance and compare the readings with the ones originally or stated in the manual. 


If by the end of this testing you determine that the voltages and electrical values check out, then the common cause is almost surely the PCM relay programming defect. 

What is the OBD II system?


The OBD II system is the acronym and term for the on-board diagnostic II, the second generation of on-board diagnostic equipment that provides monitoring capabilities. The on-board diagnostics incorporated into the hardware and software of the vehicle’s computer can allow the PCM relay to monitor every component that can affect the electronic and emissions performance of the car.


Each part of the vehicle is checked by the PCM relay and the diagnostic steps to make sure that it is functioning correctly. If a problem or a faulty sensor or malfunction is detected by the PCM relay, the OBD II system will illuminate and show a warning light on the dashboard to alert the driver of the issue in their car.


This warning light on the vehicle instrument panel will usually be the Check Engine Light or the Service Engine Soon light in the car, produced by the power of the PCM relay. Oftentimes, this illumination will also show the engine symbol. The system will also contain important information and data about any detected malfunction so that a repair technician can find the problem without having to run different codes, reduce the labor cost of the fix, and fix the problem easier. 

Why is OBD II needed?


Even though the new vehicles sold on the market today are reputable, there are millions of cars on the road that are not trustworthy and require the OBD II system and the PCM relay to keep them in check and alert the driver or potential problems. 


When the warning light comes on in your car, this lets the driver know that the PCM relay has detected an issue and there is something to fix. Since there are many different issues that can cause this light to turn on in your car, it is hard to generalize what it can be – sometimes it is very severe, and sometimes it might just be that your gas cap is open. However, since it is such a wide range, it is crucial to get this checked out quickly. 


Driving your car with a severe problem can lead to bigger issues in the future, showing how the PCM relay is crucial to diagnostics and fixing your vehicle. Certain malfunctions could even relate to your catalytic converter, which is a very expensive replacement, and needs to be detected early and serviced as soon as possible. 

How To Fix PCM Relay Trouble Code


Depending on the diagnosis ,the common repairs for the P0690 sense circuit high code is either: replacement of the PCM relay, repair of the power wiring in the PCM or electronic control module, replacement of the PCM relay fuses, replacement of the damaged battery cables, or replacement of the PCM relay completely or the electronic control module.


Make sure you clear all codes and test drive the vehicle, re-scanning as you go along to see if any of the same trouble codes reappear. 


Since this code highly affects the driving of the vehicle, this is considered a serious problem and requires immediate attention. It is recommended that this code of the PCM relay issue is addressed as soon as you see it on your screen. 


The most common diagnosis of this code is replacing the entire powertrain control module relay or the electronic control module, although you should test and repair the possible synonyms or culprits first before replacing the entire PCM relay. 

PCM Relay Replacement Cost


The powertrain control module, which powers the PCM relay, is a huge part of the performance of your car. It is in charge of controlling your car’s engine, transmission, and other driveline components. This automotive component combines the engine control unit and the transmission control unit. 


The top symptoms of a bad PCM is that there is poor performance. Meaning there is a non functioning engine and a decrease in fuel economy, shifting becomes problematic, with erratic and unnecessary changing, misfiring, spark plugs misfiring, increased emissions, meaning there will be more smoke and you might fail an emissions test, and warning lights will come on.


To fix this problem, you will need to get a powertrain control module replacement. This is a critical system that you need to take care of, so you can expect the overall PCM cost and the PCM relay price to be very high. 


The labor charges are higher than they would be with a mechanic, so an hour of work will be around $120. The average price for a powertrain control module replacement clocks in at between $500 to $1,500 for a new module, and at least $120 for the labor, since it will take at least one hour of work to install the new part. 


Of course, if you do not want to pay this much money for a replacement, you can get rid of your junk car by removing the powertrain and selling it to a local and reputable junk car buyer in your area. 

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