The Five Questions to Solve a Component Failure

Every problem, every symptom of a device, component or system is diagnosed in essentially the same way. Even if you don’t have specific diagnostic information, you can often diagnose way more than you would imagine simply by following the 5-step process.

1. What does the system do?
2. What does it need to do its job (ingredients)?
3. Where do the ingredients come from?
4. How does it do its job?
5. How does the OBD know if the system is functional?

Admittedly, the last one about OBD only pertains to those components and systems that are connected to the On-Board Diagnostic System. While you may feel like that does mean everything, it’s not quite the case.

What does the part or the system do?

We’ll be looking at two examples today, a sensor for coolant temperature, and a quick release valve. In the case of the senor, the sensor is there to detect a temperature and send the information back to the ODB. Simple enough. The quick release valve is simply there to allow air out of the brake system quickly to park safely and not have to wait.

In both cases the function is pretty straight forward and once we understand the role of each part, we can then look at what the component needs to do its job.

What does it need to do its job (ingredients)?

A sensor without voltage is an inanimate object. You will be able to measure the resistance inside the sensor in ohms but without a voltage input, the OBD system can’t receive that information. So there needs to be a voltage supply, a wire with power, going to the sensor. This would be where you begin, does the sensor receive voltage? Also the voltage coming out of the sensor needs to go somewhere. The OBD system. Here you would check voltage to the sensor, resistance in ohms in the sensor (you can also check if the ohms change with changes in temperature like an oven or fridge), and you want to see the voltage coming out and going to the OBD. You could say that the ingredients re required for the sensor and the OBD.

For the quick release valve, there needs to be a certain amount of air pressure. The air pressure needs to not go below a certain level or, if it works well, the pressure will be completely released from the air system. You first want to verify the valve is getting air pressure at all, then how much, then are there leaks, then is pressure dropped enough for the valve to finally release the remaining pressure. You would need to use a pressure gauge to guess/reason.

Where do the ingredients come from?

Ultimately the voltage for the sensor comes from the battery but also the key needs to be turned to on, and the OBD is protected by fuses and relays. While it will be rare that you have to go so far back down the stream of components, it is possible. Today there can even be software issues preventing the proper sending of voltage to the sensor. One you’ve checked one component, keep going “downstream” to find the issue. You can also go “upstream” by starting at the batteries.

Air systems ultimately start with a compressor. The compressor is reliant on the engine. The compressor fills the air tanks that supply the air systems and its valves. In a case like this you will be able to check rather easily if there is air in the system at all. If you know there is air in the system, check if the air is making its way to the valve, there could be a damaged hose or other component blocking the air. As you gain experience you will better be able to figure if you should start at the valve, or the compressor. You’ll learn quickly that a quick check of a pressure gauge in the dash of the truck or the shop air supply will prevent you from the very long task of checking an air compressor.

How does it do its job?

In both cases, how the component does its job is very similar to other questions we’ve asked and answered. Specifically, we want to know how does the component or system do its job when everything is working well? How does it behave, and what are the final results of its work? What should the sensor be giving as a reading at this temperature? How quickly, or when does the quick release valve operate?

How does the OBD know if the system is functional?

For the OBD system it may seem simple, how much voltage is getting back to the OBD, or the ECU? Ultimately the ECU is a voltmeter. All I can do is send out a certain amount of voltage, then interpret what it gets back. Every sensor has a range of acceptable voltage. Inside that range you get a reading either on the dash or it is only used for operation. Outside of that range and something else happens. It could be a lamp in your dash and it could be a reaction like a de-rate or even a shutdown. That said, every sensor and component will receive and send back different information. It’s hard to know what a reading should be without published diagnostic information but if you can compare two sensors from different vehicles, this can help. Also by changing the environment of the sensor or component and seeing how the resistance changes.

Conclusion

Diagnosing problems on vehicles is, in a sense, getting harder. Mostly because there’s so much more on a given vehicle today than ever in the past. Also, more and more components and systems are interdependent. That said, there is still a lot that you can do by working diligently through these five questions and gaining experience.