Car electrical systems – Service info and wiring diagrams

Whilst simple problems in the electrical installation can be solved by using common sense and/or experience, any tech will eventually arrive at the point where more is needed. That ‘more‘ is documentation on how the system is wired up, how components work, and how a trouble code is set. In short: wiring diagrams, service data, and technical service bulletins.

With the complexity of modern cars, nowadays, all of this data is online. You just log into your dealer account, type the VIN, and get access to OEM wiring diagrams, service data, and TSBs. Additionally, you can subscribe to services that will provide you with extra TSBs, even for the ones that are too old for the car manufacturer to bother, but are still present on the roads and that visit your workshop.

Unfortunately, and opposite to the US, the EU doesn’t have a Right To Repair legislation. This would allow consumers and any workshop to subscribe to the manufacturer’s data, such as wiring diagrams, service info, and TSBs. So if you work at a dealership, you’ll have no problem getting the data required to do the job. If you’re not working at a dealership, your chances are slimmer. But for some brands and models, you can obtain that info online on a pay-per-view basis.

Wiring diagrams:

While the wiring diagrams for my daily driver, a 1979 Cittrun AcaDiane, does fit on a single A4 sheet of paper, the diagram for our other car, a 2025 Toyota RAV4, definitely does not. Providing the diagram for all versions and with all extra equipment would more likely take a book.

With the complexity of modern wiring diagrams, using one will involve a lot of scrolling the screen and looking for reference numbers. A typical diagram for Volvo may look like this:

With the implementation of these diagrams, a legend may prove very useful indeed:

…showing a connector and module location diagram, a legend of used symbols, and a map showing where to find the wiring harness and all that’s connected to it.

Service data

Service data describes how things work in the car, as well as the basics for torque and adjustment values. The service data assumes you know a lot about the basics already and focuses on specific systems or modules.

As with the wiring diagrams, the service data will be online. It is simply too far and too expensive to print books and ship them to every workshop. You’re assumed to have a computer with an internet connection, which is how most things work these days.

Good service data will list all trouble codes and describe in detail under what circumstances that trouble code will be set. It’ll provide information like:
“If this value is less than 4.5V, or if that value is higher than 20k ohms, then trouble code Pnnnn will be set.”
This allows you to measure the parameters mentioned in the list, the 4.5V and 20k ohms in this case, and work your way towards solving the problem by the method of exclusion.

TSBs:

For cars already running on the road, problems are found, and improvements will be made. In some cases, finding the problem can be quite tricky. For such purposes, TSBs are provided by the manufacturer or, indeed, other companies. If the item happens to be a safety item, such a brakes, airbags, or safety belts, a TSB is not enough. In such cases a mandatory recall is required by the Road Authorities of your country. The owner of car will be actively contacted to make the modification, or the car is assumed to turn up for service at the dealership, at which point the dealer is expected to check for active TSBs and implement them.

In Sweden, if you book time for service at a workshop using one of the big after-market service software products, the screen will show that there are still unresolved TSBs on your car. This allows the service manager to book in the extra work, time, and parts for these jobs.

As mentioned before, after a number of years, the manufacturer will no longer actively write new TSBs for newly found defects – the car is simply too old. Luckily, some commercial actors actively keep adding new TSB data even for older cars and make it available online.

 

Paul
2025-09
Sweden

This page is part of a series. You can find the main page for the series here.

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