Car electrical systems – The art of diagnosis

In the old days, before the large-scale introduction of vehicle electronics, a seasoned technician could open the hood, listen to your engine running, and tell you exactly what the problem was. Or he’d drive the car around the block and tell you what the problem with the drive train was. Everything in the car was either mechanical or electrical.

Still in the 80-ies, when I worked in the car industry, most technicians would be able to fix most things that could go wrong with your car. They had no issues with pulling the cylinder head or inspecting the interior of your gearbox. Except for one single thing: electrical problems. Well, perhaps two: automatic gearboxes were assumed to be something for specialized companies – and in most cases, this is still the case today. But that same technician that told you, by listening to your engine, that it needed new spark plugs and breaker points, would prefer not to touch anything electrical.

Nowadays, everything in every car is either electric or electronic. Sure, the engine and gearbox are still there, but the electric part of the car has increased tremendously. With complexity and the speed of churning out new car models and accessories, the lead time for development is now shorter. And with this, the quality has not always improved at the same rate.

Where we before had wind-up windows, almost no car has these left today; they have been replaced by electric windows. Same for back mirrors, you used to be able to adjust them by hand. No more today; they’re all electric. Same with car seat adjustment, the car’s door locks, and the rear lid. Even the instrument cluster still sports dials for speed and revs, but they are either fully electric or simply an electronic display. No more instruments driven by a cable from the engine or gearbox.

So any problem brought by the customer regarding any of these items is no longer a spanner job but is now a job for a trained electrician.

The same goes under the hood. Modern cars no longer have a distributor, breaker points, or even HV leads. Nor will you find a carburettor on any modern car. No more getting your hands dirty to adjust the ignition or fix the carb. It’s all electronic now.

All and everything is nowadays controlled by one or more modules. The modules talk to each other via networks, and the only indication that something is wrong is the engine light, therefor often referred to as the money light.

Diagnosing a problem:

The easy, but potentially expensive, ‘method’ to fix any electrical problem on a car is by replacing parts. Sure, you can read the fault codes and then replace the parts as indicated by the fault codes. However, more often than not, this will not resolve the issue. And in many cases, it may only make it worse!

Instead, fixing the issue is a matter of interpreting the fault codes. Knowing the systems and how the work. Using analytical skills to determine where to start measuring values. To understand what is going on and, eventually, how to fix it. In short, you need to know what you’re doing and you need a plan.

Fun facts:

  • The practice of replacing parts without really knowing what you’re doing is referred to as ‘shooting the parts cannon and see what sticks’. The sticking part described the pure chance that part fixes the problem at hand.
  • Replacing parts with cheap alternatives, eg from amazon or e-bay, is always a risk. Scrupulous entrepeneurs rebadge cheap parts to look like original parts and resell them at a fraction of the cost of an OEM part. Guess how that’ll work out for you!

Fault categories:

Cars come into every workshop with any problem you can think of. Usually, the customer complaint, in combination with a quick fault scan, will help a good tech to know what to do next.

Some usual customer complaints:

So the first thing to do is to decide what sort of problem we have. After that, we make a plan and work that plan. Part of making the plan is doing some initial measurements and getting service info and a wiring diagram.

 

Paul
Sweden
2025-09

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

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