Faulty Alternator Diagnosis (Case Study)

In this case study, I will diagnose my faulty alternator and guide you through the whole process of fixing it.

Last week, I realized my alternator was on the way out.

Nobody likes it when a vehicle component fails (especially a car mechanic!), but it could happen to anyone.

You might find yourself in a similar situation. On this page, I’ve put together the chronological symptoms I noticed and detailed the repair cost.

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Alternator Diagnosis: Context

West Midlands United Kingdom

For context, I’m a British citizen from the West Midlands (which is in England, for those of you across the pond).

I drive – as you might expect if you know anything about our roads – a Vauxhall Corsa C (2005).

It has a tiny little 1.2-litre Twinport engine that, when new, produced 80 hp. It has a manual gearbox (transmission), gets around 40 mpg (33 US mpg), and weighs 935 kg, which is light.

It’s not much, but it’s more than enough. I love it!

(Vauxhall is known as Opel in mainland Europe and the rest of the world.)

Corsas usually have a bad reputation. They’re seen as cars for guys who like to race or as inexpensive vehicles, and that’s usually the case.

I’ve found, though, that taking good care of one means it serves you reliably for as long as you need. I have yet to hear of many people who’ve kept the same daily runner for eight years!

I take care of my car, as I always push readers to do in my articles. That – along with a bit of luck and some good guidance from Dad in the original purchase – means I’ve only spent £80 on non-service repairs since I got it in 2014. Until last week, of course. But I feel I owe it to the car by now.

Symptoms Of Alternator Failure

I’ll walk you through everything I noticed going wrong over the preceding weeks and months.

Battery Problems

12V Car Battery

The alternator should charge the battery and power the car while the engine’s running. Thus, any alternator problems usually lead to battery failure, too.

The earliest faulty alternator symptom (with hindsight) that I remember was rough starting. This happened in late summer.

On a few occasions in August, I remember worrying about the battery because of this issue. After all, it was about five years old, so I was due a new one.

The Corsa was due a full service along with its MOT, so I took it in. Here, as is routine, the mechanics did a battery and alternator test. This revealed that the battery wasn’t holding a charge, so I had a new Bosch model installed.

This may or may not have been a separate fault from the alternator. I have no real way to tell. It might have been on the way out so slightly or inconsistently that the test didn’t pick it up.

Parasitic Drain?

Turning Ignition On Car

Even with a new battery, I still noticed that the car was struggling to start more than usual. It would run fine once it got going, though, so I didn’t think the alternator was at fault.

Instead, I guessed there was probably a parasitic drain. This happens when a switch in your car stays on after you turn the engine off, drawing power.

Whenever I switched my Corsa off, I would ensure all the buttons were off.

Although there were still rough starts sometimes, this seemed to help.

I now know it was likely more to do with the idling at the end of my journey than turning anything off. (I’ll explain why that’s the case further down the page.)

Flickering Lights

Car Wont Start But The Radio And Lights Work

On a few occasions, as we headed into the middle of September, people driving behind me at night would flash their headlights.

The first couple of times, I thought it was just road rage or flashing at people on the other side of the road. As more and more started doing it, I realized there must be a problem of some sort.

With my buddy (Josh) in the car, we turned onto a side street. The car behind us continued driving without pausing to inform me about the problem – quite handy(!).

There didn’t appear to be any apparent mechanical issue from the rear, so I toyed with the lights while Josh stood behind and watched.

He noticed that my brake lights were coming on strangely. Instead of all illuminating simultaneously, they came on from left to right, one at a time.

This obviously indicated an electrical issue. However, my first guess was the Body Control Module glitching.

I’ll never know what that car behind me was trying to say. Maybe something more severe was going on. I wish they’d pulled over to let me know!

When we got home, I disconnected the battery and left the car overnight. This is a common way of resetting the ECU and its control modules.

In the morning, the Corsa started fine. The brake lights all came on at the same time, and I thought (wrongly) that I’d fixed it.

More Rough Starting

Opel Corsa
The Opel Corsa

I went to Canada (where I’m moving) for about three weeks in October. When I came home at the start of November, the Corsa was evidently struggling to start.

Again, I didn’t think much of it. It was about 7 degrees C (46 degrees F), grey, cold and damp (as always). It’s perfectly normal for batteries to struggle in these conditions.

There were no other issues, so I didn’t chase it up.

Battery Warning Light

battery warning light

Things started to get worse very quickly. Again, I went to Canada (for most of December), and when I returned, I had a red battery warning light on the dashboard.

I left the car idle for a bit (it had been started once or twice through the month but not driven). The light didn’t turn off.

This time, when driving, I noticed my dashboard lights (the odometer and the lights behind the speedo and rev meter) fading.

Here – and only here – did I realize that the alternator was probably the problem.

Diagnosing Alternator Failure With Tests

Alternator Test

How do you diagnose an alternator fault? With an alternator test! You can get these machines for home use, but it’s more financially sensible to go to a garage.

Things didn’t get any better over the next few days, so I took the car to a local mechanic I’ve used before. They did an alternator test (for free, which was very kind) and found the following:

  • 3,000 rpm (no electrical load) – 11.9 V (not charging)
  • 2,000 rpm (full electrical load) – 11.9 V (not charging)
  • Idle (no electrical load) – 14.0V (charging)

In other words, the alternator had a low output at almost everything other than idle. That wasn’t good. It meant my Corsa could have died while I was driving, taking with it all control.

I took it home, hoping that an electrical glitch might remedy itself, but no. Within five minutes, I was getting more warning lights:

  • ABS
  • Airbags
  • Power Steering

Sure enough, the steering became extremely heavy mid-roundabout (which was dangerous). Thus, I booked it straight back in with the mechanics.

Getting My Alternator Problems Repaired

Businessman turning key to start car engine

A few days later, I drove back (at the lowest rpm possible because of the test results). I parked up with a flashing odometer and all manner of warning lights.

The poor old Corsa wouldn’t start again as soon as I switched it off. There was barely a click when I turned the key. I only just made it! The alternator had utterly failed.

I left it there for the day.

The mechanics swapped the alternator out for a new one. They also found that the battery needed replacing (a complete discharge can be fatal to a car battery). The drive belt was also wearing, so I had a new one of those, too.

That evening, I went to the garage to pay (see below) and pick the vehicle up. It runs fantastically again, and I’m glad to have it back. It’s earned a little cash splash.

How Much Did It Cost Me To Repair An Alternator?


The total bill I paid was about £388. That’s about USD $460 (at the time of writing). This is a perfectly reasonable rate for the jobs completed.

This was made up as follows:

  • Total labor time (hours): 1.2 – £100
  • Alternator (Lucas) – £165
  • Drive belt – £20
  • Battery (Bosch – including battery disposal surcharge) – £75

The retail rates (what you and I would pay for the same parts) are pretty much the same. Since the shop will have bought the parts at trade prices, it will have made money. That’s good for them and good for me.

I received a ride home, returned to work, and carried on with my day. All I’ve paid additional for, in reality, is the labor. And I’m content to do that.

My thanks to the garage in question.

What Goes Wrong With A Faulty Alternator?

alternator repair cost

A faulty alternator could go wrong anywhere by definition.

In my case, it slowly became clear that it wasn’t producing a high output voltage (apart from at idle – at least during the test!).

There’s no way for me to tell for sure which specific part of the alternator was failing.

The problem with power steering caused the belt to have resistance. This indicates a potential issue with the wheel or rotor. There may have been another issue with the regulator and output.

The nick in the belt may also have come into play.

Who knows?

It works now, and that’s all I really need to know.

What Happens To The Old Alternator?

What Does An Alternator Do

When your faulty alternator is replaced at a shop, it usually gets put in the box the new one came in and sent back to the manufacturer. The alternator manufacturer, that is, not the car.

The garage will make a small amount of money back on this. More importantly (and a little cheekily), the company in question (Lucas, in this case) will find the precise fault and repair it.

My alternator will probably be back on the shelves by now as a reconditioned, ‘good as new’ part. Oh well. Just think of it as recycling.

Why It’s Important To Go To A Car Mechanic

car technician holding the wrench

It might be of interest to you, as an Auto Chimps reader, that I didn’t replace my own alternator. I have the skills, after all. Why wouldn’t I ‘save money’ and do it myself?

I’ll explain that with the following reasons.

  1. A mechanic in a garage has a vast array of tools that I simply can’t afford or find the space to keep at home. Ramps or advanced OBD II scanners, for example.
  2. If other faults are found, the car’s in the right place already.
  3. Sometimes, you run into complications at home which leave your car out of action for much longer. For instance, what if I ordered the wrong alternator and it didn’t fit? I can’t drive to the shop because I have no vehicle. Mechanics, though, can get those parts ordered to the shop in a matter of hours (or less, sometimes).
  4. Changing an alternator isn’t very labor-intensive. I paid for 1.2 hours.
  5. Garages purchase parts at trade prices. Thus, components are usually cheaper than what you can buy anyway.
  6. I strongly urge everyone to build a good working relationship with a local mechanic or garage. That way, when more severe problems inevitably come along, you have somewhere you can trust.

If I prefer to use car mechanics whenever it’s a good idea (and it usually is), so should you.

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Benjamin Kitchen

Ben is an IMI-qualified light vehicle technician from England with experience in a fast-fit garage. He aims to help drivers worldwide with common automotive problems. You’ll often find him working with his 1.2 Vauxhall Corsa. It may have a tiny engine, but in eight years it's never once let him down!