How to Fix Intermittent Dimming of Headlights on a Toyota Avalon

I have a 2001 Toyota Avalon and for over a year I had a very annoying intermittent electrical problem. Most of the time the car was OK, but occasionally when driving at night, the headlights would dim out for a second. In the day or at night the radio would cut out momentarily with the dashboard display blanking out; also, the interior clock back light would switch off. The problem seemed to be worse when braking or when putting the foot on the accelerator. When the car electrics were about to misbehave, there was usually a whining noise from the engine bay.

Sometimes the problem would go away for several weeks and at other times the cut outs would occurs every thirty seconds. Of course, the Law of Sod dictated that whenever I took the car to the garage it would work perfectly.

I went to a so called car electrical specialist. He said that it was very difficult to fix a problem if he couldn’t see it, but in his opinion the alternator needed replacing. To me, this seemed like the most plausible explanation; after all, the whining sound that accompanied the cut-outs seemed to indicate an alternator problem. Ka-ching, a couple of hundred bucks was handed over for the fitting of a reconditioned alternator. This seemed to work for a few weeks, then the problem came back. I took the car back to the specialist; he then suggested trying to changing the battery (ka-ching) to see if that fixes it.

The guy obviously didn’t have a clue what he was doing. So I decided to investigate the problem myself. After a search on the internet I discovered that this was an all too common issue. Some people had spent thousands of dollars replacing alternators, regulators and batteries all to no avail. However, there did not seem to be a definite solution.

Cause of the Intermittent Electrical Failure

On further reading, it seemed that a possible cause for this problem could be bad earthing. Earth cables carry electrical current back to the negative terminal of the car battery. If you look in the engine bay of your car, you will see a number of earthing cables connecting various components to the chassis and the battery. Good earthing is important because a car’s electrical system needs to be able to deal with very high currents.

Poor earthing is caused by a dirty contact or fraying of a cable, which increases the resistance of the electrical path back to the battery. The voltage across it will rise in proportion to the current multiplied by the resistance of the path (Ohms law). This voltage drop is what cause the headlights to dim since the more voltage that is dropped across the earth connection, the less voltage is available to power the headlights and other car components.

Ohms Law:

I.R = V

The two main components that generate and use large amounts of current are the alternator and starter motor. The starter motor only drains current when you are turning over the engine with the ignition key, so this is unlikely to be causing the issue. The current generated by the alternator varies with the engine revs. As the power failure seemed to tie in with revving the engine, it was likely the earthing of the alternator was causing the trouble.

How to Diagnose Earthing Cable Problems in Your Car

OK, we think the problem is earthing. Like me, you may have already parted with some hard earn cash and are reluctant to go around replacing all the earth cables in the car in the hope that this may fix it. What you really want to do is:-

a) Absolutely prove that earthing is the issue.

b) Work out which cable or cables in the car are causing the earthing problem.

The problem can be easily diagnosed in less than three minutes be using a simple voltmeter that can be bought from any DIY store for $10.

You will need to check the car when it is failing. I would suggest carrying the voltmeter in the car with you on journeys so that you can stop the car when it is being troublesome and test the electrics. Fortunately for me, the car started failing regularly one time when I got home.

Make sure that you have a partner or friend that can tap the accelerator to make the electrics fail.

Switch the headlights on and pop open the hood.

The first thing to do is to check that the battery is OK. A standard car battery gives out about 12 to 13 Volts, so set the range on the voltmeter to measure more than this. Put the negative black wire of the voltmeter on the negative terminal of the battery, better still, connect it to the negative battery terminal with a crocodile clip so you don’t have to hold it on. Put the positive red wire of the voltmeter on the positive terminal of the battery. You should see a fairly steady 12 Volts on the meter. Get your partner to rev the engine to make the electrics fail. On my car, the battery voltage was rock steady even when the headlights dimmed right down. This proved that there was nothing wrong with the battery. Two fingers up to the electrical expert who wanted to replace the battery (maybe just one finger in your culture).

The next thing to do is to prove that there is actually an earthing problem. If all points in the engine bay are properly grounded, current will flow back to battery without raising the voltage between the earthed point and the negative battery terminal by more than a fraction of a volt (about 0.2 volts). If there is poor earthing, then the increase in the current flowing back to the battery will cause the voltage to rise by over a volt.

Keep the black wire of the meter on the car battery terminal. Put the red wire of the meter on the metal casing of the alternator and get your friend to rev the engine. On my car, the voltage measured on the meter popped up to 3 volts. This proved that earthing was the problem.

I noticed that there was a black earth wire bolted to the body of the engine adjacent to where the battery was located (see picture). I measured the voltage drop on this wire to the battery. Again the voltage drop went up to 3 volts when the accelerator was pressed.

Faulty Avalon Grounding Cable

Faulty Avalon Grounding Cable

This wire went under the battery and connected to a junction point with another wire going from the junction point directly to the battery. Therefore we had a short, direct, all copper connection back to battery. There is no way that there should be 3 volts across this connection. The problem had to here.

How to fix the Car Earthing Connection

I unbolted the cables, cleaned the contacts for each wire using steel wool and reassembled. Unfortunately, this did not work, the electric started cutting out again within the week. There had to be a problem inside one of the cables.

I replaced both the cables with nice thick battery lead cables from the local auto store (see picture). The original ones seemed a bit thin to me considering how much current they needed to carry.

Fixed Avalon Grounding Cables

Fixed Avalon Grounding Cables


This fixed the issue permanently, six months on the car has never had any re-occurrences of the cut out.

Happy motoring.

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