Cooling down a hot and bothered Toyota

I drive an early '90s model Toyota Sprinter (4AF engine) and the car has what appears to be an electrical problem.

It initially started to overheat because the (electric) radiator fan didn't come on when it should.

At first I thought the problem was with the thermoswitch controlling the fan and I replaced it with a new one.

But the problem persisted, the engine getting so hot that it affected gaskets on the block and top.

My investigation then led to the relay box where I discovered that the relay through which the fan is switched was heating up when the engine idled.

I put in a new relay, but it, too, got hot and the problem wasn't cured.

I then unplugged the cable going from the thermoswitch to the relay, with the result that the fan now runs non-stop as soon as the engine runs.

Could the problem be with the wiring, as some seem to suggest, or could it be a symptom of something else? Please help - Thabo

Thabo, the hottest clue in your description of the symptoms is the fact the both the old and new relay in the fan circuit got hot when the engine was running.

A relay is not supposed to heat up significantly if it operates correctly.

If it gets hot, it's normally an indication of arcing at the contacts where the relay is plugged in at the relay box.

Arcing (a series of rapid sparks, usually due to a poor contact or bad connection causing a quick-fire make-and-break current which gives a small spark each time it breaks) will cause heat at the point where the sparking occurs, and this will spread by conduction to the surrounding area.

Arcing often starts very small, but the sparks will steadily erode the contacts and thus it will escalate.

If the relay experiences arcing at one of its terminals, the poor thing wouldn't know whether it should switch the fan on or not, because the signal current keeps on appearing and disappearing.

As a first step I suggest you examine the terminals on the fan relay closely. Signs of arcing can usually be seen if you look carefully.

Clean up all terminals where the relay plugs in as best you can, make sure the contacts are tight, spray some switch cleaner on them before refitting, and then see if the fan (with the unplugged cable reconnected) works as it should.

If not, I would urge you to get an auto-electrician to look at the problem as soon as possible. I don't think it will cost you too much and it might save you a lot of money in the end.

Incidentally, owners of Toyota models similar to Thabo's car will do well to keep an eye on the big three-way split in the wiring loom where the cable from the battery positive terminal divides into two branches, one going to the front of the engine and one to the rear.

That junction was not particularly well insulated and, with these cars now approaching 20 years of age, is often the hideout of electrical gremlins.

Let us know what transpires with the dodgy fan in the end.