VW Polo Mk3 Alternator light Charging fix

On older cars, the alternator only works if it receives a small priming current via the alternator warning light. After a connection failed in the instrument panel of my 1990 VW Polo, the alternator stopped charging the battery. Eventually the car died completely due to a flat battery. Fixing the instrument panel resolved everything. Do not assume that the alternator has failed until you see the alternator warning light come one with the engine running.

The problem

The first symptom was quite minor and we ignored it. Driving along the motorway one day, the fuel and temperature gauges both fell to zero. Nothing else happened and the car continued driving fine. Then, after a few minutes, the gauges came back on again. This intermittent fault continued, and got worse so that the gauges were off more of the time than on. But we thought, ‘So what? it never overheats anyway, and we already have a feel for how many miles between fill ups, so as long as we keep our eye on the odometer (which continued to work fine) we will know when to refuel. It’s just a loose connection and we will get around to fixing it one day.’ What we did not realise was that the instrument panel warning lights were part of the same circuit and were failing also. This would not be a problem itself, except that the alternator only charges the battery if it receives 12V from the instrument panel (learn about this 'exciter current' in the Wikipedia posting for alternators). So over time our battery ran down until one day, driving along happily, all the power failed suddenly, the car stopped and even the hazard lights were so faint as to be almost invisible.

The diagnosis

I assumed it was either the battery or the alternator. The battery was totally discharged, as I knew, but it charged up fine overnight (giving 14V) and stayed charged. So it wasn’t the battery. Then I thought it must be the alternator. But why hadn’t the alternator warning light come on? That was when I made the connection with the other fault, the one about the instrument panel gauges. When the ignition key was turned halfway, none of the expected lights came on: no alternator light, no temperature light, no oil light. In fact the only light that worked on the whole instrument panel was the full-beam light. So I thought, ‘before I exchange the alternator, I ought to confirm that it is broken by seeing the alternator light come on when the engine is running’. I surfed the web to see if any other VW owners had the same problem, and a site called vwvortex gave me lots of useful information. I learned all about the ‘infamous blue wire’ that takes 12V to the alternator from the instrument panel before the alternator can start charging the battery. So I guessed that if I fixed the instrument panel, the alternator would be OK. Also, I learned that other people had found a blown fuse (number 3) and fixing that had fixed the problem. Other advice was to check the voltage regulator behind the instrument panel, the earthing points and the barrel switch attached to the ignition key.

Checking the circuit

I checked the fuses visually and I checked for a circuit across all the fuse connections. All the fuses were OK. I was able to access the connector for the instrument panel, a flat, white-plastic, 14-point connector, by taking off the driver’s-side shelf. The pins in this connector are labelled T14/1 through T14/14 in the Haynes manual. When the key was in the off position, only T14/6 gave 12V (using the door frame as earth). When the key was in the halfway position, T14/5, 6, 13 and 14 gave 12V. The Haynes manual circuit diagram did not explain what T14/5, 6 and 13 were for, but it did show that T14/14 (the leftmost pin) brought 12V into the instrument panel for nearly all the circuits except the full-beam warning light. So this made me suspect the T14 connection, but the problem must be actually inside the instrument panel because everything as far as the 14-pin connector worked as expected. I removed the instrument panel as instructed in the Haynes manual (removed the steering wheel, a piece of fascia, and withdrew the instrument panel. It was a very tight fit, and one thing the Haynes manual did not mention – I had to disconnect the speedometer cable from the gearbox to get sufficient slack to withdraw the instrument panel. There is no way you can withdraw the instrument panel without doing this first.) On the back of the instrument panel is a printed circuit, sealed in green laminate plastic. It looked fine, as did the voltage regulator, a little 3-pin semiconductor component. There were no obvious burn marks or corrosion. So I set the circuit tester to ‘test circuit’ and checked for a circuit between T14/14 and the appropriate pin of the voltage regulator. There was no circuit, even though the circuit track looked absolutely fine. When I checked T13 with a distant part of the circuit (followed visually) it gave a good circuit.

The solution

My father-in-law is a professor of electronics and he soldered a wire from the T14/14 connector to the appropriate connector on the voltage stabilizer. This bypassed the (invisible) break in the circuit and now everything works fine! The instrument panel lights come on, the gauges work, and the alternator light comes on (when the key is halfway), and goes off when the engine is running, showing that the alternator is charging the battery.

Tips for the soldering

–To allow the 14-pin connector to still connect, scrape off a little of the laminate away from the connector to make the solder connection.
– Measure the piece of wire to make it as short as possible, so there is no slack.
– Place small strips of insulating tape over the soldered points and over the wire, to minimise risk of short circuits after the instrument panel is replaced.
– If you don’t have an electronics expert to hand, practice soldering first, rather than spilling solder all over the circuit.

Final tip

Disconnect the negative terminal of the battery before doing all this work. You have to reconnect occasionally to test circuits, but leave it disconnected at other times.

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very helpful!