Hi Guys

I have a Rover 75 CDT in the shop fitted with the BMW 2.0 Turbo Diesel engine. It has done about 130,000 miles and the customer complained of the Alternator warning lamp coming on intermittently when driving home from work. We had some other work to do on the car so before we did it, I took it on road test with everything switched on and the scanner connected to it. During the road test, the problem did not manifest itself (as usual!!) and we carried out the other repairs. We removed the alternator which was a Valeo unit, and on checking inside found the brushes well worn and the slip rings both well worn down, so with the belief that these could be sticking and making a poor contact an exchange alternator was ordered and fitted. When the car was restarted everything was fine, however on restarting the engine a second time, the alternator warning light was lit dimly, although with everything switched on it is still charging to 14.1 volts. I suspect there is still a problem somewhere, however I thought it worth a post to see if anyone has had this problem before on either the Rover 75 (or BMW 3 series fitted with the same engine) having fitted an exchange alternator unit? I have carried out voltage checks on both the positive and earth sides of the circuit back to the battery, and the volt drop in both cases is only 0.03volts!!By the way, there are no fault codes stored in the engine ECU!! Any ideas would be most welcome.

Thanks in advance Roger

The reason for the one volt(ish) KOEO in a "traditional" system is because the circuit in question is a series one.

There are laws that govern the way voltage behaves in different types of electrical circuits.

In a series circuit the voltage will divide across the consumers in the circuit.

As an example; In a 12 volt system if I wire two bulbs of equal wattage (resistance) in series, the 12 volts will be shared (dropped) across each bulb. In this case 6 volts will be dropped across Each bulb.

With me so far ?

If we now alter the resistance in one of the bulbs the voltage dropped across each consumer will alter as well. However the voltage dropped across both bulbs will aways equal the supply voltage. eg; 6+6 or 3+9 etc etc

The circuit in question has two consumers, the first being the bulb in the dash and the second one being the electro magnet (Rotor) in the alt. However the two consumers have different resistances.

With the Ignition on and the engine not running, the one volt that you are looking for is the voltage in between the two consumers.

Does that Help ?

Regards Mark

PS. The above hopefully explains the reason for the one volt reading on the system. But its not necessarily the best way to check the alt circuits.