RV8NOTE290, alternator fault diagnosis, RV8 Workshop Notes, V8 Register, MG Car Club

290
No charge light stays on
Adrian Brook (Woodcote 1823) from East Yorkshire posted a note on the V8BB about a difficulty he had with his no charge light and a few weeks later reported the solution. (Aug 08)

Adrian's posting on the V8BB was "I have been on a long run today and on putting the car away the no charge light has decided to come on when I switch the ignition off and remove the key. I looked at wiring diagram the light appears` to be connected` to fuse 3 with tacho, air con etc. All of these are switched off as normal. I presume the power source must be a short coming from the battery, through the alternator or alternator sensor and back to the light I have removed the bulb for the time being. Anybody had a similar problem."

A few weeks later he posted a note saying "Found the problem. It turned out to be a failed rectifier fitted in the alternator. The battery was draining through the alternator and powering the no charge light when the ignition was switched off." So how did he make the checks to determine the rectifier had failed? Well here he explains.

When I returned to the car next day the battery had drained completely. It was suggested that it might be the alternator diodes so I checked these using a multimeter and they were fine. If the diodes are defective and shorted by excessive voltage they can be tested without the engine running by setting the meter to diode function and removing the connecting wires from the alternator. Put one probe to the casing and one to the main output terminal. Note the reading, then reverse the probes note the reading again. One way it should read 0.80V and the other should over range OL or read 1.

I then tested the built in rectifier by turning the meter to milliammeter connected the probes in series between the output terminal and the alternator output cable.

Diagram from "Using your meter", a handy book available from Radio Shack. It has a useful section on automotive testing. Diagram enlarged

The leakage current should be less than 1mA - if it's higher it is probably faulty. Mine had a high reading so I took the alternator to a local specialist that reconditions and supplies alternators and starter motors to the trade and public called Sissons Garage on Burma Drive in Hull. They confirmed my diagnosis and supplied me with a reconditioned one of the same manufacture, off the shelf for £41.

What multimeter do you need?
Adrian feels any multimeter will do these tests and www.cpc.co.uk have some digital ones with a diode tester for around £4.50.