Jensen Monday Club

Charging System


Last issue I told you that my Interceptor 3 had suffered from non-charging. Mike Lotwis covered the principles of charging and some of the differences between marks of Interceptors in the 1986 summer issue, and Keith Anderson covered some fault diagnosing in the 1996 March issue. I shall try to cover the principles and also a logical fault finding process.

The alternator produces electricity by driving a rotor in-between a stator, the stator is an Electro-magnet (known as the ’field windings’) that can be turned on and off by the regulator, if there’s current in the field windings then the alternator gives electricity, if there isn’t, then no charge. On the older (pre1972 Mk11 Interceptor’s and CV-8’s) there was only one field terminal on the outside of the alternator the other was grounded or earthed inside. The one field terminal was fed with 12 volts from a mechanical regulator when required. The later Interceptors have two field terminals, one is permanently fed with 12 volts from the ignition switch, and the other is grounded to the car’s earth via an electronic regulator when required.

With these basic principles in mind we can start finding the cause of non-charging. I shall concentrate on the later cars. First we can look for a mechanical fault; are the battery connections at both ends tight and clean, is the earth strap between engine and chassis good, is the belt tight enough to turn the alternator, does the alternator turn and isn’t seized? Now if we look at the back of the alternator we can see the terminals, are all three secure and not broken? The large terminal with three large brown and white wires secured with a nut is the output, it is permanently live and connected to the battery positive. The other two are the field terminals; one is white, the other white with a purple line or tracer (it doesn’t seem to matter which way round these go, but let me know if you think differently) and have ‘spade’ connectors. A quick check we can do, with the engine running, is to earth the terminal with the white/purple wire (this is the one that goes to the regulator). I use and carry short pieces of wire with clips at both ends (available at electrical stores such as Tandy, etc.) but normal wire or just shorting the terminal with a screwdriver will do (but please be careful, the engine is running and belts turning etc.). This is in fact doing the regulator’s job. If this produces a charge, (a voltmeter reading of above 14 volts) then there is nothing wrong with the alternator and further investigation is required. This can be a good ‘get you home’ trick, you can stop and charge the battery up when required, but not permanently or for too long otherwise you will ‘cook’ the battery.

Further investigation will require a separate voltmeter or multimeter (again available quite cheaply). With the engine stopped but the ignition on, check for the battery voltage at the white wire terminal (one probe touching the terminal the other to an earth), this should be about 12volts but could be a lot less because of the run-down battery. If this is zero then there is a break in the wire between here and the ignition switch. If it’s OK then check the volts at the white/purple terminal, again this should be battery voltage, if it isn’t then there’s a break internally in the alternator field windings and means a new alternator or an overhaul. Now with voltage at the white terminal we can again start the engine (you might need jump leads) and earth the white/purple terminal, check again for a charge voltage (above 14) at the large terminal, if this is not the case then the alternator is at fault. If there is a good charge then the alternator is good and the fault lies elsewhere.

We now need to get to the regulator, this is behind the front grille, remove the grille by undoing the four large self-tappers with a long screwdriver. You will now see the voltage regulator and the electronic ignition control unit, the regulator is the one with two wires and the plug is clipped on, the control unit has five wires and the plug is screwed on. The two wires are the same as at the alternator but have changed colours at the bullet connectors about six inches from the plug. Remove the plug and with the ignition on check again for battery voltage at the two plug terminals, if all is not OK then there is a break in the wire between the plug and the alternator, the first check should be those two bullet connectors. If the voltage was OK then the voltage regulator is faulty (providing it has a good earth) and has to be replaced.

If you do find you have to replace the alternator, why not consider one of the newer 78amp models, the standard is 60 or 65 amp, and reduces to about 30 amps at tick-over. Start adding up the power required for all the accessories (lights, fans, wiper motor etc.) and you can see that the battery will have to supply a good proportion of this power during low revs, this can be reduced with a higher output alternator.

Alan Smith.