Jaguar Repair: battery charging problem, 1985 jaguar xj6, volt meter

Question
Hello. I have a 1985 jaguar XJ6 series III (4.2). A problem that I have with it is that the battery condition indicator can randomly and frequently drop down into the discharge section (red band) and can remain there or close to it for several minutes (maybe up to 15 mintues). It usually reverts back to the standard position (13 - 14 volts) and after a while go back down. This can happen with the engine running at speed or idle and with the headlights on or off. There does not appear to be any particular pattern to it. In addition, and perhaps of more concern is that the battery light on the dashboard sometimes comes on when the indicator needle drops(but generally goes out after a short period (between 3 and 20 seconds) and this light rarely comes on.

I have had the alternator removed and checked by my local garage who say it is in good condition and working ok. The belts and battery have also been checked and are fine. Have you any ideas as to what might be the cause? Perhaps the regulator? Maybe the voltage level is suppose to drop? Any help would be much appreciated. Thanks.

Answer
Hi Martin,
It is not normal for the charge rate to drop into the red and the warning light to come on for periods of time. If you were idling with the lights and AC fan on and maybe the wipers too it could drop down into the red and the warning light come on but as soon as you rev the engine it should go back to charging.

If your shop who did the alt test was correct and you are sure the belt was tight, then you need to check the ground strap from the engine to the frame, check the right and left power terminal posts on the fire wall to be sure that the large brown wire is making a continuous contact to the battery. The alternator is triggered by the charge light circuit through the bulb and a small resistor shunt around the bulb. If that gets disconnected in any way the alt will not charge at low RPMs. However, if the warning light always comes on when the volt meter goes to the red then it confirms that the charge light circuit is good because the light only lights when the regulator (inside the alternator) becomes a ground for the light. The power for the charge light circuit comes from the ignition circuit so you know that circuit is ok or the engine would shut down and it has to be ok or the light would not come on.

The fact that you say that the indicator light is not always on when the volt meter is in the red tells me the problem is most likely in the charge light circuit. The circuit is from the ignition switch (probably ok as the engine keeps running) to a multi plug connection on the instrument light cluster via a white wire, then it leaves the bulb via that same multi plug connection on the light cluster via a brown w/black tracer wire to a multi plug connector at the firewall just behind the battery and continues on to the alt.

Since this happens often it will be relatively easy to prove this is in the light circuit by running a long test lead from the (+) post of the battery to a 12v test light and on to the small post connector on the alternator. You must disconnect the existing small wire on the alternator. Your new test circuit can be taped to the top of the dash so just before starting connect up the test light and confirm that it lights the test light. Then start the engine and confirm that the light goes out. Then drive the car normally and watch the test light and volt meter and be sure to disconnect the test light after you turn of the car. You thus become the trigger circuit for the regulator / alternator.
Let me know,
Howard