The alternator on my 1991 xj6 will not start charging until the engin gets to 2000 rpm then stays on. The alternator bench checks fine and has been replaced along with several other parts.

 

When you say that it is not charging until above 2k rpm was that based on a volt meter or the dash?

Is the bench test running at the above 2k level?

 

Volt meter. I don't know the rpm of the bench check.

 

I still don't get how you know that it not charging until 2000rpm. How exactly did you test this. With a hand held volt meter accross the battery and the engine running at 2000 rpm?

Engine off, check volts accross battery = about 12.
Engine on, check volts accross battery = in the area of 13.5 to 15.1.
Then have someone turn your headlights whilst you are using the volt meter, the voltage should initially drop and come back to the above figure. = charging system working.

And then you say it stays on. What do you mean here?

If you mean that with engine on you have 13.5 to 15.1 volts this is normal.

Cheers,

 

The theory behind alternator charging in a car: PROVIDED the battery is good, the battery should read 12.6 to 12.8volt with engine off. Loading with headlamps at this stage will drop the voltage slightly. Cranking with starter motor should not drop voltage to less than abt. 9volt. If it drops to (say) 6volts, the battery is either worn out (most likely), has poor terminal connexions (common) or is in a poor state of charge. With engine running, even at idle, the battery voltage should read 14.4volt. This is a constant VOLTAGE charging system, the best for charging a lead-acid battery. The charge current in not constant, it varies a lot. It could be 50 amp after a cold start, dropping to 5amp. to balance out accessories current draw after a while. Some alternators have their control circuit inside the alternator, some outside. The reason the change from dynamos (a purely d.c. generator using carbon brushes) was due to the use of a commutator (rotating switch) in the heavy current path of the armature. Dynamos gave a poor charge at low revs. The alternator is far better at low revs., has six stud rectifiers in the ground end of the stator coils which provide the output heavy current, therefore the output is d.c. and good at engine idle. The connexion of the control current thru' armature is by slip rings, the current here is low. around 5amp. IMPORTANT; because the alternator can drive out a heavy current (like 50amp) at low engine revs., the belt MUST be good and properly TIGHT. The alternator bearings are designed for a really tight belt. Voltage checks are best done with a digital voltmter, can be had cheaply these days. Important again; a bad battery/charging system/poor connexions can and do upset any microprocessor based engine/transmission/etc. control unit. Poor connexions create small sparks around the battery terminals which can garbage date bytes stored in such computers giving false readings, odd incomprehensible effects. Also make sure the terminals are tight on starter motor, engine ground wire. Look after your battery and it's surrounding systems, it's an important part of the car.
Leedsman (yes, you've guessed, ex electronics engineer!)