1990 Volvo 240 DL Charging System Problem

[TangoBravoMike]

There's some wrong information here in the last few posts.

As I understand it -- and I'm not a mechanic -- the charging indicator light on the instrument panel DOES serve an important function. It provides current to the alternator to initiate the excitation process. An alternator uses relatively low amperage current through the rotor windings to create a magnetic field; this is called excitation. This magnetic field induces considerably greater amperage in the stator windings; it's the stator current when rectified that charges your battery. When your car is running, the excitation current is drawn from the output of the stator circuitry. But before the alternator starts putting out power, you have to "bootstrap" it somehow by giving at least a little current to the rotor. That current comes through the instrument panel's charging indicator light.

This story has a strange little twist. My ALLDATA subscription information says that if no current is supplied through the charging indicator lamp, the alternator can produce power due to residual magnetism in the rotor, but it will only begin producing power at high engine RPMs.

This is exactly the symptom that my 1989 240 has right now. My instrument panel light does NOT come on as I move the ignition key from OFF to ACCESSORY to RUN, and I only get alternator output at high RPMs. I am suspecting a burned out bulb or a wiring fault, and I'll check that before I look further for voltage regulator or alternator faults.

I really like the ALLDATA subscription by the way. I like it enough that I subscribe for 3 different cars. Recommended. (I have no connection with ALLDATA, I'm just a satisfied user.)

[Retired MVS Contributor]

Residual magnetism!!...I haven't heard that since alternators came on the market in the 60's, but it is essential for the alternator to work...In the old generators we had to apply 12 volts and run them like an electric motor to establish residual magnetism...From then on it was self sustaining...The same is true of alternators...The difference with the alternator is that the field winding is not stationary as in a generator, it is revolving in the rotor and the "armature" is stationary...

It might be conceivable that residual magnetism could be established through the lamp circuit, but that is not it's purpose and once magnetism is established it is self sustaining...This can be checked out by disconnecting the lamp...It will be seen (with a voltmeter on the battery) that the alternator will start and put out...The voltage regulator, which controls alternator output by regulating the field current, establishes magnetism...

Sharp blows can degauss magnets and that is why we had to polarize a generator after repair...It required a special impact tool to remove the field pole shoes and that degaussed the magnets...For that reason it would not be a good idea to smack the starter to free up a stuck Bendix...Starters have permanent magnets and no field windings nowadays and rapping them could degauss the magnets or shatter them since they are ceramic...

Jerry