Settle an argument plezzzzz!!

[STUpid]

Why, if the dash warning lamps dont light up on the dash board in my 940 does the charging system not work?? A mechanic friend of mine thinks i'm being filled full of s**it but i'm inclined to believe you guys as I class you lot as the experts when it comes to Volvo's!! I need a fairly technical explination to satisfy his disbelief!!!! Cheers

[craig8661]

no dash light =no charging light not working so = tickle feed to altenator not to tickle so it charges

[Glen Morangie]

The lamp is part of the charging circuit (it's the 'exciter' wire for the alternator) - when you switch on the ignition the battery makes the lamp glow, when the charge current flows in the opposite direction, it also lights the lamp.

But once the engine revs over around 1600 RPM, it will charge without it, because the alternator will 'self excite'

[STUpid]

Originally Posted by Glen Morangie

The lamp is part of the charging circuit (it's the 'exciter' wire for the alternator) - when you switch on the ignition the battery makes the lamp glow, when the charge current flows in the opposite direction, it also lights the lamp.

But once the engine revs over around 1600 RPM, it will charge without it, because the alternator will 'self excite'

Cheers buddie !! I'll show him this and tell him to put it in his pipe n smoke it!!!!!!!

[Glen Morangie]

Originally Posted by Glen Morangie

when the charge current flows in the opposite direction, it also lights the lamp.

Ooops - there's a diode on the diagram, so that can't happen. So not sure on 900 series.

[STUpid]

Is it possile for an alternator to not self exite?? intermittantly perhaps?? how would you know other than using a voltmeter??

[Glen Morangie]

Originally Posted by Glen Morangie

Ooops - there's a diode on the diagram, so that can't happen. So not sure on 900 series.

Right, having a 'senior moment' - the charge voltage from the alternator, being greater than the battery voltage, extinguishes the light, and the diode (of course) stops the lamp being illuminated by reversed polarity.

So the lamp goes out to indicate charging, if anything goes wrong it comes on again.

Sorry I don't really know if alternators can intermittently not self excite - the point in the rev range at which they will do so is variable, but if you rev an engine with the exciter disconnected to 2000 rpm, the charge should come up ok.

Originally Posted by stuartg07

how would you know other than using a voltmeter??

You would need a voltmeter to test if any charging is going on, but if you just want to know if the dash indicator is working and the wire to the alternator is OK, if you switch on the ignition and remove the small red wire from the back of the alternator, the lamp should only light when you touch it against an earth point (alternator body or bracket).

[STUpid]

Thankyou I really appreciate your input! Please could you do me one more favour?? Could you have a look at my origional thread in the 900 series entitled 'nice but dim' I am trying to establish a relevent connection between the dash warning lamps, the entire dash illumination + head lights being dim and the batt eventually giving up the ghost the other day leaving me stranded!! Put new batt on now and car is running sweet apart from the occasional non illumination of the dash warning lamps!!! Cheers, Stu