The V8 Owners Forum -

I know that my alternator is dodgy because my ignition light is permanantly on, dimly. I also have a misfire after 4000 revs at wide open throttle. It causes the exhaust to pop and flames to come out.

Could this be the cause of the problem, maybe a lack of voltage?

very possible, what voltage is your system running at?

doesnt look like the alternator is causing it although it does need changing. Battery voltage is 13.4V, voltage at batt side of ballast resistor is 12.5, so a little down, but maybe expected and coil voltage is 9V and is stable so everything seems fine in that respect.

If you know the alternator is dodgy change it before doing any more diagnosis on your problems. It may be putting out a very noisy supply which will not show on a voltmeter, a scope is needed to show it.

jw

Ok, will do but will be worth checking ignition and changing plugs anyway. cant do any harm

Alternator is just keeping battery charged, but not well.

Ile have it off

Thanks guys

The ignition warning light will come on if there's a voltage difference between the switched ignition circuit and the alternator output. In your case you know that the battery voltage is OK and the switched ignition voltage is low; this indicates an excessive voltage drop on the switched ignition circuit and not anything to do with the alternator.

Granted, but the light changes intensity with engine speed, and when idling is almost full on, ignition voltage is staying pretty steady.

ppyvabw wrote:Granted, but the light changes intensity with engine speed, and when idling is almost full on, ignition voltage is staying pretty steady.

Which voltage is changing with revs then - alternator output, or switched ignition?

Right ok, voltage diectly from the alternator is about 13.4 and incresaes with revs slightly to about 14.1. switched ignition voltage incresaes from about 12.5 to 12.8. Battery voltage as before.

I wouldn't be suprised by a slight drop in voltage at the + side of the ballast resistor because with contacts closed, the reistance of the coil and ballast resistor may be a comparable order of magnitude as the resistance of the copper wires because it is a long wire to the coil. (maybe? thats probably complete s****.)

Anyway, ive found the cause of the popping. Was just ignition timing. I adjusted my points a while ago but didnt have a timing gun so didnt bother checking timing because I didn't expect it to have moved much. I think the way the mallory dual point works makes it very sensitive to adjusting the point gaps, and alters the timing a bit when they aren't quite right. Thats fixed now but still have the alternator problem. I do suspect alternator because the battery charging properly, it hasn't got as much kick starting, but now you have mentioned this low voltage in ignition circuit im a little wary of just buying a new one.

Alternator may be OK.
Check charging circuit:

Voltmeter pos to alternator output stud, voltmeter negative to battery pos lug, with engine running and alternator output at 14.1 the voltmeter should read less than 0.5 volt.
(use the stud at the alternator and not the nut or cable lug on alternator, similarly use battery lug and not the clamp or cable, you are looking for a bad joint.

Voltmeter pos to alternator casing, voltmeter neg to battery neg lug, again while alternator is charging battery the voltmeter should read less than 0.5 volts.

Any loss of voltage arriving at the coil, either through bad joints or ignition switch ect can be chased with a voltmeter in a similar manner.

Ideally, while alternator is charging at 14.1 volts the voltage at the ign switch or coil should be within 0.5 volts of the battery voltage, ie 14.1

jw

if your getting that voltage at the alternator but not in the rest of the circuits I think you have probably got a bad connection somewhere. Might even be a bad main earth, engine to chassis.

Infact im lieing. I haven't fixed the popping its still doing it. bugger. Ile check earths tomorrow. Is there a wiring diagram on the net anywhere does anyone know. Im at uni and left my haynes manual at home

This site may be of help, under schematics.
http://www.mgb-stuff.org.uk/electricsframe.htm
Dave