2000 ford focus. I replaced bad alternator with a used one from junkyard, now the headlights and dashlights flicker at idle speed. What is causing this?
The alternator from the junkyard. More specifically, the rectifier in the alternator assembly. That’s the risk of using junkyard parts.
The people at the junkyard tested it for me before I bought it and it tested good. Could it show good if the rectifier was bad?
Most places do not test alternaters under load. The test only shows if it is putting out juice or not.
and you will soon have a dead battery.
More than likely a diode in the used alternator is defective. This can cause the lights to flicker. Either that or the brushes in the used alternator are so worn that the brush leads are now in contact with the slip rings.
IMHO it is not a rectifier diode. Usually a shorted diode will give no output and an open diode will give substantial less output but still steady.
Most likely the problem is with the brushes or runout of the slip rings. One way to diagnosis this is to turn the radio to an unused part of the AM dial and listen for random popping that is engine speed dependent. When the brushes lose contact with the slip rings they will produce a lot of RFI.
You might just get by with replacing the brushes.
Hope I have helped