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Matt, Mechanical Engineer
Category: Car
Satisfied Customers: 22933
Experience:  BEng hons Mech engineering, in auto industry 22 years
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New alternator fitted new battery still won't charge

This answer was rated:

new alternator fitted new battery still won't charge


If you're sure that the alternator isn't charging the battery ( does the battery voltage increase after engine start?) I'd start by tracing the small diameter wire back from the alternator

as the alternator is switched over to charging by the engine ECU and if the wire is broken or has a poor connection then you'll have issues

if the continuity between this small wire connection and the ECU socket is OK then its possibly the ECU is at fault and it can be sent off for repair


Customer: replied 1 year ago.
hi matt thanks for that when volt meter plugged in cig lighter shows 11.5when battery disconnected goes up to 14+


I don't quite understand

you're measuring voltage at the cigarette lighter and with the engine off its at 11.5V and when the engine is started it rises to 14V?

if so then the alternator is working fine

Matt and other Car Specialists are ready to help you
Customer: replied 1 year ago.
no the engine is running at 11.5 but when you disconnect the battery engine still running it goes up to 14+


so the battery isn't being charged then:

either the new alternator is faulty


the 'sense' or switching wire on the alternator is somehow broken so the signal isn't getting back to the ECU


The ECU is faulty

I'd start by tracing the alternator wiring back as the next step