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sean, Ford Technician
Category: Ford
Satisfied Customers: 3105
Experience:  Ford Master Technician.
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my battery light came on while driving i have re-newed the

Customer Question

hi my battery light came on while driving i have re-newed the alternator and that cured the problem two weeks have gone by and now the battery light is coming on again a minute or two after ignition any ideas what could be wrong this time also i disconnected the battery after starting the car and it cuts out telling me that the alternator isn`t working yet again
Submitted: 3 years ago.
Category: Ford
Expert:  sean replied 3 years ago.
Hello, I hope the following assists you.
You should never disconnect the battery when the engine is running, this can cause damage to various electrical components and the engine will stop as the engine management would usually shut down.
To check the alternator output correctly requires a multimeter to measure the battery voltage with the engine running at 1500 RPM, with no electrical load the battery voltage should be between 14.1 V and 15.1V. With the lights on main beam, A.C on and blower on full and the engine at 2000 RPM the reading should be at least 0.5 Volts above the battery reading when the engine is off and no electrical loads are on.
The battery warning light is controlled by the engine management (PCM) via the GEM the PCM also controls the alternator charge rate, fuse 30 in the battery fuse box is the supply for the smart charge circuit and goes to pin 3 of the small alternator plug. It's not uncommon for the small smart charge wiring loom to rub through causing problems, if the smart charge circuit fails then the alternator usually produces a set voltage of about 13.5 volts, the battery light will be on even though the battery is charging in this situation.
I hope this helps but if you have more questions regarding this let me know and I'll reply as soon as I'm able to.
Best wishes,