Issue with charging battery

[PeterGadsby]

Hi,

My son's MX5 NB has a fault where the battery is not charging, a mechanic has replaced the alternator, the belts and the battery.

However when we run the car the battery shows about 11.9volts and under load less than 8v which clearly shows the alternator is not charging.

We would like an auto electrician to have a look to help find the problem, can anyone recommend one in the Berkshire area with experience of MX5 NB's

Thanks

Pete

[Farkand]

The NB alternator needs a 12V “turn-on” signal from the charging warning light circuit. Without it, the alternator won’t produce voltage.

Test: With engine running, measure voltage at the alternator’s small L-terminal (usually a single thin wire). It should be battery voltage (~12V) . If 0V, the alternator won’t charge.

Common failure: The instrument cluster charging light bulb is burned out or the circuit trace is broken. On an NB, if the bulb fails, the alternator gets no excitation.
Quick check: Does the battery light come on with ignition ON (engine off)? If no, that’s your problem.

[PeterGadsby]

Thanks - It turned out it was the battery at fault.

We tested it by disconnecting from the Mazda battery and connected the leads to another cars battery.

We then measured the voltage at the other battery which was around 14 volts. we put the car under load by turning on everything electrical and the engine revved. We got a consistent 14 volts. So that proved the alternator was charging the battery.

So we replaced the battery and it all works fine!

Cheers

Pete