MG Car Repair: Rover battery light pops on while driving, vacuum booster, booster systems

Question
My rover 25 1.4 gl is having over heating and apart from that when ever i am driving the car the battery light comes on and goes off again.Sometimes it happens when i am using the brake and clucth together.I love the car and don't know what to do about it.A friend directed me to this site that i will find help.

Answer
Hi Chibuzor,

I don't know your car at all, but electrical problems are diagnosed about the same on all cars (except the new cars that use computers to do everything)

The charge light on most cars is also the "exciter" power for the alternator or generator. The brake light circuit usually has nothing to do with the charge light circuit except that the brake lights them selves draw a lot of current and if a charging system is not up to par the extra load of the brake lights can make a charging system be short on power and make the light come on.

If you can't duplicate the problem while the car is in the shop, do this. Run two long test leads from the battery to inside the car and connect the two leads to a volt meter. You will see 12.5 volts or a little more. Now start the engine and read the volt meter. Set the idle to a fast idle and it should read 13.8v to 14.5v (not more nor less).

Now, with it at idle and the voltage reading 12v+, step on the brakes and watch the voltmeter. It may twitch but not read less voltage.

If that is ok then drive the car like you normally do but keep an eye on the charge light and voltmeter. Any time that you can get the charge light to come on when you step on the brakes quickly read the voltmeter to see what the voltage is at the time of the light on.

It should never drop below 13 volts.

Next, does this car have a tach? If so what is the lowest idle you see when you back off on the throttle to apply the brakes? Also, does this car have a vacuum brake booster? Some vacuum booster systems can have a problem that makes the idle falter when you back off on the throttle and apply the brakes which then drops below the RPM that the charging system can operate at and cause a charge light to come on.

All of this needs to be considered when looking for a charge light fault.

let me know,

Howard