jayjay

Please could somebody help, l have a 2000 Ford Glaxy GHIA TD AUTO 1,899CC, in the past week l have had problems starting the car. if l turn key on, it makes that initial sound like its gonna start but it wont kick. l have noticed that if l jump start, it kicks right away as normal. l have checked battery voltage its 12.8V, so there is enough power for the car to start. My alternator seems to be working, its giving a voltage of 14.28V. I have checked battery overnite, there is no battery drain. What could this be? why wont it start with its own 12.8V? Why does it need jump starting?