You have just replaced your car’s alternator and battery. You are pretty sure that both are working fine yet when you park your car for an extended period, your car fails to start – no crank, no lights on the dashboard!
In this post, I will look at the 4 common reasons why your car keeps dying yet your battery and alternator are in good working condition.
1. You may have a parasitic load draining the battery
If your car battery is drained after sitting for some time then you may have a parasitic load draining the battery.
Conduct a parasitic draw test. You can have your car checked by a qualified mechanic or if you are up to it, proceed to check the parasitic drain yourself!
- With the car’s ignition turned off, using a digital multimeter (set to read current in mA), disconnect the wire connecting to the positive battery terminal.
- Next, measure the current between the battery positive terminal and the positive wire.
You should expect about 50 mA depending on the car’s model. 100 mA and more indicates a parasitic load that is draining your battery flat!
- If you suspect a parasitic load, proceed to measure the current flow at the car’s fuses box. You should be able to identify the circuit that is responsible for the current drain – Perhaps a faulty relay for AC, clutch, light left on, have you recently add a gadget to your car’s electrical system that is always on?
Disconnect the faulty circuit and measure the current flow. If the current draw drops to 50mA then you can be sure that the parasitic drain problem is solved.
2. Your car battery may not be fully charging
Your car can fail to start because your battery is not fully charging. A fully charged battery should measure about 12.6V. Assuming that your alternator and battery are fine,
possible reasons for your battery not charging fully and their fixes are shown in the table below
|No||Reasons why car battery may not charge||Fix|
|1||Battery wires not making firm contact with battery terminals.||Make sure the alternator plug is firmly plugged in its socket.|
|2||Some car models have a fuse for the alternator||Check fuse for continuity and replace if blown.|
|3||Broken cables restricting high current flow||Inspect the wires connecting to the battery and make sure they are not damaged|
3. Your car may have loose wires between the battery and the fuse box
If there is no parasitic drain and your battery is fully charged yet your car fails to start, then it may be a case of loose or damaged wires between the battery and the fuse box. You can inspect them to confirm that they are firm or have them checked by a mechanic.
4. You may have a bad alternator
I know we said that the alternator is new or good but it is possible to have bad alternator from the manufacturer. It may be able to charge but fail to produce enough voltage because of (a) faulty diodes for example.
So it is worthwhile testing it again.
You can test the alternator with a portable tester or have a qualified mechanic check it out.
In summary, if you think that your car’s alternator and battery work well but your car fails to start, check for parasitic drain, loose alternator plugs, loose battery wires.
You may also need to check that the wires between the battery and the fuse box are not damaged is your car is not able to present.
If your car continuously dies, you may need to test the alternator. A properly functioning alternator with engine started and on idle, when measured across the battery, should measure between 13.5 -15.5V