My 2000 Discovery II was running very rough, not accelerating well and service engine light was on. Changed spark plugs a couple months ago - no huge improvement. Last week battery light started coming on and eventually car would not start. This weekend my boyfriend and I changed the spark plug wires using STI wires as recommended by this site... No more running rough and the all lights were off on the dashboard. Last night the Service Engine Soon light came on again and this afternoon the battery light came on again! Help! I'm afraid to drive it and need to for work!! (got new alternator in March if that makes a difference)

 

Your "new" alternator is most likely the culprit. If you guys have a voltmeter it is simple to check. If not, buy/borrow a battery charger and charge up the battery (or take it out and take to store to have quick charged). Then drive to a large auto parts store, where they will test battery and alternator for free. Could be loose wires on battery or underhood fuse box that need cleaning up and tightening. Driving truck with battery light on means you have 30 minutes to one hour of running time with no AC and no head lights. Car will drive with dead battery, because alternator keeps up the load. If alternator is dead, battery supplies to load for a short period of time.

BTW - voltmeter test is 13.8 - 14.2 volts across the battery terminals with truck running. As you switch on lights and AC this may drop a little, should stay above 13.6. Numbers in the 12's are weak, not charging battery.

 

Thank you! We took out the battery to return to Walmart because it is under warranty. They tested it and it read 12.6 volts and that was without the car running, obviously. They would not give me a new battery because they said it tested ok. Would changing the spark plug wires this weekend have effected the alternator?

 

The battery light means that the battery is not being charged, and that the alternator is not doing its job. Truck will run for a short period of time and run down the battery, Take truck with battery to an auto parts store, like Advance, Auto Zone, etc. and they will test alternator and battery for free. Testing battery alone won't show problems with alternator.

Not related to plug wiring usually, unless you knocked a wire off the back of the alternator.

If you drive around with battery light on, truck will use up energy in battery, electric fuel pump will quit, and you will be on the side of the road waiting for a tow truck or a spare battery.