The first thing you need to do is fetch a digital voltmeter. Harbor Freight Tools has a perfectly fine one for around seven bucks. Walmart, Sears, and Radio Shack have 'em too, but don't waste your money on features you'll never use, especially auto-ranging.
Measure the battery voltage with everything turned off. It must be 12.6 volts if it's fully-charged. If it is lower than that, charge it at a slow rate for an hour, turn the head lights on for five seconds, then measure it again. Assuming the problem is still there, measure the voltage while a helper tries to crank the engine. That has to be done with the voltmeter probes right on the battery posts. Next, move each probe 1/2" to the cable clamps and measure again with the helper trying to crank the engine. We're looking for any connection where the voltage suddenly drops lower. For example, if, during cranking, you find 11.2 volts across the posts, and 10.4 volts across the cable clamps, one of those mechanical connections is loose or dirty. Normally you'll find an even bigger voltage difference.
Sometimes the chattering on and off of the starter solenoid makes digital voltmeter readings bounce all over and unreadable. You can try looking for the same drops in voltage by turning on other loads instead, like the head lights and heater fan.
Thursday, February 20th, 2014 AT 5:44 PM