Edmunds Answers


  • zaken1 06/29/10 3:11 am PST

    What usually happens in these cases is that the parts which were tested were not tested properly.

    A battery can only be tested with a load tester. It will test good with a hydrometer or a voltmeter even when it doesn't have enough reserve power to reliably start an engine.

    An alternator test must include a measurement of both the charging voltage (between 13.5 & 14.5 volts when idling) and also how much current it draws from the battery when the motor is not running. But the real test is of the alternator when under load; it sounds to me like the alternator is perhaps capable of keeping the battery charged during daytime driving, but cannot supply enough power to run the lights, accessories, and ignition; and keep the battery charged at the same time. This is often caused when one or more diodes in the alternator fail (which will reduce the maximum power output of the alternator by 33% to 66%; depending on how many diodes have failed). When that happens; the battery will gradually go dead during night driving; but may do much better during the day. The way to test the alternator is to start the engine, and first measure the voltage across the battery terminals with the engine idling and no lights or accessories running; it should be between 13.5 and 14.5 volts. Then turn on the headlights to high beam and set the heater fan to the highest speed. With the engine still idling; if the voltage drops below 13 volts during this test, and the alternator drive belt is not slipping; the alternator should be replaced. If an alternator has one or more shorted diodes in it; it will slowly drain the battery while the car is parked. A good electrical system will draw less than 50 milliamps of current from the battery while the motor is stopped and no electrical accessories are running. If it draws more than that, there is either a bad alternator diode or an electrical short somewhere.

    Battery cables are properly tested by measuring the voltage difference between the two ends of the cable while the starter is running. A good cable should have less than 0.5 volts drop through it when the starter runs. Be sure to test both the positive cable and the ground cable. It often turns out that there is substantial voltage drop at the connection between the battery cable clamps and the battery posts. Battery cable clamps will develop invisible corrosion on their inside surfaces. The best way to clean this is with a tapered reamer type battery cable service tool.

    A starter should be tested by both measuring the cranking current draw (typically between 150 and 200 amps) and also measuring the voltage drop across the solenoid during cranking. There should be less than 1 volt lost across the solenoid during cranking.

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  • bigbear6 06/29/10 6:06 am PST

    Thank you for your answer,but they check all parts at local auto parts thats why I thought there might be a relay I might I've missed. tried to jump start that wont work,maybe check starter again?

  • zaken1 06/29/10 7:53 pm PST

    Many parts stores neither have the equipment nor the expertise to properly test the items they offer to test. This is a sad but very true issue. You can test the starter yourself by turning the headlights on, and watching their brightness while you try to start the motor. If the lights stay bright (which I highly doubt will happen, judging by your report) there is a bad relay. But if the headlights dim way down or go out when you try to start; this means that either the starter is bad or the battery power is too weak to run the starter. Since you have a new battery; and a jump start has not made it any better; I would suspect a shorted starter armature. This will not show up in a casual starter test; in which they just connect a battery and see whether the starter turns with no load. The starter would need to be tested when it is either locked or under a load; and most parts stores do not have the equipment to do that kind of test. So I would suggest either buying an inexpensive inductive starter current meter, which is used by placing it against a battery cable, and reading the current while cranking the motor (should be less than 225 amps) or just replacing the starter. I need to caution you that many parts stores sell rebuilt starters which are basically junk. The only source I trust for properly remanufactured starters is NAPA auto parts stores (and even then; only when the starter is from their premium line; part # RAY 449278 for your vehicle).

  • zaken1 06/29/10 11:07 pm PST

    On reflection, it occurred to me that it is far more likely than anything else that the starter solenoid (which is mounted on the starter) probably has developed excessive resistance in its contacts; and this is what is causing the starter to run slowly. If this were the case; the lights would not dim in the headlight/starting test mentioned in my previous post. So, if the lights do not dim, I would install a new solenoid on your existing starter. I don't believe that parts stores test solenoids when they test a starter (although it would be a great service to customers if they did). Sorry it took a while to come to this realization; but my mind sometimes does not produce on demand.

  • bigbear6 07/07/10 3:31 pm PST

    Okay guys the anwser is A>C> comp. locked up would not turn over thats why the battery cables would get hot sounded like it wanted to start but comp. was bad i thought that was a dead give away cables i gave you alot of hints. Thank You beware next time for bigbear just might get you again Tech 101?

  • zaken1 07/07/10 4:08 pm PST

    Well, Big Bear; having taught electrical and engine theory, I can appreciate an occasional test. But when you don't provide all the necessary information; the test becomes invalid and unfair. YOU NEVER POSTED THE INFORMATION THAT THE CABLES BECAME HOT. This information would have been a dead give away. But you cheated by concealing (or overlooking that vital detail). So I would give your attempt at educating us an "F." Don't blame us for your failure to provide all the required information. I think you owe us an apology.


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