Expert: Brad Sears - 7/22/2011

Question
QUESTION: 1972 Ranchero, all stock, 351 Cleveland, 2bbl. Ac, automatic trans. Problem started 1 month ago, after driven approx. 10 miles and running A/C,stopped for about 30 minutes, tried to start car, battery was dead. Got a boost to start, and noticed started solenoid on positive side was arching, battery cable was loose. Tightened it, and drove home without A/C on. Next day car started ok, and did every day approx. week, then started running A/C again and after sitting at stop light car would stall, battery dead. Got a boost, drove home and parked it. Tried next AM and battery was dead. Replaced Alternator and voltage regulator, did not correct problem, removed alternator and checked positive post on alternator and it was showing output with test light, took alternator to local shop, they checked alternator and battery, and said both were ok? Reinstalled, and no change, so I bought another voltage regulator, after checking it with digital multimeter, and when turning on headlights voltage would start dropping from 12.40 to 11.11 and continue dropping until car would die. So after installing another voltmeter, at least charge would stay at 12.40 and when turning on headlights would drop to 11.10, and stay there, but after awhile start dropping very slow, as it was taking charge from battery. Pulled off positive battery cable from battery, and car would stall immediately, so still not charging at all. "HELP"

ANSWER: It is evident that the charging system is not functioning and the car is running on the battery. The alternator voltage regulator needs to see a signal that you have turned on the ignition switch in order for the system to charge. based on what you have said that you have already done I would check this system first. If the car was built with an idiot light on the dash for the charging system it receives this signal at the "I" terminal of the regulator. Using a voltmeter check for voltage there. With the key on you should see battery voltage. If there is no voltage the problem is back towards the ignition switch. There is a parallel circuit coming from the switch to this terminal. One leg goes through the idiot light bulb and the other lag goes though a 15 ohm resistor that is in the wiring harness. If the car is equipped the a factory ammeter, then the terminal that acts as the sensor for ignition on is the "S" terminal. If these test ok, then turn on the ignition and place a wrench on the pulley end of the alternator shaft. DO NOT DO THIS WITH THE ENGINE RUNNING! With the key on the end of the shaft should be magnetized and you should be able to feel the pull. If there is no magnetism felt then the regulator is defective or the alternator. However make sure that the voltage regulator has a good ground that that there is a good ground between the engine and the body. If the ground are not good and free of resistance, rust and corrosion, then all sorts of strange this will happen.
Good luck and keep me posted.


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QUESTION: when driving with a/c on, after approx. 3 miles, when accel. either from sitting at light, or accelerating onto freeway, car tries to stall, acts like running out of gas, pump accelerator, and will finally be ok, usually turn off a/c and no problems until a/c turned back on, then problem will repeat?

Answer
It could be a problem in the fuel system like a partially plugged fuel filter or low pressure from the fuel pump or a defective accelerator pump in the carburetor. Or it could be a weak ignition coil. Both of these are items that are stressed by acceleration, putting the engine under load, and the added load of the air conditioner. This is one of those gotta get the test stuff like the fuel pump pressure gauge and start testing.