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  • avatar zaken1 09/03/11 12:39 am PST

    Your suspicion that the alternator is putting out 2/3 power sounds very likely to me. The one other likelihood is that the ignition switch has developed excessive resistance in its contacts. It may be that both of those issues are present in your truck.

    Although your dashboard voltmeter sounds like it might be accurate; I would not depend on that. If you own, or can borrow or buy a digital voltmeter; it would be a preferred tool for sorting out this problem. You will need to tap into the power wire at the ignition coil in order to make voltage measurements. This coil has three terminals in the connector; and one of the two outer ones is the power lead. The way to identify the power terminal is to set the meter to read a DC voltage of about 12 volts, ground the negative meter lead to a clean bolt which threads into the engine, disconnect the wiring harness plug from the coil; turn the ignition switch to the position where the warning lights on the dashboard light up, and touch the positive meter lead to each of the two outer terminals in the coil plug in turn; while reading the voltage at each one. One of those terminals will have about 12 volts on it; while the other one should have much less voltage. The terminal with 12 volts is connected to the power wire. You'll need to tap into that wire; either by inserting a pin into the other end of the plug alongside the connector, or by using a pin to pierce the insulation of the wire at some point where it will not touch any grounded objects. Once the pin makes a secure contact with the wire; it may be helpful to tape it in place, so it doesn't fall out from vibration or movement. Attach the positive meter lead to the pin; making sure the bare portion of the meter lead is prevented from touching any metal or grounded objects.

    Start the motor, and after it has run for a few minutes with no electrical accessories on; read the voltage at the coil power wire. Then move the meter leads to the battery terminals and read the voltage there. The voltage at the battery should be between 13.5 and 14.5; while the voltage at the coil wire (when the battery voltage is within the stated limits) should not differ by more than 0.7 volts from the battery voltage. If the voltage at the coil ever differs from the battery voltage by more than 0.7 volts with the motor running, in any of the tests done here; the ignition switch has excessive resistance. If the voltage at the battery ever falls below 13.5 while the motor is running; the alternator is not producing enough power. When the battery voltage is below 13.5; the voltage at the coil should not be considered; because low battery voltage can cause an excessive voltage drop through a normal ignition switch under some conditions. It is only when the battery voltage is above 13.5, that the voltage difference with the coil becomes meaningful.

    If the readings so far have been within normal limits; turn on the headlights to high beam and repeat the test with the motor running. If those readings are normal; turn the heater fan to high speed while the headlights are on high beam, and again measure the voltage. If the battery voltage stays above 13.5 with the engine idling, while the headlights and heater fan are both on high; the alternator is probably adequate. You could also try turning on the A/C, instead of the heater fan; but I would expect the voltage would drop long before that point, if the alternator was only running on 2 phases. Be sure to measure the voltage at the coil in each of these tests. If you find that the coil voltage drops below the 0.7 volt difference; while the battery voltage still is in the normal range; then the ignition switch is defective.

Answers

  • zaken1 09/03/11 12:39 am PST

    Your suspicion that the alternator is putting out 2/3 power sounds very likely to me. The one other likelihood is that the ignition switch has developed excessive resistance in its contacts. It may be that both of those issues are present in your truck.

    Although your dashboard voltmeter sounds like it might be accurate; I would not depend on that. If you own, or can borrow or buy a digital voltmeter; it would be a preferred tool for sorting out this problem. You will need to tap into the power wire at the ignition coil in order to make voltage measurements. This coil has three terminals in the connector; and one of the two outer ones is the power lead. The way to identify the power terminal is to set the meter to read a DC voltage of about 12 volts, ground the negative meter lead to a clean bolt which threads into the engine, disconnect the wiring harness plug from the coil; turn the ignition switch to the position where the warning lights on the dashboard light up, and touch the positive meter lead to each of the two outer terminals in the coil plug in turn; while reading the voltage at each one. One of those terminals will have about 12 volts on it; while the other one should have much less voltage. The terminal with 12 volts is connected to the power wire. You'll need to tap into that wire; either by inserting a pin into the other end of the plug alongside the connector, or by using a pin to pierce the insulation of the wire at some point where it will not touch any grounded objects. Once the pin makes a secure contact with the wire; it may be helpful to tape it in place, so it doesn't fall out from vibration or movement. Attach the positive meter lead to the pin; making sure the bare portion of the meter lead is prevented from touching any metal or grounded objects.

    Start the motor, and after it has run for a few minutes with no electrical accessories on; read the voltage at the coil power wire. Then move the meter leads to the battery terminals and read the voltage there. The voltage at the battery should be between 13.5 and 14.5; while the voltage at the coil wire (when the battery voltage is within the stated limits) should not differ by more than 0.7 volts from the battery voltage. If the voltage at the coil ever differs from the battery voltage by more than 0.7 volts with the motor running, in any of the tests done here; the ignition switch has excessive resistance. If the voltage at the battery ever falls below 13.5 while the motor is running; the alternator is not producing enough power. When the battery voltage is below 13.5; the voltage at the coil should not be considered; because low battery voltage can cause an excessive voltage drop through a normal ignition switch under some conditions. It is only when the battery voltage is above 13.5, that the voltage difference with the coil becomes meaningful.

    If the readings so far have been within normal limits; turn on the headlights to high beam and repeat the test with the motor running. If those readings are normal; turn the heater fan to high speed while the headlights are on high beam, and again measure the voltage. If the battery voltage stays above 13.5 with the engine idling, while the headlights and heater fan are both on high; the alternator is probably adequate. You could also try turning on the A/C, instead of the heater fan; but I would expect the voltage would drop long before that point, if the alternator was only running on 2 phases. Be sure to measure the voltage at the coil in each of these tests. If you find that the coil voltage drops below the 0.7 volt difference; while the battery voltage still is in the normal range; then the ignition switch is defective.

  • dgarrett13 09/03/11 9:59 am PST

    Before I your answer, I did use the voltmeter to test the battery. What I found was that with only the engine running the battery was around 14.5 however, when I turned on the highbeams, ac, and radio it dropped to 12.9. I am taking the alternator to be tested. If it passes, I will follow the directions you provided to check the ignition switch.

    I really apprectiate your help.

  • dgarrett13 09/04/11 4:35 pm PST

    Ok, alternator checked good but I got another one just in case (the thread on the back of the alternator was lose when we took it off and it seemed to have a small crack in it). Well, we put it on and the voltage did seem better. However, when driving it, it shut off again when I slowed down. I started thinking about it and this only happens when my ac is on. When I turn the switch from ac to vent, my volts drop below 13 and it almost goes dead. It does go dead and the fuel pump doesn't seem to react (sometimes) when I press the gas peddle but only whend the AC is on!! Also, when my lights were dim the other morning, I had the defrost on cold sol the ac compressor was running. When I don't use the ac, the blazer runs GREAT!! So far, we have changed the rotor button, idle air control valve, throttle position sensor, ignition coil, alternator, and we had the ignition control module checked, and the throttle body plates cleaned. It does run so much better than it did one month ago. I am sure it needed the IAC valve changed and the throttle body cleaned. But now I am at a stand still. I think I will just have to take it to a 3rd mechanic!!! I am thinking this time it is more electric related than fuel related this time. Maybe a short somewhere in the AC system. Maybe O2 sensors?

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