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Charging Issue (72 Blazer)

Discussion in 'The Garage' started by PULLEY, Jan 29, 2007.

  1. PULLEY

    PULLEY Newbie

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    Just purchased a 72 K5 Blazer, and it turns out there is a charging issue. While the motor is running, I only read 12.5 volts on the system (freshly charged battery) opposed to a normal 14.x volts. I have replaced the Alt, Reg, and it has a new battery. I am told that on the alt, the F term should read 12 volts when the ignition is on. Well, I'm not getting it. On the reg, I am only getting a 12 volt reading on the number 3 terminal which I understand is the cable running from the Alt to the battery. My guess is possibly the wire running from the ignition to the regulator? What are your thoughts. I appreciate any help. Thanks in advance.
     
  2. diesel4me

    diesel4me 1 ton status Premium Member

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    old style external regulator??..

    I assume your 72 has the "old" style externally reglated Delcotron?...yeah,if it does ,it needs voltage to the feild to energize it so it'll charge...no voltage in,no charge!...

    To test the wiring you'll need a test lamp(12V)...unplug the regulator and jump the #4 terminal on the plug to ground with the test light..if the lamp lights with the key on,the wire from the ignition is good..

    Then jump the plug's "F" terminal and the #4 terminal together with the test light leads..the bulb should light up with the key on..if it doesn't the wire from the "F" terminal on the alternator to the regulator is broken or dissconnected somewhere..some trucks have a fusible link in this wire..

    Also,my 72 K5 had two fuses hidden in the ammeter wires on both sides of the corners of the radiator supports --near the headlights and battery--if those fuses blow it wont charge!..they fuse holders are molded rubber,and dont really look like fuseholders!..check them!..

    Then put the test light from the "F" terminal on the alternator (unplug the 2 prong plug first),and hook the other test light lead to the #4 terminal in the plug on the regulator..the lamp should light up with the key on--if it doesn't the feild coil in the alternator is open curcuited (junk!)...

    95% of the trucks I work on that "need an alternator" DONT!...most of the time its a wiring problem..a quick test is to jump the "F" terminal and the #4 terminal on the regulator plug with a wire ,then start it and let it idle,do not rev it up(you can pop things running it unregulated!)...if the regulator was faulty it will now charge..if not its a bad wire or alternator..

    You can jump the "F" terminal at the alternator to the battery positive while its running too--if the alternator starts charging (you'll hear the noise level change and the motor idle drop some)then you can be sure its the wire going to the feild that isn't getting juice like it should...good luck..:crazy:
     
  3. Leper

    Leper 1/2 ton status

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    Crazy good info there.
     
  4. PULLEY

    PULLEY Newbie

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    Thanks for the great info!
     
  5. PULLEY

    PULLEY Newbie

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    Thanks for the help . I ended up going with a 1 wire alt.
     
  6. K5MONSTERCHEV

    K5MONSTERCHEV 1/2 ton status

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    I had the same problem on my 72. Changed the reg, then had problems, had it tested and it was bad, put another one on, msame problem. Ended up going 1 wire.
     

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