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ANYONE KNOW HOW TO TEST AN ALTERNATOR?

Discussion in '1973-1991 K5 Blazer | Truck | Suburban' started by Brians 60s, Nov 22, 2001.

  1. Brians 60s

    Brians 60s 1/2 ton status

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    without taking it off the rig. I have a multi meter, but I am electrically illiterate, and lazy this thangsgiving. thanks electrical people. Brian

    IT'S A 1 TON THING.... IF I HAVE TO EXPLAIN IT, YOU WOULDN'T UNDERSTAND!!!
     
  2. hammer

    hammer 1/2 ton status

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    set the multimeter to 20 volts( yours might be the same as mine but anyway) check the volts at the battery with the engine off it should read 12.6 volts or really close to it. With the engine running still at the battery it shold read between 13.6 and 14.6 volts if it's to high the regulator is going or gone.
     
  3. Brians 60s

    Brians 60s 1/2 ton status

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    to high? it would be gone? what if it just isn't working at all? would it read the same when running or not? sorry for all the questions and thanks much for the help. Brian

    IT'S A 1 TON THING.... IF I HAVE TO EXPLAIN IT, YOU WOULDN'T UNDERSTAND!!!
     
  4. The easy (redneck :-)) way to do it, is to start the engine, and then pull the negative cable off the battery. If the engine dies, the alternator is bad. I've done it half a dozen times, and it works every time.

    American by birth, Marine by the grace of God
     
  5. BlueBlazer

    BlueBlazer 1/2 ton status

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    Yes, if it is bad it will just read the close to the same (like within a couple tenths of a volt) at a high idle. If it is real high like over 15 it is also bad. The voltage regulator is part of the alternator, so it must be replaced. So, if its the same or close to the same as the battery or over 15 or so I would replace it. There arent real expensive last time I checked. Also, youll want to maybe pay a couple bucks (or may be free in your area) and have the alternator and battery tested with a charging/starting system diagnostic tester. This will tell you everything you need to know about them, as long as the technician knows how to use it.

    82 K5, 6.2L Diesel, sm465, np205, 14 bolt welded, 8 lug 10 bolt lock right, 410 gears, 35" Mud Kings
    SPEAK SOFTLY AND DRIVE A BIG TRUCK
     
  6. solowookie

    solowookie 1/2 ton status

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    another thing to try that is much easier is to stick a screw driver to the back of it, and if it feels magnetic then the altenator is OK...

    <font color=blue> Jeff - may the force be with you</font color=blue>
    <a target="_blank" href=http://coloradok5.com/gallery/Jeffs-Stuff>link to k5's</a>
     
  7. EDdaTREE

    EDdaTREE 1/2 ton status

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    I bet nobody ever knew that that little hole in back of the alternator on GM's will switch it over to full charge mode as a test mode if you push it in with a probe! Could be handy info.

    9/11/01..Never forget...Never forgive
     
  8. BlueBlazer

    BlueBlazer 1/2 ton status

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    Doesnt that just bypass the regulator thereby not showing if it is good or not.

    82 K5, 6.2L Diesel, sm465, np205, 14 bolt welded, 8 lug 10 bolt lock right, 410 gears, 35" Mud Kings
    SPEAK SOFTLY AND DRIVE A BIG TRUCK
     
  9. EDdaTREE

    EDdaTREE 1/2 ton status

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    I believe it does, but it might just get you home if she's not charging.

    9/11/01..Never forget...Never forgive
     
  10. Twiz

    Twiz 1/2 ton status

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    "Street testing an alternator"

    Check cannections at the battery make sure they are clean and tight, free of corosion. check the battery for leaks, check the case to see if it's swoolen.

    Visualy check cables and fuseable links if appliciabe, also check for poroper grounds. (battery to frame, frame to body, body to block, Battery to block)

    If you have a voltag gage, check its reading with the engine off. It should be above 12v (12.4v) *Stock gages are commonaly inacurate and when the ignition switch is turned to the "run" position, there will be a extra load on the battery so the voltage reading will be drawn down further.*
    best bet is to use a multi-meter if you have one. Check the voltage at the terminals wit the key in the "off" postion, It should be 12v to 13v (12.4v) - recharge the battery if the reading is low.

    Start the engine and retest the voltage at the battery with everything turn off, It should be 12.4v or better.

    Raise engine RPM to about 2500 RPMs and retest battery voltage, It should be 13v to 16v (14.5v)

    Turn on the headlights on and to high-beam (aprox. a 30 amp load) Raise engine RPM to 2500 and retest at th battery, It should be 13v to 16v (14.5v)

    Don't remove battery cables from the battery while the engine is running. This may cause a sudden and extream load on th battery/alternator and may cause internal damage to the alternator. There is also a possiblility of a battery explosion, if the battery level is low or has a internal short.


    <font color=blue>Twiz
     
  11. Burbinator

    Burbinator 1/2 ton status

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    Like Twiz said previously, pulling the battery cable(s) is an extremely BAD idea. The alternator will sense the voltage drop and go into "full-boogie" charge mode, effectively destroying the tri-diode network in a matter of seconds from the increased and rapid current draw, possibly more.

    The learning curve is a toll road...ride with someone who has already paid!
    <a target="_blank" href=http://photos.yahoo.com/abshort1>U.S.S. Nimitz II</a>
     
  12. chevyracing

    chevyracing 1/2 ton status

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    And it could BBQ your ignition system.

    John

    Like to go sloppin' 'round in da mud in a rapid fashion....=)

    <a target="_blank" href=http://coloradok5.com/gallery/albun31?&page=1>See my pics here</a>
     
  13. solowookie

    solowookie 1/2 ton status

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    I had an altinator once that was fried because it had a loose wire going to be battery.

    <font color=blue> Jeff - may the force be with you</font color=blue>
    <a target="_blank" href=http://coloradok5.com/gallery/Jeffs-Stuff>link to k5's</a>
     
  14. high&liftedup

    high&liftedup Registered Member

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    Brian, If you have to replace it, as a recomendation, I would NOT buy a cheap alternator from places such as "Kragen" etc... I have found they just dont last... put out some $$ and purchase a new one or go to a auto electrical shop and have it rebuilt to a higher rating......

    HE will always get you out of the mire clay!!!
     
  15. solowookie

    solowookie 1/2 ton status

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    I vote for having it rebuilt! man I've had some rebuilt (not getting a rebuilt from a parts store... YUK), but taking them to a shop like you mention; man that is the ONLY way to go!

    <font color=blue> Jeff - may the force be with you</font color=blue>
    <a target="_blank" href=http://coloradok5.com/gallery/Jeffs-Stuff>link to k5's</a>
     
  16. Ryan B.

    Ryan B. 3/4 ton status GMOTM Winner

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    "but it might just get you home if she's not charging."

    I think this would be a bad idea... When you do a full field test or stick a screwdriver in the back of the alternator and ground it against the case, it bypasses the voltage regulator... You're supposed to watch your voltmeter while you do this. The voltage should go up..up...up. and not stop because you bypassed the V. regulator. All manuals will tell you that you don't want to do this for longer than a couple seconds to get a reading.
    If you tried to stick a screwdriver in there and leave it...
    It should be a matter of minutes before you see explosive Hydrogen gas smoke coming out of your battery because now it's way overcharged and cooked well-done.
    My 2 cents.
     
  17. Ryan B.

    Ryan B. 3/4 ton status GMOTM Winner

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    IF you suspect the charging system is at fault, theres a test you can do without disconnecting a thing. Just a voltmeter to the battery.

    1.Read base battery voltage with key off (for example 12.6 volts)

    2.Start engine and check battery voltage with engine @ 2000 RPM's and make sure all electrical loads are off... All the lights, electric fans OFF... etc. (Voltage should increase, but no more than 2 volts from base voltage ...example, no higher than 14.6 volts)
    IF it is higher than 2 volts above base, voltage regulator is bad.

    3. Now turn on all electrical loads... electric fan, lights, heater, radio... etc. And check voltage @ 2000 RPM's
    The reading should be somewhere between the two previous readings, BUT AT LEAST .5 volt above base voltage (example: at least 13.1 volts) This makes sure the alternator can meet the needs of all the electrical loads in the vehicle, and still have a half a volt (.5 v) left over to charge the battery.

    If your last reading is not at least .5 volt above base voltage, then you have too many electrical loads for the alternator to support and still charge the battery. IT would probally be wise to get a higher amperage output alternator.

    My 2 cents ...Hope this helps
     
  18. solowookie

    solowookie 1/2 ton status

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    man you don't have to stick a screw driver into the back just lay it across the back to see if it feels magnetic... pretty easy.

    <font color=blue> Jeff - may the force be with you</font color=blue>
    <a target="_blank" href=http://coloradok5.com/gallery/Jeffs-Stuff>link to k5's</a>
     
  19. jarheadk5

    jarheadk5 1/2 ton status

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    Pulling the cables off the battery while the engine is running is also a good way to cook the computer, for those of us with fuel injection.

    I know this trick DOES work, though (seen it done too many times...).

    "...For the land of the free, and the home of the brave"
     
  20. chvyhs

    chvyhs 1/2 ton status

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    Does your truck have an internal or external voltage regulator? Is it the original set up or has some one mixed thing up under the hood? I changed altenators and external voltage regulators more times than I could count on my '66 Chevelle. It seemed like one would go bad and cause the other to go bad in a big circle. I finally changed my car over to an internally regulated altenator. Good luck.
     

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