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ALT or battery?

Discussion in '1973-1991 K5 Blazer | Truck | Suburban' started by ftn96, Nov 29, 2001.

  1. ftn96

    ftn96 1/2 ton status

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    My voltage guage while driving shows 14-16 amps/volts whatever.

    But at night when I have the light on at a red light the guage drops down to like 9 ish and the lights start getting dim. But soon as I take off it goes back up to 13 ish.
    Im thinking this is a battery problem. But Im no good with electricity.



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  2. Blue85

    Blue85 Troll Premium Member

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    Just set the idle speed higher! [​IMG]

    The VOLTmeter on the dash measure VOLTage, not current (amps). I think ammeters went out sometime in the 70's.

    This sounds like an alternator problem, if not more. 16Volts is way too high and 9 volts is way too low. The voltage on your battery will not change really quickly unless it is really bad. But if this is all due to the battery, the truck should be cranking slow at start. Your low beams should only draw 20A or so and that shouldn't be a problem for your alternator, even at idle. The meter on the dash is not terribly accurate, so it's possible that your voltage isn't changing as much as you think.

    There was a thread a couple of days ago about checking the alt and batt without any special tools.

    <font color=green>Oh man, 4:10's are sure better than 3.08's!</font color=green>
     
  3. Burt4x4

    Burt4x4 3/4 ton status

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    I had a similar problem with my lights dimming and brighting with the RPM speed. Turned out to be my headlight ground wires that bolt to the rad. support frame. Should be one for each light. I took the ground wire off and sanded the bolt hole area to expose raw steel again and put it all back together and bam! no more light problems.....may want to double check all your ground wires!
    Good luck

    72K5[​IMG]Led Zeppelin[​IMG]Rock ON![​IMG]
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  4. DuneRunner

    DuneRunner 1/2 ton status

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    FTN96, I recently changed my Alt. due to a 9.6 reading on my Brand new Exide battry. It dieing on me. But $76 later(new alt) from NAPA Auto parts and 20 min. later to install it still didn't start. I had to charge it for 1.5 hours on the charger it started right up. The tall tail sighs to a alt going is the dimming lights on the dash/ headlights. Because of alt not charging bat. My old battery lasted 5yrs. Now with a new bat and alt I'm pulling close to 16. At idle 13...
    I hope this makes sence...

    Later KEN

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  5. ftn96

    ftn96 1/2 ton status

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    Well, folks here is a little history.
    After I had the truck for about 6 months I was bored and decided to check alot of little things. (this was before hte problem) I checked the "battery fluid" [​IMG] and it was low so I added some distilled water to top it off. Ever since then I've had this problem.
    Now, as far as in the morning even after a long cold night it starts right up.
    So one thing tells me its the battery but the later tells me its the alternator. Hell maybe I'll just replace both of them.
    Donations? [​IMG]

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  6. DuneRunner

    DuneRunner 1/2 ton status

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    Ya that is basically what I did 2 weeks ago. About 2 hun later. She's fine. If you want the peace of mind take it to a good Auto parts store (NAPA out here in CA) and have them do a test on your bat. in the vehicle with it running. That way you don't have to do the guessing game [​IMG]

    Later KEN

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  7. KRAZIE87K5

    KRAZIE87K5 1/2 ton status

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    Mine does the same thing - at idle, the lights dim, and the gauge reads 8-9 volts. I have a brand new batt and alt.

    Now what do I do?

    <font color=red>GOT MUD???</font color=red>
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  8. Executioner

    Executioner 1/2 ton status

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    You mean remanufacutered/rebuilt alt. ?
     
  9. dyeager535

    dyeager535 1 ton status Premium Member

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    Put a voltmeter on the battery when its idling, no load (lights off, etc) then turn a bunch of stuff on (heater on high, high beams, etc) and measure again.

    You will be amazed at how much difference cranking the idle up just 100 or 200 RPM's may make. (I've got a tach, I checked it) at 550RPM's, idle with load, my alt puts out about 12.2 volts (varies slightly) If I crank the idle up to 700, I'm easily at 13+ volts, load or not. I've done this "test" with both the 42amp alternator I had on the truck originally (tested before and after I rebuilt it) and also with the 67amp (could be more) alt I picked up, which was also tested before and after I rebuilt it.

    Heres a bit from a friend of mine on one of the chev car mailing lists, about the same thing:

    "The problem is simple. Older GM alternators suck and the sooner you get rid
    of it, the happier you will be. It's not going to fix itself. Also, the
    rebuilt units that are out there are usually barely functional. Bite the
    bullet and get a brand new, newer style 140 amp unit that can charge at
    idle. Even the two Powermaster 100 amp new alternators I bought didn't
    charge much at idle (Powermaster is junk, IMO, based on their products. Had
    to send one back and the other had to be combined with my old alternator
    because of extreme run-out in the shaft! Good grief!)

    Jegs part number 908-7140B 140 amp alternator. Fits 60-85 (or later)
    $173.99. Made by Tuff Stuff.

    This is the only cure except of course, to not turn the accessories on ;)

    Also, any time you create a spark by accidently touching a hot lead to
    ground, you most likely damaged or destroyed one of the diodes internally in
    the alternator. It takes a lot less than you think. If you really want to
    destroy your alternator, disconnect the battery while the engine is running.
    Say good bye to the diode trio! It make still appear to work after words,
    but the capacity will be greatly reduced and the voltage will fluctuate more
    than normal."

    Since my idle is just too low (sometimes...carb's lol) if it stayed where it should be, 700RPM or so, this would be a non issue.

    Dorian
    My tech/links page: <a target="_blank" href=http://www.dorianyeager.com/index2.html>www.dorianyeager.com/index2.html</a>
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  10. DuneRunner

    DuneRunner 1/2 ton status

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    Yes Remanufacture.
    Later KEN


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