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Thread: Borrow an idle motor?

  1. #21
    Senior Member BladeBronson's Avatar
    Join Date:  Aug 2015

    Location:  Bay Area

    Posts:    232

    My VIN:    10644

    It looks like I'm not out of the woods yet.

    When the car is completely turned off, the voltage is 12.52 (I measured the new battery last night as 12.7 at Pep Boys) At idle, it dipped to 12.4 and when I held the throttle spool, it came up to 13.9. I then turned on the headlight high beams and ran max AC on 4. At idle, the voltage was 11.8 and with the throttle, 13.5. Every time the AC compressor clicked, it dropped to 12.9 volts or so.

    Because the voltage increases with RPM, wouldn't this mean that the alternator's doing something? Could this be a case of the alternator kind of works, but isn't putting out as many amps as it once did? It has a SpecialTAuto sticker on it, but I can't discern whether it's supposed to be a 120 amp or 140 amp alternator. When Melissa took the alternator to be tested, the guy only replied that it works - he didn't give her a printout that describes the amperage rating (I'm not sure if that's common practice or not).

  2. #22
    Senior Member Bitsyncmaster's Avatar
    Join Date:  May 2011

    Location:  Leonardtown, MD

    Posts:    6,647

    My VIN:    03572

    Quote Originally Posted by BladeBronson View Post
    It looks like I'm not out of the woods yet.

    When the car is completely turned off, the voltage is 12.52 (I measured the new battery last night as 12.7 at Pep Boys) At idle, it dipped to 12.4 and when I held the throttle spool, it came up to 13.9. I then turned on the headlight high beams and ran max AC on 4. At idle, the voltage was 11.8 and with the throttle, 13.5. Every time the AC compressor clicked, it dropped to 12.9 volts or so.

    Because the voltage increases with RPM, wouldn't this mean that the alternator's doing something? Could this be a case of the alternator kind of works, but isn't putting out as many amps as it once did? It has a SpecialTAuto sticker on it, but I can't discern whether it's supposed to be a 120 amp or 140 amp alternator. When Melissa took the alternator to be tested, the guy only replied that it works - he didn't give her a printout that describes the amperage rating (I'm not sure if that's common practice or not).
    Yes the alternator is working but has low output at idle speed. I had an older Specal T alternator (I think it was a 110 amp) that worked fine on my car at idle speed. I now have a DMC 130 amp that also works fine at idle speed.
    Dave M vin 03572
    http://dm-eng.weebly.com/

  3. #23
    Senior Member BladeBronson's Avatar
    Join Date:  Aug 2015

    Location:  Bay Area

    Posts:    232

    My VIN:    10644

    I received my new alternator from DMC Houston tonight. On the website, it was listed as a 120 amp alternator, but it came with a little card that rated its output at 59 amps @ 2400 RPM and 147 amps @ 7500 RPM. I was a little concerned that it might not improve my voltage at idle.

    I took readings again with the old alternator first:
    12.55 with the engine off
    12.25 at idle, just after starting.

    I didn't let the car heat up or take it for a drive because I was going to swap the alternator immediately. After the swap, my voltages are:
    12.6 with the engine off
    14.6 at idle, just after starting!!!

    I haven't run into the 2,000 RPM at idle problem ever since I swapped out the battery with a fully charged one last week.

    Guys, thank you so much for the help on this. I really thought that this was a byproduct of a problem with the fuel mixture or the idle motor. The volt meter was right there in front of me the whole time, but I didn't know what was out of the ordinary so I didn't notice it. I think this problem might be solved.

  4. #24
    Senior Member BladeBronson's Avatar
    Join Date:  Aug 2015

    Location:  Bay Area

    Posts:    232

    My VIN:    10644

    Aaaaand just now I found the alternator dyno report in the bottom of the box. The RPMs stated in my last post are alternator RPMs, which should be 3x greater than engine RPM. Here's the sheet for anyone that's interested.

    image.jpg

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