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Thread: Borrow an idle motor?

  1. #11
    Senior Member
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    12.12V under no load is a discharged/fubared battery IMHO.

    50% duty cycle (45 degrees dwell set to 4 cylinder) suggests either your o2 sensor is shot or your mixture is so far out that it doesn't know where to start to try to correct it, so it defaults to mid scale.

    I'd make double sure that battery of yours is OK then I'd measure the base ignition advance.

  2. #12
    Senior Member BladeBronson's Avatar
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    Hmm, okay. When I bought the car (in late October), the car failed to start the first night. I bought a new battery and haven't had any starting problems since. I've only put a few hundred miles on it since then.

    What should the battery voltage read? What's the right way to ensure that the alternator is charging it?

    Thanks for all the help, everyone.

  3. #13
    Senior Member Bitsyncmaster's Avatar
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    I have a guess for you to try. Unplug the wide open throttles switch (WOT) and see if anything changes. You should get 45 deg dwell as the cold engine warms up. If the WOT switch is pressed you should get 55 deg. Those dwell numbers are the design values so they will not be perfect. I'm guessing your WOT switch is stuck on.

    Your alternator sounds like it does not provide enough voltage at idle.
    Dave M vin 03572
    http://dm-eng.weebly.com/

  4. #14
    Senior Member BladeBronson's Avatar
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    That's interesting. It might explain why this doesn't happen all of the time, and why it seems to occur after high speeds. And maybe toggling the headlights momentarily interrupts the WOT signal? I'll test tonight!

    I'll also pull the battery and alternator and take them somewhere to be tested.

  5. #15
    Senior Member BladeBronson's Avatar
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    I think you guys are on the right track with the battery and alternator. My girlfriend took the battery and alternator to be tested today. The 2 month old battery was "dead" (although the car would still start) and they are only able to charge it to 50%. I don't think I'll be able to return it to Pep Boys because I refused to give them my phone number. Oops. Autozone isn't able to test the alternator because they need some 5 digit code to enter into their tester. I have a SpecialTAuto alternator so I don't know what to tell them. Melissa's calling around to find a more old school car shop that doesn't let the computer do all the work.

  6. #16
    Senior Member Bitsyncmaster's Avatar
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    The largest battery draw with the stock systems is the door lock unit. That will draw about 12 ma. all the time. So if your not driving the car for a month or more, get a battery disconnect. The OEM clock draws about 6 ma. A FanZilla draws about 3 ma. If you have updated a radio with a clock it will draw a few ma. also.

    Most batteries have problems when they get discharged many times.
    Dave M vin 03572
    http://dm-eng.weebly.com/

  7. #17
    Senior Member Rich's Avatar
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    Quote Originally Posted by BladeBronson View Post
    I think you guys are on the right track with the battery and alternator. My girlfriend took the battery and alternator to be tested today. The 2 month old battery was "dead" (although the car would still start) and they are only able to charge it to 50%. I don't think I'll be able to return it to Pep Boys because I refused to give them my phone number. Oops. Autozone isn't able to test the alternator because they need some 5 digit code to enter into their tester. I have a SpecialTAuto alternator so I don't know what to tell them.
    That entry above plus this earlier entry:
    Hmm, okay. When I bought the car (in late October), the car failed to start the first night. I bought a new battery and haven't had any starting problems since. I've only put a few hundred miles on it since then.

    both are consistent with a car that isn't charging. The new battery probably ran the car for a month or so til recently. Now you know why it needed a new batt. in Oct.?

    The battery might be OK but you won't know til you are getting >13V when the car is running.

    The "alternator" problem may be the charge indicator lamp/connection, not the alternator....you did not mention whether that dash lamp goes on when you turn the key to ON (engine off) - with the alternator and battery connected. A blown charge lamp or a bad charge lamp connection (socket or the binnacle circuit board) will keep the alternator from charging the battery. Have you ever seen the charge lamp go on at any time since you got the car?

    And did anybody try to put an LED bulb in the charge lamp (also stops the alternator)?

    http://dmctalk.org/showthread.php?8051

    After you sort this out you can go with a disconnect like Dave suggests or you can upgrade to a digital door lock module to knock the parasitic/standby current way down. It has other advantages over the analog module but that's for later.
    March '81, 5-speed, black interior

  8. #18
    Senior Member BladeBronson's Avatar
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    Melissa was able to get the alternator tested today and was told that it's working. I voltage tested the battery when I got home and it was 12.3 volts (the person at Autozone told Melissa that the battery would only take a half charge). I was able to swap the battery for a new one at Pep Boys and the new one reads 12.7 volts. I reinstalled the alternator and installed the new battery and went for a drive tonight. I took the same drive that I did in the video that I posted and the car behaved wonderfully. It idled perfectly around 775/800 at every stop. I was sure to floor it on the highway to trip the WOT microswitch. I'll keep an eye on it, but for now, the car is running great.

    The PO added DMC's white digital door lock module and when I turn the key on, the battery light is illuminated. I remember the PO saying that he installed LED lights in the binnacle. But, when comparing the red oil, battery, and brake lights, the brightness of the battery light is different. Maybe it's wishful thinking, but I suspect that it's still an incandescent light.

    Now that I know that the alternator works on a bench, the battery is new, and the binnacle light is very likely incandescent, is there a way to verify that the alternator is in fact charging the battery?

    IMG_4861.jpg

  9. #19
    Senior Member Bitsyncmaster's Avatar
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    Quote Originally Posted by BladeBronson View Post
    Now that I know that the alternator works on a bench, the battery is new, and the binnacle light is very likely incandescent, is there a way to verify that the alternator is in fact charging the battery?

    IMG_4861.jpg
    Yes. Just need to verify your battery voltage goes to a typical 14 to 14.5 volts with the engine running. The best test of the alternator is to get the cooling fans, headlights and blower on full speed all running and verify your battery voltage is above 13 volts at idle. A weak alternator will have problems running all those loads at 775 RPM idle speed.

    You can use the dash gauge but it tends to read lower than what your really have for voltage on the battery. It's better to use a volt meter at least one time to show you what your dash gauge is reading with or without the alternator working.
    Dave M vin 03572
    http://dm-eng.weebly.com/

  10. #20
    Senior Member Rich's Avatar
    Join Date:  May 2011

    Location:  San Francisco Bay Area, Calif.

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    My VIN:    0934

    Club(s):   (NCDMC) (DCUK)

    Dave's right about the charging test.

    If you haven't found it as a new owner a handy place to get a decent system voltage reading is between the remote jump post (+) on rt side of engine bay and any metal bit on the engine. You probably found that jump post as you dealt with a flat battery already.
    Let us know what voltage you get there with the engine on both with full load and no load (lights, fans off).
    March '81, 5-speed, black interior

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