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Thread: Dmch alternator issue

  1. #11
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    You changed out a lot of different things at the same time. I bet the idle hunt and the voltage gauge going up and down are two separate issues.

    Between changing out the pressure regulator and removing the catalytic converter, you probably now need to reset the CO mixture. Enough pressures or back pressures may have changed that the system doesn't know where it is anymore. I would go through that procedure in the manual to test and see that your idle system is all working as intended, O2 sensor is fine and all the rest of that and THEN see about adjusting the mixture.

    Does your battery light come on when you first turn the key to accessories? Have you tried contacting Houston to ask specifically about how that alternator was supposed to be installed in case you missed something?


    Sept. 81, auto, black interior

  2. #12
    AKA "The Goat" Michael's Avatar
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    Quote Originally Posted by Michael View Post
    A slipping belt would be worse as the rpms rose. It looks like a dead battery to me but you said it was new.

    Get your mulitmeter and watch the voltage. Check it at the battery and at the alternator. If the battery is showing less than 12.8v while running, charge it and try again. It is possible you just got a bad alternator with a bad diode.
    My mistake. I thought you were illustrating the voltage gauge and panel lights dimming at low rpm. The idle surge would most likely have nothing to do with an alternator issue.

  3. #13
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    Quote Originally Posted by Jonathan View Post
    You changed out a lot of different things at the same time. I bet the idle hunt and the voltage gauge going up and down are two separate issues.

    Between changing out the pressure regulator and removing the catalytic converter, you probably now need to reset the CO mixture. Enough pressures or back pressures may have changed that the system doesn't know where it is anymore. I would go through that procedure in the manual to test and see that your idle system is all working as intended, O2 sensor is fine and all the rest of that and THEN see about adjusting the mixture.

    Does your battery light come on when you first turn the key to accessories? Have you tried contacting Houston to ask specifically about how that alternator was supposed to be installed in case you missed something?
    I have a new 02 sensor, I will change and then start up cold. I will record the cold start up. After that I will go in the manual to look at the procedure. Also I’m sorry it was the pressure sending unit I changed not the regulator. I’ll be back....
    Last edited by Dontilgon; 09-01-2019 at 08:55 AM. Reason: Error in original post

  4. #14
    AKA "The Goat" Michael's Avatar
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    Quote Originally Posted by Dontilgon View Post
    I have a new 02 sensor, I will change and then start up cold. I will record the cold start up. After that I will go in the manual to look at the procedure. Also Iím sorry it was the pressure sending unit I changed not the regulator. Iíll be back....
    When I was dealing with a hunt (mine was across the rpm board), O2 sensor fixed the issue.

    The O2 sensor does not affect the car in open loop so don't expect a new o2 sensor to affect cold start up.

  5. #15
    Administrator Ron's Avatar
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    $.02

    Open or closed loop, the 02 sensor has little to do with starting. Once started, a shorted or open 02 sensor would lie to the system whether the engine is hot or cold.
    I'd do the tests at D:04:15 before changing yet another thing.

    Quote Originally Posted by Dontilgon View Post
    14.9 at the battery but It obviously fluctuates with the idle.Interesting thing I started it this morning and it idled perfectly for a good minute or two,opened up the engine bay and noticed the cold start unplugged. Plugged back in and it stayed the same. I noticed has the car started heating up it seemed to have gotten worse and then it was back to its old tricks
    14.9V fluctuating is too high.

    Depending on how long it ran, it could be that it ran ~OK in closed loop then started acting up when it went into closed loop because you changed the pipe, as others have mentioned...
    Warm it up and check (only, for now) the CO dwell.

  6. #16
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    14.9 volts doesn't bother me too much unless the battery is fully charged. Once the battery is fully charged the voltage should drop down to around 13 volts. At idle the alternator can't put out it's full rated output so that is no way to test it. As long as the electrical system is over 12 volts this has no effect on your idle. Your hunting (fluctuating) idle is due to cylinder imbalance and the test pipe could have magnified it. You have 2 distinct things here. The charging system and the idle. While they have many things in common it is 2 separate problems. A new O2 sensor may not make any difference, the frequency valve is fluctuating as it should resulting in your hunting. Typically a bad O2 sensor would make the idle NOT hunt since it isn't responding to the changing oxygen content in the exhaust. Charge up the battery and check the voltage after a few minutes of running and see if it settles down to around 13 volts. Can't give you an exact number since it is affected by the ambient temperature. I would keep the cat, removing it won't give you much more power but it will make the exhaust louder. To get a little more power you have to remove the cat and change to headers. I wold not mess with the mixture adjustment to correct a hunting issue. To fix the hunting I would do a full tune-up and find and fix any vacuum leaks. Then check the A/F ratio, aka dwell, aka duty cycle.
    David Teitelbaum

  7. #17
    Administrator Ron's Avatar
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    David, if it is fluctuating @ 14.9V while idling, then it hits 15V (or more) which it should never do. And as you say, it doesn't put out fully at an idle... I agree that checking the voltage at an idle is not a proper test, but below ~13.5 V and above ~14.7 are signs of bad alternator. In this case, it should should be given several minutes to replenish the battery (from starting) and then allowed to run several more minutes to see just how high the voltage goes with everything turned off (then on, for other reasons). Or just pull it and have it properly tested.

  8. #18
    Senior Member Bitsyncmaster's Avatar
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    If the battery has developed an open cell than voltage can go above 15 volts with the alternator working. If the alternator tests good get the battery tested.

    A good battery the voltage will increase as it charges due to limiting current of the battery charging. As it charges the current drops and the voltage rises.
    Dave M vin 03572
    http://dm-eng.weebly.com/

  9. #19
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    A bad battery can make a good alternator look bad and a bad alternator can make a good battery look bad. They affect each other and they really should both be tested if a problem is suspected. The voltage regulator is supposed to limit the voltage to around 14.7 volts and if it is much above that you start to suspect the regulator, or possibly the battery or the connections between them. If you were cranking the life out of the battery and the car finally starts the output voltage of the alternator can be high initially but should start to come down once you get some of the charge back into the battery. Another possibility is you have one or more blown diodes in the rectifier bridge. That can be very bad for the battery if the alternator is outputting AC. Bottom line, if you think you have voltage problems you test the alternator AND the battery and all of the connections between them. If the battery is over 5 years old it should probably be replaced.
    David Teitelbaum

  10. #20
    Administrator Ron's Avatar
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    He already checked (replaced) the battery.....

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