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At what RPM does your alternator energize?

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  • At what RPM does your alternator energize?

    1982 Wagoneer is running and I am finalizing accessory locations (trans cooler, remote oil filter location, chassis wiring mods). So after several long idle sessions, I noticed that the battery voltage was low...come to discover my alternator doesn't energize after engine start until I blip the throttle over 1000 RPM, then I hear the alternator kick-in and the engine starts to drag down under the load. Seems odd that I have to rev the engine to excite the alternator? My '95 dodge Cummins needs no such encouragement, even on cold mornings when the intake heaters are cycling on and off. Normal? ( I don't think it is acceptable), Unique? Any fixes? All components are those provided in the R2.8 repower kit. No changes to alternator, pulley sizes, etc.

  • #2
    There is a reduction in output at idle, but not full dropout. There is a graph in the manual:

    Click image for larger version

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    The R2.8 idles at 800 rpm, so alternator would be idling at 1840 rpm, with an output of 70-90 amps. With mine it takes 15-30 secs for the battery to recover after starting before the voltage rises to 13.5-14.0 volts. After that no problems.

    I would check your system's current draw at idle and make sure it's not exceeding what the alternator can generate.
    Attached Files


    • #3
      Digger, maybe you are misinterpreting my situation. The alternator is NOT putting out charge voltage after engine start UNTIL I momentarily raise engine RPM above 1000RPM and then let it return to idle (800RPM). Let me preface this by saying I do not have an "amp clamp" for my digital multi-meter, so I can only reference DC voltage. Battery voltage will read 12.5~.7 prior to engine start and then dip down to <11.0, quickly returning to 12.5. If I do nothing, the voltage remains at 12.5 (intake heater does not cycle due to high ambient temperature. 95 degrees in AZ this weekend) for as long as I idle the engine. Once the engine reaches an operating temperature 194 degrees, the cooling fan kicks on yet the alternator will not indicate increased output (higher voltage for charging). This is what's killing my running, but no charging. SO, if I blip the throttle, the alternator kicks on, and now shows output voltage of 13.7~.9 on my meter at 800 engine RPM.
      I've been through the manual, I studied the chart you posted, I've checked out the reference/troubleshooting material on the Cummins Quickserve site, and went as far as trying a web search., all to no avail.
      So Digger, what are you seeing on your rig before and after engine start? The Murphy gauge will display volts, mind giving yours a look and see what you come up with?


      • #4
        Right after startup, the alternator is dragged down below battery voltage. I have always assumed the grid heater is kicked on because the engine is cold and it is preheating the air to improve combustion. I have a habit of bumping the throttle to warm the engine, which removes the drag after a few secs.

        If I don't rev the engine, the drag on the electrical system clears itself a little bit later, at which point the system voltage jumps to 13.5V

        Maybe Steve can chime in on this behavior, but it is similar to other diesels I've owned.


        • #5
          I find that I have to rev mine just a little to start charging. However I've done no testing, but my battery voltage won't be over 12.xx volts until it's revved up and then it's around 14V from then on. I can let it idle for a long time and it won't jump until I bump the RPMs and even under some load ie aux battery charging, fridge, accessories charging, lights on, etc at idle it'll maintain 14V easy.


          • #6
            Digger and DieselJeep, thanks for the replies...not that I want to share misery, but this looks like it's not an isolated case? I think there are more with this but they just don't notice it be they get in and go resulting breaking the rpm threshold that activates charging. I wonder if any of the Cummins reps/beta installer have any input as to why?