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OT-alt issue

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  • OT-alt issue

    I have an issue with an alternator on my Jeep. When I got it the alt would discharge the battery because it would bleed off over a few days. I put a diode in the wiring per the drawing and that cured the problem for a long time. Lately I started noticing the battery was not up to snuff if I didn't drive it everyday.

    I checked the diodes and they were ok so just for kicks I replaced them. There is still a voltage drain through the alternator. I checked the output post on the alt for a defective internal diode and they appear to test good.

    I don't really want to replace the alt unless I have to because I can disconnect the battery real easy each night.

    What puzzles me is since I have diodes to eliminate feedback how in the hell is it discharging the battery?

    I can unplug the two wire plug on the side of the alt and it won't discharge or I can disconnect the battery and it don't discharge. All this is in the alt. and I would like to know why and how. I don't have the 10 ga wire from the neg post to the alt ground but it is grounded from the engine to the frame to the battery.

    It's only ink and paper

  • #2
    Even with the diode there is a small drain around 2 to 3 tenths of volt back through it.


    • #3
      See if the light in the glovebox is on.


      • #4
        +1 on that. I had an old Thunderbird with the light on in the closed cigarette tray.
        Guaranteed not to rust, bust, collect dust, bend, chip, crack or peel


        • #5
          This is a 1951 CJ3A Jeep. There are no accessories that draw current. The current drain is about 32 ma.
          It's only ink and paper


          • #6
            Have you measured the current drain with an amp meter? That's about the easiest way to start narrowing it down.
            suspects would be anything that gets power when the ign. sw. is off.
            headlight circuit, ign. sw., interior light circuit, brake lights, radio, etc., and of course Alternator/volt regulator.
            with an amp meter showing the current drain, pull fuses and see if you lose the drain.
            If you lose the drain when you pull a fuse - that circuit is your culpret.
            If you don't lose the drain by pulling fuses - the problem is most likely in the charging circuit, or ign. sw. related.

            edit: Our posts crossed - your battery should be able to handle a 32ma drain no problem, Clock or radio?
            Have you had the battery load tested?
            Last edited by Scottike; 09-21-2011, 01:03 PM.
            I cut it twice, and it's still too short!


            • #7
              We once had an issue with a battery draining itself real quick. Could not find anything. Eventually figured out the case of the battery was bad. I could go from the terminals of the battery and touch the plastic case and I would read several volts. Had the store swap the battery and the problem went away.


              • #8
                It could be anything, I once found a problem that had been causing battery drain for two years, the owner had just given up and had installed a battery cut-out switch to disconnect the battery when the engine was not to be run for more than a couple of hours. Turns out it was caused a tiny piece of a gum wrapper that had fallen into the cigarette lighter. Finding a drain like this can sometimes be done by connecting an AMP meter between the battery and battery cable then removing fuses from the fuse box one at a time until the AMP load disappears, that will be the offending circuit. I have used this procedure several times and it works quite well.


                • #9
                  If it is the alternator, I think you'll find its the '1' or '2' wire thats drawing power, It should be disconnected by the ingition switch. (Might be faulty switch, or miswired by god knows who, or partial short along the line)

                  a small current of a couple dozen mA to about 200mA is drawn by alternators when the field coils are engaged, and alternators won't output ANY power untill those terminals are powered.

                  I once had a lawnmower + car alternator + old dead SLA.. SLA could hardly light a lightbulb anymore, but with a switch it was more then enough to get the alternator powered up.. and powering a 1000W load through an inverter.
                  Play Brutal Nature, Black Moons free to play highly realistic voxel sandbox game.


                  • #10
                    That #2 pin goes to a voltage divider (couple of resistor straight to ground) that feeds the regulator a feedback voltage. I seem to remember mine pulling about 40mA, which shouldn't be enough to kill a car battery over night. If you want to eliminate that small draw then hook it directly to the #1 pin so it is only getting power when the ignition switch is on. That #1 pin is fed by it's own internal rectifier so it will have the same voltage on it as the Bat post when the engine is running. The only concern there is the regulator oscillating since it won't be connected directly to the battery.


                    • #11
                      If it's not the alternator then start popping fuses with your gauge hooked up --- this will lead you (usually) to the right area...


                      • #12
                        Ahhh - been there, done that on a 55 CJ3B. Make sure you have the right voltage regulator because you can get both positive ground and negative ground units and some modern counter guys don't know the difference. My jeep would start and run fine for several months but slowly get to the point where the battery was about kaput. I'd charge the battery then the cycle would repeat. Turned out the wrong voltage regulator was installed - I put in the correct one and problem solved.

                        Edit - the reason this happens so frequently is that Ford tractors used the same voltage regulators as Willys/Kaiser but were positive ground. In later years, some parts suppliers combined the part numbers because nobody snapped that there was a reason two distinct ones should really exist. An old counter guy at NAPA told me this story and it sounds reasonable since that is precisely what happened on my jeep - the two regulators looked the same but had reversed polarity.
                        Last edited by HWooldridge; 09-21-2011, 03:12 PM.


                        • #13
                          Although this does not address the issue, a current draw of 32ma should definitely not drain the battery in a few days.

                          One of the design goals of sizing a battery for a modern ECU equipped car is to take into consideration the amount of background current draw for entertainment systems,ecu, etc.

                          The usual goal is to be able to have a car parked for a month and still have enough reserve capacity left in the battery to easily start.

                          Although I too wouldn't accept a 32ma draw from the alternator alone, this is typical of new car background current draw.
                          This definitely should not draw down a good battery.
                          One of my trucks for instance, has a normal background current draw of 65ma and it can sit for weeks with no starting issues.

                          Even had one car that did have starting issues in the winter after about 10 days. I traced it down to the tach driver module located on the printed circuit board of the dash. Apparently according to factory service bulletins that I was able to track down, 150ma was normal.

                          That was one that got a battery tender mounted under the hood.
                          Last edited by Willy; 09-21-2011, 07:16 PM.
                          Home, down in the valley behind the Red Angus
                          Bad Decisions Make Good Stories​

                          Location: British Columbia


                          • #14
                            Yea theres also the fact that it takes hours to fully recharge a lead acid battery. If your running it for 15~20 minutes its hardly getting charged at all.
                            Play Brutal Nature, Black Moons free to play highly realistic voxel sandbox game.


                            • #15
                              It doesn't drain the battery, I can just tell it does not have a full charge after three or four days by the way it starts. It has an internal regulator.

                              The way it's wired is shown in the opening post if some of you haven't looked at that. The two isolating diodes should isolate it from discharging when the ignition is off. The diodes are good and so is the ones in the alt.

                              I think I'll try to find an internal wiring diagram for it as that may help explain it. I threw away two big boxes of old manuals and I hope what I need was not in the boxes.
                              It's only ink and paper