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91 Coupe quattro battery voltage 8V on start

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  • 91 Coupe quattro battery voltage 8V on start



    Intermittently, the battery voltage when I start my CQ is about 8-9 Volts. The car will run, but it is clear the voltage is low.

    A minute later, voltage will jump to 12 V on its own or when I restart the car.

    All is fine for another week or two until the problem recurs again.

    Any thoughts on what can be causing this?

  • #2
    Probably the regulator, or a loose connection.

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    • #3
      Mine did that once or twice a few years back and hard tap on the voltemeter solved it, I guess the needle was getting stuck, never happened since!

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      • #4
        Hi Cgharibo,

        My thoughts are you need to gather more info, it could be anything from a loose alternator belt, to an low battery charge or any of the 3 things already mentioned above; or anything connecting them in between. Without volts at both more locations (i.e.: between various X and Y charging system locations) and at known loads (what exactly is being powered during the measurement) doesn't tell you a whole lot.

        Have a read through this entire post, it becomes especially helpful from post #23 on with tips on how and where to read voltages:

        https://www.s2forum.com/forum/techni...ery-13-3-volts
        Last edited by Lago Blue; 26 June 2021, 16:29.

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        • #5
          So when you start the car is the volt reading low.If you rev the car dose it jump up to normal charge.

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          • #6
            The jump to normal charge occurs independent of the revs. Although this may be related to the voltage regulator, the brushes are only 2/3 worn. At what point are the VR brushes considered worn?

            Lago Blue, thank you for the useful link. Is there a link to the 15V voltage regulator?
            Last edited by cgharibo; 27 June 2021, 14:25.

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            • #7
              A 15V reg, may be both a bit of a unicorn, and perhaps somewhat unwelcome, one man's experience and his reco:

              https://www.s2forum.com/forum/techni...575#post485575

              It may be best before part-swapping begins, for economy's sake, to thoroughly survey the entire charging system for both physical fault-finding and as a reference against which future changes can be compared. By survey I mean beyond the belt and batt. terminals being tight, both to make a record of voltages taken at the various locations to then gauge where any deficiencies may lie, and to do a physical dis-assembly and inspection of all the connections for condition (both for security and corrosion). You want to have shiny metal at all connections and you may want to apply something to inhibit corrosion.

              It may also be that the (lead-acid?) battery is simply needing service or is degraded, on some you can still open to replenish with distilled water, and to measure the acid's spec. grav. (with a hygrometer) to determine the state of charge; in each cell. Time, vibration, extremes of heat, or any periods of discharge combined with cold temps are all very good at shortening battery life.

              May I suggest after the survey, try this not quite D-I-Y (you need a helper) 2-day batt. test of its' ability to store volts (electron pressure) procedure:

              a) What is the best charge your battery can hold statically 12 hrs. after charging (use a CTEK brand charger overnight, refill batt. if req'd. after charging and if poss.; and then 12 hrs. later, use a DMM to measure V)? When fully (100%) charged the batt. should show 12.7V across the terminals and 1.265 S.G. in ea. cell; and
              b) What voltage does the battery drop to during cranking (use helper and again always using the DMM)? It should not drop below 10V.

              This will give you an idea of where on the service-life curve, your battery is.

              If need be, a shop can do a further load test of the battery's amperage (electron flow, a.k.a.: current) capability.

              If the bat. is fine, a next step might be what is the alt.'s output?


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              • #8
                I had a 15v regulator, it lasted about 6 months

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