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Thread: Unstable 12v power

  1. #1
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    Unstable 12v power

    My 101 has a 24v system which powers start, lights, fuel pump and original instruments. I also have a 12v system that powers wasted spark ignition (EDIS 8 module), tacho, aftermarket instruments, engine low water alarm and digital voltage gauge. At this stage the 12v system is not charged by the vehicle charging system but later will be powered via a 24/12v Dc Dc charger.

    A little while back I went to start the truck and didn't realise my smart charger was still attached to the 12v battery but it was also going through its battery rejuvenation/desulphurising program. My understanding is that it pulses current through the battery.

    After starting the engine with the battery rejuvenation system on the electronic parts of the vehicle started playing up. With just ignition on there is no issue, but with the engine running the the 12v system seems unstable in that electronic displays flicker and jump around, the low water alarm sounds with a jittery sound and when I plug in a 12v meter into a 12v socket it just flashes - switch the engine off and all returns to normal.

    I thought that maybe emf from the ignition system was jumping across the the 12v system as the main power line from the battery runs near the ignition so I moved the power line but no change.

    So looking for suggestions before I start pulling things apart.

    Thanks

    Garry
    REMLR 243

    2007 Range Rover Sport TDV6
    1977 FC 101
    1973 Haflinger AP700
    1971 Jaguar V12 E-Type Series 3 Roadster
    1957 Series 1 88"
    1957 Series 1 88" Station Wagon

  2. #2
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    Homestar is offline Super Moderator & CA manager Gold Subscriber
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    Assuming the earths are common between the 12 and 24 volt system? While it shouldnít matter, Iím wondering if there is a condenser in the back of the 24 volt alternator thatís crapped itself and thereís some noise in the system? Shouldnít make a difference but just a thought? Has the ignition system got anything in it that may cause issues like large capacitors, etc?

    Do you have an old analogue multimeter at all? This will show fluctuations in the voltage far better than a digital meter unless you happen to have an occiloscope handy?

    Just spitballing here mind you, itís a bit of a strange one if it only started after the running of the engine with the charger connected.


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  3. #3
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    Thanks Gav - I have had similar thoughts. While the 24v charging light goes out i have not actually checked it is charging the batteries - now while it is separate to the 12v system there are a number of 24v relays that come on with the ignition and provide switched 12v to things like the engine watch dog - if the 24v system is not stable maybe it is crossing to the 12v system there.

    I will break out the old multi meter and do some checking around.

    Cheers

    Garry
    REMLR 243

    2007 Range Rover Sport TDV6
    1977 FC 101
    1973 Haflinger AP700
    1971 Jaguar V12 E-Type Series 3 Roadster
    1957 Series 1 88"
    1957 Series 1 88" Station Wagon

  4. #4
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    Not had anything to do with these hybrid type of systems, but I would be looking to check that the 24 volt generator is actually putting out DC, not AC or some where in between. Your charger MAY have damaged the 24 volt regulator.

  5. #5
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    Good thoughts there - actually just checked the the alternator output and it is 28v DC. The charger was on the 12v system - charging the 12v battery so no direct cross over to the 24v system - only where a 24v relay acts as a switch to turn 12v systems on and off.

    I am leaning to the HT system which is in the 12v system is causing the variations due to inadequate shielding somewhere.

    Cheers

    Garry
    REMLR 243

    2007 Range Rover Sport TDV6
    1977 FC 101
    1973 Haflinger AP700
    1971 Jaguar V12 E-Type Series 3 Roadster
    1957 Series 1 88"
    1957 Series 1 88" Station Wagon

  6. #6
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    sounds like the charger caused a capacitor(or something) in the DC-DC convert to fail.

    I'd be looking at the DC-DC first.

    What charger did you use, ie. on the repair/desulphation mode on the 12v side?
    My understanding of this mode is that it outputs quite a high voltage, at lowish current. about 16v at 1amp.
    My NOCO does this, and from what I've read, C-Tek is similar(15.8v or whatever). And it's pulsed .. so maybe the pulse back to the DC-DC killed something within.

    What do you have at hand to test with?
    eg. do you have a 24v charger? if so, you could hook up a 24v charger to the 101 to charge it's batteries. Obviously 101 is not running. 24v charger should ouput about 28v or so and check if the 12v side is still flickering about.

    If it all runs fine, then you know that it's something on the 24v side on the 101 that's causing the spikes/fluctuations.
    Or, if it does the same thing as when the 101's 24v side is charging then more than likely the DC-DC has failed.

    I would have thought that if the 101's 24v side is acting up, you would at least see some electrical device(light, gauge, or whatnot) also flicker or play up in a similar way too.
    So why would it only direct an issue it has to the DC-DC converter to cause a flickering effect.
    Cheers,
    Arthur.

    '99 D1 300 Tdi Auto

  7. #7
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    Thanks Arthur but it does not have a DC to DC charger fitted at the moment - I charge the 12v battery at the moment via an old 240v SuperCheap smart charger. While the vehicle is driveable on the road it is a work in progress at the moment and a charged 100ah battery provides a lot of ignition pulses before needing recharging.

    While I have a leaning towards unshielded ignition pulses crossing over into the other wiring but you could also be correct that the high voltage from the battery reconditioning process could have damaged the EDIS 8 ignition module or electronics elsewhere. Whan I took it for a run yeaterday, the ignition was running rough and the tacho would not go above 1200rpm - so maybe the tacho signal module is the culprit as it gets its input on the coil pack low tension input.

    Cheers

    Garry
    REMLR 243

    2007 Range Rover Sport TDV6
    1977 FC 101
    1973 Haflinger AP700
    1971 Jaguar V12 E-Type Series 3 Roadster
    1957 Series 1 88"
    1957 Series 1 88" Station Wagon

  8. #8
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    Quote Originally Posted by 101RRS View Post
    Thanks Arthur but it does not have a DC to DC charger fitted at the moment - I charge the 12v battery at the moment via an old 240v SuperCheap smart charger. While the vehicle is driveable on the road it is a work in progress at the moment and a charged 100ah battery provides a lot of ignition pulses before needing recharging.....
    Aha!
    I mis-understood the comment about "but later will be powered by a DC-DC ..." as to mean later .. as in, once vehicle is started DC-DC will then come into action. I thought it was a funny way to describe it .. didn't occur to me that you intend to eventually install a DC-DC device.
    (my bad).

    So then, from battery to 12v devices is direct? No other devices like relays or regulators .. nothing.

    was the 12v meter plugged into the socket whilst you were 'repairing' the 12v battery? Even then, most of those plug in 12v meters can withstand more voltage than that, as many will read up to and even over 24v .. so it's flashing is weird.

    Would seem strange that if the EDIS-8 unit has died, why is the 12v meter acting up too? Or is the 12v plug for it somehow connected to the EDIS-8 too?

    Have you ground the 12v system to the chassis.
    As I'm understanding your setup it's the only common section between the 24v and 12v systems, so for the 24v system to affect the 12v system whilst the 24v system is powering up in some way(ie. engine running) the interference will most likely be via the earth(chassis).

    If the 12v system is earthed via chassis, would it be easy to eliminate it(even just temporarily) in some way? Say, disconnect the 24v from chassis, or the 12v system from chassis and use a temporary cable to any earth requirements for those devices.
    Cheers,
    Arthur.

    '99 D1 300 Tdi Auto

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