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Thread: Starter motor voltage drop

  1. #1
    are hairy Dogsballs's Avatar
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    Starter motor voltage drop

    The last week when iíve just got my car going and also having alternator issues I notice there is a decent ~1.5v drop on cranking. This is an issue as my msd ignition doesn't like low voltages. If the battery is fully charged I haven't had an issue.

    I have a big 770cca battery in the boot 2yrs old. Big zero guage cable straight to the starter terminal, and 2 big zero guage Earth's from engine to chassis.

    Is this normal or battery on its way out or other issues?
    Quote Originally Posted by 50RTD View Post
    Dave, have you uploaded the data from the carby?

  2. #2
    No I'm Not Shonky Shonky's Avatar
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    Is the voltage measured at the battery or the starter? If at battery then probably battery is the problem by 1.5v drop doesn't sound that bad measured at the battery. If the battery voltage is good but starter voltage is not, then look at wiring and connections.

    Measure the actual drop across the cable if you can. e.g. measure between battery terminal positive and starter positive to see the drop. Similarly on the negative side. Good if your meter has a min/max function to catch the peak.
    Last edited by Shonky; 12-01-17 at 05:52 PM.

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    Depends on what that 1.5V drop is starting from. If you battery isn't 12.6V to start off with then it's a bit flat and if it's less than 12.1V it's fucked.

    1.5V drop from 12.6V means 11.1V during cranking which is getting down there but not shithouse, if it's in the 10's while cranking then yeah, somethings up...
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    are hairy Dogsballs's Avatar
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    The 1.5v is at the dash, i have a voltmeter

    I know there is a 0.2v drop from the battery to the dash

    The battery is 12.6-12.4 volts after cranking for a bit (joys of e85), but ignition wont run at this level.


    It charges up to 13.4 no problem overnight and even 13.5 volts with alt
    Last edited by Dogsballs; 12-01-17 at 06:10 PM.
    Quote Originally Posted by 50RTD View Post
    Dave, have you uploaded the data from the carby?

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    Where is the MSD powered from?

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    are hairy Dogsballs's Avatar
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    Straight off starter terminal.

    But power drop is across the circuit so doesnt matter if i do it else
    Quote Originally Posted by 50RTD View Post
    Dave, have you uploaded the data from the carby?

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    Power it straight of the battery and run an earth back to the battery this is how I wire quit a lot of stuff it minimises VD. Also if you run a size of wire up from what is recommended to the battery you will pretty much be running at battery voltage.
    Last edited by Rdyno; 12-01-17 at 06:43 PM.
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    Registered User gtrboyy's Avatar
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    Still got bosch 120a on it?

    Sounds like bs but reason why I like & stick genuine bosch 85a(vs) & 120a(vt) alternators smash out half their output at low revs so can run ecu/fuel pump/thermofan/hei & headlights etc & still charge battery.

    Some aftermarket alternators apparently dont do jack untill higher rpm so basically end up slowly drained battery to point car runs like shit or wont start..effectively like have to small alternator.
    an easy fix is get a mate to drive in front of you at 60 then 80 then 100 and mark in on your speedo with some liquid paper.

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    Quote Originally Posted by gtrboyy View Post
    Still got bosch 120a on it?

    Sounds like bs but reason why I like & stick genuine bosch 85a(vs) & 120a(vt) alternators smash out half their output at low revs so can run ecu/fuel pump/thermofan/hei & headlights etc & still charge battery.

    Some aftermarket alternators apparently dont do jack untill higher rpm so basically end up slowly drained battery to point car runs like shit or wont start..effectively like have to small alternator.
    The VT alternators were changed for copo cars like police cars with the increased strain on the system while they idle for two hours roadside they are a good thing.
    Quote Originally Posted by Lord_fahrquhar View Post
    The au put the fucking tunnel in with the bullbar. Even Chuck Norris respects the ability of the au coon.

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    Quote Originally Posted by Rdyno View Post
    Power it straight of the battery and run an earth back to the battery this is how I wire quit a lot of stuff it minimises VD. Also if you run a size of wire up from what is recommended to the battery you will pretty much be running at battery voltage.
    I would definitely do this too. Should fix up your issues

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    Quote Originally Posted by gtrboyy View Post
    Still got bosch 120a on it?

    Sounds like bs but reason why I like & stick genuine bosch 85a(vs) & 120a(vt) alternators smash out half their output at low revs so can run ecu/fuel pump/thermofan/hei & headlights etc & still charge battery.

    Some aftermarket alternators apparently dont do jack untill higher rpm so basically end up slowly drained battery to point car runs like shit or wont start..effectively like have to small alternator.
    I run a mitsubishi ve 140amp alternator, it charges at 13.5v continuous

    When its charged its fine and will fire right away, also when warm its fine. Just have an issue when battery is low
    Last edited by Dogsballs; 12-01-17 at 07:06 PM.
    Quote Originally Posted by 50RTD View Post
    Dave, have you uploaded the data from the carby?

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    Half Shafted. Madhatr's Avatar
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    Short of fitting an anderson plug and a calcium battery you can keep on float charge at well over 13 volts, you aren't going to solve it without upgrading your battery capacity, or if the starter is old and drawing more current than required, replacing the starter.

    More capacity you pull from the battery, the lower the battery voltage. Increase your capacity and you won't see the battery voltage drop as low.
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    Down with ma homies Greg Rust's Avatar
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    Start at the basics. Drop your battery into a battery shop and get it load tested.
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    Quote Originally Posted by Madhatr View Post
    Short of fitting an anderson plug and a calcium battery you can keep on float charge at well over 13 volts, you aren't going to solve it without upgrading your battery capacity, or if the starter is old and drawing more current than required, replacing the starter.

    More capacity you pull from the battery, the lower the battery voltage. Increase your capacity and you won't see the battery voltage drop as low.
    770cca is a big battery it should not have any issues.
    Quote Originally Posted by Lord_fahrquhar View Post
    The au put the fucking tunnel in with the bullbar. Even Chuck Norris respects the ability of the au coon.

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    Quote Originally Posted by Greg Rust View Post
    Start at the basics. Drop your battery into a battery shop and get it load tested.
    One problem with that is most have electronic load testers you need a carbon pile load tester to be sure.
    Quote Originally Posted by Lord_fahrquhar View Post
    The au put the fucking tunnel in with the bullbar. Even Chuck Norris respects the ability of the au coon.

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    Quote Originally Posted by Madhatr View Post
    Short of fitting an anderson plug and a calcium battery you can keep on float charge at well over 13 volts, you aren't going to solve it without upgrading your battery capacity, or if the starter is old and drawing more current than required, replacing the starter.

    More capacity you pull from the battery, the lower the battery voltage. Increase your capacity and you won't see the battery voltage drop as low.
    770cca is a huge battery. I bought it specifically for multiple starts
    Quote Originally Posted by 50RTD View Post
    Dave, have you uploaded the data from the carby?

  17. #17
    Half Shafted. Madhatr's Avatar
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    Yes, but your problem is you are running the battery down, you've already said its fine when the battery is 100% charged. The lower the cell voltage, the lower the capacity, it doesn't stay the same as the battery voltage drops. So either you are prepared to keep your battery completely topped up and fix your problem, or you go with a battery with an even bigger capacity that wont see as much voltage drop. You can only pick one. You might have no problems supplying enough current to turn the motor over and start the car, but if sensitive electronics won't work due to the cell voltage dropping too far when cranking, AND if you aren't prepared to keep the battery at 100% for all starts which doesn't cause issue, then the only way to do this is to provide a battery that will provide more current at the same cell voltage, which won't cause the nominal voltage to drop as far when cranking. The difference between 12.9-13v and say, 12.5v on a battery could be 30-35% of its capacity. So if your charge state is already low, its only going to get a whole lot lower then you try to draw a lot of current from the battery during starting.

    People overlook starters too though, which is why I suggested it, just because its working doesn't mean it isn't drawing more current than it should.
    Last edited by Madhatr; 12-01-17 at 10:34 PM.
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  18. #18
    are hairy Dogsballs's Avatar
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    It's one of those stupid lhs conversion starters. As you said if it is working and keeps charge up or I charge it there are zero issues.

    I was just curious on such a big drop. I might call cae today and ask them about starter
    Last edited by Dogsballs; 13-01-17 at 04:23 AM.
    Quote Originally Posted by 50RTD View Post
    Dave, have you uploaded the data from the carby?

  19. #19
    No I'm Not Shonky Shonky's Avatar
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    I don't think you've mentioned where the actual drop is yet. Measured the battery directly whilst cranking?

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    Opens Guido's Avatar
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    Is the starter motor relayed?
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    Commodores dont run relays on the starter.

    I see the drop at the front of car and in the wiring through the dash
    Quote Originally Posted by 50RTD View Post
    Dave, have you uploaded the data from the carby?

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    Opens Guido's Avatar
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    So might be an idea to relay it.
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    No I'm Not Shonky Shonky's Avatar
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    Quote Originally Posted by Dogsballs View Post
    Commodores dont run relays on the starter.

    I see the drop at the front of car and in the wiring through the dash
    So measure it at the battery. If the battery isn't dropping then look at your wiring or connections.

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    Its a single cable of the thickest gauge!straight from the battery to the starter motor. It doesnt get any simpler, lol
    Quote Originally Posted by 50RTD View Post
    Dave, have you uploaded the data from the carby?

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    The solenoid is the relay, unless you're switching 300a through the ign switch. Maybe you want one of those reduction gear starters like a Tilton?

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  26. #26
    are hairy Dogsballs's Avatar
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    Just spoke with cae who did the left starter kit. Current starter is 1.6hp and rated to up to 12.5:1 comp. So my 15.0:1 comp is putting more load on the starter.

    For $450 I can go to 2.0hp starter and will get less voltage drop, BUT body is 10mm bigger. So will hit extractors and remote oil filter outlets on sump
    Quote Originally Posted by 50RTD View Post
    Dave, have you uploaded the data from the carby?

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    Half Shafted. Madhatr's Avatar
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    Its never easy is it? The msd has positive and negative terminals wired direct into the battery right? Then it uses something connected to 12v accessories so it doesnt stay hot continuously?

    You could check what the voltage is here too. If your switching wire (or wires) is on a circuit with many loads, youll see a lower voltage as each tries to draw current at startup. If the above is the case, id try moving it to a different circuit, even running it direct to the battery just for testing and see if it is solved.

    If it is, id look at an unloader relay that kills power to everything else but the absolute essentials, or if you want easier, leave it wired direct to the battery and just fit a switch/isolator for you to turn it on and off manually so you dont burn out the coil when the engine isnt running.
    Quote Originally Posted by Buford T. Justice
    This happens every time one of these floozies starts poontangin' around with those show folk fags.

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    Quote Originally Posted by Greg Rust View Post
    Start at the basics. Drop your battery into a battery shop and get it load tested.
    Do this. Easiest place to start. My ute is the same, battery is a couple years old now and if you don't catch it on the first couple tries the battery gets sad pretty quick. When it was new you could fuck around for ages and run fans with the car off and never have an issue.

    When the weather is cold I just gas it with carb cleaner or petrol for cold starts
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  29. #29
    No I'm Not Shonky Shonky's Avatar
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    Quote Originally Posted by Dogsballs View Post
    Its a single cable of the thickest gauge!straight from the battery to the starter motor. It doesnt get any simpler, lol
    Well no it's not a single cable. There's at least two coming from the battery with a minimum of 4 connections and 4 crimps or clamps.

    A higher hp starter can't reduce voltage drop magically. More power out means more in (assuming same level of efficiency). More load on battery means battery voltage can only really go lower. Or more current means more voltage drop in cables and connections.

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    Quote Originally Posted by Motherfucker Jones View Post
    Do this. Easiest place to start. My ute is the same, battery is a couple years old now and if you don't catch it on the first couple tries the battery gets sad pretty quick. When it was new you could fuck around for ages and run fans with the car off and never have an issue.

    When the weather is cold I just gas it with carb cleaner or petrol for cold starts
    Iíve been using unleaded to start it cold. Hundred ml down carb
    Quote Originally Posted by 50RTD View Post
    Dave, have you uploaded the data from the carby?

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