Alternator Charge Voltage at BECM?
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Thread: Alternator Charge Voltage at BECM?

  1. #1
    FRESHMAN ROVER
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    April 30th, 2010
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    Alternator Charge Voltage at BECM?

    Hey, what should the voltage going into the becm from the alternator be? I get 13.5v. It is also 13.5v at the alternator on the post for the becm wire, but 14.2 at the battery post on the back of the alt. Is the voltage regulated to go to the becm? This normal?

    Thanks
    1996 Range Rover 4.0 SE - North America

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  3. #2
    FRESHMAN ROVER
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    Location
    San Antonio, TX, USA
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    Re: Alternator Charge Voltage at BECM?

    I would think that is alright and plenty of voltage for the BECM. I am a little confused about where your measurements came from or what your problem is. The power supply voltage for the BECM comes from the fuse box and should be roughly the same as the voltage measured at the battery. It would also be helpful to know things like year, model, petrol/diesel, which you could put in your signature.

    "I get 13.5v."
    13.5v at the BECM wiring harness under the passenger seat?

    "It is also 13.5v at the alternator on the post for the becm wire"
    I thought the BECM was fed by the fuse box, but I see that the smaller wire on the back of the alternator goes to the BECM.

    "14.2 at the battery post on the back of the alt"
    Relative to which ground? The alternator case or somewhere else? Did you use the same ground when you measured the smaller wire voltage?

    Got this from the manual:
    Charging System
    When the Ignition Switch (X274) is in position II, the
    BeCM (Z23 turns the charge warning light on via
    the datalink connection to the Instrument Pack
    (Z142)
    When the Generator (Z106) starts to produce power,
    the BeCM (Z23 gets an input signal on Pin 15 and
    turns off the charge warning light.
    For the Diesel engine this signal is also used as an
    input signal for the fuel pump relay inside the Engine
    Compartment Fusebox (P125) to make sure that the
    Fuel Pump (M109) is only supplied with power when
    the engine is running.
    If the Generator (Z106) fails to produce power, the
    charge warning light remains illuminated.
    2000 Range Rover Rhino
    * EAS through 2012, Atlantic British heavy duty coils, Bilstein shocks, JKS 3100 Quicker Disconnects, 265/60R18 Goodyear Wrangler SilentArmor tires

  4. #3
    LIFETIME CONTRIBUTOR
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    Re: Alternator Charge Voltage at BECM?

    Did you read the sticky at the top of the P38A section? If you had, you wouldn't need to ask the question.

    See viewtopic.php?f=3&t=39296
    Ron Beckett
    1995 Range Rover HSE 4.6 V8 auto
    2007 Yamaha XJR1300
    2006 Suzuki DL650 V-Strom

    RIP Bucko (1979-2008 - Riding on forever)
    http://p38arover.com/lee_pix/bike_caricature-avatar.jpg

  5. #4
    FRESHMAN ROVER
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    Re: Alternator Charge Voltage at BECM?

    I'm sorry, let me try to be a little more clear.

    Connector C114(up to 99MY), C1289(after 99MY) pin number 15 is the alternator charge wire. This wire runs directly to the back of the alternator. That wire only reads 13.5v at the BECM connector and the alternator. Is this what it should read? If so, why is it not 14?
    1996 Range Rover 4.0 SE - North America

  6. #5
    JUNIOR ROVER
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    Re: Alternator Charge Voltage at BECM?

    Like Ron said, run through the rest of the standard tests. viewtopic.php?f=3&t=39296

    If you still have a problem look at the issue you mentioned.

    The voltage reported by the engine ECM is always a little lower than battery but I've never investigated why.
    Greg Hind
    http://www.skirecord.com
    1994 4.6 HSE

  7. #6
    FRESHMAN ROVER
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    Re: Alternator Charge Voltage at BECM?

    Either you all do not understand what i am asking or i just cant find the information in the thread you are directing me to. The wire on the BECM connector only shows 13.5 volts while the alternator is putting out 14.2 volts. Is this wire supposed to be 13.5 volts? That whole other thread just speaks about what the alt should be putting out to the battery which is not my problem. There is a small post on the back of the alt with a wire leading DIRECTLY to the BECM. This post only reads 13.5 volts while the battery post reads 14.2v.

    Thanks.
    1996 Range Rover 4.0 SE - North America

  8. #7
    JUNIOR ROVER
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    Re: Alternator Charge Voltage at BECM?

    Quote Originally Posted by bjohnston115
    which is not my problem.
    What is your problem. You've told us the voltage you measure but not your actual problem.

    The wire you are talking about, I think, has something to do with voltage regulation. User DJSlats had a problem with this wire years ago which took a lot to find. The wire itself needed to be replaced.

    But what is your actual problem. Hard to help fix something when you don't know what the symptoms are.
    Greg Hind
    http://www.skirecord.com
    1994 4.6 HSE

  9. #8
    JUNIOR ROVER
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    Re: Alternator Charge Voltage at BECM?

    p.s. around here, it is generally insisted that people post the results of the sticky tests when talking about ANY electrical problem. 95 times out of 100, the problem is found when we look at those results, even when the person swears the results were "OK" or those tests were not needed.
    Greg Hind
    http://www.skirecord.com
    1994 4.6 HSE

  10. #9
    FRESHMAN ROVER
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    Re: Alternator Charge Voltage at BECM?

    all im trying to figure out is what the voltage should be going to the becm from the alternator via the small wire. The terminal at the back of the alterator only reads 13.5v regardless if the wire is connected or not. THe voltage at termial with the wire going to the BATTERY is 14.2v which is fine. everything works fine, im jsut trying to figure out if this is an issue but im just not seeing any problems.
    1996 Range Rover 4.0 SE - North America

  11. #10
    Col
    Col is offline
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    Re: Alternator Charge Voltage at BECM?

    Quote Originally Posted by bjohnston115
    but im just not seeing any problems.
    Perhaps that's because you haven't got one.

    Cheers
    Col

    Beer is living proof God loves us and wants us to be Happy. - Benjamin Franklin 1706-1790

    Improvisation is the Mother of Invention

  12. #11
    JUNIOR ROVER
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    Re: Alternator Charge Voltage at BECM?

    Quote Originally Posted by bjohnston115
    but im just not seeing any problems.
    I thought so. if you are wanting to take care of electrical preventative maintenance, all you need to do is follow the electrical sticky and checkout the condition of your fuse box - see article on main site.

    In all the 15,000 people that have read the electrical stick I've only heard of one person have a problem with the wire you are referring to. All the rest had their problem fixed by the sticky.
    Greg Hind
    http://www.skirecord.com
    1994 4.6 HSE

  13. #12
    FRESHMAN ROVER
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    Re: Alternator Charge Voltage at BECM?

    I just measured mine out of curiosity. Engine cold, started engine, lights off, running about 1 minute. The alternator was replaced about 4 months ago with a remanufactured Bosch one. We have no electrical problems that I know of.

    Alternator positive output (big cable) nut to alternator ground (big cable) bolt: 14.36v
    Alternator indicator to BECM (thin cable) nut to alternator ground (big cable) bolt: 14.45v

    Hope that helps.
    2000 Range Rover Rhino
    * EAS through 2012, Atlantic British heavy duty coils, Bilstein shocks, JKS 3100 Quicker Disconnects, 265/60R18 Goodyear Wrangler SilentArmor tires

  14. #13
    JUNIOR ROVER
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    Re: Alternator Charge Voltage at BECM?

    Quote Originally Posted by hildstrom
    Alternator positive output (big cable) nut to alternator ground (big cable) bolt: 14.36v
    Alternator indicator to BECM (thin cable) nut to alternator ground (big cable) bolt: 14.45v
    That is really helpful the way you detailed exactly where you took the measurements. Most people think earth is earth (ground is ground) but it isn't. That is why I was saying to give us the electrical tests as most people don't give the reference locations in detail like you have.

    For example if the BECM wire measures 13.5 volts where is the voltage drop coming from? Most people would assume a problem with the BECM wire but it could just the same be a problem with the earth that is being used for the measurement. But anyway since the OP has no problem, I wouldn't spend too long chasing it.
    Greg Hind
    http://www.skirecord.com
    1994 4.6 HSE

  15. #14
    FRESHMAN ROVER
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    Re: Alternator Charge Voltage at BECM?

    Quote Originally Posted by hildstrom
    I just measured mine out of curiosity. Engine cold, started engine, lights off, running about 1 minute. The alternator was replaced about 4 months ago with a remanufactured Bosch one. We have no electrical problems that I know of.

    Alternator positive output (big cable) nut to alternator ground (big cable) bolt: 14.36v
    Alternator indicator to BECM (thin cable) nut to alternator ground (big cable) bolt: 14.45v

    Hope that helps.

    Thank you for doing this. Other people state i do not have a problem, but obviously if you have 14.45v and i have 13.5v there is a problem. Thank you for actually answering my question instead of jumping at me and telling me to do other things. I think i need a new alternator because the output is not correct and all the grounds are good and new.

    Thanks again.
    1996 Range Rover 4.0 SE - North America

  16. #15
    JUNIOR ROVER
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    Re: Alternator Charge Voltage at BECM?

    Quote Originally Posted by bjohnston115
    Thank you for doing this. Other people state i do not have a problem, but obviously if you have 14.45v and i have 13.5v there is a problem. Thank you for actually answering my question instead of jumping at me and telling me to do other things. I think i need a new alternator because the output is not correct and all the grounds are good and new. Thanks again.
    There are at least 3 different types of factory alternators. There are 2 totally different engine management systems. There is a possibility that a voltage measured on one year p38 at that point could be different from another year. hildstrom's data while interesting and helpful may or may not be meangful for your car.

    Enough things need fixing on these cars without doing things that don't really need to be done. If you don't have an actual problem, my recommendation is to save your money and leave that circuit as is. I don't think the voltage you have recorded on that wire is a problem - your fuel pump is being switched on and your charge light is being switched off.

    In addition, you may not be measuring this properly. Earth is not just earth and negative is not just negative. A problem in your earth circuit, which is VERY common in these cars, could account for 500mv or more (half a volt) of your problem.

    We spend a lot of time helping people with electical problems on this board. The help is almost free - the cost of admission is to do the standard tests first. This saves a lot of wasted time from the experts on here when somebody comes along and says "but that isn't my problem" "I don't need to do those tests". 95 times out of 100 it is!

    p.s. the correct earth reference point for your test is the alternator body, NOT the battery negative. your vehicle should be doing 2000rpm in neutral (no more or less) and your battery should be fully charged and probably with no substantial electrical load on. Ideally we'd have the data from the alternator postive and the wire you are trying to measure at the exact same time (2 multimeters) or done very closely together and swap back and forth to confirm they are stable.

    if you had done the electrical sticky tests and put the results here, we can interpret many of your measurements done in different ways without worring about doing it exactly as above.

    Cummon. Do the tests for us.
    Greg Hind
    http://www.skirecord.com
    1994 4.6 HSE

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