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Discussion Starter #1
Hi all,

I have bought an 140A alternator from a BMW 525TDS for my early 2.5 P38 after a recommendation.

Obviously both are different connections- mine is the 2 pin connection whereas the BMW alternator is a stud to battery and a 'push-fit' 3 pin plug. I know what all the connections are on each unit but I am now struggling to think of a SAFE solution for bodging the BMW alternator into my wiring harness.

Has anybody done this modification and has any advice of how they managed it? I have found a 3-pin plug that I can use to splice into my wiring but would this be acceptable or am I going to have to keep the fire brigade on speed dial?

Many thanks in advance!

Greg
 

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Do you know what are the circuit connections going to other two wires?
Shoudn't be a issue unless you directly earth a +ve wire.
From the existing wires, the smaller one runs the in-tank pump when the engine is running and also directly connected to BECM for the operation of charge light. Take care.

I'm alos curious and this should be a good way to increase charging capacity as the original alternator is 105A.
 

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Discussion Starter #3
Hi Kapilamuni,
The 3 pins in the push in plug are:
To computer
To warning lamp
To ignition

So I should be able to attach the smaller cable to all 3 pins?

I was recommended this alternator, apparently it is a common upgrade but I am struggling to find anyone who has done it! I am fitting a split charge system to run accessories for travelling but I also use it to drive to work (15 minute drive) and I have noticed (along with my current alternator starting to go bad) that the battery has been struggling especially during the winter with the cold weather! So I am hoping this alternator will give a bit more charge and cure my low voltage! I was also told it can help improve MPG (being as everything is struggling with the low voltage) I am not sure if that is true but we shall see!!
 

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I'm sorry that I can't advise you on this matter as I'm not a qualified auto electrician.
However, If you look at the RAVE elecrtical section and circuit on Starting & Charging, you will see that P38 diesel alternator has two wires connected, one is thick wire from Bat+ and the other small wire coming from BECM and it's also spliced to intank pump relay-12.
So the small wire has Computer & IGN . The charging light is different and coming from BECM.

Without verifying do not connect any wire as errors will be costly with BECM involved.

Can you tell the alternator model so that we can search for the specific wiring circuit ?

As suggested by Richard-G, it's no waste to have a proper capacity battery installed even before this modification. The diesel P38 needs about 110AH battery, my problem here is we don't have that capacity battey with dimensions locally. What we can get is 95AH max, to fit that space.
 

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Hi,
what's the final outcome on this?
 

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Discussion Starter #7
I already have the Hankook battery, it is the alternator at fault of that I am 100% certain. The alternator I have upgraded to was recommended to me by a P38 guru as being compatible, all I needed to know was whether I need to now reroute any wiring or just connect the 3 pin plug from the alternator to my 1 signal cable from the old alternator. Luckily I have a spare BECM anyway (I keep spares for common issue parts for all my LRs 😂)

Outcome was- everything was fine, no issues yet!
 

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Great. Tell us what/how are the connections you made
 

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Yeah. Why leave out the most important part of your post?

How to get it to work.
And not only: “It works.”
 

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You didn't tell us what are the wiring connections.
or tell us the alternator model, at least.
I also want to upgrade the alternator.
 

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I don't understand why you would want to upgrade from the original alternator. The car is never going to draw anywhere near the maximum current of the original alternator while running. The only scenario I can see where this might be usefull would be if you expected to do a lot of heavy winching. In that scenario you could run your battery down to the point where it would not start the car if you stalled it so a fast charge would be advantageous
 

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and going from 120A to 140A is so minimal the difference isn't going to be noticeable anyway.
 

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The diesel alternator is rated at 105A.
I want to install an electric fan inplace of VC fan. One I'm looking at is BMW 520D fan which is rated at 600W.
That gives, fan starting current as 50A. With our RR s,say night drive in rain, head lights, wipers, AC, ABS pump, EAS pump et etc, will exceed the capacity of original alternator.
 

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The condition you describe of every electrical system in the car working simultaneously is extremely rare and even if it happens it will only be for a few seconds.
The steady state current draw of the fan will be a lot lower than the starting current, so again you will be within spec of the original alternator.
Even if you momentarily exceed the alternator capacity, the battery will cover for the excess draw and then it will be recharged by the alternator.
 

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It's not going to be extremely rare. In heavy traffic where you stop & go for every 2-3 metres, within 10 meters ABS pump would come in. Simillar frequency would be with wipers too as it steps down when car stops. Take the AC . It cycles the compressor. Two blower fans& front cooler fans always ON. Add the propsed radiator fan and you are facing a Delorean Special scenario.
The battery available here is 95AH maximum to fit the RR space. Even now, if I srtart & stop within short time thrice, say 20-30 minutes. then for the next start, battery will say NO. It's a new battery replaced in last December.and the alternator out put is 14-14.2 V.
My point is why leave space for trouble and fitting a 140A alternator will not harm anything.

Also, why would LR put 150A alternator on the petrol car?
I guess for the diesel, they took the whole lot from BMW that came with the engine.
 

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Petrol version has to run coils so a bit of extra load there.
The current draw of the fan is probably going to be about 25-30A when running, the 50A will only be for a couple of seconds when starting.

Fair enough though, if your driving scenario is mostly being stuck in traffic then I guess what you say makes sense.
For the average user though the situation you describe would be an extreme case senario.
 

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Even now, if I srtart & stop within short time thrice, say 20-30 minutes. then for the next start, battery will say NO. It's a new battery replaced in last December.and the alternator out put is 14-14.2 V.
My point is why leave space for trouble and fitting a 140A alternator will not harm anything.
That is not normal behaviour, unless your battery never gets properly charged. A bigger alternator will of course help, but could be just masking an underlying problem. Like a battery drain.
You are correct that a bigger alternator will do no harm. It just seems like a lot of effort, especially combined with the electric fan, whereas the standard viscous fan will do a better job than even a 600W electric fan and will put no extra load on the electrical system that is by your own account already appears at its limits. If you want to thinker and change because you can, by all means do so. And share with us, we all like a good tech story. But purely from an operational and reliability point of view, I'd say KISS. ;-)

Filip
 

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Quote
..............That is not normal behaviour, unless your battery never gets properly charged
...........

Battery gets charged but on short runs with AC , it seems marginal.
Other reason I want to get rid of VC fan is the noise. When hot , just start to move, it roars. Even when standstill and just press the throttle, it roars. Drawing attention like I'm driving a big truck.

Yes, I expect the transition will be simple, once I sort out the alternator.

I even got a BMW M57 fan (these are low noise(?) as it has a circular piece connecting all blades) , and got it attached to M51 VC coupling. Gave up as it requires enlarging the cowling.
 

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If you can start normally and after a few minutes running the battery seems to be down so it won't start again, that points to a dying starter, failing when hot. The VC roar should only happen when you first start from cold until the VC unlocks after a few seconds. After that it should only lock up when it gets hot, really hot.
 
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