So,
with all the little problems i have had with my car one of them is a voltage drop between alternator and battery.

i have installed a volt meter direct onto back of alt and another few connected to aux power.

what i notice is system voltage drops sometimes quite low even when alternator voltage is high.
my alternator will be reading lets say 13.8v and my battery/aux power will be reading only 13.1v

.7v loss between alt and battery seems excessive.
i have already doubled up my ground leads.

so i need to find which circuit/circuits is causing this drop.
so i suppose the best way is to remove fuses and check when the voltage becomes same same.
now obviously some fuses when removed will make car stop.
any fuses i must avoid?

should i remove them one by one and then replace them before removing the next?
or remove them and leave them removed incase the fault is on a ground affecting multiple circuits?

 

13.8 to 13.1 is not bad

13.8 at the alternator B+ is on the low side to begin with

You are approaching it as a draw ( drain) between the 2 points , but it is more of a restriction of the battery cable runs between the alternator and the B+ post and the battery B+ post

If you had a parasitic drain like from the alarm system not going to sleep you would notice a dead battery ( eventualy )

You will have the natural restriction of the long battery cable length with a huge gauge size I bought some from a battery cable build up shop ) , a draw from the radiator fans , fuel pump , and engine regulation , and then the large terminal post along the way as restrictions ( and those can only be made so good as a fresh ,clean ,tight design )

As long as the battery receives more voltage than a fully charged battery of 12.75 volts , this has a relationship in how fast the battery recharges after a start cycle

On all fuse boxes # 1 -9 are hot at all times

Fuse # 10 , 12 , 14 , 16 require the relay in the corner to close

So that leaves the rest to be hot at all times # 11 , 13 , 15 , 17

With your approach , as a non - running engine these hot at all times fuses should not have a current flow with the exception of clock and alarm system ( that should not include door , trunk lock mechanisms and their fuses after alarm goes to sleep / all locks go to lock position) to negate that variable of consideration just leave car unlocked during test , and very small knob on steering collum to off ( seat and steering collum to automatic exit position )

 

I understand you. But .7 volt drop seems a lot.

do others have this? Cus i have never heard it as a thing.

there are likely a couple issues in my wiring. But either way i need to find it.
its getting worse.
atm at traffic lights it is dropping to 12.2!

can only try and attack one problem at a time

Alt and battery came from car where volts were reading full 14.5 all day.

noticing a .1 to .2 volt drop with fog lights, a/c, headlights.
yes i can get the alt rebuilt to a bigger spec, but should not need to and also if there is a problem elsewhere i still need to fix it.
maybe battery is not great but it tests ok . Yes i can buy another but would be dissapointing to spend hundreds and still have same problem

 

The alternator guts can be fine but the voltage regulator that in itself controls the alternator field windings excitation ( takes power to make power ) which controls the alternator output voltage

Rule of thumb 13.5 to 14,5 good

VR # IN435 , 35 dollars US but you have to remove alternator to dig it out unless you can remove alternator back shell on car

Rebuilt to a larger spec should not be an issue as the original is 120 amp unless you a installing a huge woofer speaker

12.2 not good

Once the engine is turning the battery is superseded by the alternator higher voltage so your 12.2 is the alternator has dropped below the battery voltage and you are reading the battery and it's charge state

fog lights, a/c ( blower fans is the bigger factor ) , headlights. is a natural expected voltage drop as current voltage trade off and your 0.1 to 0.2 just fine

This is someone's X300 pic of a good alternator on a running engine

 

To improve what you have ( 13.8 but that changes in RPM dropping in traffic from your observation ) for free is to clean and tighten the battery cable connections which are 3 or 4 which include the tie point at the starter solenoid upper terminal post

Digging up someone's pics and then bedtime before dialysis in the morning

 

I will note tho, my trips to and from work are 20mins each way, so short trips, and i have never had any need to jump start my car. So it is always in a state of net positive charge.

 

Are you testing right on the alternator B+ terminal or the instrument cluster gauge ?

One would expect an alternator light on the dash at 12.2 volts





Can't be staying at 12,2 volts otherwise you would expect the battery would not be fully charged and would show up in starter performance

But then with my large starter sag it still has plenty of crank

Do you get a change in the instrument cluster voltage reading in different driving / engine speeds and bits of tape on the gauge help keep high and low readings ?

It in reality only related to engine speed so this test can be done in the driveway after the alternator has warmed up

If you do than it points at the voltage regulator not stable , and this goes against your observation at the time it was on the other donor car

Cleaned the 2 wire round rubber plug on the alternator ? there are a couple of points to check and clean upstream of the round rubber 2 wire plug

 

On my 1995 X-300 with the in line six I found a short where the battery cable enters the engine compartment right front firewall. The rubber insulator around the terminal post was rotted and allowing electricity to pass through to the firewall. Replaced the rubber gasket and problem solved.

 

With that you would expect battery drain to dead , correct ?

the insulator prevents the battery cables from going to car frame ground path back to battery

 

certainly not staying there. That was a momentary drop,
that voltage was read from the obd adapter port via obd fusion app.
the volt guage at that point looked under 12.

i have installed volt gauges inder bonnet so can easy see both battery and alt volts but can only do that when car is stopped.

the volts are not stable. And does change with engine load.
and yes it does not seem right as this alt was stable in the other car

 

Last night i checked the battery and it was 12.5. This morning it was 12.3
i understand from previous my stereo might be causing a parasitic drain.
buf the low voltage whilst running i believe is another issue.
Gotta love old car problems

will upload a video to show later….
it is always a little low.
and it does increase with engine speed but only to a point.
at idle in d it drops.
but idle speed in p or n it is at its maximum.

below 700 it drops.
above 700 it sits more stable.

its odd.
in the images you can see the gauge reads even lower than my obd. It is innacurate. The obd reads same as multimeter on battery l.

 

The Data reader reads the power from the right rear fuse box , fuse # 10 ) and it's battery cable connection to the fuse box and running engine current / voltage trade off relationship

This is a power pin in the OBD 2 socket , same fuse # 10 for the fuel pump relay " control " or pulling coil

The gauge is a question to me as coming from the alternator round plug " fault alternator light wire " or the power sources to the instrument cluster or other ( ?)

Fault alternator light wire would then not be a discrete value as on or off but a voltage value and is determined inside the cluster if in range to give you the light

Will be watching the video for the alternator light

 

I was having these same issues with my old alt before i swapped.
i need to get my alt into the other car and check it out and see.

 

Here in the States the auto parts stores have an alternator spin up machine with readouts like diode package ( not suspect ? , if it is in play ) you can't read

This is minus real world heat buildup and maybe electrical real load ( just coasting )

To know you have a bad diode package do the AM radio station sound test , check to see first if it can vary the speed on the machine as your output voltage is unstable

 

i don’t think bench test will help.
as i know if i leave my car in the driveway idling all is good.
it is certainly only affected in real world scenarios.
and certainly worse on heat build up.
on a cold start for example alt is def 14.5v.
now its summer there is no fully cold start tho. Lol.

will look at that alt test.

i just really don’t wanna pay for a rebuild if its not gunna fix my issues.
but will do a rebuild if i need to.

 

Back in 2010 I had a Series 3 complete with original Lucas 70A alternator which in busy city traffic

suffered similar problems, I used to sit at the lights watching the battery gauge drop.

It was all about the amps (not volts), I was simply using much more than the poor old Lucas could produce, especially at idle speed. The main culprit being the Falcon twin electric fans which did enjoy a few amps, especially with engine at idle sitting at the traffic lights on a hot day & the A/C going full bore.

Have you tallied up your total amp draw, especially your aftermarket stereo.

It's a simple case of what's coming out has to be replaced (by the alternator) and the conditions you encounter on your morning commute to work are probably not ideal, heavy traffic, much idling etc.

Your initial hit on the starter drawing BIG amps (200amps plus) needs to be replenished as soon as possible, is the 20-minute commute to work together with perhaps a continual large amp draw being met.

My solution was to fit a 108A Bosch alternator from a Falcon EL.

Not only did it have a higher amp output, but also the output curve was heaps better suited to low-speed operation than the Lucas.

Whilst our alternator may be 120A, it's in your case the output at idle that may be of more importance.

I have attached JLR article on Battery drain -



John Herbert

1996 XJR

 

Yeah i totally follow you.
from what i have been able to tell there is not too much difference with stuff on or off.
I will pull out my multimeter and check across each fuse to see the draw on each circuit.
the only thing that i have seen a noticeable drop with is .3 volts or so when i turn on the fog lights.
other things like headlights, a/c etc may have an initial drop when first turned on but then goes back how it was.
i will get my partner to watch the alternator gauge whilst i turn stuff on and off also.

it does not feel like anything i turn on and off makes a difference tho
i have installed a cut off for my stereo and it makes no difference when i fully cut it off
i feel more like there is a short in a circuit somewhere drawing a constant load it should not……

it just looks like even at its best its not quite as it should be
going to get a clamp meter to check total current draw and

 

Alternator is definitely losing voltage.
only thing that makes sense is alternator
Having a bad voltage regulator.
but like i have said that makes no sense as it was working in another car
But i dunno what else
cold start puts it at 14.4v
by the time i got home it was 13.8v
but.
that is dependant.
that is the voltage whilst in park.

put the car in drive and it drops down lower again to 13.2
put the revs up and it climbs again to 13.8
put the ac on and it drops to 13.2.

mind you this is alt voltage.
battery/system voltage is even lower.



those pics are the voltage when car first started.
guage cluster
alt voltage
batter voltage

next pics are in park once car is warmed.

 

so i am not noticing a voltage difference between alternator connectors and anywhere else in the system when car is off.
only noticing it when car is on and running.
alt puts out lets say 14v only 13.5 is registered at battery.

where exactly is the centre bulkhead positive terminal?
the one the starter motor and therefore alternator is connected to?

 

If it was so easy everyone would be doing the work

Intermittent faults are the hardest to resolve as not a solid fault

Timing in when the suspect alternator failed ( timewise ) did not correlate to the time you swapped it , there may have been other factors involved as you disturbed like dropping the part and jarring something wrong

I was only as good as my parts in my Carrer

" so i am not noticing a voltage difference between alternator connectors and anywhere else in the system when car is off.
only noticing it when car is on and running. "

You have that elusive relationship of current / voltage trade off in a restriction which is different than a load draw ( device working like a cabin blower ) to ground

An extreme example of this is the starter draw but the only set of eyes you have is voltage and not current as you a missing a sence of hearing ( so to say )

The alternator spin up machine test in the states is free as they want to eventually sell you a part based on the paper readout you get and the trust that is developed that they are your " go to "

" where exactly is the centre bulkhead positive terminal? "

This is possibly a terminal post / pass through between the cabin and the very lower rear right wheel well panel ( there is a British term for the American Quarter Panel )

This terminal does not show on the wiring guide and Watto700 discovered it in his though going through the heavy battery cable runs from front to back

I have yet to look for it on mine and put my eyeballs on it , You can see how an error was made in how they manufactured the car in large pieces , and the prototype may have been a no terminal post solid cable pass through grommet and a change was made that was not communicated to the technical writes ( the usually non - engineers that makes the docs )