P38A Alternator upgrade 4.6 Gems

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awkwardbob

awkwardbob

Well-Known Member
Posts
578
Location
Glasgow
Hi,

last one for now, I promise...

Ive been having some battery grief over the last wee while and it’s been pointed out that my original spec alternator only pumps out 13.8V which will struggle to charge a battery to 14.5v.

I have the heated screen and noticed that while that was on, the voltage from the alternator dropped to 12.6V.

I know I can change the voltage regulator to get it to output 14.7 but if it’s struggling to deliver 13.8V under load, how is it going to make 14.7 ?

is there a beefier alternator available that won’t ruin me (I saw a 175A Wosp one but it was £550!)

All advice very welcome !

thanks again

Bob
 
An alternator pumps out high voltage if left unchecked, the electronics regulate the output to clip the voltage.
How are you measuring volts? The most efficient way I'd with a DVM between the output post and the alternator body, anywhere else and you could have connection/earth/cable problems
I had similar problems and eventually the alternator pecked up and stops charging at all, replaced replaced with another 150A off eBay and it's been fine, no issues with voltage drop as you increase loads
 
Search my posts to check, but from memory mobiletron vr-153a will fit to an a127m alternator cage ie lucas/ magneti marelli. I have two similar but different MMs, for one i needed to take a file to the VR to make it fit around a fixing stud for the cage, but both churn out +/- 14.4 quite happily. remove your cage and read off the part no for the VR, or upload a pic
 
romanrob said:
Search my posts to check, but from memory mobiletron vr-153a will fit to an a127m alternator cage ie lucas/ magneti marelli. I have two similar but different MMs, for one i needed to take a file to the VR to make it fit around a fixing stud for the cage, but both churn out +/- 14.4 quite happily. remove your cage and read off the part no for the VR, or upload a pic
Click to expand...

thanks, I’d found that in an earlier post and should have one waiting for me when I get home.

Was the changing of it pretty straightforward ?

thanks again

Bob
 
Reasonably yes.... If you're doing it for the first time I'd recommend removing the alternator, and then replacing the VR on a bench, as some of the fixing screws for the VR are a bit fiddly to remove. If you want to remove the cage only (and not the alternator) that is also possible, but you may need to cut an unused bracket off the fuel rail to be able to do so, and you will need a good light and a telescopic mirror will also come in handy.
 
romanrob said:
Reasonably yes.... If you're doing it for the first time I'd recommend removing the alternator, and then replacing the VR on a bench, as some of the fixing screws for the VR are a bit fiddly to remove. If you want to remove the cage only (and not the alternator) that is also possible, but you may need to cut an unused bracket off the fuel rail to be able to do so, and you will need a good light and a telescopic mirror will also come in handy.
Click to expand...

Finally, getting around to this amidst a slew of other jobs. Had a wee look at the replacement VR yesterday and the protruding thin metal thing scared me a little.

Sorry to be a pain but was it a screw off/on job or do I need to break out the soldering iron ?

Thanks again

Bob
 
awkwardbob said:
Finally, getting around to this amidst a slew of other jobs. Had a wee look at the replacement VR yesterday and the protruding thin metal thing scared me a little.

Sorry to be a pain but was it a screw off/on job or do I need to break out the soldering iron ?

Thanks again

Bob
Click to expand...

On the doozle it just screws in and out. I assume it is the same on the 4.6?
 
awkwardbob said:
Finally, getting around to this amidst a slew of other jobs. Had a wee look at the replacement VR yesterday and the protruding thin metal thing scared me a little.

Sorry to be a pain but was it a screw off/on job or do I need to break out the soldering iron ?

Thanks again

Bob
Click to expand...
Just unscrews
 
This 14.7 battery volts is a myth.

The alternator won't charge the battery to 14.7, it will charge at 14.7 which means it is pumping current into the battery at a higher current than a 13.8 volt output, so will charge more and faster. When you stop, the battery will immediately settle to its natural 12.6 volts. If the battery is running at 12.6 volts when you are driving with all electrics on, then the alternator is just balanced with the load but is not further charging the battery, which is not healthy.

(P38 4.0SE) My 072 battery charges at 13.8 volts off the MM alternator and that is all absolutely fine - it is as the guys in Solihull designed it.
That is not the perceived solution on here though, where the recommendation is an oversized Korean truck battery being overcharged at 14.6 volts. It means that you can abandon the car for ages and then still start it. Apparently - I wouldn't know. But looks like you have beefed up your system, which can only be for the better.
 
P38 Monkey said:
This 14.7 battery volts is a myth.

The alternator won't charge the battery to 14.7, it will charge at 14.7 which means it is pumping current into the battery at a higher current than a 13.8 volt output, so will charge more and faster. When you stop, the battery will immediately settle to its natural 12.6 volts. If the battery is running at 12.6 volts when you are driving with all electrics on, then the alternator is just balanced with the load but is not further charging the battery, which is not healthy.

(P38 4.0SE) My 072 battery charges at 13.8 volts off the MM alternator and that is all absolutely fine - it is as the guys in Solihull designed it.
That is not the perceived solution on here though, where the recommendation is an oversized Korean truck battery being overcharged at 14.6 volts. It means that you can abandon the car for ages and then still start it. Apparently - I wouldn't know. But looks like you have beefed up your system, which can only be for the better.
Click to expand...
The recommended charging voltage for all lead calcium batteries is 14.7/14.8 volts, a fully charged battery after the surface charge has dissipated is 12.7 volts not 12.6v.
13.8 volts will never fully charge a lead calcium battery in a reasonable length of time.
Given the constant drain on a P38 battery of 30/35ma plus the regular EAS self leveling drain, the bigger the battery the better and there is also the addition drain of the BECM waking up on cars with MK1 or MK2 RF receivers.
 
Datatek said:
The recommended charging voltage for all lead calcium batteries is 14.7/14.8 volts, a fully charged battery after the surface charge has dissipated is 12.7 volts not 12.6v.
13.8 volts will never fully charge a lead calcium battery in a reasonable length of time.
Given the constant drain on a P38 battery of 30/35ma plus the regular EAS self leveling drain, the bigger the battery the better and there is also the addition drain of the BECM waking up on cars with MK1 or MK2 RF receivers.
Click to expand...
I don't have a Diesel engine. I don't have EAS. I have a Marty RF Filter.
So my BeCM slumbers gently.
My battery fits neatly and my alternator is not stressed.
If I don't use my P38, I top the battery up using a Ctek smart charger plugged into a permanent connector once a week.
Landyzone info is key to my setup and I am as ever eternally grateful for the detail I have received, not least (in fact, most) from your good self..!!!
Horses for courses. I'll get my coat.
 
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