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Engine Swap Electric Woes, Battery Not Charging


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Everytime i post on here i seem to always be asking questions! Hopefully one day i will be able to give back with my accumulated knowledge but not today haha!



I'm struggling with some fundamentals of automotive electricals and today is the charging system. I would be very grateful if someone could possible try work through a problem i'm having as of all the key areas in 02's its not my strong point!



had my m44 running a few months ago when i first swapped it in and got some of the basic wiring done(so i thought) and left it at that while i attacked the multitude of other work i needed to do. I have now gone back to the engine as i'm now in the tidy up stage and have noticed a big problem.


With the battery charged the car starts fine, runs and idles. However battery voltage begins to drop and the engine eventually dies and theres isnt enough voltage left to turn the starter properly.


So could be a bad battery, as it has been sat since april however it worked fine before with the m10 or more likely i have not done the wiring correctly.


I struggled here though as the Jake B guide purely shows how he sets up the cars and doesnt take into account understanding of the electrical system, its just a attach wire here etc and as i have an m44 a lot didnt apply.


Then using the 2002 electrical diagrams i have the issue that the m10 alternator has 5 connections(including ground through block) and the m44 has only 3.


So looking at my current situation i have 3 connections on my alternator, the ground through the block, a large red lead,and a smaller blue lead. Both connect to the starter. I have the square type 3 terminal voltage regulator with a black wire, a brown wire(ground) and then 3 green wires(12v+) from the last terminal.



This is where my basic understanding of electricity is bad. The 12v from the battery obviously needs to connect to the starter to provide cranking power aswell as to the electrical system for spark etc. But what is the relationship of the voltage regulator to the alternator? should the 12v feed from the alternator go to the regulator to prevent the voltage exceeding 12v before joining the rest of the system?


I'm finding it hard to apply this to the wiring i have available....


I have routed the battery to the rear but have connected my extension wires to the same wires that had the clamps on. I have so far tried to avoid undoing the bundles underneath the relays so far if that helps



many thanks for any help




*edit* is it possible that the m44 alternator has a built in regulator? meaning i dont need the original 02's?


*edit* google says the z3 (my m44) has an external voltage regulator

Edited by DaveBMX
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Your battery is not getting recharged properly by the car's charging system.


The alternator's job is to try and keep the battery charged at near 100%.


All the voltage regulator does is to regulate the voltage of the current coming out of the alternator that is recharging the battery, so that it is high enough, but not too high (as the engine spins up the alternator will produce more current and voltage if not otherwise throttled by the regulator).


The electrical needs of the starter and the rest of the car should always be driven off the battery itself, never directly from the alternator.


Since you can recharge the battery (presumably using external charger) and it works again, the battery is probably fine.

Either your alternator, your voltage regulator, or wires in between are defective or wired wrong.


Here is a really extensive primer on charging systems: http://www.ratwell.com/technical/ChargingSystem.html

You can use search to find the specific way your system should be wired up.

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thanks hc2002 having a look through that page.


thanks Eurotrash if this is true for the m44 what did you do with the stock voltage regulator? Did you just disconnect it or did you need to alter the wiring?



If its just a case of unplugging it then i can test this scenario easily.


Ive been doing testing and with the battery at 14v with the engine run for 5 minutes i can see that voltage has dropped to 12v. checking the fat red alt wire i get just over 12v at idle. My digitial instrument cluster shows system voltage and it displays 11.5v



if it requires the system to be at around 14v to charge the battery then it appears my alternator is not making enough power?







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ok took the cover off the alternator, has its own regulator!!


so need to work how removing the stock reg affects the system as in do i need to bridge wires etc


and or has the m44 regulator worn out as i have seen that that is a problem with them and that they are replaceable!


eurotrash where does your blue wire go? Does your red go to the starter or to a 12v junction to the battery?







Edited by DaveBMX
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You should see 13-14V at idle, mid 12v at rest. To test battery, disconnect ground and hook up volt meter with engine off. It should hold at mid 12. If it slowly drops, battery is suspect. If it holds, hook up ground strap and do test again, comparing numbers after say 24hrs. This is to look for parasitic drain. You can also use a 10amp tester to look for draw. It should be minimal.

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I just fixed a charging problem on my E3, maybe you've got the same problem. The alternator needs battery voltage on the D+ terminal to "excite" the alternator, i.e. create a magnetic field. Without it, the alternator will not create current to charge the battery. The D+ terminal gets it's voltage from the instrument panel battery/charging indicator lamp circuit. When you switch on the key, the lamp lights and provides excitation current to the alternator. After the alternator output voltage exceeds the battery voltage, the circuit is broken and the indicator light goes off.

Make sure you've got battery voltage at the D+ terminal when the key is on position. In my case, I had an open in the circuit between the instrument panel and the alternator. I ran a bypass wire and fixed the problem.

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Yep, as you've figured out, ditch the 2002 regulator and most of its wiring.


You can wire the 2002's charge light to the D+ post on the alternator  It's the blue

wire that comes off the regulator, and if you want to test it, just ground it- the

charge light inside the car will light up.  If it doesn't, it's either burned out

or you've got the wrong wire!  (but you want the blue wire, I'm 99% sure)


Then everything should work as stock, including the alternator wire warning

you when the alternator's not working, and the system not charging when

the bulb burns out!  You'll note, below, that the 318 has a resistor to cover that

base.  You could add one too, if you wanted, or just note, before you crank, that

the alt light comes on in the dash.


D+ is traced blue in the 2002 diagram.


This is easy to fix, after all you've gotten through!








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Hi Dave, read Zeebuck's instructions on the E30 alternator upgrade here and basically do all of the same stuff.  In short, cut off that three connector plug, big red wire goes to B+ terminal on the alt., and blue wire (free now that you just decapitated the connector) goes to the D+ terminal.  Take the old 2002 regulator to work and leave it on your desk to confuse people.




P.S. Depending on your exact setup, there is a small chance that you'll experience engine run-on after you  turn the key off.  I did after the E30 alt upgrade with Megasquirt; turned out that 'L' charging light on the dash was enough current to keep my MS and ignition brains alive. Easy fix with a diode,but just PM in the rare case that you experience this too with the M44 setup.

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Wow, guys i would never have known that about the D+ excite field!


shows exactly how amazing this forum is! i would have been going around in circles indefinately.


Will have another look now and report back after i read that post Vespa guy!!!


kind regards






Toby and John i may need some clarification on the bulb issue. I have replaced the entire dash unit with a digital one and im using just plain 12v bulbs for the warning lights. battery light is grounded through the brown wire in the instrument cluster plug and is connected to the blue wire which would have gone to the D+ on the regulator. If i just connect the blue wire bundle from the bottom of the old reg connector to the D+ on the alternator will the bulb work properly? or did the instrument cluster have some kind of circuitry for the battery light?





Edited by DaveBMX
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And with the new alternator/regulator, the dash light stays on until you rev past 1200 rpm or so.  Like an EFI car does at start up.  that 'excite' thing.    So's ya know. Well, if you had a stock motor...  so this is for anyone that might stumble across this thread.



I left my old reg in there, just to appear stock.  I'm dumb.

Edited by eurotrash
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