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Alternator test and dash red light issues


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Hi all,

 

1973 RHD 2002 Baur


I'm know this has been discussed here a lot! but I've searched the forum and can't find a definitive answer... so I'll pick brains again here if I may.

I have read the BMW2002FAQ Alternator troubleshoot article, but i'm still not clear.

 

Recently my dashboard charging red light isn't always illuminated at key start... sometimes is but more often not?

When it does illuminate (and goes off at start) i get a 13.8v +- charge at the battery, but when the red light doesn't illuminate at key run I don't get an alternator charge?

 

I've checked the running Alternator (1500+rpm lights on) with a multimeter on the B post and alternator body and all I get (no red light) is less than 12v so i suspect the Alternator is at fault?

 

However, I'm sure I've read here that if the Red dash light isn't on at start, then the Alternator won't energize to charge??

 

What is strange is that my battery always reads a decent 12.4v at terminals and engine cranks okay at start?
Regulator seems okay too?

 

I've removed the alternator from the car... so are there any definitive tests i can do with removed Alternator to test?

I'm confused if its critical for the dash red light to be illuminated at start for the Alternator to function or the isolated test on the alternator is a good test? 

 

And finally, if the Alternator is bad, what options do i have to replace... I've heard later model alternators versions fit... but I do want a straight swap, please.

 

As always, any nuggets of wisdom are appreciated! 

:D 

Edited by 2002FjordBaur
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20 minutes ago, dlacey said:

Why do you think that?  Its the regulator that provides the field voltage that controls the  alt output (assuming original external reg setup)

I tested the regulator as the trouble shoot article and all seemed okay.., but would a faulty regulator cause my dash light intermittent issue and give me a non charging result even with the B post, alternator body test? Maybe I should double check the regulator again?

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It's true that the voltage regulator could be on the fritz, although I think some of these alternators have internal voltage regulators and changing them isn't that trivial. If the VR is external, remove it and check out long long the poles are, they may be completely worn down. If so, changing the VR is the first thing to do, it's cheap and easy. Not sure about out-of-the-car tests on the alternator, but a multimeter across the two output ports should give you very close to 14 volts. As far as replacements alternators, I bought and installed the 35 amp Bosch from FCP Euro and have been happy with it.

 

Nonetheless, don't go too far from the house until you have complete confidence in your charging system, I speak from experience.

 

- Ric 

Edited by ricsix
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Basic wiring is like this:

IMG_20220109_073204.thumb.jpg.c84223ea50125b946bb40bcb0e8beff4.jpg

It's the external regulator that is turning the light on (path to earth)

 

The wire colours mentioned here are for a Porsche I think...maybe different on a BMW...but conceptually the same on our BMWs

Edited by dlacey
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Quote

 previous owners may have changed to alternators have internal voltage regulators and changing them isn't that trivial

What Ray said-

 

first check bulb, wire, fuse and connections, especially the fuse connection.

From DLacey's excellent schematic, above, a bit of metering around will narrow it down quickly.

 

On the externally regulated cars, I've had the wires fail right where they come

out of the alternator plug.

 

t

 

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AFAIK the alaternator wiring (between alt, VR and dashboard charge light) isn't in a fused circuit.  

 

Didja check the external ground wire that runs from the alternator body to the engine block.  It's a thick brown wire with ring terminals on both ends.  They've been known to separate under the insulator sleeves so that they look intact, but aren't.  A bad ground wire will cause wonky problems with the alternator, as the only other ground path is via bolts running through rubber sleeves--not the greatest of grounds...

 

And...make sure the 3 prong plug on the alternator's backside is nice and tight and held in place with the factory wire bail.  Same with the plug at the VR, minus the bail.  Corrosion or looseness will also affect the whole charging system.

 

mike

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3 hours ago, Chris_B said:

12.4 V is not really a great charge reading at your battery. You should see +12.6V on the battery at rest. What is the reading when the motor is running?

So my charging system isnt charging at the moment... i get less than 12v at the battery and running is no different.
As mentioned, I've checked all connections and grounds and all seem okay, but my main issue is that red dash light doesnt illuminate at key run. I've checked the bulb and that is okay.
What does confuse me is that that dash light does sometimes illuminate (and go off when i start).

But still my main question is about that dash light... if the light isnt illuminated then apparently the whole charging system wont charge... the light needs to be in the circuit to energize the alternator - yes?

So my main investigation should be why that light isnt working?
Or would the dash light not work because the alternator/ext regulator is/are faulty?
I'm trying to work out the chicken and egg scenario??
 

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1 hour ago, 2002FjordBaur said:

But still my main question is about that dash light... if the light isnt illuminated then apparently the whole charging system wont charge... the light needs to be in the circuit to energize the alternator - yes?

So my main investigation should be why that light isnt working?

Yes, exactly this. The current through the dash light wakes up the alternator. Since it's working sometimes it sounds like a bad connection somewhere in the wake up circuit.

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9 minutes ago, Tommy said:

Yes, exactly this. The current through the dash light wakes up the alternator. Since it's working sometimes it sounds like a bad connection somewhere in the wake up circuit.

Understood.
So I'll revisit all connections... and if i start going down the road of replacing parts...
I've found this Regulator... seems very cheap, compared to my usual UK suppliers... is this correct spec?
Regulator

 

 

 

Edited by 2002FjordBaur
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The dash light indicates whether you have +12V on the d+ circuit, which is required to charge the field coils in your alt and make output. If your light is not on when your ignition switch is in the run position, then the bulb is burned out, you have an open circuit (broken wire [unlikely], bad connection at alt or regulator, or lack of +12v on that circuit. Check your bulb first (they burn out) and if bulb is good, check for +12V on that circuit. 

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45 minutes ago, Chris_B said:

The dash light indicates whether you have +12V on the d+ circuit, which is required to charge the field coils in your alt and make output. If your light is not on when your ignition switch is in the run position, then the bulb is burned out, you have an open circuit (broken wire [unlikely], bad connection at alt or regulator, or lack of +12v on that circuit. Check your bulb first (they burn out) and if bulb is good, check for +12V on that circuit. 

understood... all makes sense now thanks!
I've checked the bulb and that was fine, so now I'll check 12v through the system... time to swat-up on multimeter use!

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