Alternator Not Charging and Light Not Coming On

Sydney · Padawan Joined: 22 Sep 2012 Posts: 55

EJ20 in a bay.
Following a recent issue with my fuel pump not priming, I realised today that my alternator light doesn't come on when I switch the ignition on. I checked the bulb and it was Ok.

I started the engine and found that the battery wasn't charging (remained at around 12V) even after revving it, but the alternator light still didn't come on.

Is there a single fault that would cause the alternator to not charge the battery and also stop the alternator light coming on?

Cheers

Kai · Posted: Sat May 09, 2015 11:19 pm Post subject:

My memory is very vague on this as it's been a while.

When the alternator has no charge is provides a ground for the alternator light. The light is lit via the ground provided to by the alternator and a +12v from the fuse box.

So when the alternator is charging it sends +12v to the battery light. The two +12v (one via the fuse box and the other via the alternator + charge ) causes the light to go out.

Does the light come on with the ignition? It should instantly go out when the engine starts running. Apparently not.

So, depending on your wiring of your conversion is the charge light wired to the alternator correctly? If so then your alternator is possibly providing enough juice to cancel the light out but not enough to charge the battery.

Either way, as you have said that the alternator is not charging it wants replacing.

I've had a similar thing happen to me with a Mitsubishi Pajero, thing had this alternators fitted. Neither was charging enough to satisfy the battery but was enough to turn the charge light off. Wasn't enough to keep the engine running which came as a surprise lol
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Sydney · Padawan Joined: 22 Sep 2012 Posts: 55

Thanks Kai,

Mine is conventional bay wiring at the front and conventional EJ20 wiring at the rear so it works the way you describe. It's been working well over the last 2 years or so.

I've now managed to do some more investigation (now I've finally found my multimeter). With the ignition on and the engine not running I get 12V on both sides of the bulb but the bulb stays unlit, so it would seem that there is an open circuit or no ground at the alternator.. I need to check the alternator wire now but I ran out of time.

I read somewhere that an open circuit in the alternator bulb circuit stops the alternator charging the battery. Do you know if this is true for the EJ20 alternator? If so it could explain my problem.
Cheers

Kai · Posted: Mon May 11, 2015 8:39 pm Post subject:

That's an advanced question that I honestly don't know the answer too.

The bulb is has the same physics as a piece of wire. Your test with the ignition on giving +12v is correct. The +12v is coming from the fuse box to the bulb and travelling to the alternator. The alternator when working correctly should be providing a ground when the engine is not running but ignition on (ie no charge) The ground is done via the alternator body. For what ever reason your alternator is not providing the ground to extinguish the bulb. The best test for you to commence is to test the alternator bulb wire with the ignition on engine off, and then another test on the bulb wore with the engine running. With the engine running it should be grounded.

I imagine, as you should have a Subaru plug for the alternator there is a join between the VW wiring and this Subaru plug. Check this. The plug should contain a +12v (possibly switched) and the bulb circuit. Also there are some alternators with one extra additional wiring wire, it's use is beyond my knowledge.

An alternator requires a voltage to produce a voltage so check that the plug is functioning correctly.
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Sydney · Padawan Joined: 22 Sep 2012 Posts: 55

Right, finally had a bit of time to do a bit more checking. I only have two wires on the alternator (apart from the big red charging wires).
With the ignition off I get 0V on both alternator wires.
With the ignition on and engine off I get 12V on both alternator wires but the alternator light still won't light. The oil light does.
Ground the alternator light wire and the light comes on nice and bright. I assume the other wire is the switched 12V from the ignition.
Turn the engine on and I am now getting around 13V from the alternator. Which seems a bit low, I expected at least 13.5V.
At least the alternator appears to be producing something now but I'm worried it will stop again. Because the alternator light still refuses to come on I suspect the alternator is faulty.. I've cleaned up the alternator attachments to improve the grounding but it made no difference.
I've taken the battery out now to charge it and then see if the alternator can producie a higher voltage.

Kai · Posted: Mon May 18, 2015 10:47 am Post subject:

Sounds like your alternator is knackered. It should be kicking out 14.5v

Have you got a spare? I have your welcome to purchase from me. £25?
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Sydney · Padawan Joined: 22 Sep 2012 Posts: 55

I feared that might be the case. I might well take you up on your offer.
Will it fit a 98 (possibly 99) EJ20 that has only 2 wires in the connector plug?

Thanks.

Kai · Posted: Tue May 19, 2015 6:23 am Post subject:

You can change the plug. Let me get you a picture
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Sydney · Padawan Joined: 22 Sep 2012 Posts: 55

If the alternator has more than 2 pins and I have only the warning light and ignition wires to connect to it, will the alternator still work with nothing connected to the other pin(s)?

Sydney · Padawan Joined: 22 Sep 2012 Posts: 55

Hi Kai, took the alternator off to clean all the grounds and terminals, but one thing I noticed was that the resistance between the light connector and the alternator body ground was 22kOhms. I can't find out what the resistance should be but this seems way too high. So whatever electronics inside that provide the ground when the alternator is stationary have failed.

Did you manage to check the number of connector pins on your spare?
Cheers