Charging system seems weak

[jimf]

Hi folks, need a little guidance here. I have had a couple of breakdowns in my '87 325i with a flat battery sometimes straight after a run. After a full charge of the battery which was sitting at 12.9v with engine off, I checked the running voltage and it measured a touch over 13 volts and appeared to be dropping. Today I changed the alternator for a 2nd hand unit but the fault remains. The alternator earth 'looks' ok but not sure how I can actually check it. I 'think' the battery is ok. Any ideas on what I can do?

[maggspower]

A freshly fully charged battery should be putting out a bit more than 12.9v.

However, when you test the alternator at the battery you are testing the wiring as well. Now test the alterator at its positive output and and earth. This will test the alternator only. and narrow down the problem.

Be careful testing it with the engine running!

HTH

[jimf]

Oh I tried that, forgot to mention. 13.6v was the result which higher than the measurement at the battery. I assume it should be much more. Have I fitted another duff alternator or could it perhaps be an earthing issue?

[Speedtouch]

How good is the battery? Also check the engine-to-body earth strap and the earthing points of the alternator against the block.

[jimf]

I take it these are just physical checks, the alternator to block seems ok. Where is the engine to body generally? I suppose the battery could be a bit tired, I don't know how old it is, even if the battery was weak would the measument at the alternator read normal. I'm I right in saying it should be over 14v

[Speedtouch]

Yes, under normal charging conditions, you'll see around 14V at the battery terminals, but if the battery is shot, you'll see less.

The engine-to-body earth strap is usually under the nearside front chassis rail.

It is worth using a wire brush to thoroughly clean the alternator mounts to ensure a good earth.

[jimf]

Just come in from the car, found the main engine to chassis strap. It's solid. I'm running the risk of looking stupid here but how is the alternator earthed? There is the big terminal with two red wires and the small terminal with two blue wires, that's it.

[maggspower]

The alterator is earthed through its body.

[jimf]

The alternator is only kicking out 13.6v from its positive terminal, a weak battery wouldn't cause that would it? I'm guessing I've got another duff alternator.

[Speedtouch]

The alternator only supplies what the battery requires, so if you have a battery of reduced capacity, then it will only supply as much as the battery will accept.

I would suggest trying another battery, before condemning the alternator.

[jimf]

Oh I didn't realise that, cool, will take the battery out of my e46 tomorrow and try that in it. Electrics are not my strong point, alway learning though. Many thanks, will report back tomorrow.

[daimlerman]

Be aware that some alternator's are mounted in rubber bushes,this type require a seperate earth from the casing to the engine mount.
A nice new earth lead from the engine to the car frame will only cost two or three pounds and may cure your woes....

[Billwill]

Maybe you need to replace the voltage regulator/brush pack which sits at the rear of the alternator. The brushes get worn badly. This pack is very easy to remove from the rear of the alternator..get a Bosch replacement.

Your "second hand" unit may have had worn brushes!

[jimf]

Hey folks, a quick update on this with improvement but not really a cure I don't think. I tried a battery that I knew was good which made no difference. Next I removed the alternator then cleaned and sanded the points that meet the mounting bracket. I also removed the engine to chassis earth strap and did the same. Once back together and running it now runs at approx. 13.85 volts measured at the battery. This is better than before and although not over 14v which i believe it should be it seems enough to charge the battery. After a run the battery has definitely not lost any voltage. However when I put lights or fan on the voltage drops and doesn't really increase as it should. When I turn them off the voltage seems to creep back up to enough to charge the system.

[jimf]

Where can I buy replacement Bosch brushes?

[Speedtouch]

Try your nearest Bosch service centre, or most decent motor factors will probably be able to source some.

[jimf]

Expensive?

[Speedtouch]

Depends where you go; try a quote from the BMW dealer or Cotswolds on here, because they may be surprisingly reasonable...

[Brianmoooore]

Remove the regulator pack from the back of the alternator, see wherever brush is the shortest, and give it a tug. The brush is held in its holder by a flexible copper braid, and pulling it will straighten/stretch the braid slightly, allowing the brush to come out slightly further than it did before.
If, after you've refitted the pack, everything works properly, you have a month or two to find a replacement pack.
I've done this procedure by the roadside. Takes about ten minutes, using only the tools in the standard E30 tool kit.

[jimf]

Thanks Brian. Just tried that and the brushes were in very good nick. Have them a little pull anyway and everything is still the same.

[Brianmoooore]

In that case it's likely to be either an alternator earthing problem (measure for any voltage between the battery negative terminal and a cleaned off part of the alternator body with the engine running), or one of the diodes has failed in the diode pack inside the alternator.
An alternator actually produces three separate outputs, which are combined together by three pairs of diodes. If one of these fails open circuit, then the average output voltage will be low, which won't actually show up on an 'ordinary' meter, since the DC voltage will have a strong AC component in it which will confuse the meter.

[jimf]

Ok, tried checking for any voltage between battery negative and alternator housing. The meter reads -0.01v. No idea if that means anything. Is the diode pack a changeable part?

[Speedtouch]

1/100th of a volt voltage drop between battery negative and the alternator is nothing to worry about - just normal resistance in the leads.

The diode pack is changeable, but at the price it is likely to cost, you may be better off sourcing another used alternator, available from around £20.

[jimf]

I have a couple of other Bosch alternators lying about but for E34's. The brushes were interchangeable so perhaps I can try the other diode pack too.

[Speedtouch]

Indeed, or fit the whole alternator if it will fit.

Don't worry about the extra output rating; it will only deliver as much as required by the system.

[Brianmoooore]

Changing the diode pack involves heavy duty soldering after opening the alternator right up. Far easier to change the whole thing.
Alternators come in LH and RH types (position of the adjuster strap), but apart from that, and possibly needing a change of pulley, many are interchangeable.