Alternator/battery Problem

[GLPoomobile 958]

I know I'm committing a cardinal sin here, as all of this will be covered in previous topics, but I just want to check that what I suspect to be obvious, is indeed correct.

 

Before I left London yesterday, I did all my routine checks, one of which was to check my battery voltage as it does tend to lose a bit if left standing. It was showing just a little over 12v, and when running was at about 14.5v (measured at the battery).

 

I completed the journey up to Edinburgh with no problems.

 

Then this morning I've started it up and noticed the battery light glowing dimly (wouldn't have noticed it if not for the fact that I was in an undergraound car park so it was a bit dark down there).

 

So I checked the battery again and it was showing just under 12v (yet it started OK!). I started the engine and there was no change at all, exactly the same voltage. Gave it a rev, and no change. So I took it down the road, opened it up to get the revs up just in case it was a problem with the excitor wire, pulled over, checked it all again and no change bar a small voltage drop.

 

Got home and parked up. Managed to get the multimeter probe down on to the alternator +VE connection and connected the -VE to the - terminal on the battery. With the engine running the voltage is all over the place. Minimum was around 15+ and hunting up and down to over 16v and sometimes spiking for a millisecond at around 19v.

 

So my diagnosis is that the alternator is fubar and overcharging, which has killed the battery. It's obviously running OK as the alternator is supplying enough (more than enough!) voltage, but it's slowly killing the battery. I'm surprised it still starts on less then 12v, but it definately didn't sound quite so eager on the last start!

 

So before I go to Halfords to get the battery replaced (think it's still under warranety), I'm going to replace the alternator with the known working spare I have. I just want to check that you guys are in agreement that it sounds like the alternator voltage regulator is at fault before I go to the hassle of replacing it, or if there is anything else likely to be causing the issue.

[MerlinGTI 3]

I would be doing what your about to.

 

I would fit and test the replacement alternator before sticking your new battery tho. Just to prevent damage to the new battery in the unlikely event the replacement alternator is also dodgy.

[Rob Thomson 6]

How come it's suddenly generating ~16V when you got home after not generating anything on your test drive?

 

Before you go to the hassle/expense of swapping things try exciting the alternator. To the best of my knowledge these old alternators don't self-excite as do more modern ones. You need a wire with a bulb in it, and connect between the battery +ve and the excitor terminal on the alternator. The bulb should be on before you start the engine and go out when you start, just like the warning bulb in the dash. DON'T try this without a bulb, it acts as a current regulator. If you can get it to excite then test the battery voltage. If still all over the place I think your diagnosis is correct, otherwise check the wiring.

 

I drove mine round for months with a wire/bulb straddling the engine bay, having to connect/disconnect at the start and finish of every journey. Fun.

[Anthony 1,003]

Before you go to the hassle/expense of swapping things try exciting the alternator. To the best of my knowledge these old alternators don't self-excite as do more modern ones.

They should self-excite with enough revs, something like 3500-4000rpm or so. Certainly I've successfully done this on a few 205's when there's been something wrong with the wiring for the warning light.

[CaptainP 2]

I had an alternator that decided to perform the overcharging trick and inturn boiled the battery. I Had the alternator tested and it was indeed faulty, the motor factors I bought them both from just replaced them. The replacements were absolutely fine and the problem has never resurfaced.

[GLPoomobile 958]

How come it's suddenly generating ~16V when you got home after not generating anything on your test drive?

 

I'm not sure what you mean by "suddenly" When I checked it this morning after seeing the warning light on, I only checked it at the battery, and I was seeing no extra voltage with the engine running. I tried to get the alternator to self excite by getting the revs up to 4k rpm, and still no change at the battery end. It was then that I checked it at the alternator and saw it was charging at ~16v, but then moving the multimeter back to the battery I was not seeing that same voltage. So it wasn't a sudden change. The failure seems to have occurred on the 400 mile journey up from London.

 

What does confuse me though, and this might be a vital clue, is why I'm not seeing the same voltage at the battery as I am at the alternator. I mean even if the battery is goosed and not taking charge then surely I should still see the same voltage there as at the alt when running. Although I was probing the battery posts rather than the clamps.

Edited September 5, 2010 by GLPoomobile

[Anthony 1,003]

Do a resistance check between the alternator terminal and the battery positive - should be nigh-on zero ohms

 

A faulty/loose lead could account for a difference in voltage that you're seeing between the two.

[Henry Yorke 269 3 Cars]

I've also had a voltage regulator on an alternator fail and boil the battery, pushing acid over my ignition amp (mounted nearby on mine) which took out 2 cylinders, pumped fuel down the exhaust which ignited and caused the cat to glow red!

 

I believe you could just change the voltage regulator but not sure where you get spares from, though I've (not looked on servicebox to see if they are a separate part

[welshpug 1,657]

find a bosch/lucas stockist, most can supply the parts to sort alternators out.

[GLPoomobile 958]

I've got a spare known working alternator so I'm all set to go on that front. But I think I need to do some more checks before I swap it out.

[large 33]

What does confuse me though, and this might be a vital clue, is why I'm not seeing the same voltage at the battery as I am at the alternator. I mean even if the battery is goosed and not taking charge then surely I should still see the same voltage there as at the alt when running. Although I was probing the battery posts rather than the clamps.

 

 

Do a resistance check between the alternator terminal and the battery positive - should be nigh-on zero ohms

A faulty/loose lead could account for a difference in voltage that you're seeing between the two.

 

As Anthony has said and you yourself have picked up on. You should be seeing the same voltage more or less on the battery as you have on the alternator.

 

I would defiantly do as Anthony suggests, and would have a good look at the cabling from the alternator and the battery before replacing the alternator.

[GLPoomobile 958]

I've not investigated this any further, but just having a think about it.....

 

How does the voltage regulator work? How does it keep to circa 14v under normal operation?

 

Or, to be more specific, if the alternator is merrily charging away, but it can't 'see' the voltage at the battery, would that explain why it's fluctuatuing all over the shop?

 

I'm only asking as it seems highly unlikely, that I've suddenly developed 2 seperate problems out of the blue - one being the regulator packing up causing the alternator to charge too high, and the other being a probable wiring fault that is stopping the battery getting more than 11.9v from the alternator.

 

Since the car starts fine, I can rule out a problem with the +VE from battery to starter. So I'm wondering if the problem lies with the cable that jumps from the starter to alternator. Since I replaced the starter quite recently, perhaps the connection has worked loose.

 

If that was the case, and the battery wasn't receiving a full charge from the alternator, would it cause the voltage regulator to do funny things?

Edited September 21, 2010 by GLPoomobile

[large 33]

Now stay calm! I am only asking, have you got an auto ranging multi meter? If so is it possible you were reading 1.9 or .19 volts? .

 

I should have said I did not write the above.

[GLPoomobile 958]

And we have a winner!

 

My hunch was right. Bloody nut has worked itself off the starter terminal so the lead from the starter to the alternator is just hanging on the stud Typical. You fix one problem (replaced the starter) and create another. It's got the right washers on it and I did it up tight, but it's still worked loose.

 

I'm glad I know what it is now, but it's a sod that something so simple is probably going to be an arse to fix. Trying to get to it and get the nut on tight is going to be a mission without taking the manifold off again.

[large 33]

I normally do it with a socket on a couple of long extension bars going in from above the alternator.

[GLPoomobile 958]

Well that was fun

 

It wasn't the nut that had worked loose, it was the damn ring terminal that had sheared off. THe only thing holding it in place was the rubber boot over the top (which in itself is lucky, as it wouldn't have been too good if the bare end had come away and made contact with something metal!).

 

Took that whole length of cable off the car, and then discovered I had no spare ring terminals that would fit the starter stud, but plenty in the next size down So I just drilled a smaller one out to 8mm. Not happy with the overall quality of the repair, but after an hour or two of faffing it's back together and working as it should.