Time for a new alternator?

Hi

I put a new battery in my '81 mk1 gti a few months back recently I've had trouble starting the car but after a few attempts it sprang to life. Now it wont turn over at all - all the signs of a dead battery.

I will charge the battery over the weekend and see if that does the trick. But being mechanically inept I am guessing I may have a duff alternator? What's the best way to test this? I've read a few threads on here and am thinking I should measure the voltage across the battery when engine is off then start the car and take another reading - I should see an improvement right? If the reading gradually dwindles the alternator isn't doing its job?

Should I then buy a new one and change the old one? Remember I am mechanically inept but I really want to learn (without making things worse) is this an easy job for me to tackle?

Thanks

Matt

heres a quick guide!

 you should first check the level of the battery with the earth lead off.  should be about 12.6v at least.

then connect the earth and check it.  should be exactly the same as before. if it isn't then you have a voltage drop somewhere i.e a stereo thats not turning off or a permanently on heated screen.

If it's the same then turn the ignition on and take a reading. should drop a bit but not much.

start it and take an idle reading. should be over 13v if it isn't read on and i'll get to the fix.

get a glamorous (or minging doesn't really make a differance…) assistant to hold the car at fast idle (about 2-3k rpm) and it should read about 14v.  if it does get said assistant to turn every switch on one at a time. if it drops much below 13 the battery wont charge properly and you need a bigger alternator. I had this when i fitted a four headlight grill! if i was in winter mode (wipers, full beam, fog light etc)  then i dropped down to 11.2…

Right.
If at any point before the full electrics test the voltage stays at 12.6ish, goes down or doesn't rise a little with the revs then check haynes to see how the brushes fit and pop them out. I'm betting at least one is below the minimum 5mm exposed.  they will need replacing.

if the voltage goes through the roof (i.e above 14v stably, it is normal to flash 15v for a moment before it levels,) at any point then the regulator is overcharging and this can also kill batteries/fuses/the coil etc. that means the regulator is dead. In your case this is unlikely.

the motorola alternator has the brushes underneath the regulator.  the bosch has a one piece brush.regulator unit.

hope i haven't missed anything? if i have someone will tell me i'm sure lol

I had to do all this with mine the other week.

remember also if the alt hasn't been charging properly for whatever reason the battery is probably shot from constant discharge so replacing it would be a good plan.  The cold weather we've had isn't good for batteries either.

Jon.

Thats brilliant thanks for your excellent response, I will give it a bash over the weekend.

Cheers

Matt

Step 1, ignore anything that mentions voltages and anyone who mentions voltages. :roll: Sick of saying it, all it tells you is you have a problem, which you already know cause otherwise you wouldnt be checking it.  

Step 2, search for "battery draining" and look for one of my posts outlining how to check.  :wink:

so all the technical manuals and training i've done on car electrics was useless??

lol I know voltage drop only indicates a problem, but voltage drop at certain times tells you exactly where the problem is.

you could spend hours resistance checking or checking the ampage of every switch/componant.  check the voltage, find out a rough idea and go from there.  I find that voltage drops are often down to something simple such as a stereo not having the off button held down to turn it off, or an alarm not working.  sometimes it can even be the heated screen switch earthing.  On most of the cars i've worked on i haven't found a voltage reg sapping power.  Then again if it's a bosche alt then the reg is attached to the brushes which i find cause 90% of charging issues.

doing the above sorted my issues and every other car/motorcycle with charging issues i've worked on…

especially for those who don't know much about electrical testing.   I am a firm believer in k.i.s.s (keep it simple stupid, not the band. well maybe a little…)

Jon.

So far as I'm aware it doesnt tell you to check the voltages in the manual either

There's some useful information here.
http://www.autoelectrics.net/alternator.html

thats a fair point lol

it is a good indicator though!

Jon.

Its all very well until you realise that an alternator produces an alternating voltage, and that information was written before digital multimeter were the norm Hence my reasoning. If your using an oscilloscope or even an analog meter then you have a point.

Measuring the voltage with a digital multi-meter or an analogue meter would make no difference would it?

Yep, an analog meter would show you if it was fluctuating, digital wont.  

Unless you have a fancy digital one that give you max and min voltages the digital just gives you an RMS voltage which will always be slightly high, when reading a faulty alternator, ie when the voltage reg has gone. Because what you get with a faulty alternator is AC ripple on a DC voltage, which is what causes problems and which is why reading voltage won't tell you a thing

Madferret, Please don't think I'm disputing what you say, I'm just interested. Surely if the voltage regulator were faulty you would get a high or low DC voltage. If the diode pack were faulty you may get an AC ripple but the battery would smooth that although the voltage would still be affected depending on whether a diode had gone either short or open circuit.

Thanks for all the info guys - it all sounds straightforward in writing but when opening the bonnet I always feel overwhelmed I'm sure there has been a series of electrical fixes and bodges on my MK1 as there are a jumble of wires hanging off the battery either with fuses in or taped up!

Any way I had some help from someone with a bit of knowledge (and a multimeter) with the car off the battery was putting out about 12.5 with the car running and revving it went to about 13.4 ish ….

I took the alternator out (partly to prove I'm not totally incapable) and will replace it (depending on cost) any ideas of cost of new one? where to buy? Is it worth slotting in a new diode back and brushes to start?

I'll replace it and see what happens - at least I will feel I have achieved something!

I really do think the jumble of tangled wires probably doesn't help!?

Thanks

Matt

cedar said

Madferret, Please don't think I'm disputing what you say, I'm just interested. Surely if the voltage regulator were faulty you would get a high or low DC voltage. If the diode pack were faulty you may get an AC ripple but the battery would smooth that although the voltage would still be affected depending on whether a diode had gone either short or open circuit.

The battery does smooth yes, but it lags because it has an internal *edit* resistance *edit*. TBH I can't remeber if alternators have half wave rectification or full wave and that maybe down to manufacturers design, but you will, if the diode pack goes faulty, get some sort of AC ripple on a DC offset and depending on where the offset is it will give you bad readings that you just cant see on a bog standard £10 digital multimeter. Which is why I say Oscilloscope or Ammeter is the way to go, even ammeter's arnt great and a load bench is the only true method.

I know what you mean by measuring voltage but there's so many variables that may or may not give you a good reading that you may as well not even bother, as you already know your battery is going flat, and that can only be, either an ancillory item drawing a lot of current (As was the case with Pedder Immobiliser), Poor Charging (i.e voltage regulator/bad wiring), or a Bad Battery which in most cases is caused by a bad charging circuit (or in some cases just lifespan).

A diode can go either short or open circuit too which adds to the problem as they are what push the voltage in the right direction, sometime they dont completly fail and leak, its not so simple.

And that is my reasoning, rightly or wrongly

Hey guys,

I've had an intermitant fault with the electrics in my Cabby for a while now.  A couple of months ago I broke down and couldn't start the car again.  Just before I came to a halt, the radio cut itself out, then the dash stopped working (speedo was OK though) before it finally curled up and died.  Ive had a couple of people look at it, the first time round we isolated a connection fault with the Alternator.  Second time round all seeed fine and the guy just did a general under-bonnet tidy up.  Took it out for the first time since the repair, and on the way home the dials dropped again (I just have the speedo), but the car is still motoring on, and everything else is working fine.

I checed the voltage over the battery, and it stayed around 12 volts, whether I revved the engine, turned on switches or what.  Has never deviated from 12v since, and seems to start on the button, every time.  I even gave it 4 or 5 starts to see if that drained the battery.  Again, all stayed around 12v.  The alternator appears to be running - it is the Bosch unit.

My limited understanding is that if there is charge going to the battery from the alternator, then the voltage should be 13-14v.  Mine isn't, but is displaying the characteritics of a good battery, even when I try to run it down.

Does this sound like a duff alternator, bad bushes or just a faulty connection behind the dials?

Thanks.

neo283 said

Hey guys,

I've had an intermitant fault with the electrics in my Cabby for a while now.  A couple of months ago I broke down and couldn't start the car again.  Just before I came to a halt, the radio cut itself out, then the dash stopped working (speedo was OK though) before it finally curled up and died.  Ive had a couple of people look at it, the first time round we isolated a connection fault with the Alternator.  Second time round all seeed fine and the guy just did a general under-bonnet tidy up.  Took it out for the first time since the repair, and on the way home the dials dropped again (I just have the speedo), but the car is still motoring on, and everything else is working fine.

I checed the voltage over the battery, and it stayed around 12 volts, whether I revved the engine, turned on switches or what.  Has never deviated from 12v since, and seems to start on the button, every time.  I even gave it 4 or 5 starts to see if that drained the battery.  Again, all stayed around 12v.  The alternator appears to be running - it is the Bosch unit.

My limited understanding is that if there is charge going to the battery from the alternator, then the voltage should be 13-14v.  Mine isn't, but is displaying the characteritics of a good battery, even when I try to run it down.

Does this sound like a duff alternator, bad bushes or just a faulty connection behind the dials?

Thanks.

Can you measure the voltages more accurately than 0 decimal places?

12.3
12.4

Think the highest I got was 12.6?

sounds like the alternator is running a bit low, but enough to charge the battery a reasonable amount.

May be worth changing the brushes to see if that makes a difference?

It really should read between 13-13.5 unloaded, and about 12.9 with everything on.

Better to read resistance and amps, though don't ask me how lol.

Jon.

A fully charged battery should show 12.8V with no loads on it. When being charged, should be around 13-14V to get any decent amount of charge into it. Note that after being charged, there will be a "surface voltage" which must be taken into account when you measure & check.

Amps is the best way to determine if the alternator is charging but you'd need to either use an ammeter in series (the traditional type) or some kind of inductive pickup. The problem will be connecting a multimeter to measure it and not exceeding its 10A max rating if the alternator does work.