33 replies to this topic

[john2502]

Hi all
My 2000 MPI mini has been a little sluggish this winter starting. Has had a recent new battery after old one failed and when replacing I've noticed at idle it was only charging at 13.3V. Seems a little low to me if expect just over 14ish.

It gets used almost daily but often for multiple little short journeys here and there as required (school run, then nursery drop off etc) so I think combine this with the good old rover security system drain and I don't feel 100% its going to manage to start (I need to be able to rely on it as its my pretty much daily).
When I turn the lights on it drops to 13V so combine that with when the wipers and demister is on and could easily be hardly breaking even (so to speak).
There seems a little slack in the belt (is a little flappy when running but not excessive), with the later MPI Minis is the tension all controlled or does it need adjusting?
Or is it most likely alternator so worth getting a new belt too?
Thanks for any help/advice
John

[Ethel]

Where are you measuring the voltage? Closer to the alternator will eliminate poor earths & battery cables.

[john2502]

Good point, was checking on the battery, I'll check from the alternator itself. The cables don't look very old on the battery itself but needs investigation I also need to check when higher reving but I was Billy no mates today and my legs aren't THAT long!

[Spider]

I've moved this thread here as it's a more appropriate place for it to get the response you are looking for ;)

[Steam]

You will need to get the correct tension on the belt before going much further, refer to tech manual for info on this.
Then it is a matter of testing output as close to the alternator as possible, no need for high revs.
If the voltage is down at the alternator then it will need interogating, if it is good (14v etc) at alt then work your way towards the battery testing all the way.

[john2502]

Little update, I've tried taking a reading from the alternator and it is very low. I assume I'm doing it correctly by taking positive from the larger cable coming from the alternator to any point on the engine as negative. I got 11ish at one point but mostly it reads below 1V so assume alternator is knackered.

[Ethel]

Is the charge (ignition) light working? With output that low it should light.

[john2502]

That is what threw me at first, it does light when the ignition is on but then goes off when car is started (as you'd expect it to). I wondered if i just wasn't getting good connection but I've tried and tried again and seems to be below 1V. the connection itself looks a little rusty so am wondering if some electrical cleaner and a wire brush to clean it up and reconnect it might help things?

 

would the light come on if the connection was rubbish, or is that through the other smaller wire so therefore a separate connection? if that makes sense

[coopertaz]

the ignition light is turned of by the alternator swithing on when it senses voltage so if alternator is gone light will not always come on. best way is a voltage check like you are doing. may just be regulator on alternator though these are easy to change if the small bolts holding it don't shear

[Ethel]

The warning light is in the "exciter" supply that energises the rotor. When the alternator produces high enough voltage in the stator it effectively tries to flow in the opposite direction through the bulb. Not enough current can flow in either direction if both the battery & alternator are close enough to the same voltage so the bulb goes out.

 

The bulb's more than an on/off indicator:

 

It's brightness is an indication of the difference in alternator & battery voltage.

 

If the battery is faulty it tends to be brighter at low revs because the battery can't make up the shortfall in power from a slow spinning alternator.

 

If the light gets brighter with engine revs the alternator's regulator has failed and it's voltage increases to rise.

 

If the light remains lit with the ignition off, it's the regulator again - a diode is allowing current to flow the wrong way from the permanent live output connection via the bulb to earth through another ignition switched circuit.

 

It could be the rotor brushes are on their last legs, but that's a guess.

[john2502]

Thanks for the replies guys. Not had a chance to look at this as family is down with a bug again including me.
Is the regulator being easy to change also the case on MPI minis as it seems is quite different to previous ones from a quick look?

When i test the voltage at the alternator it kind of flickers around, have seen 14ish a cpuple of times but mostly its less than 1v.
The light goes totally off as you'd expect when all working fine and the volt gauge reads around 13V when lights etc off and drops to mid 12s when lights and heate are on

[stuart bowes]

the stuff you learn on here is amazing 

 

if only I could memorise it all for future reference I'd be set

[john2502]

Yeah great resource isn't it!

 

So I'm just about getting over this bug going around so now looking at this again. I have now managed to get 14V from the alternator by really pointing the multimeter leads in there. So am figuring this is ok. I still only get about 13V at the battery BUT this is also about what i get when i charge it directly in the car using my Halfords charger on "fast".

 

So am wondering if the battery is just not able to accept the charge. Thing is I've had the car since Jan last year, about a month in it needed a new battery, that one then lasted until about Nov last year (RAC computer confirmed it was battery), and then I got a replacement one of course so this one has only lasted 3 or so months!

 

So if it is the battery either:

 

1. I've just been unlucky (the ones I've got have been both Napa 065 which i understand is a good brand!)

2. Something else is killing the battery's, although i can't think what. Does intermittent charging on/off a lot cause this possibly? So if the alternator or connection was intermittent it could wear the charging ability of the battery prematurely so to speak?

 

thanks all for any help in advance.

[bpirie1000]

I would suggest checking the connections at the starter solenoid and check the earth at the top engine steady. Physically remove these and clean up,with some sand paper..
Connections at solenoid corrode and snap seen this may times on the mpi minis.

Good luck

[weef]

Not only do you need to measure voltage but current is equally important , so an amp clamp meter is useful but as not everyone has one it is possible to reach a good diagnosis just using a voltmeter so you need to start with some basic checks.

The checks I describe only use a voltmeter set to measure DC volts

 

Ensure the battery is fully charged, 12.5 volt minimum, and is in good condition. I personally find all Napa products dissappointing.

Do a volt drop test across the main charging cables, negative battery terminal to alternator body and positive battery terminal to the alternator output trminal, niether should exceed 0.2 volt and if they do rectify as required checking cable and terminal integrity.

Check fuse A4, 10 amp, in the passenger compartment fusebox, this is the supply for the charge light and alternator exciter.

Start the engine an allow to idle, measure the voltage across the battery terminals, expect 13.2 to 14.6 volts for a good system.

If the voltage is lower than expected move to the alternator output terminal. As you have already ensured the alternator body has a good earth, volt drop test, measure the voltage output at engine idle speed , measureing between the alternator body and the alternator output terminal. Expect anything between 13.2 and 14.6 volts, preferably at the high end, if not suspect an alternator fault and repair/replace as required.

 

As you have checked the cable integrity and found/ repaired to specification the fault should be found using the above checks.

Thats about it really carrying out these checks hopefully will point you in the right direction for a fix.