Alternators

[Stanmo]

Hello once again! Last week was having trouble starting the car after replacing dash bulbs, turned out with your help it was the battery. Anyway got it going went for a spin and the fan belt snapped. RAC came to help and bodged up the broken belt with some tape. Have waited all week for replacement belt from ESM only to discover now the alternator is not charging , and red light is staying on. I thought to check wiring in case something was dislodge but can’t figure out how to get the connector off from the back. Could someone help with how I remove this and any other fault diagnoses I should perform to find the cause? Have checked fan belt tension and this seems good, also have checked battery with voltmeter and it sits at 12.1 even when revving? Help?

[ManyMinors]

I would think you should pull the wire clip in a downward direction so that it is out of the way of the wiring connector and then the connector should pull free? My car has a dynamo so is not like yours.

[Stanmo]

I tried dropping the clip and pulling out but doesn’t seem to want to come. Didn’t want to force it in case I made things worse. Wasn’t sure if there was a screw hidden I had to remove first?

[JOWETTJAVELIN]

Looks like the standard AMP plug so should just pull out yes clip out of the way, sometimes they are a bit stubborn.

[Stanmo]

Finally have taken it out. All wires look secure. If using a multimeter to check how would I go about this?

[Stanmo]

I’ve checked current by using a multimeter with live in the fat brown and neg on battery earth. Reading didn’t change when running and when not , stayed around 12V. I’ve checked all terminals, checked belt slackness which is around 1/2 inch on longest length. I’m at a loss? All worked fine until belt broke. Could this be the brushes? Can they be changed and Do I need to take off the alternator to access? Thanks in advance.

[oliver90owner]

The alternator requires a current through the rotor to start it charging. Without the feed from the ignition lamp, it will not be energised. Using a multimeter in that situation will end in tears if the alternator does start charging!

Current is in Amps, voltage is in Volts. Could be different sockets on the multimeter and certainly a different range.

It is usually only the outer brush that wears but, if the track is worn on the commutator, a change of brush may be a poor move on your part. New commutators are cheap and easily changed if you are any good with a soldering iron. Your choice on whether to simply change the brush in situ or check it out properly.

[Stanmo]

Thanks Oliver,
Commutator??? I’m lost now. I’ll have to do some research.

So am I right to remove the unit or is there anything else I should check prior?

[myoldjalopy]

You say it all worked fine before the belt broke. You also say you previously had problems starting the car due to a flat battery and that the battery only shows 12.1v when revving the engine. So it wasn't all fine before the belt broke, otherwise the battery wouldn't have gone flat - I suspect a faulty alternator.......

[geoberni]

I’ve checked current by using a multimeter with live in the fat brown and neg on battery earth. Reading didn’t change when running and when not , stayed around 12V.

No, you haven't checked current. You've checked Voltage.
To check current requires the meter to be set to Amps and to be 'in-line' with the circuit. At which point, if the alternator is working, a regular Multimeter will blow it's fuse (if it has one, not all cheap ones do) , or it might start to melt. A typical multimeter can only handle up to 10amps. There are ways around it that start getting complicated, using Shunt Resistors to take the excess output, so let's forget that.

As myoldjalopy has just said, the battery was flat, so very likely it wasn't 'all fine' at all.

As you mentioned right at the beginning, you were previously messing around with the dash bulbs....
viewtopic.php?f=1&t=73121&hilit=dash#p663338

Did you change the IGN lamp for an LED!
If you did that's the problem.
Put an LED in the IGN lamp and the Alternator wont work!
The initial excitation current of the Alternator is supplied by the IGN warning lamp.
The higher the wattage of the bulb the greater the initial excitation current. If the bulb wattage is too low the alternator will not generate any output, will never self excite, and even with the engine running flat out it will not charge the battery at all.
For a 12 Volt system a 2 Watt bulb is normally used.
If you have put an LED in the IGN Lamp position, you have put in something of far too low a wattage.

It ain't ever going to work.

[Stanmo]

Cheers Geoberni. I assumed which I know you never should that the bulb would be a straight swap. Last week I changed the bulbs and rubber seals on doors. The battery dropped below 11 volts and I put that down to testing lights and working with door open etc. I used a jump start battery to fire her up and then all was fine with no red light. Took her for a spin and still no red light until fan belt snapped. RAC stuck a fix on which lasted about a mile (I was 4 miles from home) so had to gingerly get back , it was late so lights were on. Again I assumed this drive with no alternator charging had sucked battery dead. I’ve charged battery full put new belt on and now the red light. This is why I never considered the bulb as it was fine before. Should I change bulb back as a first step. I never really understood the alternator function other than to charge, I now realise there’s a bit more too it?

[Stanmo]

Geoberni, I changed bulb and it’s gone out and battery charging, on Rev it shoots up to 14V.
Again a simple fix which has come from this forum. Can’t thank you enough.

Just one question......How and Why?? I can’t believe a simple bulb changed fixed it and if it is the case why do they sell them ,

Learning more everyday

Thanks again.

[geoberni]

Just one question......How and Why?? I can’t believe a simple bulb changed fixed it and if it is the case why do they sell them ,

Well I pretty much explained it earlier.
I don't really want to write reams about the hows and whys of electrical generation, but in simple bullet points, missing out a great deal of technical stuff.......

+ Generating electricity (as far as your car is concerned) generally requires magnetic fields and moving windings of thin wire.
+ A Dynamo has some residual magnetic field in it's fixed coils and that enables it to generate once it starts moving.
+ An alternator does not have sufficient residual magnetic field, so it relies on the current flow through the warning lamp to generate some.
+ If you put an LED in place of the traditional lamp, there is insufficient current flow, thus insufficient magnetic field.

As to why they sell LED lamps, that's because they have no control over what you're going to do with it.
That's like asking why they make 21W/5W Stop/tail light bulbs in 6V and 12V versions.
Many Classic car & motorbike owners would greatly appreciate the 6V one, but if you fit one to a Minor, it'll likely last a few milliseconds at most.