Alternator packing up?

[james butler]

I've got a problem with the rocko that's been getting worse over the years and I don't know why. In 7 years of ownership I've gone through 2 alternators and 3 batteries which temporarily cures the problem.
Once the engine is warm if I turn the engine off and leave it 20 minutes when I restart the engine the battery light is on solid. But the engine is idling perfectly around 1000 rpm if I rev the car above 2000 rpm the battery light goes out and the engine revs drop to 600rpm and is running a bit rough. I've also noticed my interior light at idle is pulsing if I leave it on at idle.
I've got a feeling one of the diodes that forms the bridge rectifier in the alternator is failing.
Hence why the battery light doesn't turn off immediately after starting.
What do you guys think?
The cars a H reg rocco on weber carb the alternator is a 90 amp version,
I've replaced the red power wires to the battery and starter motor and changed the brushes in the alternator, and I've fitted extra earth wires just incase a bad earth might be to blame.

[Nate]

only a thought... does the car have lots of electrickery fitted (large stereo, electric seats etc) could be that you'd be better with the 65A alty?

are you fitting cheapo batteries? if yes, 3 in 7 years would seem about right, they generally only last a year or two.

same question with the alt, decent branded one or a cheapy? could try just swapping the regulator?

i'm assuming you've checked for volt drop across the earths and power cables?

[james butler]

The car has large stereo but that's about it but sytoms are still there even with all the electrical stuff off.
Out of curiosity why would a 65amp be better?

[james butler]

I tested the little blue wire that goes to the alternator today, it has 12 volt with the ignition on with the blue wire disconnected but when it's reconnected the voltage drops to 3 volt. I tried a spare brush pack but the voltage dropped to 1 volt at this blue wire. I'm starting to think there's a short circuit within the alternator.
I'm going to grab a new brush pack on Tuesday.
But what I can't understand is that to reduce the voltage of this blue wire like that to 1-3volt would take quite a conciderable short circuit.

[Nate]

the blue wire connects to the alt warning light. the other side of the light is connected to ignition live. not sure what voltage you should see with this connected and engine on. you could try making up a temporary replacement using a new bit of wire and a small bulb, such as a sidelight bulb connected straight to the battery. this bulb should now work as your alt warning light and you can do your voltage test again and compare results.

i don't know what voltage you should see on this wire, but i've got a feeling it should basically be an earth

[go-for-it1]

I thought the blue wire was for excitation of the alternator?

[james butler]

The blue wire is for excitation but for some reason when I disconnect it from the alternator with the ignition on I get full 12 volt but if I reconnect it I only get 1 or 2 volt which shouldn't happen.
This will be why the light on my dash stays on at idle. But I've no idea why.
I tried another wire with a bulb in going to the same location on the alternator but I get the same result. Bulb stays on untill I rev the engine.
I did a resistance measure on the brush pack between the two actual brushes and I get a reading. But the readings differ between brush packs, I don't know if there is a correct reading for this?

[Nate]

For the alt bulb to light up, it gets a 12v ignition feed and earths thru the alternator. When the alternator starts charging, this becomes a live feed too, so the bulb goes out.

Incidentally, trying to suss out why I didn't have a working alt light on the mk1, I took some volts readings from a known working mk2 that helpfully had called round for tea and cake.

The readings were the same as yours.

Start from the beginning. When and how did this problem start? What work had been done to the car immediately prior to the fault starting? I have a feeling you might be looking in the wrong place...

[Lukeh17]

I also have a problem with my alternator. Twice since I bought the car in Feb I have been driving and the car literally dies and it seems dead. I have had a battery change so now I know it must be the alternator. It is 4 years old but we couldn't see anything wrong straight away. It is now with an alternator specialist awaiting a test but I think its a connection thing. Once I have jumped the car I can carry on driving as I was. It does seem very strange and my local mechanic is also confused by it but just waiting to see what the guy says with the alternator.

[Nate]

earths and a battery drop test would be my first port of call

[Lukeh17]

Thanks Nate I am driving the car to where the alternator was tested and they are going to check connections etc. I will also ask them to look at the battery too.

[james butler]

I found out the problem.
It would seem there is identical looking brush packs that have different resistance readings for different amped alternators.
Most will work in different alternator outputs but if you you put the wrong type in the 90 amp nor it gives you this result.
Only found this out as eurocar parts stock 3 types so ended up buying all three to see what happens when I put each In.
The higher power type brush pack has a resister soldered between the brush terminals but the lesser ones don't.
This apparently is what makes the difference.
What I must have had in the first place is the brush pack from a lesser power alternator.

[go-for-it1]

So, got a part number for the correct one?

[james butler]

I can't find a part number on the brush pack so i will ring up euro car parts and ask them.
The only differance seems to be this resister wired over the brush terminals.
I will try get the part numbers for all the types of alternator.