DDK - Die Deutschen Klassiker

Looks like it, and will no doubt work as it should. I would search for the datasheet and check the dimensions and pinouts match the one on your car, unless you have already confirmed that the Hella part number is compatible with your car...

I bought that one Gary. Pins didn't match but some strands of wire helped hook it up. Wasn't that so i bought this from ebay.
https://www.ebay.co.uk/itm/143452924610
Hopefully yours is a bad earth.

Thanks both, I'll check before connecting anything.

As you say probably a bad earth, but everything was clean as a shiny thing when I rebuilt it!

Probably stating the obvious Gary, but worth checking you have the correct tension on your fan belt in case it is slightly slipping.
Changing the regulator is a cheap fix, otherwise it is probably the alternator.

Hope you get it sorted.

So…
checked the belt tension.
did the earth leads.
Replaced the voltage regulator
Wasn’t any of those!

So new alternator going in.

Anyone know which wire goes where… I’ve contacted the seller but if anyone can help quicker that would be awesome!

Gary71 wrote:Definitely low at idle, and up to 2k, hence the light being on.

I’m going to do a voltage drop test later and make sure the earths are all good before I start pulling anything apart.

If you did these tests today (in the 'heatwave') then I disagree, 12.6 is fine at idle. If anything, I actually think your 15+ voltage is too high. The Bosch CDI will destroy itself above 18V so not much margin there.

Couple of things:

Battery voltage decreases with ambient temperature. Also, if the battery is healthy and fully charged, then the voltage will generally be lower when charging.

If is quite normal for an older 911 with a standard alternator to sit around 12.5 at idle with accessories on (lights, wipers, rear defogger etc).

Thanks Johnny,

The voltage was measured a few weeks back when it wasn’t this hot, and there were no accessories on.

What voltage would you say the charge light should come on at?

With the new alternator I understand now it has the voltage regulator built in so I’ll bench test it later (with a drill!) to try and work out the connections and go from there.

If I can work it out I’ll bang the old one back in and drive faster

You usually have to be under 12v for the light to come on. The light comes on when there is a voltage difference across it (one end is connected to battery, the other to the alternator output). Basically, any difference between battery voltage and alternator output voltage will light the lamp.

I would check the voltage drop between alternator and battery. To do this test make up some very long wires for your meter. Connect one end to the starter cable at the starter motor (this is close enough to alternator voltage). Connect the other end to the battery positive. You are now measuring how many volts are being lost down the cable. You should do this under as much load as possible so turn lights, wipers, defogger on. Ideally, you don't want more that 0.5V loss. If there is more than this, review the cable connections, clean and refit. Check ground straps also.