I’ve fitted and new alternator (mx5 engine) and it is not charging, I know very little about testing alternators so with the engine running I put a Volt meter across the output terminal and the alternator body and got 11.5V, I did the same for the battery and got the same.

I disconnected the battery cable from the alternator got the engine running and but the volt meter across the alternator output and body again and got 2v reducing to 0.5 volts.

Can anyone help ideas of what I can test or check?

Do a youtube search for the tests and that will tell you a lot.

Essential to have a good main feed to the alternator - this is the direct heavy cable from battery to alternator, the alternator will not charge if it doesn't have a feed from the battery in the first place. Some alternators also don't work if you don't have the warning light connected & working - do you have warning light connected?

If main feed is good & warning light connected & working then with engine running at fast idle you should have at least 12.5V across the battery terminals, increasing to around 14.5 as engine speed rises

If all wiring as above is fitted & intact & fan belt is tight & spinning alternator then it only leaves the unit itself to be at fault

The warning lighting might be the problem, I have an Acewell 7659 which doesn’t have a direct connection from the alternator.

I've found this thread https://www.locostbuilders.co.uk/viewthread.php?tid=200512

Possible I've wired the 2 pin pug the wrong way round so the alterantor is not energized.

[Edited on 18/3/19 by Charlie C]

I've got the alternate charging by connecting the alternator warning lamp circuit to a switched live supply. the problem is now the belt is slipping badly the tension is to bad but it like there is a large load on the alternator.

If the battery is a little flat, quite likely as the alternator hasn't been charging, there will be a large load on it to top the battery back up. If you have a charger then manually charge the battery b4 running again, that said, belt really should be tight enough not to slip even when alternator is putting out full load

Firstly thanks for all your help.

I'm charging the battery now and i'll adjust the belt before i start the engine again.

Don't forget the NA (Mk 1) MX5 engines use a regulator which is inside the alternator and the NB (Mk 2 - 2.5) uses the ECU to control the charge rate of the alternator

[Edited on 18/3/19 by ianhurley20]