mx5 alternator testing

[RichardH]

my 1.8 mx5 engine 1999 is not charging, i suspect the alternator is faulty but there isnt much about testing them , i have 12v on the heavy cable which comes from the battery via main fuse, i would expect this to rise to 14v ish on startup but it doesnt.
the plug onto the alternator has two wires a grey and a grey/red, i have 5v on the grey and nothing on the grey/red, from the web i reckon the grey/red should have 3 v AC ???? as a signal to the ecu. before i cough up for another alternator anyone any ideas

[Ianr]

My alternator was ecu controlled and would not charge. I traced it to a broken wire somewhere in the loom by checking continuity from the ecu to the alternator. Wire replaced and off it went!

[flyerncle]

Ford/Jag have smart charge too,usually broken wire in harness causes non charge.

[RichardH]

thanks guys the plot thickens. spent hours trawling the net last night about this, the mev forum had a bit about non charging and i found two things, there is a sensor in the airbox that measures temp before the air reaches the maf sensor i hadnt plugged that in as i have no airbox, plugging that in made the charge light go out when running. but still no charge, another post mentioned a plug that lives in the boot of mx5 that needs to be plugged in, found that this is the grey/red from the alternator, i runs from the alternator up the drivers side to the boot, then plugs into the lighting loom and comes all the way back to the front to the ecu. plugging that in now gives me a small charge, battery increases by about 0.5 volts, with the car running i am getting 12.9v of it drops to 12.3 ish.
still not sure if the alternator is gubbed or the ecu is still telling it not to charge as much.
i have continuity from the alternator to the ecu on both wires but there is feeds off them doing other things not in the wiring diagrams

[RichardH]

so after further testing i am wondering if something isnt switching on the charging of the alternator or alternator istself , i have checked continuity again from alternator to ecu all ok, i have 5 v on the grey and nothing on the grey red, if i link the wires together so there is 5v on both wires the alternator charges high 14volts, if i feed the red/grey 12volts the alternator charges up 16v.
my thinking is the 5v to the alternator should come back on the other wire as a signal/voltage to regulate the charging, therefore new alternator?

[flyerncle]

They do "overcharge" on Ford/Jag due to temp and high discharge on starting etc and ecu controls this and you will see 14/15 1/2 volts when cold,also needs silver calcium battery or voltage will fry normal battery.

[RichardH]

so another alternator arrived from ebay today, problem solved. the last gauranteed one is knackered, hopefully i can get the regulator part and swap it in the duff alternator and keep as a spare