Alternator - Help, i cant work this one out on my own

Datatek said:

Disconnecting the battery with the alternator running will destroy diodes in the alternator unless you are very lucky.

Click to expand...

Oops, wont be doing that again.

Just tried with a jump lead from alternator casing to negative and a lead from alternator output straight to battery. No charge. With the output cable off the battery its reading 14v. The second it touches the battery it dropps to 12v, even when revved hard it only reaches 12.5v.

I've tried using both the output terminal and the b+ stud on 2 alternators, some outcome.

Right, I might of got somewhere.

I've disconnected the glow plug relay, I've got the one that connects straight to the battery, and it's reading 13.3v across the battery when revved. Its not the 14v i want but it might have to do.

Would it be the relay drawing the volts? or another part of the glow plug circuit.
Is there anything else you could think of that might draw volts?

Try a voltage check on your glowplugs, the relay may be stuck on and keeping the plugs lit which will drag a lot of power from your battery/alternator

billiam said:

Right, I might of got somewhere.

I've disconnected the glow plug relay, I've got the one that connects straight to the battery, and it's reading 13.3v across the battery when revved. Its not the 14v i want but it might have to do.

Would it be the relay drawing the volts? or another part of the glow plug circuit.
Is there anything else you could think of that might draw volts?

Click to expand...

The alternator and other bits would get bloody hot if it's supplying enough amps to pull the voltage down to 12.5.
Modern batteries need 14.2 to 14.8 volts to charge fully, 13.3 is nowhere near enough.
If you think the alternator is being loaded, run it for a few minutes with everything on and see how hot it gets, turn the headlights on and see if they get brighter when you rev it.

Ok. So with the glow plugs disconnected, and ALL the fuses pulled out, i'm getting 13.3v.
I cant think of anything else electrical on the thing. What else would drag volts from the alternator?

did you trickle charge the bat?