Altenator Troubleshooting

[cinquecento]

Have a A127IM AMR4247 Marelli 100 amp altenator (96 Griff 500) that is not charging the battery. I have pulled it apart..bushes, rotor coil resitance check out OK.. so I suspect the voltage regulator. Alternator has no volage output yet 12v is present on the exciter wire.

Is there a way of full fielding or bypassing the VR to see if I get voltage out of the altenator. Alternatively, what's the best way of troubleshooting this?

[clive f]

as Adrian says, also I replaced the voltage regulator, cost around £25 from Lucas.

(are they beack open yet)?

[cinquecento]

Yes that's fine. I am not getting any ouput from the altenator.

Clive.. how did you know the regulator was stuffed?

[Barreti]

Hey
Can one of you wiring guru's write up what you're doing to check these things. I think it would make very interesting reading.

[Del 203]

Barreti said:

Hey
Can one of you wiring guru's write up what you're doing to check these things. I think it would make very interesting reading.

In very plain non electrickery English pls

[clive f]

cinquecento said:

Yes that's fine. I am not getting any ouput from the altenator.

Clive.. how did you know the regulator was stuffed?

from memory, it was a whiole ago now, the voltage meter in the car would be reading 15volts, even with heater and headlights on, and the little red light between the speedo and rev counter would flash intermittently, so I took the alternator into lucas, who tested it and replaced the regulator, job done.

[GreenV8S]

I've seen two sets of symptoms from a failed electronics module:

1. output voltage varying with load and going over 15V when unloaded
2. no output voltage, 'no charge' warning light stays on with ignition off and goes off with ignition on (engine stopped)

You can test the 100A fuse even if you can't find it, by looking for a voltage difference between the alternator output terminal and the battery 12V terminal with the engine running. However, if the fuse has failed mechanically (as seems common) the problem may be intermittent as the broken parts of the fuse may still be touching.

[crackedfinger]

GreenV8S said:

I've seen two sets of symptoms from a failed electronics module:

1. output voltage varying with load and going over 15V when unloaded
2. no output voltage, 'no charge' warning light stays on with ignition off and goes off with ignition on (engine stopped)

You can test the 100A fuse even if you can't find it, by looking for a voltage difference between the alternator output terminal and the battery 12V terminal with the engine running. However, if the fuse has failed mechanically (as seems common) the problem may be intermittent as the broken parts of the fuse may still be touching.

It was number 2 that left the car stranded in Le mans this year. The volt meter will start from 12v and work its way down till there isn't enough to run the car any more. Initially the red light came on, but after a while it turned off.

[cinquecento]

Problem solved.

Many thanks to all who replied. What had happened is despite the 0 ohms resistance from the + battery to the altenator, the 100 amp fuse has dissolved itself under the small brass bolt in the black fuse holder. There was a small piece of melted black box blocked by the thickness of the battery cable.

Peter's 2nd suggestion pointed me into the right direction.. I should have looked for a voltage drop, not a resistance drop. Thanks Pete...