Technical Alternator

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Technical Alternator


New member
Mar 11, 2006
Help! Alternator

:confused: My Battery is draining very fast and I have been having problems of of engine misfire and cutting off ... I decided to test the Alternator by measuring the voltage across the Battery while the engine is off and while its running.
My multimetre shows the newly recharged battery as 12.9 v ( which is normal for a newly recharged battery) while the Engine is off.

but when I start the engine, the difference in Voltage increase is a negligible 13.1 v.
I increased the engine rev and the difference was again very small (13.15)
my third test I switched the headlights on also the heater to maximum together with the stereo.

I then noticed a drop in the voltage to about 12.9.. and when I increased the speed again it dropped even further.

Does this indicate a faulty Alternator? and could the misfiring and the cut off at high speed be associated with a faulty alternator too ?
I appreciate any help ...thanks
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the alternator must fill about 14V so i think your's is fine!
maybe something is draining your battery while the car is not working!
maybe i'm wrong but it is much cheeper to check that first!
Hellcat said:
If left for a couple of days does the battery flatten?

I am not absolutely certain, anyway I haven't used the car the past 24 hours, I will test it tomorrow afternoon to see if the battery I put in is drained, and i also know it was measuring 12.9 V when I put it in, so if the voltage drops without me using it this perhaps indicate a drain somewhere ..
Hellcat said:
Be best if we could get another 1.2 16v Brava owner to post their off/running/running with lights+heater voltages.
yeah that is a brilliant idea .. it would give a much better indication to the condition of my Alternator
sometimes the voltage regulator is faulty, that can cause a constant drop of voltage under all conditions. as a general rule i expect to see 14.4v at idle and no less that 13.99v when under high load (rear demister, full heaters, fullbeam lights, stereo on). if both these values were reduced by roughly the same amount (e.g. 1v drop = 13.4v and 12.99v) then i'd suspect the voltage regulator, if both these values are reduced to the same amount and that amount is below 13.99v then i'd suspect worn alternator bushes, if the voltage stays above 14.4v for more than a few secs under any conditions you also suspect a voltage regulator fault, and expect to see the battery boil.

your figures suggest worn alternator bushes more than anything else, although those figures should not be low enough to affect engine performance, you would expect to drain the battery if you did a lot of night driving in traffic but i've seen worse cases where the car ran fine for months to come. if your battery is draining very fast (how fast is that?) then its more likely that you are suffering from discharge because a permanently live curcuit is connected to earth (chassis).

disconnect the battery negative earth lead and connect a multimeter to the battery and cable, put negative on batt and positive on cable, measure the amps, thats your drain. what does it measure? hope for a low number.
oh one more thing, make sure you have everything turned off when u try to measure the drain, i know thats obvious but thought i'd mention it.
jug said:
oh one more thing, make sure you have everything turned off when u try to measure the drain, i know thats obvious but thought i'd mention it.

Right! I went and did the earth test and it looks Negative because when I connect the lamp to the Battery (-)and to the (-) earth lead I get one initial bright spike but it quickly fades and the lamps goes off in a matter of Few seconds.
So I am assuming Now that the Alternator may be at fault.
Another possible reason is the starter motor using far to much power to start the engine because it always starts to turn very slowly and then turns fast enough to start the engine ? ( a solenoid prob perhaps).

but I think the likely reason is drainage while using the car with the Alternator unable to provide enough power to run the car and spare some current to charge the battery.

By the way I can also rule out the battery because I’ve got a normal 43 ah and a new bosh 55 ah which is a bit more powerful and lasts a bit longer.

The funny thing is the battery light on the dash always goes off when the engine starts and it never shows up while I'm driving .?
I would say at just over 13v with the engine running that the alternator as almost deffinetly on its way out. If the alternator has an internal fault it could be draining the battery even when the ignition is off as the wire from the battery to the alternator is constantly live. The voltage running should be near if not above 14v and if you are sure the battery is good then a new alternator it is.
yeah the Alternator looks to be the prime suspect, I am surprised there was no warning light coming up while I'm driving at night and using maximum output (lights,heater ..etc) ?
I phoned my local Car spares shop today and they quoted me around 160 pounds :( .. I didn't know they were so expensive.. So I am wondering if perhaps I could change the Brushes .. if they can be changed.
go to the electrician and ask him how much would repaire cost? i think that it could be much cheeper! he can't change the brushes but he can change the regulator! try!
omot said:
go to the electrician and ask him how much would repaire cost? i think that it could be much cheeper! he can't change the brushes but he can change the regulator! try!

Can't the Brushes be changed in these Alternators? Haynes Manual says it is ...
It looks like the Alternator was just a symptom that is part of another problem, I’ve seen the alternator output go up to 14V today when I touched few wires.... its all explained in the thread I started today " give us a clue"
Its worth reading because it shows just how difficult and confusing to try isolating an electrical problem that seems to touch ignition, charging, cooling and fuel systems...
Problem Solved :)

Believe it or not all my problem were caused by a leaking clutch slave cylinder :(
What happened was the clutch fluid was dropping on the clutch/transmission earth Strap and corroded it very badly so all I had to do is clean it and find another dry place to bolted in… and that solved all the problems of misfiring, crazy gauges, counters and the draining of the battery of course.

I am so pleased it’s now solved and thank you all for your feedback (y)