alternator fun

neilly · Mar 24, 2014

Hi All,

So I changed the alternator on my Old mans disco 300tdi. As the last one gave up the ghost, no electrickery coming from it.

Fitted a magneti marelli 100AMP unit which I got from someone,

The prob is on engine start up it takes a few minutes for the dash battery light to go out. But once it has gone out, then whenever you restart the car within a few hours it goes out straight away.

when the dash light is on there is no charge from the alti. When the dash light goes out I have 14.4 volts at the battery. Alti has been fitted for a month now and has not changed.

Any ideas on what is wrong with it on the initial start up? bushes and slip ring look fine. Not sure how to define if it is regulator / rectifier or internal wiring breaking down.

would like a bit of info before I take it off and run it into Halfrauds for one of their free checks.

Other option is too strip old broken one and replacement one to see if i can make one good one.

Disco does not do high mileage, so there is no panic to fix, hence the questions rather than just go and spend a £100 on a new one.

Cheers

Neil

jackmac · Mar 24, 2014

Does it go out straight away if you give the throttle a blip?

neilly · Mar 24, 2014

Hi JackMac,

No, at start up it does not, rev it as much as you like and the red light stays on,also I forgot to mention that the rev counter also does not work when the red light shows, then after a a few minutes you can goose the throttle and the battery light will go out and the rev counter will work, intermittently, then after another minute the red light goes out permanently and the rev will work.

Cheers

Neil

jackmac · Mar 24, 2014

More likely that the regulator pack has a problem I would have thought. If you manage to get a replacement one, let me know. For the life of me I couldn't find a replacement brush pack for mine when it went, ended up with a new alt

neilly · Mar 24, 2014

I might have to do a bit of pick and mix with old alti and the replacement one.

I have seen regulators on ebay for about £23 for the magneti marelli's , a lot more expensive then the lucas parts.

Cheers

neil

Rasher · Mar 24, 2014

is the live feed to the alternator fitted correctly. it sounds like its not getting power until its making its own.

neilly · Mar 24, 2014

Hi Rasher,

I checked the continuity between the battery / earth and the alti, all seems OK.

Do you mean the 12V from the ignition to the exciter then i will go and check it to be sure, But IIRC it was all OK. but its worth taking a look again.

Cheers

neil

Rasher · Mar 24, 2014

I'm not totally clued up on them but I had a problem on a car once, where the rev counter was acting like you said. it turned out to be a broken wire that should provide the alternator with 12v when the ignition was turned on.

trax · Mar 24, 2014

exciter wire goes to alt to provide field coil energy via a bulb.

initially this goes to ground in the alt and the bulb goes on.

when alt makes leccy, it sends 12v up and they cancel each other out.. no light.

sounds like it isn't energising via the 12v feed and is self energising when spun up a bit.

i'm not sure, but does RTC5670 work with all the different versions of a127?

Rasher · Mar 24, 2014

That the right way to say what I meant.

neilly · Mar 24, 2014

Hi Trax,

Not sure about rtc5670. looks different to the one on the magnetti alti . So do not know if this is just a visual difference or a mechanical difference.
But it appears to only be 65amp, not the 100 amp I have at the moment.

Your description sounds the same as my thoughts have been going, which parts would be at fault on the alti and need changing?

Cheers

Neil

neilly · Mar 24, 2014

So,

Just went out to check some things and the old girl started and the red light went straight out / rev counter worked. This is afetr being sat for approx 4 hours.

checked some volts and ohms.

this is what i found. three different states

IGN OFF, IGN ON , motor running

Batt charge 12.32 V , 12.09 V, 14.2 V,
Batt Earth to Alt Earth 0.25 OHMS, 3.46 OHMS, 20 OHMS,
Batt POS to ALT 0.01 V, 0.015 V, 0.076V,
Batt POS to ALT 0.06 OHMS, 12.20 OHMS, 72 OHMS,
B+ Alt to Earth 12.3 V, 12.06 V , 14 V,
W to Earth 0.009 V, 0.004 V, 6.1 V,
d to Earth 0.009V, 1.031V, 13.8V
B+ ALT to W 12.35V, 12.06V, 8.2 V,
B+ ALT to D 12.35V, 11.06V, 0.6V,

Looks Like i may have got some strange results. Anyone pass some light on this.

Cheers

Neil

trax · Mar 24, 2014

that battery is too low, probably due to the bad alt you switched out.

go stick it on an overnight charge, leave for a few hours and check again.

should be over 1.4, but you ideally want close to 12.7

(ps the 300tdi will have the glows kick in on a timer when you turn the ignition on)

while waiting, you can take the alt off and strip it. should be a few vids on resting the reg and diodes on youtube.

also grab the part numbers and makes from the bits that are broke.

also, stick a jump lead from batty neg to engine block, test resistance between alt and batt earths again. Do this first, before mucking with anything else, bad earths can do lots of funny things.

Datatek · Mar 24, 2014

neilly said:
So,

Just went out to check some things and the old girl started and the red light went straight out / rev counter worked. This is afetr being sat for approx 4 hours.

checked some volts and ohms.

this is what i found. three different states

IGN OFF, IGN ON , motor running

Batt charge 12.32 V , 12.09 V, 14.2 V,
Batt Earth to Alt Earth 0.25 OHMS, 3.46 OHMS, 20 OHMS,
Batt POS to ALT 0.01 V, 0.015 V, 0.076V,
Batt POS to ALT 0.06 OHMS, 12.20 OHMS, 72 OHMS,
B+ Alt to Earth 12.3 V, 12.06 V , 14 V,
W to Earth 0.009 V, 0.004 V, 6.1 V,
d to Earth 0.009V, 1.031V, 13.8V
B+ ALT to W 12.35V, 12.06V, 8.2 V,
B+ ALT to D 12.35V, 11.06V, 0.6V,

Looks Like i may have got some strange results. Anyone pass some light on this.

Cheers

Neil

Looks like you have a bad earth possibly from the engine to the chassis.

The energising voltage to the alternator is not that critical, the current is. Anything over 12 volts should kick it into life. A bad earth will affect this. You could tray running a fairly heavy wire from the alternator case to the battery negative.

neilly · Mar 24, 2014

Thanks Datatrek,

will try it with the jump leads,.

Cheers

Neil

trax · Mar 24, 2014

neilly said:
Thanks Datatrek,

will try it with the jump leads,.

Cheers

Neil

if you try to the alt casing and it works, then also do it to the engine block as the alt grounds via the block, so as long as it's not corroded, then it's engine/gearbox straps to chassis/battery time.

neilly · Mar 24, 2014

Well,

I tried jump lead from battery earth to alternator casing and it made no difference to any of the measurements I had earlier.

The problem is that since I had it running this morning the red light on the dash now goes out straight away.

The more i think of this the more I am thinking it is something to do with the exciter wire. I have put the battery on charge for the hell of it. Will see what tomorrow shows.

what in the alternator does the exciter wire actually go to? could this possibly be knackered? Should I have 12V at the exciter wire when the ignition is turned on?

Cheers

Neil

Datatek · Mar 24, 2014

neilly said:
Well,

I tried jump lead from battery earth to alternator casing and it made no difference to any of the measurements I had earlier.

The problem is that since I had it running this morning the red light on the dash now goes out straight away.

The more i think of this the more I am thinking it is something to do with the exciter wire. I have put the battery on charge for the hell of it. Will see what tomorrow shows.

what in the alternator does the exciter wire actually go to? could this possibly be knackered? Should I have 12V at the exciter wire when the ignition is turned on?

Cheers

Neil

The exciter voltage goes though a diode and the armature coils to ground, without it there's is no magnetic field in the armature to start generating.
The exciter wire should have 12 volts at the alternator if it's not connected, when connected it is effectively held at ground potential so lighting the lamp on the dash which is fed with 12 volts when the ignition is turned on. When the alternator starts generating, there will be 12 volts on the exciter wire at the alternator which will extinguish the dash lamp as it will have 12 volts on both sides.

neilly · Mar 25, 2014

So,

Today, one charged battery.

started the Disco, red light on dash did not go out.

checked the back of the alti.

got the following readings.

W to Earth 0.4V,
D to Earth 1.0V,
compared to what I had yesterday IGN OFF, IGN ON, Engine Running checked after red light has gone out.

W to Earth 0.009 V, 0.004 V, 6.1 V,
d to Earth 0.009V, 1.031V, 13.8V

seems like the alti is not giving out a signal from W or D until it has run for a while.

which part of the Alti do i need to be looking at changing?

Cheers

Neil

trax · Mar 25, 2014

forget W, that's the rev counter and it'll only work when the alt does.

do the basics. pull the D+ warning lamp connection from the alt and measure the voltage and current of the wire when the ignition is on.

You seem to be saying it's 1v, if so then that's an issue. follow the wire up into the dash looking for bad connections. measure voltage on any connection you come across.

also, place your multi on the alts b+ measuring voltage, start engine and apply 12v from the battery to d+, does the alt start kicking out 13v+ ?

alternator fun

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