Alternator voltage

[Hibbo]

Hello everyone,

Can anyone give me a bit of advice as to the electrics on my 300D? I noticed that the good/charge/check indicator on the battery always shows 'charge'. (The indicator was always green when the battery was in the 190, but is blank now it's in the 300). I put a voltmeter across the battery, and with the engine idling it read 12.8v, rising very slightly to about 12.9-13v when I revved the engine. I thought it should be at about 14v with the engine at speed?

A few months ago I did whack the back of the alternator with a grolly bar whilst trying to adjust the steering box, and it attempted to weld itself to it. I fear this may have knacked the alternator somewhat.

So my questions are; Can I test the rectifier pack and voltage regulator? Can these be replaced or is a whole new alternator required?

Thanks for any help.

 

[television]

This is a difficult question, to check the charge depends on the condition and the state of the battery,

The car runs on the alternator and this takes 30% of the output, the next 30% is for in car, heater,defrost,aircon,radio and etc, the rest goes to the battery.

You can give volts, but not amps (current), as the battery will take what it needs and no more, otherwise you could cook the battery. The voltage (presure) is determined by the battery its self. Sometimes a cell can go partial short circuit, causing the voltage to be lower than the normal 12volt,. Under these conditions it will never get up to the normal 13.8 volts (engine running 1k).
We have had a few later cars where when this has happened causing ABS and other lamps to come on, though the battery held its charge OK

You could mesure the voltage one morning before you start the car, turn the headlamps on and mesure.
There are 4 different ways to tell what diode has gone, if one has gone, but it is irelivant since they all come in a pack, cheap to buy easy to fit.

Hope this helps a little.

malcolm

 

[Hibbo]

Thanks for the reply Malcolm,

The car runs on the alternator and this takes 30% of the output,

Please remember that this is a diesel, so I don't think that is the case.

I'm pretty sure the battery is ok, and has been trickle charged overnight after it ran flat (lights and radio on whilst working on the car).

As I understand it, the battery cannot attain a fully charged state as there is not enough of a voltage differential to cause a charging current to flow. It is charging as the battery light goes out as soon as the engine starts and I have not had any (other) flat battery problems. It is just not getting fully charged.

Does the rectifier pack use zeners and thus provide voltage regulating itself, or is there a sperate regulator? How much would I expect to pay for these? are the pretty much standard items common to many alternators, or specific?

Thanks again

 

[mattkh]

Hi
Is it possible to put a picture of the rear of the alternator in your car.

 

[television]

The only thing that you have not got on a diesel is the ignition or around 8 amps less the a petrol, than on diesels the battery is often larger than that of a petrol. Youn still have ABS and a fuel pump and a high lift pump that uses quite a lot.
You can use OHM's law to calculate the max charge. for a start the voltage can never go beyond 13.8 is it is held down by all of the other components on the car. this is why a alernator should never be used to charge a flat battery, it is not designed for that. So the MAX charge that you can get is over the differencial of 1.8 volt over the 12.

There are no zeners in the pack its just a simple 4 diode bridge. All bosch agents keep them. they used to cost £5

Malcolm

 

[Bolide]

There's no electric lift pump on a W124 300D. The rectifier pack is part of the brush assembly and it plugs in in 2 minutes. About £15 nowadays

Nick Froome
www.w124.co.uk

 

[Hibbo]

Thanks again for the replies,

The only thing that you have not got on a diesel is the ignition or around 8 amps less the a petrol, than on diesels the battery is often larger than that of a petrol. Youn still have ABS and a fuel pump and a high lift pump that uses quite a lot.

Please remember that this is a 1983 w123 300D, the injection pump and lift pump are fully mechanical. The car doesn't have ABS. I'm pretty sure this engine (and transmission) would run perfectly with the battery and alternator disconnected. Once it's started of course! No fuel or temp reading obviously.

I have uploaded a picture of the back of the alternator. I assume that the back plate unscrews and the brush/rectifier pack slides out? Can this replacement be done in situ, or would I have to remove the alternator from the car?
The yellow arrow shows where (I think) the grolly bar hit and caused a short (the other end was in contact with a good earth). Would this tally with knacking a diode or two?

Thanks again, all your advice is greatly appreciated!

 

[sixpack]

Alternators

Hi Hibbo,
in answer to your question about zener diodes, no the main 3 diodes are normal silicon diodes. There may be more than 3, but they are used for other things like overvoltage protection. the charge current is controlled by a regulator circuit that monitors alternator output, which is the same as battery voltage. this regulator varies field current to maintain the output about 13.8 / 14v, more field current, more output. Remember that the rotating part of the alternator is the field. The field current is fed via slip rings, or more usually a flat type of carbon contact. The diodes are fed from 3 fixed coils around the outside, i.e. 3 phase. If you have damaged a diode it would have gone short circuit. You may have damaged the field winding itself. Either of these would cause the alternator to run very hot. If you can get the diode pack out, you can test it by connecting an ohm meter across each diode, You should read high resistance one way and low the other. You can use either an analog or digital meter, but a digital meter may give readings that are harder to understand. Its possible you have damaged one cell in the battery. its quite hard to diagnose a battery fault, easier to borrow one to try.

At the risk of rabbitting on a bit, its worth pointing out the importance of having a good connection between bat. neg, and engine block, and bat pos. to starter and alternator output. The reason being that the alternator regulator is trying to look at battery voltage via the battery cables. If there is any voltage drop on these cables, the alternator does not see true battery voltage, so cannot adjust charge properly.

Hope this helps
Peter

 

[Hibbo]

Thanks very much Peter, good info.

Would there not be 6 diodes, giving full-wave rectification from the 3phase stator output?
If a diode had gone short, am I right in thinking that this would lower the DC output as the negative half cycle would be getting through?

What rating of diode would I need to replace the knacked one, think I've got some kicking around somewhere. Would it be the full rating of the alternator, or one third of it?

Cheers again for the reply

 

[sixpack]

Alternator

Hi Hibbo,

Your right, 6 diodes are needed to make a 3 phase, full wave alternator. Ive tried to attach a drawing from an old Haynes book. This one uses another 3 diodes to supply the field. I dont think you can replace diodes separately, all ive seen had them as one unit. Anyway its good to know how it works!

Regards
Peter

 

[Hibbo]

Checked the alternator after I drove home last night, it was fookin hot! Too hot to hold anyway! I'm now hoping that a replacement diode(s) will be enough, and that I haven't damaged any of the windings. How much heat can these tolerate?

Going to whip the back off it and prod around with the multimeter...

 

[290Gee]

Ignition light

Hello all

I have read the threads with interests, as I have a charging problem with my Merc G Wagen. When I start my truck the ignition light goes out. But in the dark it gives off a very dim glow. I have measured the voltage across my battery with the truck running and it shows a consistent 12.7-8 volts. I have changed the regulator only to find the voltage to be the same, but the light is still glowing dimly.

The battery is a new Odyssey starting/deep cycle battery.

Any ideas please

 

[television]

I have answered your first thread.

Malcolm