W124 charging problem

[Willthechippie]

My 1991 300TD is not charging the battery up. There is no battery light glowing when running (working on start-up) but whilst driving the electrics all fade rapidly. i've changed the alternator with the correct exchange Bosch one (which does work, as i tested it myself), but still barely get 12V across the battery terminals (without load). i seem to have voltage from the battery to the alternator but no amps (ghost voltage) which leads me to think that there is some kind of heavy duty fuse between the battery and alternator that may have blown. Is this right? if so where is this located? or could it be something else? battery good. alternator good.
thanks
Will

 

[television]

Your fault must be in the exciter circuit, this is all part of the lamp in the dash that indicate no charge,, for the 124 its worth getting the Haines book as the diagrams in in there

 

[Ellsy Tanners]

As malcolm, is the battery light on the instrument cluster with the ignition on, blown bulb in cluster or fault with igntion feed to cluster.

Do you get 12v at the blue wire at the alternator with ignition on?

 

[Willthechippie]

The battery light on the instrument panel is working as it should. i.e. on when ignition switched on and goes out when engine starts.
i don't know if there is a blue wire at the alternator! there is one cable that goes into the plug that has 3 connectors and it's impossible to see what colour any of these may be. i've checked the voltage at this plug and there is 12V - will check again this morning.
Did manage to get back from work and here again this morning, however i was late and very cold as too paranoid to put heater or lights on!

 

[wireman]

You might be suffering the corroded crimp syndrome on the heavy cable that connects directly betwween the alternators output and battery + terminals. It is not normal to have a fuse in this cable.

To check the cable try a test lamp connected to body/frame/earth and the fat cable on the alternator (you will need to remove the back cover off the alternator cable plug to access the tag), Use a head lamp bulb that will draw a decent current it should illuminate fully.

How did you test the alternator?

 

[Willthechippie]

Alternator tested on an old Cafco alternator tester (big metal box with various lights, dials, loading levers, pulleys etc.) that a colleague of mine has.

However: Problem now solved! yippee
Very close to the fusebox there is a fuseable connector block (no fuse, just a black connector block) for the cable that runs from battery to alternator. Upon close inspection this looked a bit corroded and when tested was not passing charge properly. Dismantled, cleaned and 15 seconds of emery cloth rubbing the connection was refitted and now passing current and am now getting 13.7 volts between battery terminals.
I shall now be able to drive home with my heater on.
Very happy owner although £115 worse off for initially buying a new unnecessary alternator.
Thanks all for your posts.

 

['89 250TD]

Hi folks,
I've a related issue maybe you can help me with...
Removed battery from car to charge it as it had little charge.
Connected it to car with jump leads when charged as charger indicated it just needed a trickle charge & I thought battery was shot...
Managed to connect it incorrectly for a short time...
Charged battery again & put it in car...
Car starts OK now but battery charge light doesn't go out...
Will I have blown a fuse or done some damage by connecting battery incorrectly that will be causing battery charge light to come on now...

Thanks for all suggestions...

~ Mark

 

[television]

I am not sure if there is a fuse in the charging circuit,,see if I can find out

 

['89 250TD]

battery charge light on...

OK, thanks - I'll check the main fuses & see if I can spot any duff ones...

I am not sure if there is a fuse in the charging circuit,,see if I can find out

 

[wireman]

If you connected the battery to the car backwards even for a millisecond your alternator diodes will be fried, sorry to be the bearer of bad news but reverse polarity is a killer for semiconductor devices, I hope that the damage is limited to the alternator your heater controls, ABS and radio could be damaged as well if they were on at the time...

The alternators output cable is connected directly to the block next to the battery, there is no fuse since the diodes in its rectifier will blow in less than 1 mSec and a fuse would take a lot longer to even think about failing.

 

['89 250TD]

diodes fry-up

alternator diodes will be fried

Thanks for the info, even if it's bad news...
So that's a replacement alternator or can the diodes themselves be replaced...
See from yr other post/s that they're inside the alternator itself...

 

[wireman]

You need to find a decent auto electrician, there are loads of them around some are mad keen on doing the bitty jobs others want to fit exchange units. The rectifier diodes are pressed into heatsink plates within the generator itself and may be available as seperate diodes or as assemblies of 3 diodes ready to fit (soldering required). There are a total of nine diodes in the rectifier, 6 big ones for the chaing current and 3 smaller for the regulator circuit. The melted ones are almost certainly in the charging circuit.

It may be best to fit an exchange unit, you will get good bearings, regulator & brushes i.e. all the bits which wear out. Into the bargain there is usualy a warranty on the thing.

 

[hotrodder]

Recently had a 90A bosch alternator rebuilt for a '93 320te- died the day after i bought the car. Rectifier rebuilt with new diodes and regulator, tested and warrentied for £48. If it had been a manky old alternator i would have got a recon or good secondhand but the alternator was clearly a recent exchange unit- looked brand new, bearings and brushes good just had been reconed badly (maybe beer o'clock on a Fri or something but the place that rebuilt it said the red oxide coat on the windings was a telltale of a cheap recon).

I used tmf batteries FWIW