A question for the sparkies out there....

[Jode]

Ok, the 300 TDI has a problem. I recently replaced the 65A alternator with a 90A unit in order to cover our fridge/freezer and other mods. This decisions was helped along by the original alt not charging properly (the charge light wouldn't go out once the engine started).

But Sod's luck, the truck exhibits the same fault after installation of the new alternator: the alt won't "excite" at turnover (the warning light STILL won't go out). After installing the new alt I did a bit more research and found that I'd get a charge if I applied 12V manually to the D+ post. This suggests that the excitation circuit is open, as the warning light stays on, despite the alt charging the batteries.

So far so clear, however I've clearly missed something: when I measure the voltage at the D+ post once the alternator is charging I only get around 3V. I would have thought that I'd get 12V if the exciter circuit is closed, or - if it was open - 12V from the alternator itself.

Can someone kindly explain why I'm only getting 3V?

And I presume that I've got an open circuit somewhere from the ignition switch (white wire to warning light), to the "non-return" diode (not sure which colour wire), to the D+ post (brown wire with yellow trace).

[Scouse]

IIRC, this was linked to a blown fuse somewhere along the line. Do your brake lights work?
I think it may be on the later Td5s but it might affect the earlier ones too.

[Blknight.aus]

check the engine compartment fuse to make sure it hasnt blown.

do voltage drop check from the main terminal of the alternator to the battery.

check the voltage at the D+ terminal both with and without it connected to the alternator you should get about 3v with it connected and nearly battery voltage (+/- ~.5v) without it connected

that the light is iluminating means that the excitation system is fine up to and including the bulb.

If the light is not coming on at full brightness with the ignition on and the engine not running means that the connection between the bulb and the alternator or an earth connection to the alternator is not being made appropriately.

If the light goes on full with the ignition on and the engine not running but only dims when the engine is running means all the alternator output is not making it to the battery.

Thats the basics, theres a bit more to it but that will get you into the ball park for diagnosing.

oh, whats the tacho do when the engine is running?

[manofaus]

This might sound crazy but your bulb for the warning light on your dash is likely the culprit. If the wattage of the bulb is too high then the voltage to the exciter is too low. Some alternators need only 8v to work. Take the lead off the exciter coil on dash and with ignition on measure voltage going to alternator. I know dead set...... change your bulb out for a lower wattage is the solution.

[oldyella 76]

I had a similar issue with my td5, the alternator would not charge, but there was no charging light on. Took it to the dealer to be fixed under warranty, they took the alt out and after 6 hours found the brake light fuse had blown, replaced it and fault fixed. Apparently the brake light circuits also control the ABS and the traction control, as well as the exitation of the alt. As yours is a 300 it could be different but it threw them. So look outside the square.
Lindsay.

[Jode]

check the engine compartment fuse to make sure it hasnt blown.

do voltage drop check from the main terminal of the alternator to the battery.

check the voltage at the D+ terminal both with and without it connected to the alternator you should get about 3v with it connected and nearly battery voltage (+/- ~.5v) without it connected

that the light is iluminating means that the excitation system is fine up to and including the bulb.

If the light is not coming on at full brightness with the ignition on and the engine not running means that the connection between the bulb and the alternator or an earth connection to the alternator is not being made appropriately.

If the light goes on full with the ignition on and the engine not running but only dims when the engine is running means all the alternator output is not making it to the battery.

Thats the basics, theres a bit more to it but that will get you into the ball park for diagnosing.

oh, whats the tacho do when the engine is running?

Dave -
Thanks for the tips, I'll go through them as soon as I can (having a braai today so that'll likely be tmorrow).

But one question: what's the relevance of the tacho reading? If the alt is charging, then the windings should be outputting normally and the tacho would reflect the correct rpm, wouldn't it?

[Jode]

check the engine compartment fuse to make sure it hasnt blown.

do voltage drop check from the main terminal of the alternator to the battery.

check the voltage at the D+ terminal both with and without it connected to the alternator you should get about 3v with it connected and nearly battery voltage (+/- ~.5v) without it connected

that the light is iluminating means that the excitation system is fine up to and including the bulb.

If the light is not coming on at full brightness with the ignition on and the engine not running means that the connection between the bulb and the alternator or an earth connection to the alternator is not being made appropriately.

If the light goes on full with the ignition on and the engine not running but only dims when the engine is running means all the alternator output is not making it to the battery.

Thats the basics, theres a bit more to it but that will get you into the ball park for diagnosing.

oh, whats the tacho do when the engine is running?

This might sound crazy but your bulb for the warning light on your dash is likely the culprit. If the wattage of the bulb is too high then the voltage to the exciter is too low. Some alternators need only 8v to work. Take the lead off the exciter coil on dash and with ignition on measure voltage going to alternator. I know dead set...... change your bulb out for a lower wattage is the solution.

Thanks, Manofaus, I'll check that out. As the bulb was in the vehicle when we bought it some 50K ago, it seems odd to think that it's wattage has increased. Having said that, if we've got more resistance in that portion of the circuit (loose connection?) then the problem could be as you suggest.

[Jode]

IIRC, this was linked to a blown fuse somewhere along the line. Do your brake lights work?
I think it may be on the later Td5s but it might affect the earlier ones too.

Scott -
Thanks for the heads-up. Lights seem to be working and haven't found any blown fuses, but once it's sorted I'll post back an update.

Jode

[Jode]

I had a similar issue with my td5, the alternator would not charge, but there was no charging light on. Took it to the dealer to be fixed under warranty, they took the alt out and after 6 hours found the brake light fuse had blown, replaced it and fault fixed. Apparently the brake light circuits also control the ABS and the traction control, as well as the exitation of the alt. As yours is a 300 it could be different but it threw them. So look outside the square.
Lindsay.

Thanks, Lindsay, but my problem is a bit different: the no-charge light comes on (in fact it doesn't want to go out unless I turn the ignition off). But your advice to break out of the box is sharp.

Jode

[Blknight.aus]

the tacho is triggered off of the +5v point (lifted to around 12v by the resistor in the circuit) of one of the 3 phases on the alternator before its all rectified back to AC, if you have no tacho action and there is no charge thats usually an indication of no output from the alternator (other than reading battery voltage) Be mindful that this is usually only a reliable indication on start up as once the alternator is excited it will remain excited and operating bar a failure of the slip rings or rotor winding.

theres other things that can cause it but the normal diagnosis of the alterntor charge warning light on full and the tacho suddenly dead is a regulator failure to earth or the d+ wire shorting to earth.

(tacho stopping or intermittant is usually the w lead coming loose)

oh the 3V thing...

if you only get 3v at the alternator d+ post thats the voltage left coming through the bulb in the dash, the bulb is there as a current limiting device to prevent the winding in the rotor from overdoing it during startup while everythings trying to get sorted before the regulator kicks in and takes over.

what happens if you start the vehicle and bring the engine up to about 2k Rpm?
What happens if you start the vehicle, let it idle and then just touch 12v onto the d+ post momentarily.

The answer to your problem may infact be your new alternator. if your new alternator is expecting to see 12V to excite the alternator and its only getting 3 through the bulb then its not going to start charging. The old one may have failed but was setup to work of the existing supply.