TDV8 Sport alternator query , one for the gurus I think.

[Nomad9]

Hi There,
2007 TDV8, 161,000 klms on the clock, I have already posted about this issue before, however the vehicle was delivered back this afternoon by the RAC and I now have a few more queries. I sort of know the final outcome however I would just like a bit of verification.

The guy in Denham fitted a battery to get some basic functionality back, door locks, handbrake and gear selection, not the right battery. However when the battery was installed the RR started, this allowed the vehicle to me moved with the camper trailer etc and put on the trailer.

This afternoon getting the vehicle off the transporter with the undersized battery fitted vehicle wouldn't start so I fitted the new correct battery. RR started and ran perfectly, even the battery light went off. At this point I started to think that maybe this was just a battery fault and maybe not the alternator........ don't I wish.

Ran the camper trailer up to the shed, unhitched the camper and put the RR in the shed. Battery light still off, everything working fine, no dash lights on etc. Thought I would check the battery terminal voltage, 13.58 volts, steady. However and this is a bit of a biggie, there was a bit of an electrical smell. Might just be my paranoia........ but I was sure I could smell something electrical.

Turned the engine off and connected the CTek charger, the battery was it appeared a 1/2 to 2/3 discharged according to the charger, the battery voltage at the terminals with the charger attached was 12.91 volts and climbing as I suspect the charger was kicking in.

I'm committed to changing the alternator I believe this is definitely at fault, I would just like to try and understand the failure mechanism. Would it be possible for the alternator to be "rooted" and yet still keep the alternator or at least the battery light extinguished until the battery drew its last breath? I'm also trying to determine if I originally had a battery fault or an alternator fault which then totally "Ronnied" the battery, the battery was a Century 850 CCA about 18 months old.

When the problem originally happened the battery light came on and stayed on until 23 to 25 klms later when the vehicle just stopped with the majority of the dash lights on. After that there wasn't enough power to turn the interior lights on, the battery voltage read 0 volts.

Since fitting the new battery the battery light on the dash extinguishes immediately after the engine starts.

Any constructive enlightenment is welcomed.

Cheers Marty.

[Graeme]

The battery could have a broken internal connection causing intermittent low voltage. However these alternators fail because the Zener/avalanche diodes deteriorate with usage whereby their trigger voltage that shuts-down (short circuits) excessive voltage to protect the vehicle's electronics gradually drops from the original 30V to normal charging voltage. Hence a low charging voltage doesn't trigger the reverse current flow but when the voltage rises just a little more, the diode that has deteriorated the most short-circuits.

On a different tact, there is a noise suppression capacitor attached to the outside of the gearbox that is known to fail short-circuit thereby drawing excessive current.

Edit: The reverse-trigger voltage of the diodes is affected by heat, thereby altering the voltage at which the short-circuit occurs and possibly causing more than just a single diode to short-circuit.

A single short-circuiting diode (only 1 phase affected) may be overpowered by the other 2 phases still functioning properly allowing voltage to be maintained with the engine running but the battery will soon be drained by the short-circuiting diode once the engine is stopped.

[Nomad9]

Hi Graeme,
The noise suppression capacitor attached to the outside of the gearbox, are you possibly suggesting that this might have failed (as well)? Is this a common failure mode, what does this look like and where abouts on the gearbox is this gizmo, how do I know if it has failed or not? Does it get warm / hot?

Do I simply replace the alternator and battery and then see if everything is OK? Does this attachment on the gearbox have a fault code?

Thanks for the assistance, appreciated.

Cheers Marty

[Graeme]

The gearbox capacitor can be disconnected and tested for short circuit and something to be checked if a check of the alternator current draw as suggested below doesn't implicate the alternator. However the symptoms described are most typical of failing alternator diodes. That the temporary battery was flat when about to unload really indicates that the original battery is not the cause and therefore most unlikely to have an internal break, so something is drawing lots of current.

I would disconnect the starter cable from the battery (I suspect that the alternator connects to the starter but only guessing), put some charge into the battery then remotely test reconnect the starter cable, first fitting a jumper lead to the battery then to the cable to keep possible sparks away from the battery. If sparks occur then the starter/alternator are drawing current yet they should not and therefore basically proved that the alternator has at least 1 short-circuiting diode. You would have to disconnect the starter cable at the fusible link to leave the rest of the vehicle powered to know that current is indeed being drawn on the starter/alternator cable.

[Nomad9]

Hi Graeme,
I have started replacing the alternator so I have various bits disconnected now, I'm committed to fitting a new alternator, this seems to be the most likely bit that is broken according to others comments. I have the alternator and I have the new battery so I'll soldier on.

Where abouts is the gearbox bit you talk about? Is it easily removable or do I need the anatomical features of a new-born to reach it?

Cheers Marty

[sheerluck]

Marty, it's on the right hand (driver's side) of the transmission, and looks like this:


Very easy to see and get to once the transmission shield is removed.

[Nomad9]

Hi Dave,
Thank you, I'll have a look when I get the RR up on the hoist. My wifes birthday today so I'm not sure now is the right time to be grovelling under Rex, time and a place, thanks for the photos, makes things much easier. Are these a common failing point? Would you happen to have a part number?

Cheers Marty

[sheerluck]

I wouldn't say common. The one on my D3 died, and judging by the posts on Disco3, it happens occasionally. At least they are inexpensive and easy to replace.

[Nomad9]

Hi Dave,
How did the failure show itself? Also do you have a part number you could throw my way...... please?

[sheerluck]

Hi Dave,
How did the failure show itself? Also do you have a part number you could throw my way...... please?

Mine was easy to diagnose, it had fallen apart.

Part number is NNW502820 or LR020488