75ZT Community

As I work through the issues on a new acquired 04 plate CDTi saloon (with under bonnet pump)

I now find the alternator charges around 13.2 volts which goes down to 12.2 volts when loaded by headlights, heated windscreens and heater fan.

A parasitic drain test showed a problem with multimeter on 10 amp setting read 0.69. Should I assume this may well be the alternator? I took the easy to access coloured relay and fuses in the engine bay out and it did not affect the reading of 0.69. I could not find a tool to easily pull the 4 flat shaped low profile 50 amp to 100 amp fuses.

We know the diode on the alternator can cause fail and cause this kind of parasitic drain? But would changing the regulator/rectifier pack be a fix? Is changing the regulator/rectifier pack the best way to fix? Buy cheap from ebay for around £30? The car has only done 86k miles.

0.69 sounds like 690mA which isn't massively high (though I'd expect lower). How long did you wait after switching off ignition, removing keys closing doors / bot / bonnet and so on? Some systems take a suprisingly long time to go properly to sleep. I remember on another car with some BMW based parts, the heater controls took up to 20 minutes to sleep fully. They are not the ones used on a 75 though, so I couldn't tell you how long you would need to wait.

13.2 at no load sounds low, but what state is the battery in? If low charge, it will drag the alternator down by sucking a lot of current until the charge builds up somewhat. 12.2v certainly sounds like the alternator is struggling somewhat. Have yo tried bringing the RPM up to say 1000 and see if the volts come up? The alternator is capable of providing up to 120A, but not at low RPM. Itcould also be helpful to check the alternator current but you would need a clamp meter, and specifically a DC clamp meter as many are AC only.

The replacement pack, is a regulator / brush box. It Doesn't include the rectifiers. Rectifiers used to be a problem on the old Lucas ACR alternators but I've never seen a problem on newer stuff like ours. The regulators and brushes can cause problems, usually if someone has dripped engine oil on it while servicing, but I'd expect there to be a charge warning light on, though I wouldn't guarantee it. If it is the rectifier pack you would be better swapping to a new alternator. I'll also say that my alternator failed. For speed I dropped in a new alternator. When I diagnosed the old one, the regulator / brush box was fine, but the coil on the armature (rotor) was open circuit.

Thanks Duncan.
I have just opened the boot to rear seat ski hatch and even when I close the boot and start the car and switch off with the boot closed the boot light does not go out. After removing the boot light bulb the reading was the same.

In case I am doing the parasitic test incorrectly - with the bonnet open I left a weight on the bonnet alarm switch so that the car would think the bonnet is closed. Locked the car. Waited 50 mins. On the multimeter I placed the multimeter red lead into the multimeter left plug socket and switched to 10amp. On the car - I and then put the multimeter between the battery negative terminal and negative lead to get a reading of 0.69

Posts on the web suggest I should unplug the lead to the alternator to tell whether the alternator is the cause of the drain.

I am confused by the label on the Alternator saying Bosch exchange remanufactured as photo.

Second hand one been fitted then not the original, i would say.

The boot light is on a timer. Not directly controlled by the boot switch. The BCU (Body Control Unit) supplies power for the interior lights, map lights etc. From what I can tell, the boot lamp is fed the same way. So it will go out when the supply from the BCU to the map lamps is switched off. It's done this way to prevent you accidentally flattening the battery by leaving a map lamp on.

Regarding the test. I'm not sure of your sequence. Do you let the car sleep, then disconnect the battery and put in the meter? Or is the meter already in the circuit when you let it sleep?

Testing the alternator. If it's the diodes simply unplugging won't test that. You would probably be best removing the relevant large 150A fusible link (link 1) in the underbonnet fusebox. Unplugging the regulator would test that, but it has no permanent feeds according to RAVE. It has an ignition feed, the warning light connection to the instruments, and a connection to the engine management.

Thanks Steve and Duncan,

I read on the web a drain can be caused by the of the modules developing a fault and not going to sleep. In which case my boot light staying on may mean i have the same issue with the BCM as Daveyk1976

Thanks for detail about the alternator plug that saves me time.

When doing the parasitic drain test on the negative lead my sequence was. 1) open bonnet and place a weight on the bonnet alarm switch so the car thinks the bonnet is closed. 2) with the battery connected as normal lock the car. 3) wait an hour 4) placed the multimeter leads one on the negative battery terminal one on the negative clamp and slip off the negative battery lead without breaking continuity.

Ed3 wrote:Thanks Steve and Duncan,

I read on the web a drain can be caused by the of the modules developing a fault and not going to sleep. In which case my boot light staying on may mean i have the same issue with the BCM as Daveyk1976

Thanks for detail about the alternator plug that saves me time.

When doing the parasitic drain test on the negative lead my sequence was. 1) open bonnet and place a weight on the bonnet alarm switch so the car thinks the bonnet is closed. 2) with the battery connected as normal lock the car. 3) wait an hour 4) placed the multimeter leads one on the negative battery terminal one on the negative clamp and slip off the negative battery lead without breaking continuity.

That's fine. When I used to test this at work, I made up a lead with a switch to go between the -ve lead and battery -ve. Also had a fuse in the lead, as the wires were thin. Only opened the switch when the car was asleep, to measure the current. This protected the fuse in the multimeter. Or at least it did until someone marched in and without even acknowledging me, opened the car door. Popped the fuse in the meter (we were on the mA range) and wrecked the test. Thanks Simon!