Cerbera Alterator diode pack

[Mad Mark]

How easy is it to change the diode pack on a Cerb 4.5 alternator? or is it easier to get it reconditioned?

I recently fitted a new/recon alternator to my Cerbera but it seems the diode pack is faulty. Trouble is I bought it from online over a year ago but only fitted it this winter. I taxed the car this week took it for a drive and then noticed the battery would be flat within a day. It charges fine at around 14.40 volts but when left stationary it drains the battery out. Disconnecting the live from the alternator and the battery stays charged.

[EGB]

Methinks. If your alternator is charging 14 volts then your diode and slip ring is ok. Suspect the battery? Also a rapid battery drain is happening when connected. Suspect a drain somewhere. The imobiliser is well known for this. Surprised it happens in one day!

[Mad Mark]

I think my last line might have been a little confusing i meant..
If I disconnect the live from the alternator but leave everything else connected the battery stays charged. It only discharges when I connect up the alternator.

[EGB]

Does the alternator still feel hot (drain discharging battery)after engine has cooled. Have read somewhere this happening. Try another alternator to test.

[Mad Mark]

Have you seen the position of the alternator on the Cerbera!! lol
It's in the 'V' under the fuel rails, injectors and air intake pipes. It's a bit of a mare to change. But it looks like that is what I am going to have to do again.
I changed the alternator because the bearings had gone in the old one and was noisy. Although the old one is now useless to use due to taking a beating getting the pulley off I may still be able to plug it in and see if the drain continues. And who knows maybe i can swap the diode pack over if need be.

[scotty_d]

Have you checked the power draw from the car at all see how many amps are being pulled when it is switched off?

I would do that first with the alarm and immob on and if it is pulling something silly it should only be a few milliamps at most i would think, try pulling a fuse at a time to see what is pulling the power.

That is where i would start.

Edited by scotty_d on Monday 11th March 14:24

[Cerbieherts]

Exactly. You need to measure the parasitic draw and then trace the cause. You would need upward of 80 milliamps to have the battery draining that quickly, so it should be relatively easy to trace...

[Mad Mark]

I already know it's the alternator. If I disconnect the live feed to the alternator (between the big fuse and alternator) the battery remains charged.
I have read that sometimes a diode can fail/short allowing it still to charge but then discharges the battery as soon as its turned off.

I was wondering how easy it was to change the diodes and if a the diodes have different ratings etc.

[glow worm]

My alternator was knackered (windings) and it drained the battery over night ... I though it was just the diodes but not and had to buy a reconditioned one. The diodes were integal to the alternator on both the original and reconditioned one.

[Mad Mark]

The diodes are part of the rectifier and are pretty easy to change. After searching the web I found this video. The internals are pretty much identical to the Cerbera / Range Rover unit.
http://www.youtube.com/watch?v=8tvXocfXyic
the guy doing the video has a spooky resemblance to Michael Schumacher lol

My plan is to swap the rectifier from my old alternator to the new one and see how it goes from there. Hopefully it wont be the windings shorting out but apparently most faulty alternators are caused by the rectifier.

[Sardonicus]

A drain through the alt is always diode related the windings can not do this on their own you need a faulty diode pack for this to happen.

[FarmyardPants]

If you can't face taking the alternator out again perhaps you could wire in the diode pack from your old alternator in line with the big fuse . (Roger the bodger)

[Mad Mark]

FarmyardPants said:

If you can't face taking the alternator out again perhaps you could wire in the diode pack from your old alternator in line with the big fuse . (Roger the bodger)

lol Probably more effort doing that than doing it properly.
Started stripping back today but in this cold weather only doing a little a day. I must be getting old lol

[Mad Mark]

I am just wondering whether I may have connected the smaller black wire to the wrong terminal, there are two connections but one wire. I am pretty sure I haven't but thought it worth asking before pulling the alternator back out or making the same mistake twice.

So which one should it be connected to? currently I have the smaller black connected closest to the main live.

What would the consequences be if I plugged it to the wrong one?