Can diodes be replaced?

[mriley]

I've stupidly blown the diode(s) on my alternator by wiring it up to the battery incorrectly. Alternator still seems to charge OK, but when engine off battery drains v quickly. The question is, does anyone know if the diodes can be replaced as part of an alternator re-build? Are they a standard item?

 

[Anthony]

Yes normally they can be, however the symptom you describe is not typical of faulty alternator diodes. If the main alternator diodes are blown then you will not normally get any output from it.

Are you sure there are no other loads / faults that could be draining the battery? When you say it drains quickly, what are you using to measure this, and over what time scale? Have you tried turning off the isolation switches for the engine and domestic, i,e, if nothing is connected to it does it still appear to discharge quickly?

You stated you connected it to the battery incorrectly, do you mean that you reversed the polarity? if so was the alternator run in such a configuration? Just wondering if its possible that the battery got damaged rather than the alternator.

Difficult to tell without seeing it / knowing more details, but at this stage dont presume that it must be the alternator diodes at fault. If you take the alternator to an alternator repairer they will normally happily test it for you for a small fee, or sometimes just a polite thank you, obviously this would help rule it out.

Anthony

 

[mikehibb]

Yes they can.

But you will need a good old fashioned auto electrician to do it, Try yellow pages.

I run an old Land rover, now living in former soviet states, Georgia and Azerbaijan, we still have people over here that repair things, had the diodes replaced last year by a local "Master", he could not find the correct replacements, so he used some diodes from an old russian TV set, the Alternator now pumps out even more charge.

I have said this before on other threads, find a good auto electrician before they all disappear, this is a trade that we will soon lose due to the modern throw away and buy a new part mentality.

 

[halcyon]

Yes, the neg diodes will conduct through the stator, and back out the pos diodes.
What are you using to say it's charging ?
Disconnect alternator, and check there is no drain.

Brian

 

[mriley]

I should have given more info. Alternator was smoking and got hot when I got polarity wrong. Just a few seconds of not thinking straight. Guessed I'd damaged it - you don't get a second chance. Battery voltage indicator seems to indicate battery charging OK when engine running, but discharging quickly when not. Presume diodes shot with voltage going in wrong way. Thanks for the advice so far - I'd rather get things fixed anyday than get them replaced.

 

[halcyon]

Could be insulation failed, giving you an ouput, but leaking part of it via insulation fault, or you could have one diode failed, and partial charge.
Get it checked, LSUK, or similar, they need a test rig, and check it is faulty before changing / repair.
It may just not be that.

Brian

 

[colinroybarrett]

Most (that I know of) engine alternators have three ‘windings’ in a star configuration and individual ‘diodes’ to produce the DC output from each winding.

It is quite possible (and likely in the example given) that one or more diodes have gone short-circuit to ground. When the engine is running the working coil(s) and good diode(s) will produce a reduced, but adequate amount of power to battery charge, but as soon as the engine is stopped, the faulty diode(s) discharge the battery(ies) again.

It is no big deal to replace alternator diodes, ‘ Radiospares ’ now called RS-Components, methinks, sell ALL of the required equipment to do same.

In desperation, they will sell to private buyers (look on the web-site for your local ‘Trade Counter’)

You need to understand a bit about electronics and a lot about mechanics, but it shouldn’t be rocket science, these things were first used in the mid-1950’s!!!

 

[stephenmartin]

Many years ago I had the same problem on my car....battery discharged overnight or in the car park....and needed to be bump started....Numpty here asked the garage and they said new battery before alternator.....50 quid later same thing....

I took the alternator to an auto electrician and he said one of the diodes had a short on it....voila...he put a new diode bank in and job done....

As far as i know it is still working fine....somewhere....sold it

Try a breakers for another Alternator...or a good auto electrician