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I've got an alternator whose diodes are going bad as evidenced by excessive AC output from the alternator.

The problem is that the diode assembly is a discontinued part, which I can't even order at the Nissan dealership. My other options are to buy a 20 year old alternator from a junkyard, or to buy the same thing from the one shop within an hours drive which "refurbishes" junkyard alternators by giving them a cursory check and a six month warranty.

So I'd really like to know if it's possible to just replace the actual diodes in the rectifier.

This is a 99 Nissan Almera 1.6L GA16DE and the rectifier part number is 231240M011.

  • If you know what you are doing you could make an external rectifier... – Solar Mike Feb 26 '18 at 11:03
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    have you really tried to search for this part? I readily find it on dozens of sites. – agentp Feb 26 '18 at 13:06
  • @agentp The only site I found it on was in Russian. I don't speak or read Russian, and the few English sites that seemed to have either don't actually have it or listed it for close to $200USD. If you found a site in English that has it in stock for a decent price I'd love to have the link. – Robert S. Barnes Feb 27 '18 at 4:40
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Possible, sure. But you need some beefy diodes that can handle an amperage of at least 50A, and at the right spec.(think about type, voltage drop, switching time, reverse blocking specs, temperature...) Otherwise you might get some unexpected magic happening in your electronics, or release the magic smoke in them.

I'd rather look for a rectifier unit as a whole, or just a new alternator altogether. If it physically fits right, and the pulley size is about the same, you can take any alternator you want. It looks like a very common one, so it should be easy to find a fitting replacement.

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    Also the right format - not the same as diodes you tend to find on a pcb... – Solar Mike Feb 26 '18 at 12:25
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    @SolarMike It's not really the format/size that matters, but the inherent dissipation capacity, defined by resistance, voltage drop and current ratings. If the current ratings and other spec requirements are met, then any size would do fine. PCB mountable diodes can get really beefy, and may just be fit for the job. Nearly anything can be fitted on a PCB anyway. – Bart Feb 26 '18 at 19:18
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    if they don't fit properly in the heat sink then you need to make a different heatsink as cooling is definitely an issue given the under-hood temperatures... – Solar Mike Feb 26 '18 at 21:05
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It is possible to replace all the parts of the alternator, including the diodes in the rectifier plate. The only sure dead of an alternator is a breakage of the case. Even a defect in one of the windings is curable, although it requires expensive rewinding.

The main problem is that the diodes are pressed into the diode plate and look sturdier than they actually are, making replacement a complicated issue. The press fit is a bit of a challenge as a bad contact with the carrier plate results in heat-death of the diodes under full load, combined with spurious electronic problems. Also it could be that acquiring matching replacement diodes is difficult.

Additional problems: The slip ring is most likely consumed, in some cases it is possible to repair it by turning it with a lathe. Also the brushes would need replacement. The bearings should be easy to replace, opt for rubber sealed C3 bearings if you have the possibility.

Repair of all these things is labor intensive and prone for errors. If I where in your situation I'd get a new/refurbished one.

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    Some alternators have slip rings that are easily replaced... – Solar Mike Feb 26 '18 at 15:10
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If you know the basics of how an alternator works, you can replace the bridge by pulling the wires from the stator to a 3 phase diode bridge like the MDS100A (which you can get for around 11 bucks from the usual suspects.), and removing the original bridge.

Some (most) alternators have an extra half bridge going to the regulator and the warning light, you could use 3 10 amps diodes to replace this one if it's borked too.

If the regulator isn't behaving properly either, you could make one using an op amp, a transistor, and a couple of passive components (turn the rotor on when the voltage is lower than 14.2V, shut it down if it's higher)

Note that you should be careful to avoid making stuff that isn't plugged into the ground touch the chassis.

These fixes should work, but if you can afford to spend 50 bucks more on a refurbished alternator, and won't have fun making your own, do it.

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I wasn't able to find a picture of that part online, but drawing of the alternator parts shows it is a typical heat sink with diodes on it. Some of the alternators that I have seen have button diodes that are soldered in to holes that are drilled in the heat sink. The entire thing is heated up to melt the solder and then the diodes come out. More modern designs seem to have the diode dies mounted directly to the heat sink. They are covered with some hardened protective coating that was transparent in my case.

I attempted to repair one of these directly mounted die rectifiers by using button diodes and drilling out holes for them. I didn't use an end mill so the bottom was not flat but angled. It worked but solder doesn't conduct heat as well as aluminum and at full load it only took a few minutes for the diodes to melt the lead solder pool at the bottom and disconnect themselves.

Automotive alternators are still using an old design that is very inefficient and uses passive inductive over current protection. This means that the open circuit voltage at full load and high RPM is very high. The important thing to remember is that the diodes used in an automotive alternators are actually TVS diodes that clamp the voltage to a maximum of 32-40 Volts in the event that the battery goes open circuit while the alternator is at full load. This protects the car from over voltage in the short time (milliseconds) that it takes for the regulator to reduce output of the alternator. Since the rectifier diodes are also TVS diodes, it's a fail safe system. Failed TVS diodes also mean that the rectifier diodes are failed so the alternator can't put out any current.

If you chose to bring out the 3 phase AC alternator output you'll need a 3 phase rectifier that can handle around 100 Amps. That's about 200 Watts generated and heat sink needed without considering the high temperature of the engine compartment, so it's actually closer to 400W of heat sink for normal temperatures. You still have to have TVS diodes to protect the output from voltage spikes. Also, keep in mind that the wire connecting the rectifier to the battery may be a fusible link to prevent a melt down and fire if the alternator rectifier shorts out.

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