Alternator Diode Replacement How-to

[telford dorr]

This post show how a broken diode in a Bosch AL75X 55 amp alternator (as found on type IV engines) is replaced. To do this, you will need suitable large soldering equipment and a small arbor press. Don't try it with a hammer - you'll likely destroy the diode pounding on it.

First, remove the front air cover from the alternator, and unplug and disconnect the wiring. Now remove the two nuts holding the diode plate in place and carefully pull it up. In this alternator, one of the wire leads on a diode was broken, so it had to be replaced. Use a sharp knife and remove the insulation from the junctions where the wires connect to the diodes, and unsolder them. [Note: originally, the flexible wires were spot welded to the diode lead. If yours are still that way, you'll have to unsolder the diode wire from the diode trio assembly. These diodes were replaced somewhere along the line and were soldered.] Once the diode plate is free, press out the offending diode. Support the plate on the press with a suitable socket, and press the diode, from the lead end, out of the plate.

If you have to replace a negative diode, you'll have to fully disassemble the alternator. This involves unsoldering the stator wires and negative diode wires from the diode trio assembly. Use a good-sized soldering iron and a solder sucker to remove as much solder as possible from the joints. Then use needle-nosed pliers to unfasten the wires from the terminals. Be careful! The diode trio terminals ARE NOT STRONG and won't take much force!

In this example, a diode is being replaced because it physically broke, so only it was replaced. If you're replacing diodes because of shorts or burnout, replace ALL of them, even if they test OK, as they may be damaged.

Now check the new diode with an ohmmeter (use the diode range) to verify that the polarity of the diode is correct (compare to the other diodes on the plate). Diodes come in two polarities - "positive" (the wire lead is the anode), and "negative" (the wire lead is the cathode). Positive diodes mount on the metal plate; negative diodes mount into the alternator body on this alternator. On other alternators, there are two sets of mounting plates.

Once this sanity check is complete, press the new diode (NTE, in this case) into the plate. Diodes always press in from the flat side of the plate. Support the plate with a suitably sized deep socket. Press the diode in flush with the plate surface.

Now bend the lead on the new diode at a right angle to the body. IMPORTANT: grip the wire lead at the top of the diode body with needle-nosed pliers such that the diode does not take any force from the bending. You don't want to snap the lead off of the diode! Bend the lead in a direction which won't interfere with anything else inside the alternator, and trim it to maybe 3/16". Solder the three wires back onto the diodes using rosin cored 60/40 electrical solder. Make a good solder joint, but try to minimize the amount of heat you dump into the diode. Their ability to take a joke is limited. Don't forget the shrink tubing! Shrink it to insulate the diode lead.

Now reinstall the diode plate onto the alternator. Position the diode wires such that they don't get pinched or squished between anything. Reinstall the mounting hardware: small washer - diode plate - big washer - star washer - nut. Note that the big washers have been ground flat on one edge so that they better clear the metal air cover. When grinding, you will develop a burr on one edge of the washer. Leave it - it helps keep the washer from rotating as you tighten the nut.

Now reinstall the wire harness.

Install the rear cover and gasket. Use your ohmmeter (diode range) to check for any shorts from the big red wire to the alternator case.

At this point, I mount the alternator assembly in my test jig and run it. You need a 12 volt battery (I use one of those [otherwise useless] battery jump boxes that have a 12 volt gel cell battery in them), a test light (to emulate the dash light), some jumpers, and a 500 amp carbon pile tester (Harbor Freight). Note that the test jig also holds the regulator, grounded to the base plate. I spin the alternator up and check for proper charge voltage, then load it to 50 amps (for a short time only - you don't have the normal forced air cooling connected, and you don't want to cook a diode!)

If all is well, reinstall on the engine.

Part numbers (I believe to be correct, but...):

OEM - 021 903 357 (positive), 021 903 355 (negative), both NLA

Aftermarket - WAI/Transpo 32-1040900 (positive), 32-1040901 (negative), both hard to get in the US, but easy in Europe. These guys also sell the diode trio assembly IB112

NTE5934 (positive), NTE5935 (negative), available in the US Note: NTE is, for the most part, a component repackager who sell other mfg's parts in small quantities. As such, their parts will cost more than direct from the mfg, but that's the cost of convenience... http://www.nteinc.com

Note: the OEM diodes are rated at 300 volt, 35 amps. The NTE parts are rated at 400 volts, 75 amps, but are physically the same size! Diode current ratings are continuous current ratings, but they typically have quite high short duration peak current ratings. Any one diode conducts in an alternator only 33% of the time. This is why a 35 amp diode can be used in a 55 amp alternator.

If you substitute other diodes, be aware that some diodes have a flange at the wire lead end of the diode body (most US alternators), and others have a flange at the base end of the diode (Bosch). This flange must be flush against the flat side of the diode mounting plate, and thus govern which way the diodes orient on the plate. NTE diodes have no flange at all.
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'71 panel, now with FI
'Experience' is the ability to recognize a mistake when you're making it again - Franklin P. Jones
In theory, theory works in practice; in practice, it doesn't - William T. Harbaugh
When you're dead, you don't know you're dead. The pain is only felt by others.
Same thing happens when you're stupid. - Philippe Geluck
More VW electrical at http://telforddorr.com/ (available 9am to 9pm PST)

[SGKent]

I know how hard it was to locate those diodes.
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“Most people don’t know what they’re doing, and a lot of them are really good at it.” - George Carlin

[telford dorr]

Steve helped a lot researching diode part numbers and sources - thanks again, Steve!
_________________
'71 panel, now with FI
'Experience' is the ability to recognize a mistake when you're making it again - Franklin P. Jones
In theory, theory works in practice; in practice, it doesn't - William T. Harbaugh
When you're dead, you don't know you're dead. The pain is only felt by others.
Same thing happens when you're stupid. - Philippe Geluck
More VW electrical at http://telforddorr.com/ (available 9am to 9pm PST)

[SGKent]

Great - now if anything goes wrong he'll blame me. I wasn't looking for a thank you. I still owe you for your time on a dozen other things. Steve
_________________
“Most people don’t know what they’re doing, and a lot of them are really good at it.” - George Carlin

[ivwshane]

How do you know if your diode is broken?



Thanks for the write up!
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77 westy 2.0 FI
69 ghia coup 1600dp
70 single cab

[SGKent]

[telford dorr]

It charges poorly, if at all. One diode out unbalances the three-diode bridge, causing the voltage fed to the regulator to be weird, messing up the charging. In this case, when I pulled the diode plate as part of a thorough inspection, the wire was hanging loose. Probably bent too many times over the years. The junction between the lead and diode innards is weak at best, and won't take much stress. That's why holding that lead with pliers while bending it is critical!
_________________
'71 panel, now with FI
'Experience' is the ability to recognize a mistake when you're making it again - Franklin P. Jones
In theory, theory works in practice; in practice, it doesn't - William T. Harbaugh
When you're dead, you don't know you're dead. The pain is only felt by others.
Same thing happens when you're stupid. - Philippe Geluck
More VW electrical at http://telforddorr.com/ (available 9am to 9pm PST)

[telford dorr]

[mark d]

would it be doable to change a 35 amp alternator into a 75 amp ?

thks
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1972ish westfalia

"what good are tractors without violins ?" jose figueres

[TomWesty]

[telford dorr]

Actually, less but bigger wire. Output current is mainly limited by the inductance of the stator windings, which varies with the number of wire turns. Unfortunately, there's lots of other factors which make the mod a bit complex. Not a DYI project, unless you have lots of time, materials, and a test jig...
_________________
'71 panel, now with FI
'Experience' is the ability to recognize a mistake when you're making it again - Franklin P. Jones
In theory, theory works in practice; in practice, it doesn't - William T. Harbaugh
When you're dead, you don't know you're dead. The pain is only felt by others.
Same thing happens when you're stupid. - Philippe Geluck
More VW electrical at http://telforddorr.com/ (available 9am to 9pm PST)