This modification is for the plastic case electronic voltage regulator that Wherle made, with its name on the case. I have only modified a few of these.  IT IS POSSIBLE THAT YOURS MIGHT LOOK DIFFERENT INSIDE, as different versions have been made.

This modification will increase the charge voltage to better charge the battery and possibly give longer battery life.  In-town commuters may be particularly interested. This modification will be useful in instances where the voltage at the battery terminals (engine running well above idle rpm) is found to be at or below 13.9. This modification essentially turns your voltage regulator into the pricier BMW-sold higher voltage Authorities (Police) regulator, and does this very cheaply. TWO methods are described, so that at least one of the parts needed will be available locally.  All Wehrle regulators are not the same.  Be sure yours is the same as the one described here, before modifying it.

Cost: I will assume you have the tools, and will do the labor yourself. The parts cost will be from 10 cents to $3.00, depending on what part(s) you purchase.

Time involved: Once you have the parts and tools and regulator on your bench, allow a few minutes if you are experienced, and half an hour if not, and hour or more if exceedingly unfamiliar with electronics items internally.

Skill and tools, caution:

Must know how to de-solder and solder teensy solder joints. Must have a small tip soldering iron, not over perhaps 25-35 watts. Tip MUST be small…perhaps 1/8″ or so.  You will need a few inches of 60-40 rosin core solder; and, a short piece of de-soldering braid, rosin impregnated or some soldering rosin, would be nice to have. You will need to remove the gas tank for access to the regulator, and must unplug the regulator and remove its two mounting screws, and then pry off the red top carefully. There is a chance you might need to drill out two very small diameter holes, very slightly larger. You need an accurate digital voltmeter to check your work. Fully charged battery helpful.
You can ruin your regulator by being hamfisted with it.

arts needed: ONE part from #1 or #2, but you need only ONE part!:

#1. ANY common silicon diode of the lower power variety, rated at one ampere or more. Voltage rating UNimportant. Available at Radio Shack in all sorts of ways.

Example: #276-1101; 276-1102; 276-1103, etc. These are same as common industry types 1N4001, 1N4002, ETC. You COULD get a whole little bag of 25 of assorted diodes from RadioShack, their part number 276-1653 $2.29 for the bag. There is NOTHING critical about this part, it is in a very low power circuit.

#2: ***NOTE: This method is neater, as you will replace the existing part, not adding another part, BUT, there is a potential problem, in that the common parts described here have TOLERANCES, and you MIGHT just get one that is a bit too high in ACTUAL voltage rating, which MIGHT boost your regulated voltage to a bit too high a value for your battery. Most any common silicon diode of the type called a “Zener Diode”, you will need one rated at 8.2 volts (that is a STANDARD value). 5% or smaller % tolerance. Common number is 1N5237B…and others. RadioShack has an 8.2 volt silicon diode, package of 2 is $0.89, part RSU11928413. RSU parts are not normally stocked. You could also use almost any 8.2 volt zener diode from your local TV repair store or electronics components store. Rated at 500 mw (milliwatts)…the same as 1/2 watt. Or, you could fit a 1 watt one too. SOME 1 watt units have leads a bit larger in diameter than the PC circuit holes in the regulator …this is true for all the parts in section #1 and section #2. This will require you to enlarge the existing holes a teensy amount.

My suggestion is for you to look at your regulator innards, see what the existing part looks like, and then you will have a better idea of what to get. ***NOTE that the part you purchase may well look SLIGHTLY different…perhaps larger, or marked a bit differently, or colored solid black, or whatever. It will ALWAYS have a LINE marked nearer to one end.

Doing the modification: Pry off the red plastic cap…it is held by small projections in the plastic, centrally, each of 4 sides. Once the cap is removed, you can now gently press, alternately, the three connection male spade lugs and the metal heat sink sides, all of which protrude through the bottom…and the innards will slide out as an assembly. Looking at the side of the printed circuit board that has the parts, very close to the center of the board, and located close to a ‘row of 6 resistors which have various colored stripes on them’ will be the diode described. There is another diode, larger, on the board, but it is NOT near the 6 resistor row. One Wherle voltage regulator I looked at had this diode as a glass package, and it had marking on it of ZPD7V5. Interpreting that for you, it is a zener diode (a voltage reference device) rated at 7.5 volts.

What you are going to do is either replace that part with a 8.2 volt part, or ADD a part that will raise its effective value to 8.2 (approximately) (nerdy types need NOT point out the small voltage error here, nor the tolerances of the diodes, etc.)

NOTICE that the diode is mounted vertically (as are the row of 6 resistors). It is VERY important that you NOTICE that ONE end of that diode has a LINE on it. Be SURE to mark the board with a pencil: so that the lead of the diode end that has that line, is so marked, by a line on the board. It is possible for EITHER end of the diode to be the longer leaded end. Another way of thinking about this: The LINE end of the existing diode will be towards the END of the board that has the heat sink and larger squarish looking transistor that has three leads coming out of it and mounted to the heat sink.

You now have to install your new diode. IF..IF…you have a common diode, NOT one of the #2 section, above, zener diodes, then follow THIS method:

1. De-solder the LINE end, ONLY,  of the existing diode. Shorten the lead of the added common silicone diode at ITS line end, and carefully solder the line ends of BOTH diodes together, quickly.

2. Solder the NON-line end of the added diode, to the place on the board you previously de-soldered the line end of the existing diode.

3. Use fairly short leads for this…to avoid stress from vibration.

IF you purchased a zener diode: Remove the old diode, replace it with the new 8.2 volt diode, line end same as original. Reassemble the unit, after making sure your work was neat, nothing touches anything else, or can short circuit, and soldering was neat, and you did NOT short the adjacent soldering pads on the soldering side of the board. If you want to clean off any excess rosin, common alcohol on a Q-tip or other swab will do nicely.

Testing: Rather than give a table of values for temperatures, I will assume that the engine is started at perhaps 50-100°F ENGINE TEMPERATURE, and that you do NOT allow the engine to develop such heat that it rises and warms the regulator. Optimum would be 70°F.

With a digital voltmeter meter across the battery, start the engine, and within two minutes slowly raise the rpm, watching the battery voltage rise as it recharges, and noting where the voltage ends up at at high rpm. If the voltage ends up around 14-14.3, that is fine. If it does not charge, or the voltage is well over 14.5, you did something wrong. 14.5 is the limit you can use. If you use a slosh battery, 14.5 setting on longer trips will cause the battery to use water faster, although it probably won’t injure the battery. That voltage is also near the limit for the Panasonic/WestCo batteries. Note that as the REGULATOR temperature rises, the voltage regulator will automatically adjust DOWNward, and the reverse is true for colder. If you use the fairing voltmeter for the testing, understand that in a good system with a good connections….the fairing voltmeter will normally read about .3 volt LESS than the actual battery voltage, so a fairing meter reading of about 13.7-14 is just fine.  Don’t depend on the fairing voltmeter for precision checking the VR; DO use a digital multimeter with the test leads connected AT the battery.

For a vast number of technical articles, and an expanded version of the above information:

https://bmwmotorcycletech.info/index.html
Snowbum (#1843)
Copyright, June 2021, R. Fleischer