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BMW Airhead Motorcycles:

Increasing the output voltage of the Wehrle PLASTIC CASE voltage regulator

copyright 2020, R. Fleischer

This article applies only to the PLASTIC box Wehrle regulator; and likely not to all versions of them.  For the METAL case Wehrle, see the article on Bosch and Wehrle regulators.

Read this article entirely before deciding to proceed!   This article is primarily for tinkerers and nerdy types; as one CAN purchase, quite reasonably, a good adjustable voltage regulator from various places such as: and

This article was originally written while working with a Wehrle RED top regulator, model 55-990-001.   I have seen other Wehrle versions that had a rubbery potting compound, and these were slightly different.  One was a RED topped model 55-990-002.   There was a fairly different looking BLACK topped model I looked at, fairly different layout, also potted, and it was model 55-990-006.  THAT particular unit is possibly one of the Authorities (Police) types, as its voltage setting was about 14.3, when the others are usually closer to 13.8.   I believe there are some even later versions of these various Wehrle regulators.

Because of the potting, a nuisance to remove, and the different layout ....and particularly since almost any regulator with the same three terminal layout (as from auto-parts store or junk yard or?) will "sort-of work", and and sell new adjustable ones at quite reasonable prices may not be worth it to you to modifying these Wehrle regulators for higher voltage output, since in many instances the cost for a replacement regulator (and even some higher voltage fixed setting ones) MIGHT be more attractive than spending your labor on this project.  The PART you need to modify the plastic case regulator is CHEAP.   Some are interested in learning, having some fun workbench time, etc.   YOUR choice.     I had a bit of fun developing this article.


You will need to have the same physical layout VR as I used, or, enough electronics knowledge to find the diode involved.  You will need a piece of round 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 might need some very small drills.   You WILL need an accurate digital voltmeter.  If your battery in the bike is not fully charged, put it on a charger so it IS fully charged by the time you are done with the modification.

You will need to remove the gas tank for access to the voltage regulator, and you must remove the harness plug from the regulator and remove the two allen head mounting screws of the VR.  It is not necessary to disconnect the battery.  Once you have the regulator on your workbench, pry off the red top carefully.

Special Part needed:
ONE part from #1 below or #2 below, but you need only ONE part!   You should read this entire procedure through and decide if you are going to use method #1, or method #2.  You might have to visit your local RadioShack or other electronics parts store to make the final decision.

#1.  Any common silicon diode of the power variety, rated at one ampere or more. Voltage rating is not important.  Probably available at Radio Shack in various types.

Example: Radio Shack #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:  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 get one that is a bit too high in ACTUAL voltage rating, which MIGHT boost your regulated voltage to a bit too high of a value for your battery. What you need is most any common silicon diode of the type called a "Zener Diode".  You 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. That will require you to enlarge the existing holes a teensy amount.

I suggest 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. The part you purchase may well look SLIGHTLY different ...perhaps larger, or marked a bit differently, or colored solid black, or whatever. It should have a LINE marked nearer to one end. Pay attention to the physical orientation of that diode line in your VR, as the line is there to indicate the polarity of one end of that diode.

Doing the modification:

Pry off the red plastic cap 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. Look carefully at the side of the printed circuit board that has the parts, and notice that 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 we will describe. There is another diode on the board, likely physically larger, but it is NOT near the 6 row. The Wehrle 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 above zener diode with a 8.2 volt part (described as #2 above), or add a part described as #1 above. Either method will raise the effective value of the original to ~8.2 volts.  Nerdy types need NOT point out to me 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 note WHERE that line end connects to!!!    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. Being vertically mounted, it is possible that during manufacturing,  EITHER end of the diode was the longer leaded end be SURE you mark that board!   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.

Follow THIS method if you are using method #1, ADDING a diode:
1. De-solder the LINE end of the existing diode. MARK THE BOARD! Shorten the lead of the new to be added common silicone diode at ITS line end, and carefully solder the line-marked ends of BOTH diodes together.

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 moderately short leads for this avoid stress from road vibration.  Do NOT break the leads/glass!!  Sharp bends right at the diode will crack it!

Follow THIS method if you purchased a zener diode, and are REPLACING the original part:
Remove the old diode, replace it with the new 8.2 volt diode.  Be SURE to have the LINE of the diode connected at the SAME place as the original!

Reassemble the unit, after making sure your work was neat, nothing touches anything else, or can short circuit, 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, but methanol works better. Snap on the top, and install the regulator.


Rather than give a table of values for temperatures, which are in the article, I will assume that the engine is at ambient temperature, perhaps 50-100F; and, that you do NOT allow engine heat from running the engine to rise up and warm the regulator hardly at all.  Optimum would be ~70F at the VOLTAGE REGULATOR case. I also assume that your electrical system is in good condition, with good rotor brushes, solid tight shiny connections, good grounds for the diode board.  Failure to have good connections will defeat your work.  If you have rubber diode board mounts, I strongly suggest you change them to metal types.  Refer to and be sure the battery is fully charged and has good connections/wiring.

With a digital voltmeter meter connected directly to the battery, start the engine, and within a minute, begin to slowly raise the rpm, watching the battery voltage rise as it recharges, and noting where the voltage ends up at; all of which should not take long. If the voltage ends up around 13.9-14.4, that is OK. If it does not charge, or the voltage is over 14.7, you did something wrong.

14.7 is my recommended safe maximum limit you should use with any lead-acid, flooded or sealed battery, although more properly it is actually 14.9 with the Panasonic/WestCo or Gel or AGM types, even the Odyssey. If you use a slosh (flooded) battery, 14.5 or somewhat higher, even up to perhaps 14.7 setting on longer trips, will cause a flooded type battery to use water faster, although it probably won't injure the battery.  Note that as the REGULATOR temperature rises, the voltage regulator should automatically adjust DOWNward (a few tenths of a volt), and the reverse is true for colder. If you use the fairing voltmeter (best NOT TO) for monitoring your system while riding, understand that in a good system with good connections ...the fairing voltmeter will normally read about .3 volt LESS than the actual battery terminal voltage.

09/16/2003:  Minor clarifications, and information on the other Wehrle regulators at top of article.
12/10/2003:  Add PLASTIC to type of regulator for this page and the associated note.
01/16/2010:  Clarifications, cleanup.
04/13/2010:  Emphasis, plus small, relatively minor changes, especially in the last paragraph.
12/07/2010:  Minor stuff, nothing of importance.
10/15/2012:  Add QR code, add language button, update Google Ad-Sense code.
Sometime in 2013, remove language button, as it caused problems with some browsers.
04/27/2016:  Update metacode, layout, add script for editing/checking.
01/13/2017:  Metacode, scripts, slight layout and fonts, commentary improvements.
06/01/2018:  Cleanup.

Copyright 2020, R. Fleischer

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Last edit/check: Monday, December 14, 2020