Engine Electrical System – Airheads Beemer Club

Why Diode Boards Fail

At 51,000 miles, I noticed the alternator light on my ‘92 ‘GS glowing at idle. Time for a new diode
board (again), I thought. When I removed the board, I found that both of the upper rubber mounting posts were broken–the stud end bolted to the board had come unbonded from the rubber. It was clear from the discoloration of the metal that those studs had gotten very hot. This occurrence is not uncommon as many Airhead owners know, but others who have written on the subject seem not to have noticed what I have.
The problem has been discussed extensively, but my observation suggests that solid mounts and a
bunch of ground wires with soldered lugs are not really going to fix it.

In the world of electricity, current times resistance equals power. If the resistance is in a bad
connection, power is wasted there as heat. Power from the alternator is supposed to replace the
power supplied to the bike and therefore keep the battery charged. But the bad connection I refer
to is at the point where the diode board is bolted to the studs.The voltage drop there causes the
the regulator to flog the alternator more to maintain the desired voltage at the battery. And that makes even more heat at the bad connection.

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Wherle Electronic Voltage Regulator

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 other versions could have been made.

This modification will increase the charge voltage to better charge the battery and give longer life. This will be useful in instances where the voltage at the battery terminals is found to be at or below 13.9, at approximately 70°F at the regulator. This modification will ensure better battery charging, and longer battery life, especially for in-town commuters. This modification essentially turns your voltage regulator into the pricier BMW-sold high voltage Authorities (Police) regulator, 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.

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Testing Voltage Regulators

If you have a charging problem, you may want to find out if the VR (Voltage Regulator) is the problem. A faulty VR can cause various problems, the most common are no charging or too high a voltage during charging. NOTE that the stock factory VR internal setting is such that the battery terminal voltage is typically 13.8 volts during cruising (the higher voltage Authorities…Police…regulators are about 14.2). The 13.8v is a reasonable compromise for water use on flooded fillable batteries, but really too low for best battery life, particularly on AGM/VRLA batteries.

There are TWO very easy ways to test a voltage regulator that do NOT involve the technical methods in the rest of this article. These two methods are:

1. Substituting a known good voltage regulator (any 1970+ BMW airhead regulator, or any three terminal automotive regulator that fits the airhead plug). If whatever the problem was now disappears, then the VR was faulty.

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Starter Removal Instructions

BMW starters are not too difficult to remove and install, but some care in fitting, and some hints are helpful

Disconnect any and all battery negative (-) cables. Make sure they are well out of the way and won’t make contact with the battery (-) terminal.
Remove fuel tank.
Remove top engine cover.
Remove front engine cover.
Remove all feed wires to the starter solenoid unit.
Remove the two (2) main starter mounting bolts. You may need a 1/4″ drive to get a skinny enough socket onto the nuts/bolts. Some starters do not fit all that well, and interchanging Valeo and Bosch or Denso starters CAN BE particular problems. Very little work is needed to make for a good safe installation, however.
An article that covers some overhaul work, but near the end has photos of what the fit problems are, and how to fix it, is here:
http://bmwmotorcycletech.info/boschvaleostarter.htm The heads are 13mm and sometimes difficult to get a box end or ratchet wrench on. Some replace these bolts with Allen head bolts because it is easier to insert an Allen wrench between the engine case and the starter to hold the head of the bolt.

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Starting & Starter Problems

Functioning,starter circuits & associated parts:

The starter motor is a powerful electrically operated motor. The starter motor may have to provide upwards of 1/2 to 1 horsepower. In cold weather more power from the starter motor will required. The starter motors in our Airheads, like all vehicle starters, are not very efficient motors. The Valeo is more efficient than the Bosch. Quite a lot of amperes might be needed under some rather common conditions. Starter motors are, at best, 60% efficient due to magnetic field losses, friction losses, etc.

745.7 watts is DEFINED as ONE horsepower. If the system is a nominal 12 volts during cranking (typical, with good battery, wires, and connections, voltage as read at the starter terminals), then 745.7 divided by 12 equals 62 amperes. Due to the efficiency losses, & the need for many more amperes to BEGIN engine rotation, it is NOT uncommon to require twice that number of amperes. The power rating of the most powerful of the three Bosch units used on the Airheads was 0.7 KW. 0.7Kw is 700 watts; divided by 12 volts is 58 amperes. Notice that the starter is also rated at 320 amperes. That is the supposed maximum drain under a severe load. That is equivalent to 3,840 watts….and is equivalent to a bit over FIVE horsepower. The battery, cold day, thick oil, so-so starter, ETC., may need to provide a LOT of power! If the battery is marginal, it may not have enough power to cause the starter motor to rotate the engine properly for starting ….possibly not rotate it at all.

There must be a means of switching on and off the large amount of electrical current to the starter. A heavy-duty solenoid-operated switch is physically located on the starter motor itself. It is a fairly large round cylinder with two electric terminals of the bolt/threads type and one small spade type which supplies a modest amount of electricity to the solenoid from the starter relay, located along the backbone of the motorcycle.

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Spark Plugs – Reading Them

Numerous problems can occur if your engine’s spark plugs are not correct or not working properly; or, there are problems such as tight valve clearances, improper carburetor mixture adjustments or jetting, needle settings, worn engine items creating excessive oil burning, ETC. Spark plugs are ‘read’ by looking at the combustion chamber end of the plug. You can look at the color and deposits from combustion and determine a fair amount…with some practice. It is not just the condition of the normally near white or light tan or orange color of the central electrode insulator, but the condition of the outer and central electrode and the color and look to the metal on the flat end portion of the spark plug, which is about 1/8th inch wide. Once you gain experience [it doesn’t hurt to ask SEVERAL supposedly experienced mechanics to confirm your analysis], you will find that much can be learned from a look-see. Spark plugs are looked at to determine if they are worn to needing replacement; to see if the engine seems to be operating correctly; and to get some idea of specific problems. Reading spark plugs is somewhat of an art, requiring experience. Experience is especially needed with modern unleaded fuels, most of which contains alcohol and other ingredients that makes reading spark plugs even more difficult, as the ‘old colors’ do not always apply. SOME things never change, however, so, read on. You should have a working knowledge of spark plug heat ranges, proper torque setttings, whether or not to use an antiseize compound on the spark plug threads; and what the proper spark plug caps are for YOUR motorcycle.

An in-depth article on spark plugs, problems with newer Bosch Spark Plugs; and chart of all three Bosch spark plug numbering systems; chart of Bosch versus NGK; and a listing of NGK spark plug caps, and much more, such as use of antiseize compound, etc., is here:

http://bmwmotorcycletech.info/sparkplugs.htm

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Spark Plug Caps

On the early Airhead points type models, spark plug caps contained a resistor of 1000 or 1200 ohms. Using a resistor inside the caps is a better method than using highly variable (between manufacturer’s especially) “resistor” type spark plugs, in which some of those resistors were actually coils. The resistance caps reduced Radio Frequency Interference, and helped form the proper type of electrical spark itself. Later Airhead points models and all electronic ignition models used 5000 ohm caps for even lower RFI, with added safety benefit for the 1981+ electronics ignition…described later here.

The spark plug cap resistance has more than one purpose:
It reduces spark plug tip and ground electrode erosion, and therefore greatly reduces any gap change over the life of the spark plug, and does this by reducing the electrical current flow. It does NOT reduce the applied voltage. Reduces some types of radio interference. Works in a complicated electrical way with the coil to produce a quality spark at the spark plug for good fuel mixture igniting. There are some other beneficial and rather complicated effects as well. Note that if the resistance is too large a value, then the EFFECTIVE spark ENERGY (combined voltage, current, and with SOME time effect) will be lessened too much, and, this means that using resistor spark plugs WITH resistor caps is a BAD IDEA.

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Servicing a Valeo Starter

The chances are very good that your starter problem is relatively cheap and easy to fix. Perhaps your starter has the same symptoms as stated below. If not, you can still check things out with the part marked “Testing”, and once a problem is isolated, the remaining portion covers disassembly and re-assembly of the Valeo.

Reviewing the symptoms: The starter operates correctly most of the time. When the starter does operate, the engine is cranked over at a reasonable speed and it does not appear that the starter motor is struggling as if the battery is nearly dead. When it does fail, you still hear a clicking noise, every time you press the starter switch. This clicking noise is a rather strong sound from under the starter cavity cover (not to be confused with a small clicking noise which is the starter relay). Most of the time, when it is failing, several successive starting attempts will get the starter motor to eventually operate.

If the above wordy sequence is true, your problem most likely has to do with the starter solenoid (rides piggyback on the starter motor).

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Making the Bosch (and Wehrle) Metal Can Electronic Voltage Regulators Adjustable.

I wrote this article because a few people asked for it. You can purchase an adjustable voltage regulator quite reasonably from http://www.RockyPointCycle.com or http://euromotoelectrics.com. But, the stock metal can regulators are not difficult to modify, the cost is very low, and you get to keep the original stock appearance …if you want-to. The metal-can VR’s are very reliable ….and are relatively easy to repair. They can even be modified to work with very high powered alternators for the Airheads (by changing the power transistor inside). These metal can regulators, no matter the name on them, MIGHT have been made by Bosch or Wehrle.

The exact same procedure is involved….(and the same hole drilling and adding a grommet, if you want to). Here is a photo of the Wehrle METAL CAN Electronic Regulator, with the potentiometer, with factory sealing paint. This is a how-to article, the purpose is to make your Bosch or Wehrle ELECTRONIC metal can unit easily adjustable. This article is NOT for the similar (but taller) metal can mechanical Bosch regulator…see http://bmwmotorcycletech.info/boschmechreg.htm.

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Ignition Systems; historical & modern

The following article is a SIMPLIFIED explanation of ignition systems, although lengthy. There is CONSIDERABLY MORE technical information, including repairs, etc., in the author’s website, particularly here:
http://bmwmotorcycletech.info/ignition.htm

A common type of ignition in very early cars consisted, typically, of wooden boxes, with a vibrating electrical contact which sent battery energy into a coil of a modest number of turns of wire wound on an iron core located inside the box. There was another coil of wire wound on that iron core, with thousands of turns, and these turns ‘transformed’ the lower voltage of the ‘primary winding’, fed by the battery, to a few thousand volts, and this was applied to the spark plugs. In most early engines the spark was applied continuously. Somewhat later a rotating switch was used. It had a contact or close by contact, one for each cylinder, and this was called a ‘distributor’. The only problem with the vibrating contact system is that the spark output can not be set to ‘fire’ the spark plug at a very specific piston position, although the distributor method helped SLIGHTLY. With the very low compression ratio and very low power output of these engines, together with the very long stroke and large diameter pistons, these ignition methods worked OK. As engines improved in various areas, the need for better and more precise ignition came about.

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