Understanding the Ignition System

by Richard Atwell
(c) Copyright 2005-2011


The stock ignition system...well, it's in need of improvement. It's not like any other Bosch equipped 70s vehicle is any better nor would I recommend you replace the Bosch/Beru components 1:1 with another brand but there are some improvements you can make to restore some of the power waiting to be released in that 2L bus engine. That's a big torquey engine compared to your average Type 1 sewing machine motor although it has to lug around a 3200 lbs chassis.

When my bus is in tune it's a real pleasure to drive. When it's not you quickly notice it because the power you have is a precious commodity. In other words there is just barely enough. The smaller displacement Type 4 engines are only suitable for 411s and 914s. Even the purists can't help themselves from including a little cc boost during a rebuild.

The stock Bosch L-Jetronic fuel injection is a dream compared to a set of carbs. There's nothing to adjust except for the idle mixture and it stays in tune. However it is just as susceptible to weak ignition and the FI is always the first to blame but it's often an ignition problem. Unless we are talking about the 79 CA model with electronic ignition, the parts are the same from 72-78.

Here's what I've learned about the system and what I've done to make the best of it. Sometimes all you need is a new part you didn't realize you needed. Sometimes there's a better alternative.

Note: VW used both Bosch and Beru for parts. Beru is considered a 2nd tier vendor although the quality is still very good. Since Bosch is #1 and easiest to find I generally recommend Bosch. Bremi is another OEM but I have not seem parts for our buses from them.

How it works:

The Bosch ignition system in your bus would be easily recognizable if an automotive engineer time travelled from 1910 to the present and looked in your engine compartment.

The one nice aspect about the stock system is that the parts are dirt cheap. If you haven't priced ignition parts for new cars (especially other 4-cyl engines), I can guarantee that you that you won't be able to replace the ENTIRE system (minus distributor itself) with new OEM parts for the $75 the VW system costs. Even pricing aftermarket parts for a modern car wouldn't be as inexpensive.

If you happen to need a new Bosch distributor it's only $129 and it includes points, cap, rotor and condenser! Now that's a bargain compared to almost any other make of vehicle which is largely due to the fact that VW's were low cost, rarely changed (for better or worse) and were made in the millions over decades.

What you get is largely what you pay for but that's a problem with the design of the system, not the quality of the parts.

Unless you are broken down on a road trip, AVOID using stock style ignition parts from a company other than Bosch or Beru, the two OEMs that VW used to produce millions of reliable quality made parts. Those parts work great, fit the best and are inexpensive. You can't beat their value.

The Bosch ignition system is based on a design by Charles F. Kettering and is sometimes simply referred to as points. His system is compact, multi-tasking and is still the foundation for factory ignition systems installed in most cars. At first that might seem shocking but remember the gasoline otto engine is also that old.

Every Kettering based ignition system is made up of a coil, distributor and spark plugs/wires. I'll explain the function of each of these. Open this diagram in another window and refer to it as you read along.

Boch diagram

1. The Coil:

The coil is the black or blue canister that bolts to the fan shroud. Inside the can are two sets of wiring coiled around an iron core suspended in epoxy (the earlier cans were oil filled). The filler acts as a di-electric that also serves to dissipate the heat that is produced as the system operates.

On top are 3 terminals which Bosch labels Terminals 1, 4 and 15.

It's an efficiency of design that a Terminal 1 is shared (otherwise two "ground" terminals would be required).

12v power is applied to Terminal 15 and is grounded at Terminal 1 by the action of the moving components inside the distributor. The current that flows through the coil generates a magnetic field because it's an electrical inductor. When power is removed from the coil (disconnect Terminal 1 from ground) the field collapses and this induces a voltage in the secondary winding which generates a current at Terminal 4, called the high tension lead.

The coil is both an energy storage device and a step-up transformer which converts a small voltage (12v) and high current (4A) into a very high voltage (6kV) and a very small current (8mA) to ignite the air/fuel mixture.

2. The Distributor:

The high voltage from the coil leaves Terminal 4 and enters the top of the distributor:

3. Spark plugs and wires:

Each wire connects the coil to each cylinder through the rotor under the distributor cap to deliver the electrical energy to the spark plugs. Each cylinder wire is made from stranded copper or alloy wire fitted to a connector that embeds a 1.0 or 1.4 kohm resistor inside depending on the length of the connector. The resistor damps the voltage of the ignition pulse when the spark is cutoff and is necessary to limit EMI/RFI.

The coil wire is made from a carbon conductor and has a much higher resistance (10-18kohm) than the other wires to limit EMI/RFI. To be effective these resistors must be physical near to the gaps which are generating the EM pulses. It is also a current limiting device. If the other wires don't rust their connectors from age, the coil wire usually burns out and fails first.

Each spark plug is threaded into the cylinder head and protrudes slightly into the combustion chamber. The stock plugs have a copper conductor and come in one or two suitable heat ranges. Some plugs protrude more into the combustion chamber than others but the threads of the plug never do. The energy produced by the ignition system manifests itself as a high voltage spark that jumps from the center electrode to ground by ionizing the air in between. When the spark occurs, the current that flows ignites the air fuel mixture.

Problems with points:

So now that we know what parts are in the system, let's go over their flaws and understanding how the various parts work to see what we can benefit from replacing.

The biggest weakness of the system are the points. They are not designed to last the 15k mile service interval as they are often in terrible shape by then.


  1. The rubbing block of the points wears on the cam. The cam is supposed be to lubricated with a heavy grease (Bosch 5 700 002 005) to reduce wear but many owners who perform their own maintenance fail to do this. Even with greasing, the block wears down and as it does the contacts grow closer together. When this happens the time that the points are closed is extended increasing the dwell and it has the effect of slowly retarding the timing.

    You may have noticed two types of rubbing blocks: white and brown. The white one is made from polyamide (nylon) and the brown is made from resitex. Polyamide is the newer material but apparently a bad batch of them sent people hunting for the old style a couple of years ago. IIRC, the block broke off too easily. Some VW owners recommend the 01 030 points used in some vintage Porsches because they have the brown rubbing block as well as a stronger spring than the 01 011 set. I personally don't think the difference is enough to matter when fit in the bus.

  2. Every time the current that passes through the points is interrupted a little bit of the metal from one contact is transferred to the other. This has the cumulative effect of creating a pit on one side and tit on the other over time. The contacts are radiused to begin with so that the closest point to each other is centered and as the shape of the points change so does the distance and the location from which the spark plugs. This changes the gap and the dwell.

    The contacts are tungsten plated for added durability. When the tungsten wears out the points pit more quickly. If you file the points flat to resurface them, you will remove the tungsten coating and burn out the points faster.

  3. When the condenser fails the points pit quickly because of the high voltage that is not supposed to flow through them when the spark occurs in the secondary. The result is a weak spark or even lack of spark.



Eliminate the points altogether. For $60 from CIP1.com you can purchase an electronic ignition that fits inside the distributor. It's completely stealth and you'll never have to adjust or replace the points ever again. Limiting the maintenance of the distributor to oiling the wick is a godsend because the distributor is in an awkward position to work on.

The stock distributor requires the Pertronix Ignitor 1847V (sometimes 1847 but never 1847A which is for the 009). Installation takes about 30 minutes (less if the distributor is removed from the engine) and you simply remove the points AND condenser from the distributor, fit the Ignitor inside and then connect the two wires to the coil. Do not reverse the wires otherwise you will fry it before you've even tried it out. Red goes to 12V (Terminal 15) and black goes to the other post (Terminal 1). Other than that, the unit is absolutely dependable. Because this style of ignition has special circuitry to switch the coil on/off unlike the points which have to conduct several amps through them, it's a long lasting upgrade.


The first thing you notice after performing this upgrade is that the bus idles better, accelerates more smoothly and revs higher. Pertronix claims your gas mileage will increase and it does but don't expect miraculous. Expect 2-3 mpg increase on average.

Distributor wear:


If your engine has been fit with the 009 (Bosch 0 231 178 009) distributor instead of the stock distributor you are missing out. The problem with the 009 is a basic one: it ruins the drivability of your bus even though it's working perfectly:

  1. Apart from never being fit to any production VW, the total advance is too little under most conditions for a VW engine (only 22-24 degrees). Not only that but the advance comes in too fast. To compensate for this, the timing at idle has to be set higher than normal.

  2. There is no vacuum can! Because partial throttle produces lean mixtures which are harder to burn, the spark occurs too late for maximum torque during most of the travel of the pedal!

The end results of this behavior is that there is a flat spot just off idle and inadequate advance as you accelerate. Some people think the surge of power that comes later is cool but all they are really achieving is abrupt acceleration, a larger gasoline bill and more emissions. Maybe it works on a beetle in a tailwind with no passengers but in a bus? No way.

new Bosch SVDA

Having bashed the 009 suitably, the original distributor won't last forever. If it's worn in any of these ways it needs replacement:


Unless you've also switched to a set of carbs that cannot power the vacuum can on a stock vacuum (SVDA) distributor you should switch back to the stock system. Luckily a Mexican Bosch vacuum distributor (043-905-205ZB) is now available (meant for beetle but perfect for 75-79 FI bus).

Note: aside from replacing worn out parts and setting the points gap, the top of the distributor shaft has a piece of felt that must be oiled regularly to prevent sticking of the advance mechanism. Overtime the inside of the distributor will become gummed up with dirt and old hardened grease and will require disassembly, cleaning and lubrication.

Which Coil?

The engine originally came with a Bosch coil in a black case. This part is no longer available and the replacement is a Bosch blue coil. Often just called the blue coil, it's thought of as being a "hotter coil" probably because it's been marketed under the words super and screamer in the past. Hot is equated with hotter spark but is it true? I've always been skeptical of this claim so I decided to put it under scrutiny.

Bosch blue coil

The energy stored in the coil can be mathematically represented using the energy storage formula for an inductor:

Energy = 1/2 L I^2

...or read using the units of measure, Joules = Henries * Amps * Amps. What's important to notice about that formula is that the energy level increases as the square of the current compared to a linear increase by the inductance. In order words boosting the current will do more good as long as the coil is designed for it. This is why low resistance coils exist (they are designed to pass more current and dissipate the heat generated from it).

The windings of the stock coils are connected to the terminals in series with a ballast resistor measuring only a few ohms. How does the Blue coil compare? It has a higher primary resistance (lower current) than the black coil but is that due to the windings or the ballast resistor? The secondary windings have a lower resistance so they are clearly different. You'd have to ruin both by taking them apart to figure out how much resistance is attributed to the ballast. Even without a ballast you cannot measure the resistance of the coil and infer from it the number of primary and secondary windings which will determine how the spark will perform (the step-up transformer function).

rotors under oscilloscope

We could measure this current flowing into the primary using an ammeter (the higher the current level the more energy for the coil) but comparing the inductance between the two coils is difficult without specialized equipment so we can't draw any conclusions even though the current level is more important according to the equation. Rather than try to measure the minuscule current level in the secondary, we can measure the voltage level in the secondary to witness the effect of switching coils. Because the energy level is conserved across the windings (minus losses) we should see higher voltages in the secondary representing lower currents (weaker sparks) in the primary. We can also assume the the voltage required to jump the spark is fixed, so once we've exceeded the threshold voltage the current flowing only depends on what's available all other components being equal.

The exact amount of current will be affected by the number of windings in the secondary and the wiring between the coil and the spark plug but we don't care about resistance or counting turns of wiring, just the results witnessed at the spark plug. Here's the trace from my oscilloscope:

As can be seen, the peak voltages are the same when switching coils except in the case where the rotor resistor is 1 kohm (this seems to destabilize the circuit anyway). The portion we are interested in is the burn voltage and duration (the raised part of the signal after the spike marked by the solid horizontal line until the oscillation begins). In the graphic, the Blue coil is on the left and the Black coil on the right. These traces are averages over time which is why the blue coils have an oscillation during the burn voltage and why there is no harmonic damping witnessed as the spark burns out for either coil.

The duration is about 1.2ms at 6kV for the Blue coil and the burn voltage is about 10kV for the Black coil. Since the burn voltage level of the Blue coil is lower we can assume the current level is higher and we're producing a better spark using the Blue coil. Current is the essence of spark although total power (watts) is important too. The burn duration is also similar so overall we are ahead using the Blue coil.


The differences are slight: the Black and Blue coils are essentially identical and the Blue one produces a stronger spark but you won't see any improvement in driving or at the gas pump from the Blue coil because the spark from the Black coil is already enough. Of course the Black coil is NLA and you have no choice when you need to buy a new one but I just wanted to debunk some of the hot coil legend and save you $25 on the unnecessary purchase of a Blue coil if your Black coil is still working. Paint it blue and don't tell your friends it's really black.

What about the Compufire/Pertronix chrome coils (the "hot coils" with a primary that measures 3 ohms and it's secondary 9 kohm)?. They spark no better than the Blue coil. Save your money and stick with stock.

Which Spark Plugs?

spark plugs

Which plugs work best? They vary by heat range, length (19mm reach), threads (M14x1.25), ground electrodes and internal construction.

The stock plug is either a Bosch W7CC or W8CC plug (aka Bosch Super with copper electrode). The former is recommended for highway usage, hot weather and FI while the latter is suitable for all year round plug use and idling in heavy traffic. Neither of these plugs have a protruded nose or multiple electrodes. You might be tempted to purchase platinum plugs or buy a plug with multiple electrodes but you'd be throwing your money away.

Platinum plugs last 120k miles in new model vehicles. They are designed and for a modern high compression combustion chamber which makes them unsuitable for use in a VW aircooled engine. They are also meant for a more modern electronic ignition system. I've never come across a bus owner who fit platinum plugs and saw any improvement. Given their high cost, the money is better spent elsewhere.

Multiple electrodes are a marketing gimmick. The spark can only jump from the core electrode to one ground electrode. Since the center electrode wears much quicker than the ground electrodes there is no benefit and the other electrodes simple serve to block the flame produced by the ignition event. No manufacturer that I'm aware of fits multiple electrode plugs in their cars. So you may ask why does a reputable company like Bosch do it? Marketing is a powerful tool and they must feel that they need to compete with Champion, Autolite and the others.

Finding a Bosch plug that was Made in Germany can be difficult. I've never had a problem when shopping at smaller shops but the large chains carry plugs from all parts of the world. I recently switched to NGK B6ES plugs on the advice of Type2.com list members. This plug is still made in Japan or assembled in the US from Japanese parts. I cannot yet say whether or not it's a longer lasting plug than Bosch (which last about 13k miles before going downhill) but the plugs seems to burn cleaner.

The biggest benefit of using a NGK plug is that the body is slightly narrower allowing easier fitment of a cylinder head temperature (CHT) sender under the spark plug. If you don't have a CHT gauge in your bus you maybe overheating it without knowing it because there is no radiator to boil over. Under ideal operating conditions, the engine is designed to stay cool but the gauge is your only warning that something is wrong before the engine seizes or breaks.

Stick with any of the plugs mentioned above. Make sure the spark plug gap is set to 0.7mm.

What are resistor plugs? These are spark plugs with a 5 kohm resistor inside. They are used on ignition systems that lack the RFI supressing resistors elsewhere in the wiring. Generally, you don't need them but if you were to use wiring that lacked sufficient suppression you could use these type of plugs. One exception are CDI ignition. They don't repsond well to extra resistances just as the rotor doesn't (see below).

Which Spark Plug Wires?

spark plug wires

You can't beat the stock plug wires for cost or quality. They are 7mm silicone jacketed wires (at least from Bosch). If the jacket is a hard rubber instead of silicone then you've got a really old set you should replace.

When you start upgrading your ignition system, it's tempting to upgrade the spark plug wires. Like spark plugs with multiple electrodes and so called high voltage coils, you don't need to upgrade the spark plug wires with a stock ignition system because you cannot take advantage of them: the voltage required to jump the plug gap is all that will be generated by the ignition system and no more. As the gap increases from wear, the voltage levels will increase but it will still be within the capability of the stock components. Sorting out the marketing messages from the science behind wiring design is so defeating that you simply have to experiment to get any answers. Most often, replacing worn out part accounts for 95% of the improvement people see replacing their ignition system components (the placebo effect).

If you try an aftermarket set of wires you may find they either lack the engine tin seals or lack the correct size spark plug connectors or both. I can find Bosch 09171 sets online for $21 so it makes no sense to buy anything else. These sets are now made in Mexico but I find the quality the equal of the German made sets. Stick with Bosch whenever possible because they are the correct length and fit so well. They also last a long time. The weakness of the set is the resistor in the spark plugs connector which eventually burns out and presents such a high resistance to the coil, the result is a weak spark.

Bosch 7mm wires have more than enough insulation to handle the higher voltages from upgraded ignition system. If you are tempted to use an 8mm or 10mm jacketed wire keep in mind that these wider jackets are too wide to fit the Bosch spark plug connectors. If the aftermarket wire set has its own connectors but doesn't have a long enough connector you will have a harder time removing the plugs wires. Even CDI ignition system (from marketing departments who rate the output at 45kV) do not produce those kind of voltages under normal operating conditions that require wires with larger jackets.

Bosch has changed their box. While it says Opti-layer mag core wiring, there is a disclaimer that only Japanese and Domestic models apply. European wire sets, like VW are stranded copper conductor with resistive connectors.

Which Rotor?

If you have a Bosch distributor you should be using a Bosch or Beru rotor. They are the best made parts you can buy that are not only inexpensive but well made and long lasting. Only buy a rotor from the local auto parts chain in an emergency. Better yet, carry a Bosch/Beru spare.

If you have a stock ignition system your rotor is probably Bosch 04033 (or perhaps the 04016 rev limiting version). These rotors have an embedded 5 kohm resistor. The resistor serves several purposes:

  • Limits current to reduce erosion of the spark plug electrode
  • Reduce radio frequency interference (RFI)
  • Works with coil to increase the burn duration of the spark
  • Limit reverse current to protect the ignition switch

If you have a CA electronic ignition system from a 79 bus the rotor will contain a 1 kohm resistor. It uses a smaller resistor because the special coil used has different primary and secondary resistances to match.

If you normally required a 5 kohm resistor and switch to a lower resistance rotor you may notice a slight difference in engine firing. Compare the spark produced by the 5 kohm resistor in the first trace to the traces for a 1 kohm resistor and one that has had the resistor removed (0 kohm). The 5 kohm trace has a very sharp fall from the peak voltage required to fire the spark plug, to the burn voltage level where the bulk of the current flows. This results in the longest burning spark. As the resistance of the rotor decreases, the length of time it takes to fall from peak voltage increases and the shorter the spark. Don't try to outsmart VW/Bosch: they knew what they are doing when they specified which rotor to use.

Is it ok to substitute the 1 kohm version?. I'm not sure. It may damage your Pertronix ignition because the resistor also serves to limit that current that flows in reverse when the coil collapses (maybe they've put an internal diode inside for protection). It doesn't seem to be a problem for the factory CA ignition. Did you also notice the end spike in the 1 kohm trace? That peak may be heard on your radio in which case you'll have to switch back as well.

5k ohm 1k ohm zero ohm

Capacitive Discharge Ignition:

CDI is probably the most important upgrade you can make even if your stock system is performing at 100%.

Unlike conventional systems that store their energy in the coil, CDI systems store their energy in a capacitor and only use the coil to step-up the voltage. The advantage of this is the incredibly quick charge time of the capacitor. On its own this system has drawbacks but when combined with circuitry that allows multiple sparks (so called multi-sparking CDI systems) the result is more complete ignition compared to the conventional system. This can be witnessed through lower tailpipe emissions.

Instead of 12V applied to the primary, CDI applies hundreds of volts. Spark plugs can be gapped much wider with CDI due to these higher voltage levels that can be generated. The main benefit is that a wider gap can be utilized to produce a taller spark which can ionize more air/fuel mixture. The higher primary and secondary voltage means the output current is lower vs. the stock (inductive) ignition system. You can think of the spark in terms of wattage consumption just like an electric motor (if you can run your motor at 230V you can halve the current requirements).

When a manufacturer chooses the spark plug gap, they factor that the combustion chamber works a certain way. Like many parts of the motor, the design is a compromise and a single type of spark (for design simplicity and lower cost) must suffice for all conditions. At idle, the type of spark preferred is long and slow burning. This is because the mixture has less assistance from the incoming air charge to create the turbulence required that helps the flame produced by the ignition event to propagate. At high rpm, a spark need only be brief because of the flame propagation assist from the intake charge but the spark must still be strong enough not to be blown out by the turbulence therefore there is a limit to the size of the gap.

When CDI systems first appeared they were't good enough to replace the conventional inductive designs. The main reason was that the low current levels were not sufficient at idle which otherwise require a long steady spark. At higher rpms this isn't a factor and so this wasn't a problem for a race car that is running WOT all the time where the technology originated but for the street it sucked. To offset for this weakness many modern CDI boxes compensate with the production of multiple sparks.

Multi-sparking is limited to about 3k rpm. This is because these units spark through 20 degrees of crank rotation only. At idle this is enough time for 2-3 extra sparks but after the cutoff, there is no time to charge the capacitor and produce extra sparks so the system turns itself off.

With the inductive system, the designers have to pick a small gap, deal with a slow coil charging time, and limit the dwell because of the points "bounce" at higher rpms when not using an electronic trigger like Pertronix. For high revving race engines, the conventional system cannot keep pace but even with our low revving bus engines, the weak spark at higher rpm contributes to lost power. CDI is the cure for the weak stock ignition system because the spark is taller, is harder to blow out, packs more energy and is numerous.

Want to learn more. Look for a future article on CDI...



02/06/05 - Created
11/23/05 - Updated with field testing
09/08/11 - Fixed broken photos, added translate button, updated footer