Rebuilding the 308 electronic ignition (SM805A distributor, Marelli AEI200 Ignition)

Late model 308 GT4s and 308 GTB/GTS cars have a factory installed electronic ignition based on a single Marelli SM805A distributor with two VR sensors, and two Marelli AEI200 transistor ignitions (one for each bank).
This setup works quite well when in good shape and delivers a strong spark due to the 6 Ampere coil current (which is 50% more than a points, Ignitor I or Crane XR700 setup, and similar in output to an Ignitor II or Crane XR3000 setup), so it makes good sense to rebuild it to factory condition.

When repairing/restoring an ignition setup, make sure you clean all contacts and terminals with a good industrial grade contact cleaner which removes the oxidation (WD40 is not a contact cleaner even it is sometimes advertised as one).

A quick test can be done using a current clamp and measuring the positive (yellow wire) feed going to each coil. At idle you should see around 4-6 Amperes of current (depending on the type of current clamp, e.g. RMS type or standard). If the current is below 4 Ampere for both coils, then there is most probably a problem with the voltage supply (check terminals, wires, ground and fuses). if the current is different between the two coils, then the sensor gap is most probably not set correctly or one of the coils is defective.

• VR Sensors
The VR Sensors tend to fail due to a defect of the coil windings or a cable defect. A sensor does not necessarily need to fail completely, it can also partially fail (as example only when warming up). Fortunately, replacement sensors are still easily available, as example from Superformance.

A simple test is to measure the resistance of the VR meter using a multimeter. With a healthy VR sensor you should see a resistance between 700-770Ω.

What is often done wrong is the installation of the replacement VR sensors itself. The gap between the VR sensor and the reluctor wheel needs to be set at 0.25-0.4mm, also it is quite important that the gap is identical for both sensors as it controls the dwell (coil charge) timing.

A larger gap leads to a shorter dwell, which in turn lowers the coils output, leading to a weaker spark with a shorter duration.

A simple test with the distributor removed from the engine is to slowly turn the distributor axle by hand. If the gap is set correctly, you should feel a tiny bit of resistance (due to the magnetic field) each time a tooth of the reluctor wheel passes a VR sensor (e.g. eight times for a 360º turn of the axle). If you don't feel any resistance when turning the axle, then the gap distance is too large, leading to a lower coil current and weak spark output.

The easiest way to check and set the gap is by installing the distributor on a distributor testing machine and connecting the output of the two VR sensors to an oscilloscope (black=ground, blue=positive). At 800 RPM you should get an amplitude of at least 8-12 Volts if the gap is set correctly. Also both outputs should have the same amplitude +-2 Volts.

• Ignition Modules (Amplifiers)
The factory Marelli BKL 1A (GM 468 in earlier units) ignition module is similar to a standard four terminal HEI module, but not all HEI modules can be used as a replacement for the BKL 1A. Most if not all of the HEI modules on the market are specified for an eight cylinder engine, the HEI modules available nowadays as OEM replacement typically start to accept triggers around 600 RPM at eight cylinders, and the ignition output current is often set at 5A or below.

In the 308 setup, each ignition module handles a single bank, e.g. four cylinders, so the idle RPM is just around 400 RPM (800/2). Using an eight cylinder HEI module in this setup can lead to ignition dropouts and bank failures at idle, so it is important to choose a replacement module which is compatible with the lower RPMs and is able to deliver 6A coil current to the ignition coil. Unfortunately, even dedicated shops which specialize in Ferrari replacement parts often sell standard HEI modules as replacement for the 308 ignition.

The following modules have been fully tested, work with a four cylinder (single bank) setup and are able to deliver 6 Ampere as in the factory setup (sorted in order of preference):

Hella 5DA 006 623-05
Beru ZM005
Pertronix D2000


• Ignition Coils
The original Marelli BAE 207A coil installed in the AEI200 ignition is a high power transistor ignition coil with a primary resistance of 0.8Ω and a secondary resistance of 9KΩ.
A defective coil can either stop working at once (cold, or only when heating up in the engine compartment), but can also develop defects in the secondary winding, leading to a lower spark output.

Using a standard points coil such as the Marelli BK2A is not a good idea as the internal resistance limits the coil current to around 3.5-4 Ampere, which leads to a weak and short spark. The ideal replacement is a transistor ignition coil with similar parameters as the original BAE 207A coil

The following coils have been tested and are similar to the factory BAE 207A coil (sorted in order of preference):

Bosch 0 221 122 334
Beru ZS 120
Pertronix 40011


Rebuilding an SM805/AEI200 based ignition with the parts mentioned above and setting the VR gap correctly will restore your ignition setup to factory specifications.

Good luck,
Adrian

 

Every non US carb 308 GT4/GTS/GTB made from 1978 on would have this setup. Not sure about the 1978-1980 308 US carb models, chances are they kept the points based ignition until the introduction of the GTBi/GTSi cars.

Accell 35361 HEI modules should be fine, the one i tested worked nicely at lower RPMS, it had a bit less output but should do nicely.

Around 800 ohms is still acceptable. You might want to check the terminals in the plastic connectors for corrosion/dirt and clean them if required.

As Andy already posted, unfortunately you have to disassemble the distributor to exchange the sensors. Remove the six screws at the bottom and the three screws at the top, then gently hit the top end of the axle (where the rotor was mounted) with a rubber (!) hammer until the axle with the advance mechanism pops out from the bottom, after that the top plate with the VR sensors can easily be removed.
Mark the position of the bottom plate (six screws) against the body with a Sharpie to be able to reinstall it in the exact same position.

I recommend to exchange the factory installed slotted-head screws against stainless steel crosshead screws, as the slotted-head screws can easily get damaged and then are difficult to remove.

Exactly, each AEI200 ignition unit runs the bank next to it.

 

Rotate the distributor body CCW to advance the timing

 

The electrical defects a VR sensor can have are either a break of one of the windings (no output anymore and infinite resistance) or a partial failure of the insulation varnish due to age and vibrations (lower resistance). Both defects can be identified by measuring the resistance using a standard multimeter. If the sensors resistance is within the specs, e.g. 700Ω-770Ω for the type discussed here, then it will output a valid signal. A higher resistance than normal usually indicates corroded connectors.

Partial shortened wire turns due to a insulation varnish failure lead to a lower resistance and a lower amplitude, but the sensor will still work as the ignition module triggers on the signal crossing zero and not on the amplitude, so even partially damaged sensors will still work.
A sensor with a partial insulation varnish failure (e.g. lower resistance than normal) will deliver a signal similar to a too large reluctor/sensor gap.

Best,
Adrian

 

Sure, here are some photos:

1. Magneti Marelli SM805A Distributor, shown from the engine mount side
2. SM805A inside view, showing the two VR sensors (black) mounted to the top plate
3. Marelli AEI200 Ignition (left side), Coils: Bosch, Beru, Pertronix / Ignition Modules: Hella, Beru, Pertronix
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Sure, but please create a new thread so others can participate too

Best,
Adrian

 

The dwell angle is more relevant for setting up the points gap, it shows how long in "distributor shaft degrees" the points are closed to charge the coil.

With the AEI200 the dwell angle is controlled by the distance of the VR sensor to the reluctor wheel. A higher distance gives a lower dwell angle measurement.

16º sounds a bit low, for an four cylinder engine (the SM805A setup behaves like two independent four cylinder engines with an AEI200 engine for each bank) you would usually see a dwell angle of around 54º. The effect of the lower dwell angle would be a much shorter spark duration.

Any chance the sensor/reluctor wheel gap is too large? Did you notice any "friction" due to the magnetic field when spinning the distributor axle by hand?