308 QV ignition problem - Back bank will not continue to fire

Easy things to start with:

1. Use your spark tester on the wire going from the coil to the distributor. They had a problem with the QV distributor rotor shorting thru the plastic to the top of the distributor shaft (so the spark goes from the coil to the distributor, but not from the distributor to the spark plugs).

2. Try swapping the Digiplex ECUs (you don't need to physically move them, just swap the connectors) and see if the fault moves to the other bank. If the problem does move to the other bank = bad sign for that DigiPlex ECU (they got smarter when they did the subsequent MicroPlex ignition system and made the high-current, current switching output transistor module a separately replaceable component).

3. Check/reaseat the connections and operation of the Flywheel TDC sensors -- they should produce a small AC voltage between their two terminals (like 0.1~0.2 V AC) when plugged in and the engine is running or the starter motor is cranking. Since you have two, you can compare them to determine if one seems bad/wacky. (Not the Flywheel RPM sensor as when that one fails = both banks go dead.)

 

Your "miata" link doesn't work for me (as I don't want to sign up ). You've probably got what you need, but here's a photo showing the three flywheel sensor locationss on a 308QV and how they are shown in the technical documents:
Image Unavailable, Please Login
Image Unavailable, Please Login

 

That would be my thread over there: https://forum.miata.net/vb/showthread.php?t=507999 I've owned a Miata for 23 years; when I bought the 308 QV in 2013, there was a lot of interest on Miata.net forum to document and share the Ferrari ownership experience, so I've kept that thread fairly up to date.

I have a maintenance thread here in this section, https://www.ferrarichat.com/forum/threads/bought-an-84-euro-308gts-qv-today-july-2013-now-a-maintenance-thread.414276/ , but I haven't updated it in a while - I seem to keep the thread on Miata.net more current.

On that Miata.net thread, posts 287, 288, 299 discuss replacing the crank sensors, with more photos and descriptions as follows:

There are three flywheel pulse unit / crank sensors on a 308 QV - one for each ignition unit providing a trigger signal for each bank, and one for top dead centre, shared by the two ignition systems. These are known to eventually fail, causing a misfire (if only one bank's sensor) or a no-run if the shared TDC sensor fails. I had decided to replace these as a preventative measure, as they are around $40 each and much easier to do now than on the side of the road somewhere...

The forward bank's sensor is the most difficult to access - blind, located below the forward distributor and a coolant pipe - here's a photo of the sensor location, looking straight down from over the flywheel - look for the glimpse of white in the purple circle:

Image Unavailable, Please Login

It's held by two 8mm nuts, sunken below the level of the surrounding block and it took a strange combination of 8mm 1/4" drive socket, flexible 1/4" extension, adapter, or a gear-drive stubby socket wrench to get the 8mm nuts off.

With the sensor unplugged, unbolted, and removed, it appeared that this swap was more than just a good idea - at some point, the sensor cable had broken and been repaired. Under a wrap of electrical tape was the patch on the white cable, compare to the black cable on the new sensor:

Image Unavailable, Please Login

On fitting the new sensor and finding it wouldn't seat over the two studs, a closer look found that the lug hole spacing was too wide! I have no idea if the spec changed on later cars, these sensors were listed specifically for the QV engine. I checked the resistance of the old and new units and they were identical, the connectors were identical, so rather than mess with return shipping and waits to send back to the UK (and they wouldn't have any different stock anyway), I decided to make them work.

A dremel grinding bit ovaled out the holes by 1.8mm each, and then metal shears took care of the excess overall width of the mounting plate. After a successful test fit, it was then the fight to get the two 8mm nuts back on the studs and tightened, a 10 minute struggle. Fortunately, the engine started and ran properly, so a successful replacement. there's 2 more to do tonight, but those are much more accessible.

(Later post) A bit of research showed that these are the currently correct replacement sensors - the original sensor with the correct hole spacing in the mounting plate is no longer available, and these are one letter off in the part number and are available. Used on a few Euro Ford models, apparently. So, for the last 10 years, the standard sensor replacement process includes filing the holes and trimming the plate ends! The nice thing is that they are available at all, and they're not big money. Apparently I could have found them on eBay for $35 each - I paid a bit more, and if you buy from a bigger name Ferrari vendor, they're over $100 each!

(Later post) Modified 2 more new crank sensors, and installed them tonight. If looking end-on at the engine (flywheel end), the sensor replaced last night was at about the 11:00 position. The other Ignition crank sensor is 2:30, in this now-familiar view of the engine:

Image Unavailable, Please Login

The third, shared RPM crank sensor is at about 6:30 position, behind the clutch lever, accessed from directly underneath. Here's a photo looking straight up, at the top of the photo is the clutch lever pivot from the clutch cable. This sensor was recessed like the first one, but is far easier to get at!

Image Unavailable, Please Login
​

So there's photos of all 3 crank sensors, not just the one you quoted in your post above.