Have you lost power in one bank of cylinders?

Check this first!

I got a call from a buddy needing help with his 85 308 QV. While driving back from his mechanic's shop his car lost power a mile from home resulting in the usual symptoms including a red hot exhaust. This immediately indicated to me a loss of ignition on one bank. A quick check and I find the coil wire for bank 1-4 had separated from the crimped metal connector at the coil. Yay! Problem solved right? Nope, red herring. B1-4 still dead. Time for a sensor check.

The sensor voltage check showed that the one for B1-4 an order of magnitude lower than the tach and B5-8 sensors. The reading was only 0.05VAC vs. 0.5-1VAC for the other two. Yay! He purchased three new sensors because as long as we are at it, might as well minimize risk of having to come back and change the others. Time to dive in.

Unfortunately, the sensor for B1-4 is in the worst possible location under the coolant pipe forward side of the bell housing. He found info on FC that suggested we pull the distributor cap for B5-8 for better access and decided to pull the inner fender as well. It was pretty much a PIA but managed the change out of all three.

Time to test the repair. Yay! Problem solved right? Nope, red herring #2, B1-4 still dead. This is where things got real interesting. Checked VAC for all new sensors, 1-1.3VAC for all three, check. New coil wire and new distributor cap (his broke while removing it) resistance okay, check. Resistance from new cap terminals through all spark plug wires, okay, check. Resistance across rotor essentially zip, check. Inspected new cap and find the radius from center post to terminal pick-ups is larger than stock Marelli cap, leaving a gap of about 1/8" from end of rotor to terminal. Superglued Marelli cap back together and reinstalled....Yay? Nope...B1-4 still dead.

After much wrangling and rechecking coils, Digiplexes, wires and sanity it became apparent that something was amiss downstream of the coils. I said it must be the rotor..nah, can't be the rotor, they never fail. Rechecked resistance from coil contact to rotor end, as expected, zero ohms. Even checked the resistance from the coil contact area to the metal spring that holds the rotor onto the camshaft end...as expected, it was open circuit.

Then something special happened....as I finished up the resistance check on the rotor, I notice that the spring clip had come loose. Hmmmm...pulled the spring out and noted what looked to be black carbon on the underside..looking into hole for mounting the rotor onto the shaft and see a black burn spot. Hmmm....put my ohmmeter probe on the spot and the other on the rotor end and eureka....zip ohms. Plastic under the coil contact point had actually burned through thus provided a path for the spark to arc directly to ground at the camshaft, thus loss of spark to the plugs but enough of a gap prevent a direct reading to ground!

In the pics below you can see the Marelli rotor with spring in place, with the spring removed and finally, the dark spot of ignition death...

His 308 is up and running very well with the simple change to a good rotor..So if your 308 loses ignition in one of the banks, be sure to carefully check the rotors as part of your diagnostic procedure....never assume rotors cannot go bad.

Brock
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Apparently the sensors were purchased off of eBay from a Polish source. My buddy purchased them so I am a little unclear as to who it was. He said they were very reasonably priced and they are Marelli branded. Cable length was identical as was sensor size, however, the flanges and hole spacing were slightly different which required a small round file to elongate the holes. Once that was done, they fit perfectly. Apparently, they are spec'd for an Alfa.

It depends on your engine as to how many are needed. In general, if you have two Digiplex ignition units, you will need a total of 3: one for each Digiplex and one for the tach. His 308QV was a Digiplex setup. I think this is true for the 3 liter engines. If you have a 3.2QV with the Multiplex ignition (a single module unit) then you would only need 2 sensors: one for the Multiplex and one for the tach.

I took to heart the fact that the sensors do fail and because I am in the middle of performing a major on my 86 Mondial QV, decided to be proactive and replace both while things were apart and relatively accessible. The sensors are all located on the bell housing where it is bolted to the engine and basically pick up signal from various points on the circumference on the flywheel. For his 308, looking from the left side of the vehicle along the crank axis, the approximate sensor locations are at 4, 7 and 10 o'clock on the bell housing. For my 3.2 they are at 4 and 10.

Installing the sensors require no adjustment...plug and play, bolt in place... just be careful as to which sensor is plugged where, it does matter!

The following pics depict locations on my 3.2 both without a sensor in place and then with the new one installed. The top pair of pics are the 10 o'clock (Multiplex) and the bottom pair the tach (which are wired first to the Digiplex/Multiplex). All pics were taken from above although the 4 o'clock can be accessed through the wheel well. Much easier access from the top with out distributor caps in place....

As an aside, I am willing to bet that the most common failure is the sensor at 10 o'clock. This would kill bank 1-4 on a Digiplex system. This one is forward, buried a bit in the housing and not that far from bank 5-8 exhaust manifold...ie, it probably gets hot. While they have no moving parts, the sensor itself has a magnet and high heat cycles will cause them to lose magnetism. The 10 o'clock sensor on my buddy's 308 was only putting out 0.05VAC indicating a loss in magnetic flux. All of his new ones put out 1-1.3VAC.

Hope this helps...

Brock
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A word of warning...don't go by sensor resistance alone....all of my buddy's sensors read in the neighborhood of 700ohms and initially thought they were all good. You have to measure the voltage (AC) at the same terminals you mentioned while the engine is being cranked. He had 2 at 0.5VAC and the one at 0.05VAC. Minimum is stated to be 0.5VAC.

Brock