SOLUTION: No Power/Ignition In One Cylinder Bank

Long post, but hoping others may benefit from a very frustrating problem we just solved on my '85 308 QV.

As I drove car home from a recent service at Scuderia Rampante, it lost power and the exhaust system heated up and began smoking. I discussed with neighbor and fellow Ferrari owner Brock O’Kelly and we agreed it sounded like an ignition problem. So, in order of the things we tried, here’s what we did and learned along the way.

Spoiler alert: The problem ended up being a faulty distributor rotor, but along the way we discovered a suspect timing sensor that took us down a blind alley before we arrived at our final definitive diagnosis. Take-away: test the rotor first, before you try the other stuff we did!

1. Coil Wire. The coil plug on the rear bank wire (cylinders 1-4) on the car had pulled free of the wire, remaining in the coil – so we assumed the problem was with this bank, and replaced this with an extra wire Brock had for his 1986 Mondial 3.2. Problem remained.

2. Distributor Cap. We removed the coolant reservoir tank from its mounts and bungeed it out of the way to ease access. We opened it up to check the rotor and wire connections and cracked the OEM Marelli cap in the process. Replaced it with a $100 aftermarket cap from AllFerrariParts, securely attached all the wires, sanded the face of the rotor, reinstalled. Problem remained. (We later discovered that the gaps between the contact points and rotor are wider than OEM spec, so we super-glued the original cap (worked great) and reinstalled it. Dave Helms at Scuderia Rampante that this is an acceptable fix, and superior to any of the aftermarket caps out there. Do not buy non-Marelli distributor caps!

3. Coils. We used a timing light to confirm that no current was coming from the rear bank coil (should have done this first!) so we consulted the wiring diagram in the car’s manual and swapped the low-voltage wiring and the coil wires to check the coils (snapping a photo to make sure we rewired correctly). We confirmed that the lack of high voltage was associated with the physical bank rather than the coil, so both coils were fine. Problem remained.

4. Digiplex ECU. The electronic ignition control units are mounted to the bottom of a plate in the left side of the trunk (the Jetronic fuel injection controller sits on top). There are two ECUs – one side-agnostic unit for each bank. We swapped the large wiring harnesses on each and concluded the problem was associated with the physical bank rather than the ECU (thank goodness, because these are very expensive). Problem remained.

5. Timing Sensors. On my car there are three of these, all with the same part number (119502); one for the tachometer, and one each for the front and rear banks. (They are called many different things; Flywheel Pulse Unit Crank Sensor, Angular Speed Sensor, TDC Sensor, etc.). We used a photo posted on FerrariChat to confirm the function and location of each sensor, focusing on the one for the rear bank (cylinders 1-4, at about 11 o’clock on the bell housing). We traced the wires for each sensor to a large 6-position Molex plug about 10 inches to the rear of the rear coil. We tested the voltage and resistance of each using guidance from deeprivergarage on F-Chat:

a. Not cranking, key off: the sensor resistance should be 750+/- ohms. This is what we found.

b. Cranking: sensor voltage should be 0.5 volt to 1 volt AC. We found the voltages for the front and tach units around 0.5V, but the voltage for the rear unit could barely muster .05V. Hmm, we may have found the culprit. We did the same check on Brock’s car (which only has two sensors, the one for the tach a different part number and the other the same as mine). Resistance was in the indicated range, as was cranking voltage. Now we’re starting to get convinced we’ve isolated the problem. And Brock decided to replace all his while he has his engine apart for a major service.

I ordered replacement sensors for both our cars from a Polish seller on eBay (Crank Sensor TDC - Magneti Marelli SEN 8D For Ford, Fiat, Lancia – ordered five for €100 total); we both figured while we’re in there we might as well replace them all, as they seem prone to failure and this is a pretty big job gaining access to them. Also, deeprivergarage said that experts say a bad tach sensor will cause both electronic ignition modules to quit at the same time.

6. Replacement. We followed the consensus opinion on F-Chat that the best access is obtained from the LR wheel well – which is actually pretty easy to remove. Put the car on a jack stand, remove the wheel and liner, and you have pretty decent access to the sensors and the bottom screws on both distributor caps. The rear sensor is below the rear distributor. The front sensor was easiest to remove by first removing the front distributor cap. When removing the sensors, use a magnetic probe to remove each nut AND washer; consider replacing the washers with LOCK washers. The new sensors have slightly different mounting holes from the original spec, so you need to expand each hole on the mounting plate toward the sensor using a Dremel grinder bit (tricky) or a circular file (easier).

7. The Culprit: Rotor. The problem remained, so we went back to the drawing board. We tested continuity on all wiring between the sensors and the ECU and found no problems. We pulled and reattached the sensor plugs 3X each (at Dave Helms’ suggestion) to be sure the issue was not with plug contacts. We tested for high voltage at the distributor cap by wrapping a stripped wire around the center of the rotor contact inside the distributor and placed the other unstripped end near an engine ground, cranked the motor and confirmed spark. At this point we concluded that the problem was between the distributor cap and spark plugs. We tested continuity on the plug wires and found all were fine. We tested for high voltage at the plugs using the wire-spark trick above (jumping the test wire from the center of the rotor contact inside the distributor to the plug wire, and plugging in an extra spark plug on the plug end and resting it against an engine ground) and confirmed spark at two of the four, at which point we decided the problem might be the rotor. We swapped the rotor from the front to the rear, and voila, problem solved. Brock pulled the metal insert out of the suspect rotor and we discovered a carbon-blasted hole at the bottom of the mounting hole that clearly was caused by the high voltage arcing from the distributor contact point on the rotor directly to the mounting shaft on the distributor, shorting the entire secondary ignition at this point, preventing current from getting to the plugs, and causing a lack of ignition (as confirmed with absence of timing light pulses even between the coil and distributor cap). Dave Helms was kind enough to give me a used rotor he had on hand to get the car running, and I ordered a couple extra rotors to keep in the car at all times, because Dave said this is a common problem with them. 

 

What the heck is that "fluff?"