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What to test if 308 is running on 4 cylinders and how?

Discussion in 'Technical Q&A' started by 308GTS, Nov 1, 2003.

  1. 308GTS

    308GTS Formula 3

    Dec 27, 2001
    2,220
    TN
    I have read many threads about 308s Injected running on 4 cylinders and it being linked to a bad coil, ignition box, flywheel sensor. How do you go through each, test each to figure out which is the one giving the problem? I can see that it is a no spark issue but are there resistance values for the flywheel sensors so they can be tested and if so what are? Where is the best place to begin on this, order of operation. Thanks.
     
  2. Doc

    Doc Formula Junior

    Sep 13, 2001
    886
    Latham, New York
    Full Name:
    Bill Van Dyne
    I don't know the resistance values of the sensors offhand, but they are listed in the owners manual under the electrical schematic section for the igntion system. Re: where to start to dx this problem, I'd first try the coil wires--ie switching them to see if the problem moves to the other bank ( actually move the entire wire from each coil and distributer and install on the opposite banks). Also check all of the electrical connections running from the Digiplex units to the engine.
     
  3. 308GTS

    308GTS Formula 3

    Dec 27, 2001
    2,220
    TN
    Thanks. Can you switch coil wires that way without harming the digiplexes or getting the engine to fire at incorrect times? Still though this would show that you didn't have spark. I just trying to figure out if you switch the connection and it goes to the other side what does this show? I know I am missing something. Then how could you tell if it was the coil, the pickup or the ignition box? My main point is how could you tell which component failed without replacing them?
     
  4. Doc

    Doc Formula Junior

    Sep 13, 2001
    886
    Latham, New York
    Full Name:
    Bill Van Dyne
    You can't just criss-cross the coil wires--that would cause major problems! One way to test if it's the coil wire is to disconnect each coil wire from both its respective coil and distributer. Then, install the wire which was on the front coil/dist onto the rear coil/distrib and visa versa. If the bank failure then changes , after you switched the wires, then you know it's a bad coil wire problem. If the bank failure doesn't change after you switched the coil wires, then it has to be another component--such as the flywheel sensor, digiplex, or the wiring. I'm not an expert at testing the other components, but resistences can be checked by way of an ohm meter , connections can be cleaned and tested, etc. I'd call a dealer/ Ferrari mechanic re: testing the digiplex.
     
  5. 308GTS

    308GTS Formula 3

    Dec 27, 2001
    2,220
    TN
    I guess it is just best to have a spare coil and then you can put the new coil in place of the old coil to test it. Then test the resistance of the coil wires with a DMM. This gets rid of those. Now find the values of the flywheel sensors and go through them. Now on the Digiplex this could be more difficult. Without the tool that goes in the diagnostic socket I am not sure how to test them. There must be a way to test the coil. Does anyone have a resistance value for the coils, acceptable values? My point is that this problem occurs a lot on 2V injs. and QVs. I was looking for a definite way to test the components without just replacing parts until it works.
     
  6. Doc

    Doc Formula Junior

    Sep 13, 2001
    886
    Latham, New York
    Full Name:
    Bill Van Dyne
    I believe that the coils can also be tested w an ohmmeter. I'm sure some of the experts on board to provide the specifics. Unfortunately, I'm not one of them! By the way, I have an '85 308 and this happened to me a year ago. It was a flywheel sensor and a case of old wires. A dealer dx'd the flywheel sensor problem.
     
  7. 308GTS

    308GTS Formula 3

    Dec 27, 2001
    2,220
    TN
    Thanks. Maybe we can get some of the values.
     
  8. Verell

    Verell F1 Veteran
    Owner Consultant

    May 5, 2001
    6,951
    Groton, MA
    Full Name:
    Verell Boaen
    There have been several threads on this in the past 3 years or so. One of them had sensor resistances, another had at least some of the pins for the diagnostic connector identified.

    Try searching the old Tech Q&A archives for 'diagnostic socket' 'diagnostic connector' 'sensor' 'resistance'...
     
  9. Owens84QV

    Owens84QV F1 Rookie
    Silver Subscribed

    Oct 2, 2001
    4,473
    Somewhere in NC
    Full Name:
    Greg
    308GTS...the problem you are writing about has been the single most frustrating issue with my 84-QV. Living down in Florida has only made it more agrivating. I've found that I have to routinely check all of my connections to the coils for corrosion. If I keep those very clean and tight, she runs like a champ.

    About two months ago, I replaced EVERYTHING including coils, distributor caps & rotors, wires, spark plugs...everything.
     
  10. 308GTS

    308GTS Formula 3

    Dec 27, 2001
    2,220
    TN
    Verrell I agree but most of them never get solved or if so are plug and chug. Replace, replace, it works. I am going to look for the resistance values. Did it have the for the RPM and TDC sensor or just one. They should have different values. Thanks.
     
  11. hardtop

    hardtop F1 World Champ

    Jan 31, 2002
    11,211
    Colorado
    Full Name:
    Dave
    You can check the digiplexes by trying them one at a time. Figure out which bank is working (the tach is connected to the 1-4 digilex, so if the tach is working, 1-4 is firing. The boxes are located under a plate on the driver's side of the luggage compartment. Simply pull out the connectors and move them to the other box. If both boxes fire the working bank then you know it's not the digiplex.

    The coils can be switched to check them, but it is necessary to switch ALL the wires attached to them, so you pretty much have to take them out and switch positions. Make notes on which color wires are connected to each terminal first.

    If these checks don't work, it gets harder unfortunately to check sensors, wires, etc.

    Dave
     
  12. Verell

    Verell F1 Veteran
    Owner Consultant

    May 5, 2001
    6,951
    Groton, MA
    Full Name:
    Verell Boaen
    OK, did the search myself.
    TDC sensors should measure 800 to 1.2K Ohms.

    Most common failure will be opening up, not being way off on resistance. Other common failure would be shorted turns which usually aren't detectable with an Ohmmeter.
     

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