Tach Operates Erratically...Then Car Backfires | FerrariChat

Tach Operates Erratically...Then Car Backfires

Discussion in 'Technical Q&A' started by Owens84QV, May 27, 2007.

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  1. Owens84QV

    Owens84QV F1 Rookie

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    Greg
    Ok…I was out in the 308 a couple of weeks back and 8-10 minutes into the drive, after pulling away from a stop sign, the tach began to operate erratically such that very light taps of the throttle would result in the tach jumping from 1K to 7K almost instantly…then the car would backfire.

    Last October, the tach did the exact same thing and I replaced a faulty tach flywheel sensor. This fixed the problem then.

    This time, I ohm’d each of the flywheel sensors and everything checked-out OK at 700-Ohms. I checked a brand-new one and it registered 700-Ohms. I checked and cleaned all of the connections, I put everything back together and now the car idles at 2K for the first ~5 minutes after start-up. The tach still jumps from 1-2K to 7K instantly and still backfires.

    If one of the ECUs or coils were the issue, either the front-bank or rear-bank would run rough or erratically which they do not. Like I said, the car runs smooth…until you touch the throttle. As I mentioned earlier…the car now idles at 2K for the first 5-8 minutes of start-up.

    What I don’t know is where to check next? Is there another sensor for the tach that could be faulty?

    This one’s got me perplexed. Appreciate everyone's help!

    EDIT: While the tach is showing an idle of 2K...I believe the car is really idling at 1K so, the 2K idle reading is incorrect.
     
  2. No Doubt

    No Doubt Seven Time F1 World Champ

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    I don't know about your 308, but when the tach acts up on the 348 it's often the alternator going out.
     
  3. hardtop

    hardtop F1 World Champ

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    You may have an ECU going bad. The tach is connected to one of them (I forget which one), so that should be a clue as to which bank is causing the problem. One thing you can do is switch the ECU's. Simply change the wire plugs from one to the other. If the car still backfires, but the tach stays normal, you will be on your way to tracking it down.

    Dave
     
  4. Owens84QV

    Owens84QV F1 Rookie

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    Dave / No Doubt...appreciate your responses.

    I swapped the ECUs and also added Paul Sloans recommended ground wire. Neither made any difference.

    The flywheel sensors for the front and rear banks are brand new. The rear-most flywheel sensor for the tach was replaced in October.

    Usually these trouble-shooting tasks begin to lead one down the correct path...this is yielding no information.
     
  5. Mark 328

    Mark 328 Formula Junior

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    I would try pulling the tach wires out of the plug at the coil and the ECU. On the ECU, a thin object can be inserted in the plug to release the barb that holds the individual wires from the plug assembly. The coil may have the same set-up to remove wires.

    Mark
     
  6. Owens84QV

    Owens84QV F1 Rookie

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    Mark...my 308 is a Euro so the coils wires are the spade type. At the ECU, it's probably like all ECUs. Is the purpose to clean each one? I sprayed fast drying electrical contact cleaner into each ECU and coil plug.

    Appreciate the help.
     

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