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flashing SDL stumping me.

Discussion in '348/355' started by yelcab, Jun 28, 2025.

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  1. yelcab

    yelcab F1 World Champ
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    Mitchell Le
    #1 yelcab, Jun 28, 2025
    I have a 96-355 with 5.2 ECU. The slow down light comes on flashing immediately after starting up with a cold engine. So... with the engine running, and the back of the 88 pin ECU connector opened up, I measured ...

    1. The three outputs from the three TCUs, pin 14/15 and 73, referenced to chassis ground at the ECU location: all three are 0.500 V, to be expected at the room temperature.

    2. I measured the output from the ECU that goes to the SDL, and the signal is flashing at the frequency of the SDL flashing light. So the ECU is telling the light to flash.

    3. Chassis ground at each of the three TCU connectors are there, within an ohm of zero. Ground is good at the TCU.

    4. Ground is also good at pin 71 of the 88 pin connector.

    All inputs are good, output is bad. Am I to consider that the ECU is at fault? I tried clearing the CEL and it does not seem to help. What am I missing?
     
  2. 308 GTB

    308 GTB F1 World Champ
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    Barry Wolinsky
    #2 308 GTB, Jun 28, 2025
    ShineKen and flash32 like this.
  3. yelcab

    yelcab F1 World Champ
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    #3 yelcab, Jun 28, 2025
    I have a spare ECU that I will swap in this week to see if it's the ECU or the wiring. As of now, it points to ECU
     
  4. Qavion

    Qavion F1 World Champ
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    #4 Qavion, Jun 28, 2025
    Unfortunately, there are precedents for this.

    Not the best, but it may not affect the output of the TCUs too much. The diagnostic sheets do give a value for 200milliohms for grounds with respect to engine management stuff. I cleaned mine and got them down to close to that value.

    I had a flashing light with key on a few weeks ago... I reset the TCU plugs and started the engine a few times and it went away. I did make up a TCU bypass for diagnostic purposes many years ago, but I either lost it or gave it away.

    https://www.ferrarichat.com/forum/posts/145451782/
     
  5. yelcab

    yelcab F1 World Champ
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    #5 yelcab, Jun 28, 2025
    What do you mean?

    I don't have to make a bypass, the TCU output voltages that are input to the ECU were measured at the ECU 88 pin connector to be 500mV. That is the correct voltage for the thermal probe at room temperature as it should be with the stone cold engine.
     
  6. Qavion

    Qavion F1 World Champ
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    #6 Qavion, Jun 28, 2025
    Last edited: Jun 28, 2025
    The Motronic ECUs have caused this before.

    Fitting "bypass plugs" (as per the link in my last post) simulates the correct signal for a not too hot, not too cold exhaust (without you having to scope the wires for correct signals). Doesn't work for the exhaust bypass though (on all cars). You can create a bypass with a diode and resistor. You don't need to go to the bother of creating a plug housing (unless perhaps if the ECU side of the TCU connector is male).
     
  7. yelcab

    yelcab F1 World Champ
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    #7 yelcab, Jun 28, 2025
    And I will know in a few days...
    If it is the problem, I don't think any of the US board rebuilders will know how to fix it. All they ever do is change a few capacitors and resolder a few joints for $500 -- offering no guaranty.
     
  8. m.stojanovic

    m.stojanovic F1 Rookie
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    #8 m.stojanovic, Jun 29, 2025
    Many cheaper multimeters will read 0.6 to 1.0 ohm even when their probes are connected to each other. This "residual ohm" value should be deducted from the readings of any low ohm resistances. If you are using a somewhat "higher end" Fluke multimeter, it will give you exact resistance of whatever low resistance you are measuring so you don't need to do any deduction. It seems that Fluke multimeters have a built-in automatic compensation for the residual resistances in the probe connectors and along the probe wires.
     
    Qavion likes this.
  9. Qavion

    Qavion F1 World Champ
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    #9 Qavion, Jun 29, 2025
    Indeed. My higher end Fluke doesn't read resistances above 5K ohms, focussing on the lower end (resistances) so to speak.
     
  10. yelcab

    yelcab F1 World Champ
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    #10 yelcab, Jul 1, 2025
    Confirmed. A different ECU installed and the problem went away. Now ... who's got a ECU for sale?
     
    Qavion likes this.

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