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Question about slow down light…

Discussion in '348/355' started by vinny, Oct 24, 2016.

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  1. vinny

    vinny Karting

    Nov 1, 2003
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    Vincent Pitocco
    #1 vinny, Oct 24, 2016
    Yesterday I was in traffic and noticed the slow down light 1-4 come on. I was approaching a red light and as I was stopping for the light it stalled. I started it up and slow down light was still on then about 1/8 mile later light went off ( didn't notice any power difference when light was on). Parked the car and started it tonight and the light is on again..so my question is why is the light on when the car is cold? I thought the light warns you if the cat is overheated..thanks in advance, Vinny
     
  2. cuneo

    cuneo Formula 3
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    #2 cuneo, Oct 24, 2016
    Cat ecu is probably shot, common problem.
     
  3. ///Mike

    ///Mike F1 Veteran

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    #3 ///Mike, Oct 24, 2016
    Cuneo is probably correct but worth checking for ECU error codes just in case.
     
  4. Dave rocks

    Dave rocks F1 World Champ
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    #4 Dave rocks, Oct 24, 2016
    Bingo. If it's black potted TCU, even more suspect.
     
  5. vinny

    vinny Karting

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    #5 vinny, Oct 24, 2016
    So basically the cat never overheated, just a false reading from the bad ecu?
     
  6. Motob

    Motob Formula 3
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    Nov 11, 2003
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    #6 Motob, Oct 24, 2016
    If it came on cold then ECU is most likely bad. Very common issue (do a search). Old//bad ECU's have black potting material. Better/new ECUs have green potting material.
     
  7. yelcab

    yelcab F1 World Champ
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    #7 yelcab, Oct 25, 2016
    I would not assume that the cat was never overheated, but the first solution is to change that TCU if it is black.
     
  8. ///Mike

    ///Mike F1 Veteran

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    #8 ///Mike, Oct 25, 2016
    Why not diagnose it first before just throwing parts at the car? There are certainly occasions where changing parts makes more sense than spending a lot of time on diagnostics, but as easy as it is to switch the TCUs from side to side seems like it'd be worth doing in order to confirm the assumption that it's bad. After all, the thermocouples can cause the same symptoms and they go bad as well.
     
  9. vracer

    vracer Formula 3

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    #9 vracer, Oct 25, 2016
    It sounds like it might be time for a code reader, but I agree about the black/green ECUs. However, I have a question. Slow down 1-4? My memory says there is only one SDL. Is my brain SDL blinking again?
     
  10. yelcab

    yelcab F1 World Champ
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    #10 yelcab, Oct 25, 2016
    95 355 have 2 slow down lights.
     
  11. hercfe

    hercfe Karting

    Apr 21, 2015
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    #11 hercfe, Oct 25, 2016
    Last edited by a moderator: Sep 7, 2017
    I had the same issue. I had plans to take the CAT's out anyway, so I just replaced them with test pipes, and replaced the O2 sensors as well. Completely took out the CAT ECU's and thermocouples as there was no need for them any longer. Haven't had a problem since. What I did do prior to all that was ensure that the CAT's were not actually overheating. I shot them with an infrared heat gun after driving till I got the SDL lights. Both were well with range, 500f. So, I was confident the engine/or CAT's weren't the culprit. I would say that is the easiest and cheapest way to troubleshoot this, without throwing parts at it. FYI, before taking the CAT's out, make sure you will still pass whatever inspection criteria there is for your State.
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  12. hercfe

    hercfe Karting

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    #12 hercfe, Oct 25, 2016
    P.S. I have a 94' 348 Spider; but understand it is the same system.
     
  13. ///Mike

    ///Mike F1 Veteran

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    #13 ///Mike, Oct 25, 2016
    The OP's profile lists a '91 348TS. Presumably he's asking about that car, in which case there are two SDLs.

    Removing the TCUs apparently doesn't set a CEL on early cars, but does on mine.

    Agreed that test pipes are the best/easiest solution when you can get away with them. Gets rid of a ton of heat and some weight. Plus, it improves performance. Win/win/win. :)
     
  14. vinny

    vinny Karting

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    #14 vinny, Oct 25, 2016
    Thanks for all the replies! I will switch them around and see happens.…thanks again!
     
  15. vinny

    vinny Karting

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    #15 vinny, Oct 25, 2016
    Mine is a '91
     
  16. redwedge

    redwedge Formula Junior

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    #16 redwedge, Oct 25, 2016
    Vinny, the thermocouple ECUs aren't particularly expensive for a F-part (although they are expensive for what they are!), however if you do find that one of yours is on the blink, it can be brought back to life.

    I placed my faulty unit in the airing cupboard in a bowl of dry rice for a day to dry it out, then resealed the potting, which had shrunk away from the plastic casing, with silicone sealant. I refitted the part and it has been fine for something like three years now.
     

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