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Fixing my Slow Down Light problem

Discussion in '308/328' started by Kidasters, Mar 21, 2021.

  1. Kidasters

    Kidasters Formula Junior
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    Oct 4, 2013
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    Ken
    When I bought my car - one thing that it did that was puzzling is that the 1-4 "slow down" light would come on randomly. Sometimes it doesn't turn off when I turn the key to accessory. Sometimes it randomly comes on and then goes off while driving. When my car was being worked on at a shop last, they said they couldn't find the problem. Not knowing what to ask, I didn't ask where they looked. And - with the other issues I've had with the car, it's just sat.

    Now that I'm getting the carbs sorted, I also want to get the "Slow Down" lights sorted too. Here's what I'm thinking: Because this happens when the car is cold, and the key is turned to "on", I don't think this is a problem with heat/thermocouples. If it is - I've ordered two replacements (from the Replacement Parts thread), and I'll access the cats to try and sort that.

    What I really think is that it's related to the voltage that one of the "ECU"'s in the passenger footwell is getting. So - today, I'll open the footwell panel up and try to remove both and give them a good cleaning.

    If I can sort this issue, it's a huge personal victory for me.

    More to come later.
     
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  3. mike32

    mike32 F1 Rookie

    May 13, 2016
    3,343
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    Dont know the actual setup, but these things are more like a bad ground or a cable fault. Something is switching to cause it to come on and off
     
  4. mike996

    mike996 F1 Veteran

    Jun 14, 2008
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    The slow down light is a a routine problem on these cars. Mine (328) began doing weird stuff - turning on/off at random and finally quit illuminating at all when the ignition was turned on. I did all the usual troubleshooting - grounds/connections, etc but determined the control unit itself had failed - common issue. So it COULD simply be connections but don't be surprised if it's not.

    I think Ricambi now has new control units. But if you have removed the cat converter(s) on the car, as many people do, there is no need for the slow-down system to work at all. ;)
     
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  5. Kidasters

    Kidasters Formula Junior
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    #4 Kidasters, Mar 21, 2021
    Last edited: Mar 21, 2021
    Ha. Yeah - I found a "used" unit. If that temporarily fixes it, I may buy the new units and replace. Or not. Ricambi wants 545 for one!!!
     
  6. Kidasters

    Kidasters Formula Junior
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    Update.

    I finally got a chance to get into the garage and tackle this problem.

    This thread was so helpful: https://www.ferrarichat.com/forum/threads/replacing-exhaust-thermocouple-probe.629102/

    Got the panel open and identified which unit was 1-4 and which one was 5-8. Checked resistance on both - 5-8 was reading 9 ohms, 1-4 was reading 0 and the light was on. Switched the wires to see if it was the unit or the probe. When I moved the 1-4 wires to the 5-8 side, that light came on. That said probe not unit.

    Car up in the air, and pulled the old thermocouple out of the bottom cat. Temporarily wired in the new thermocouple, and checked resistance in the footwell again. Now - 5-8 reading 9 ohms, and 1-4 reading 8.7 ohms.

    Ordered a new brass crush sleeve from McMaster-Carr, and when that gets here I will clean up the wiring - do some permanent splices and button it back up. Happy to have yet another problem solved.
     
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  8. Brian A

    Brian A Formula 3

    Dec 21, 2012
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    Did you find/buy a thermocouple that fits the stock location and with the correct threaded nut? If so, what is the vendor and part number?
     
  9. Kidasters

    Kidasters Formula Junior
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    Yes.

    Thermocouple is here: https://www.ferrarichat.com/forum/posts/147431729/

    I cut the old thermocouple and removed the compression nut from the old part.

    I ordered the compression sleeve from McMaster-Carr

    Compression Sleeve for 1/8" Copper Tube OD
    50915K611

    It's brass, just like the one on the factory part. I measured it's dimensions, and they are similar to the factory compression sleeve.
     
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  10. Brian A

    Brian A Formula 3

    Dec 21, 2012
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    Very cool. Thank you.

    ... Did you confirm your thermocouple splice still triggers Slow Down light at the correct temperature?
     
  11. Kidasters

    Kidasters Formula Junior
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    #9 Kidasters, Apr 11, 2021
    Last edited: Apr 11, 2021
    No - Refer back to the link I put in the second post. I'm taking it at face value that this thermocouple will be fine. But - the resistance numbers make me feel pretty good about it.
     
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  13. mike996

    mike996 F1 Veteran

    Jun 14, 2008
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    When you turn on the key do you get the normal slow down light illumination for a few seconds then off? If so, and with the proper resistance showing on the thermocouple, it sounds like it's working properly.

    The typical failure of the control unit is characterized by random illumination of the light while driving at any speed and/or no illumination of the light when turning the ignition key on.
     
  14. Kidasters

    Kidasters Formula Junior
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    Oct 4, 2013
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    Yes - both lights go through the startup routine now and act the same. My McMaster-Carr bag of 10 crush sleeves arrives tomorrow. Maybe I can get the car sorted this weekend.
     

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