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Slow Down Light voltage weirdness

Discussion in '308/328' started by Brian A, Jan 12, 2018.

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  1. Brian A

    Brian A F1 Rookie

    Dec 21, 2012
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    1983 US 308 GTS QV
    #1 Brian A, Jan 12, 2018
    I just replaced one of my slow-down light thermocouples with a used replacement on my 1983 US-spec 308 GTS Quattrovalvole (the 308 with two catalytic converters).

    Threading these things from the back of the car, through the center console and under the passenger footrest is a major pain, but I got it done. Dimensionally, it fit fine: it threaded into the cat perfectly, the shielded wire was the right length, etc.

    Upon test driving the car, the associated “Slow Down Light” is now lighting intermittently. The engine is running well and the cats are new. I’ve measured the temperatures of the cats with my IR thermometer and they are within 50F of each other (approx. 550 F). I’ve swapped the thermocouple wires on the two control modules and the problem moved to the other light.

    From this testing, it looks like the replacement thermocouple is causing false positives, which is weird because that means it is generating too much voltage. I measured the voltage at the modules when the cats were hot and determined that the old working thermocouple is delivering about 18 voltage units (Microvolts? Minivolts? Maxivolts? Demipicovolts?) while the rogue thermocouple is delivering 55.

    I’m not so good at this electricity stuff. Any recommendations on what I may have done wrong? Can I just put in a resistor into the rogue circuit to reduce the voltage to match the functional one? If so, what value of resistor?

    If I can't find a better solution, I may just have to disconnect the rogue thermocouple from the Slow Down Light module, which would be a shame because it is there, serves a useful purpose and I like having stuff working.
     
  2. kcabpilot

    kcabpilot Formula 3

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    #2 kcabpilot, Jan 13, 2018
    Hate to say it Brian but you got a bad thermocoupler, no point in trying to analyze or troubleshoot any further, you've already done the one thing (swap) that you could to identify the culprit. The result of that test is what's known as a "smoking gun". We have them at work, they are used as engine fire detectors and they go bad and get replaced all the time. Not sure where you got it but I wouldn't buy a used one.
     
  3. moysiuan

    moysiuan F1 Rookie
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    #3 moysiuan, Jan 13, 2018
    Could also be a nicked or otherwise damaged/internally corroded thermocouple wire. When you remove the offending one, check to make sure the wire insulation is not broken anywhere and that connectors are clean, with good crimps. And clean the thermocouple itself gently with a brass wire brush, surprising small amounts of deposits can alter the reading.

    Probably the thermocouple is worn out as noted. But might be worth one last meticulous clean and check effort.
     
  4. kcabpilot

    kcabpilot Formula 3

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    #4 kcabpilot, Jan 13, 2018
    You could try any generic cromel/alumel k-type thermocouple, problem is getting one with the right threads for the bung in your Cat. Surely someone has already been down this path, no?
     
  5. Brian A

    Brian A F1 Rookie

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    #5 Brian A, Jan 13, 2018
    You make me hopeful. As far as I can tell, no one has found a replacement. Are thermocouples really that generic (hope, hope)?

    The threads on the thermocouple are SAE: 5/16" fine thread (5/16-24). (Flats on the thermocouple nut are 3/8" = 9.5mm).

    Page 75 of the 1983 Owner's Manual explains the operation of the Slow-Down light system. The light starts blinking when the cats hit 900C +/- 20C and light steadily when the cats hit 940C +/- 20C. It makes no note of corresponding thermocouple voltages however.

    It would be GREAT to find a inexpensive generic thermocouple that fits (and works).
     
  6. kcabpilot

    kcabpilot Formula 3

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    #6 kcabpilot, Jan 14, 2018
    Voltages are standard, they produce Millivolts so at 1000 deg C it’s something like 40 millivolts or 0.04 volt. Here’s a chart:

    https://www.thermocoupleinfo.com/type-k-thermocouple.htm

    You can find them on Amazon for twenty bucks or so thing is thread size and length of probe, not to mention length of the leads since you have to go all the way to the footwell.
     
  7. Brian A

    Brian A F1 Rookie

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    #7 Brian A, Jan 28, 2018
    I have measured one of my 308 thermocouples: leads are 154”, probe is 1/8” in diameter and probe length is 2 ¼”. The threads on the thermocouple are (surprisingly) SAE: 5/16" fine thread (5/16-24).

    If anyone knows a source of a thermocouple with this physical specification, please post the source for it here.
     
  8. moysiuan

    moysiuan F1 Rookie
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    #8 moysiuan, Jan 28, 2018
    This place would probably custom make something if all the specs are known. Might have something off the shelf in this online catalogue, but probably easier to just call them. https://www.omega.ca/pptst_eng/M12PROBES.html
     

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