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Slow Down Light - Please De-mystify

Discussion in '308/328' started by Adaptel, Nov 16, 2007.

  1. Adaptel

    Adaptel Karting

    Jul 21, 2007
    89
    Concord, NH USA
    Full Name:
    Kevin Pearson
    I have an 85 308 GTS QV that has the Slow Down light stuck on pretty much all the time. Every once and a while, the light will go out on accelleration, but then quickly comes back on again. I cannot find any decent schematics that outline thsi system, so I am wondering if anyone can help, as I want to rule this out as a cause of performance issues that I am troubleshooting. This car has the K-jetronic with lambda control, and when the O2 sensor is plugged in, the mixture seems spot-on when at lower RPMs, but seems to run exceedingly rich past 3500. A disconnection of the sensor causes the car to run normally with constant accelleration thorughout the RPM range. My 02 sensor has recently been replaced, and I am starting to think that I may have issues with my Lambda ECU, or frequency valve, but need assistance on how to check these systems.

    From what I can tell, the thermocouple in the Cat is at the core of the "slow down" circuit, and it leads to an epoxy-coated box in the right section of the luggage compartment. From there, four wires join the rest of the wires that are fed into the lambda ECU. My first quesiton is what are the parameters to look for to test the thermocouple? Does its resistance vary with temperature, or does it produce milivolts when heated, and what are the specs of one in good shape? Secondly, what function does the epoxy-coated box provide, and what kind of output does it produce? Lastly, when the system detects an overtemperature conditoin, does this affect the behavior of the lambda system's mixture calculations for safety reasons, or does it simply turn-on the "Slow Down" light?

    From what I have read, the frequency valve goes into a "safe mode" when the 02 sensor is unplugged, and is obviously running when I do so. Does the lambda computer have any input as to what control pressure it is creating in this "safe mode", or does it revert to a fixed frequency(control pressure bypassed). Can any valuable mixture data be obtained by hooking an oscilloscope up to the valve, or in other words, can conditoins where the car is tryijng to be "more rich" or "more lean" be observed by seeing the frequency that the valve is operating at? Thanks in advance.
     
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  3. dinogt4guy

    dinogt4guy F1 Rookie

    Oct 31, 2004
    3,197
    Hewitt, Tx.
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    Dino Tifosi
    I believe the "slow down" lights have something to do with the "cats", thats where I would start looking.

    Cheers!
     
  4. Badman

    Badman Formula 3

    Mar 4, 2007
    1,115
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    Bruce Wayne
    The slow down light is tied to the thermocouple in your cat. The thermocouple sends the differential between the ambient temperature and the temperature in the cat to the ECU (that epoxy box you mention). The ECU will then blink the 'slow down' light if the cat temperature is near the limit of acceptable, and will turn the 'slow down' light on solid if the temperature is too hot.

    My guess is that you've got a broken thermocouple wire, and you need a new thermocouple. You can try putting a jumper on the ECU connections where the thermocouple wire goes. This will simulate an ambient temperature reading, and so should turn that light off (and if not, it's a probably bad ECU as Tillman talks about below)
     
  5. GrigioGuy

    GrigioGuy Splenda Daddy
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    Nov 26, 2001
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    Does your slow down light immediately when you start the car, and not turn off? That's the classic symptom of a bad ECU. You can either try to find another ECU, or just pull the cat off and disconnect the wire harness at the ECU. (Disclaimer, that's a federal crime, EPA, global warming, yadda yadda)
     
  6. Adaptel

    Adaptel Karting

    Jul 21, 2007
    89
    Concord, NH USA
    Full Name:
    Kevin Pearson
    Yes, the slow-down light is always on solid, never blinks. It is on solid when the key is turned on (before cranking), when the motor is initially started, and at any engine temp. I am positive that there is a bad component somewhere in this system, but I wish I had more info on it's operation so that I can do some more "logical" troubleshooting.

    I have a feeling that finding an ECU will pose a challenge.... Does anyone know what other vehicles may have used the K-Jetronic w/lambda & would most likely have used the same ECU?
     
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  8. Badman

    Badman Formula 3

    Mar 4, 2007
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    It's a pretty simple circuit, I don't know what other info you're looking for. The thermocouple goes directly into the ECU, and the ECU goes directly to the 'slow down' light. So if the 'slow down' light is always on, it's either a bad ECU or a bad thermocouple (or I suppose there's a remote possibility that the light itself is shorted to ground, since it looks on the schematic like it's low side switching, but I'm looking at the badly scanned version from the online version of the manual).

    My jumper test mentioned above should quickly and easily determine whether it's the ECU or the thermocouple that's the problem. I haven't actually tried it, so I'm making the assumption that the ECU will turn the 'slow down' light on with an open thermocouple, and wouldn't be smart enough to turn it on with a shorted thermocouple. But I can go out and try it on my car tomorrow (also an '85) and confirm.

    Hey Tillman (or anybody else), is it common for those ECUs to go bad? If so, and they are really hard to get, I could look into manufacturing new ones. I've been working on manufacturing the thermocouples themselves for a while, just been held up trying to find the right compression fittings for them. Also recently discovered that the thermocouple sheaths for at least some of the 328s seem to be longer than the ones for the 308s.
     
  9. eulk328

    eulk328 F1 Rookie

    Feb 18, 2005
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    F683
    to answer just one of your questions..... they produce a small voltage when heated. Thermistors on the other hand, used for oil temp., water temp. etc. change resistance when heated.

     
  10. Badman

    Badman Formula 3

    Mar 4, 2007
    1,115
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    Bruce Wayne
    Just to be slightly more accurate, they produce a small voltage not when heated, but when there is a temperature difference between the tip of the thermocouple wire and the other end. This is different than a thermistor, which will change resistance depending upon the temperature of the element, regardless of the temperature at the other end of the wire. So they really measure temperature differential, not absolute temperature. This is important to realize when testing one if you're trying to compare it to thermocouple charts.

    The other funny thing about thermocouples, the red wire is always negative and the other wire is positive. Thermocouples were invented way back in the 19th century, before red became associated with positive! I've noticed on the original thermocouples for these Ferraris that it's hard to see which is the red wire. Sometimes I've seen a red strip woven into the insulation, other times it seems there's nothing to distinguish!

    Anyway, when the car is cold, it's going to read very close to 0mV, because there's no temperature differential and so no output from the thermocouple. I try to discourage people form looking up a thermocouple chart to try and see if the thermocouple is working, that usually just confuses people who aren't familiar working with thermocouples. But it's such a simple device (just two dissimilar metals welded together at one end) that it either works or it doesn't, so there's not a lot of point in using a chart to diagnose it. If you really want to see if it's working, disconnect it from the ECU and measure the ends with a voltmeter. It should be near 0mV. Start the car and the mV should start to climb (if they drop instead, you've got the positive and negative wires backward). If that happens, the thermocouple is working.

    But if you really, really want the actual temperature specs, it is a "J-Type" thermocouple and here is a temperature differential to mV chart: http://www.pyromation.com/downloads/data/emfj_c.pdf. Look at the mV coming out of the thermocouple and match it to a temperature on the chart. Add about 25'C to that number and you've got the approximate temperature of the cat.
     
  11. Badman

    Badman Formula 3

    Mar 4, 2007
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    BTW, just to finish the information... the ECU trip points are at 900'C (flashing) and 940'C (solid). Those are both spec'ed as (+/- 20'C). I'm sure this is because the ECU doesn't compensate for ambient temperature, which means in very hot or cold weather it would actually trip the light technically out of spec. It doesn't really matter, it's just a warning light so it's close enough. However, if I built replacement units I'd definitely add cold junction (ambient) compensation so I could tighten up that spec to within about (+/- 2'C) just because I'm anal :D

    Based on those ECU trip points, you can completely test the functioning of the ECU by the following:

    Attach a mV source to the ECU.
    Set it to 0mV.
    Start the car.
    'Slow down' light should come on, then go off after a few seconds.
    Set mV source to 52mV.
    'Slow down' light should be flashing.
    Set mV source to 55mV.
    'Slow down' light should be on solid.
    Set mV source to 50mV.
    'Slow down' light should be off.

    The problem, of course, is getting yourself an accurate mV source. You could also get a thermocouple simulator and run a similar test, but those aren't exactly cheap. If anybody is really interested, I could quickly design a custom test box to generate the mVs for the above test.
     
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  13. Adaptel

    Adaptel Karting

    Jul 21, 2007
    89
    Concord, NH USA
    Full Name:
    Kevin Pearson
    I have done some simple tests on my suspect termocouple, and it is indeed open-circuited. Shorting the terminals on the epoxy-box (does anyhone know the techinical name for this guy?) causes the slow-down light to go out. Logic tells me that this is simply a case of a bad thermocouple. Are these still available anywhere? I never get a warm fuzzy feeling when ANY system is disabled in my 308.

    Does this system do anything more than simply warn the driver that he is going overtemp, or if does anything to try to help correct it? It seems to me that if it was simply a "warning light', it wouldn't have to get the lambda ECU involved. Since the output signal of the thermocouple goes into the ECU, it would make sense that an overtemp condition might cause such things as the exhaust air-injection system to close, and the mixture to richen (by decreasing the fuel distributor's control pressure) to try to help cool things down. I may be thinking way more into this, but it would be nice to know.

    Could you please provide a link to this version you are looking at. All schematics I have, are slightly older, and omit the ECU system. Also, nice explaination on thermocouples, that is great info!
     
  14. Badman

    Badman Formula 3

    Mar 4, 2007
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    Bruce Wayne
    Yes, they are available. But they list for $800!!! Ricambi can probably get that closer to $500, but it's still too much to pay. I'm working on manufacturing them myself and they'll cost about $100. I've got the whole thermocouple assemblies, but I'm having trouble finding the correct compression fittings to finish the job.

    The thermocouple ECU is a seperate unit from the lambda ECU. Two different ECUs. The epoxy box that you're looking for the name of is the thermocouple ECU. The slow-down circuit is not tied into the lambda circuit at all. And yes, it is just to control the dash warning light, the thermocouple does nothing else.


    You can get any owners manual you want from ferrariowners.com, which is where I got the one I was looking at. There is no '85 manual, but the '84 manual has the schematic with the lambda and slow-down circuits. You've got to sign up as an owner with your VIN before you can get access to them though, so I can't send you a direct link.
     
  15. Badman

    Badman Formula 3

    Mar 4, 2007
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    BTW, I think you're right not to want to disable it. If the car is running normally, disabling the slow-down circuit won't effect anything. But without it, if you lose a bank or similar and your cat gets too hot you won't know till you see the flames!
     
  16. GrigioGuy

    GrigioGuy Splenda Daddy
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    Correct. Of course, if the ECU is bad the system is already disabled. That's why I said remove the cat and disconnect the harness -- you have to do both to stay safe.

    If Ricambi has these I need to call them. When I was looking for one 5 years ago nobody had them at any price. I've been running catless and ecu-less for the past 48,000 miles.
     
  17. Badman

    Badman Formula 3

    Mar 4, 2007
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    Yeah, removing the cat solves lots of problems :) In some of the more goofy states <cough>MA</cough> though you can't get by inspection without the cat.
     
  18. Adaptel

    Adaptel Karting

    Jul 21, 2007
    89
    Concord, NH USA
    Full Name:
    Kevin Pearson
    When people talk about "removing the cat", are they referring to changing it over to using one of those hollowed-out cats? Maybee I'm crazy, but I found that this makes the exhaust note sound slightly "off"....sorta weird sounding that most other people don't notice. How restrictive is the cat anyway?

    I used to live in MA, and what a crazy overkill inspection that state had!!!! Do you still have to strap your car to a rotating barrel so they can sniff your exhaust while the wheels are running at highway speeds? Having moved to NH, they usually just check that all the lights work, peek at the brakes, and slap a sticker on. No emissions checks whatsoever! In NH, we live free or die (of breathing toxic emissions gasses).
     
  19. Helmut

    Helmut Formula Junior

    Dec 11, 2004
    635
    Has anybody ever found reasonably priced replacement thermocouples for a 78 308?

    Helmut
     
  20. ramosel

    ramosel Formula 3

    Sep 11, 2004
    1,236
    Meadow Vista, CA
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    R Moseley
    "reasonably" is such a hard word to swallow in the Ferrari parts arena. I just got two NOS ones (111703) from Patrick at FerrParts. I think they were around $200 each.

    Reasonable? dunno - depends on your budget. I was just not looking forward to having the car all back together soon and having one slow down light still take 10 minutes to go off.

    Rick
     
  21. Helmut

    Helmut Formula Junior

    Dec 11, 2004
    635
    Hi Rick, Thanks for the info, I will call Russ and see if he still has some. I was wondering though, does it have anything to do with the thermocouples wether or not the slow down lights go out? I think they just go out after a few seconds once you turn the ignition on, its switched by a relay.

    Helmut
     
  22. Paul_308

    Paul_308 Formula 3

    Mar 12, 2004
    2,335
    Sounds like you need the inexpensive readable schematics package for the 84-85 US QV. http://www.ferrari308gtbi.com/


    ________________________________
     
  23. Birdman

    Birdman F1 Veteran

    Jun 20, 2003
    6,687
    North shore, MA
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    THE Birdman
    I had the cat ECU die on my Mondial a few years ago, rather than the thermocouple. Since there are two ECUs in the '83 Mondial, I was able to swap them to discover that the error went with the ECU not the thermocouple. Fortunately, I was able to get a replacement from an fchatter. I agree that you should have these warning lights if you have cats. Not worth the risk of not having an early warning if something goes funky with the ignition and a cat overheats.

    Jeremy, you really need to finish building your replacement thermocouples...I'm sure there is a great market for them not just here but Ricambi and others would probably love to get a decent price on them.

    What about a service to swap them out for dead OEM ones to recycle the fittings?

    Birdman
     
  24. smg2

    smg2 F1 World Champ
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    Apr 1, 2004
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    Here's some more tech knowledge for you all. Cats that originally came on these cars are no longer used, the 2-way converter. it's now a 3-way catalyst and those operate at different temps and for control of Nx. and with closed loop control vs the open loop.

    some white paper info:
    so a couple things pop into mind, the early two bed converters are no longer used as EFI and OBD II controls became the norm and the TWC-converter is std. early TWC's used an aux air injection to help with light off. cat temp is highest at the exit and colder near the entrance of exhaust gases. the Lambda feed back CIS can employ the TWC with little change. the earlier non-lambda systems run with a higher percentage of HC and exceed the chemical reaction efficiency design of the TWC converters. however the TWC's as noted above will tolerate and operate at higher temps than the previous 2 bed ones. so if your carb'd or non-lambda system is within spec I wouldn't be surprised to see the slowdown lights kick on based on the above specs.

    I'd also check to see where the thermocouple probe is in reference to the exhaust stream in the cat, up or down. and if you have an open loop system use a two-way converter.

    The thermocouples I've seen are the yellow/red wire ones which would be the K type, seems both are employed. the wiring diagrams I have either show yellow/red or no color at all. even my mondial has both, yep somebody put in another set and left the others in place. industry use K types are about $50 but are not long enough and have the wrong end. not real helpfull other than they are not worth the $$$ Ferrari wants. personally I'd wire in a wideband sensor and display, tune the engine with that and use one that can trip the light based on too rich or lean.
     
  25. smg2

    smg2 F1 World Champ
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  26. smg2

    smg2 F1 World Champ
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  27. Bad Dogg

    Bad Dogg Formula Junior

    Sep 29, 2006
    422
    The Burgh
    Full Name:
    Howard
    I have the same issue but due to a mouse eating the wires in my ecu. Slow down light is now on intermittently.. If there is a way to get another ECU out there, I would be all for it!
     

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