Thermocouples, slow down lights, related CEL’s, and how to conquer them...

No I know you’re blowing smoke in the garage It is impossible to reverse engineer a system without a complete understanding of its inner working. You can’t just say ok I need something to do this, this, that when there’s an issue and go build a device that does this, this, that and all is ok. Designers have to account for unwanted operation when this, this and that happen. Moreover the design must have the ability to................... You need to have depth not just understanding how it works.

I posted on this very subject on another thread awhile ago. Members said they no longer have nuisance SDL because they did something to the TU’s or install aftermarket units. I asked, “How do you know the TU’s work.” I didn’t get a reasonable answer. Because I didn’t get an from you, Dr. K or anyone else implies no one knew. Beside, the intend of my question was to confirm or debunk the method(fix). Nothing is more scary than driving a 355 around thinking the meltdown safety net is working.

I’ll ask you, John, what causes SDL to flash?

 

There is nothing to reverse engineer. The main ECU receives a voltage signal representative of the exhaust gas temp and processes it through an algorithm. What causes the SDL to flash? That signal being between two voltage limits. How does the ECU actually flash the SDL? Who cares. I don't need to know how an audio amplifier works to know that if I input and audio signal to it, and it's output is connected to a speaker, sound comes out. I don't need to know how an IC engine works to drive a car.

Look, as I said, I don't need to reverse engineer anything about the ECU. I could connect a VM to the output of the TCU and heat the TC with a torch and see at what voltage the SDL flashes, what voltage it come on solid and at what voltage it shuts down a bank. Once I do that I would know the min and max voltage range over which the SDL stays out. Then I can build a device that provides a constant voltage in that range to the ECU to keep the SDL out. If you want to call that reverse engineering, OK. But it doesn't require knowing anything about the circuits in the TCU or ECU.

 

Roth, if you asked the same question of OEM parts, would you expect a reasonable answer? How do we know OEM parts work? I bought an OEM/factory engine harness through a reputable dealer. I expected it to work. It didn't. I had to rewire it myself so it did. The aftermarket TCUs (I'm aware of) were sold through reputable Ferrari parts dealers. We expect them to work. As I understand it, these aftermarket TCUs were basically a copy of the original using more reliable components. They are/were clearly marked for cat and non-cat cars.

Without a circuit diagram of the Motronics ECU, all we know is that the ECU produces an earth to put on the light. On the 5.2 car, this lamp and Check Engine lamp are provided with a common battery voltage via relay E13 in the passenger footwell.

 

Gentlemen, please do t get too excited. This is not the political rally or a porn stars convention. We are here to discuss issues that are actually important and hopefully get a legs up on the problems. Recall the question, “How do you know it works” The reason I asked, if you found a fixing or came up with something better than OEM TUs and works PROPERLY, then you have a deep understanding of the system inner working. You should be able to explain how it works and better yet post the circuitry. Now, I don’t read every thread or every post in the thread. I’m sure I read enough to say no one knows the exact thing causing nuisance SDL flashing. I maybe missing something. Please prove me wrong example. Im confident I can confirm or debunk it. I’m willing to put my reputation on the line. Either way I got a dozen donuts and a happy meal voucher going your way.


John, nothing in your post is wrong. Nine out of ten is not a perfect score though. Read your latest post. Paragraph one. Sentence number 4. Everyone in the 355 community knows by now the ECU flashes the SDL and shut down engine. There’s more to it than what we see. As to your drawing, it’s spot on if anyone wants to know how the system works. However, it is not useful if you want to reverse engineer Or add a device alongside the original TU’s.


Ian, OEM parts come from the factory and the folks who designed the car. The materials used, the method maybe the weaker link but I rest assured the parts do what intended. I can’t say the same for after market fixes unless there’s literatures. We have to remember the line between a TU that works properly and one that doesn’t is small invisible to the eyes.

 

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I did read on one of the forums, that one of the problems with the OEM thermocouple ECUs was the potting compound used. It appears to deteriorate prematurely, leading to components being exposed to the elements. Materials have improved over the years. Gothspeed (at least) used improved potting compounds on his cat/non-cat/switchable type ECUs.

By the way, I don't think we are trying to attack your good reputation as an engineer. I (for one) am just trying to fully understand the logic/thought processes behind what you are trying to do. I don't know exactly what you've read on the forums.

 

This seems to be the appropriate place to obtain help. My 98 355 OEM bypass valve needed replacement. Since Ferrari doesn't make the valve anymore, I bought an aftermarket valve which was put in. Unfortunately, the aftermarket cats I put on the car, because the OEM cats went bad, run cool. Apparently this causes the sensor at the bypass valve to misread the sensors running to the cats so that the position of the bypass valve is misread and the check engine light goes off. Happens when the car is at idle but not when driven hard, presumably because the cats are hotter when the car is driven hard. There is apparently an electric "box" that can be placed on the car that fools the sensors so the CEL won't go off. I got one of those which worked for a few months until that box shorted out. I also understand there is some device in non USA cars that is not allowed in the US which can "fix" the problem. That is about the extent of my understanding of the entire ridiculous mess. The car runs great and I have been told to just drive with the CEL on but not really comfortable with that solution. Quite sure I am not the only person who has had the CEL go off for "bypass valve" issues even though the valve is working. Hoping there is a cure

 

I haven’t heard of any device but available in the US before. This topic has been vetted in many threads over many years in here.

The BP valve location has been problematic for some people on not others who have hi flow cats installed. I for one have them and have had them for 5 years and have not had the recurring sdl that you have. I had an issue with that location but it was a problem with the thermocouple not the SDECU or the cats. I would verify that your issue is not from components completely before assuming it’s the cats.

Some who have had straight pipes installed have had this problem and used Goths constant voltage box to fake the SDL algo. Skipp is one who can comment on the success of that as I think it worked for awhile but came back?

Anyway, I have your set up and I do drive my car , about 3000 miles a year, and have no sdl due to hi flow cats. Again, trouble shoot the components and the output of the SDECU from that location before assuming it’s your set up.

 

Ehrst, when you said you changed your cats, do you mean the small cats in your bypass system? I can't see how the main cats would affect temperature in the bypass system.

Does the bypass thermocouple have several functions? Does it tell the ECU if the bypass valve is operating properly and detect overheat?

As Bob says, I would make sure that the SD light is not due to some other factor. Something may have been installed incorrectly or damaged during the installation of the new cats. Have you ever tested the thermocouple itself at any point? By the way, do you recall which brand bypass valve and cats you fitted? Also, what brand "electric box"?

 

I dont think that is the problem.
I think it has nothing to do with the bypass
The O2 sensors are very sensitive.
Put extenders or better yet minicats inbetween the sensors and the cats.

 

I have had hyperflows for over 10 years and had a intermittent problem before the mini cats.
Since then no cel for many many years

 

Grant, Ehrst was talking about SD lights, not CEL's . AFAIK, minicats won't help with SD lights.


Until we know the timeline of the component changes and what was actually changed, diagnosis is a little harder.

 

Grant there is no relationship between 02 sensors and SDL at all. SDL take their cue from thermocouples and SDECU only.

The hi flow cat/ straight pipe connection to the SDL issue relates to the cat temp change detected as a result of the altered OEM set up that alters the voltage outputs of the TC and SDECU complex.

In the case of the BP location on 5.7 cars, this change in dynamic sometimes alters the temps at the BP TC such that a SDL is thrown.

Now a rich fueL condition could lead to a hot cat which could throw a SDL due to a high voltage at the SDECU detected by the TC. You would probably have a CEL associated with this for fuel trim but again this would be at the cat locations not the BP location.

I would again focus on the components and if the OP wanted to definitively diagnose this, measure the actual voltage output of the BP SDECU real time and he will have his answer.

 

Hmmm. Was it not Skipp getting CEL based on bypass valve? Perhaps my memory is off.

 

Ugh... If true, perhaps the ECU is alerting the driver to two different things .... e.g. if the bypass thermocouple is not detecting any temperature rise when the valve is open, maybe it generates a CEL to tell you the valve is faulty. If the bypass valve opens normally but the temperature is too high due to a bypass cat failure, does the ECU generate an SD light?

(EDIT: if the solenoid valve for the bypass valve was faulty. This should generate a CEL. Perhaps Skipp's problem was this?).

I have no cats in my bypass line, but the only time I got an SD light was when I accidentally had my bypass and right cat TCU plugs swapped over (no CEL). I still can't figure out the logic behind this.

 

Did i read it wrong i saw cel posted

 

I only read above you guys
Am i missing something
This is why i thought he meant cel

 

LOL.. Mass confusion. I just saw temperature mentioned... so I assumed .

Maybe Ehrst can clarify. Did he check the ECU code generated?

 

Sorry about the confusion. Probably due to my lack of knowledge. My problem is with the "check engine light" CEL. I have not had any problem with the slow down lights. Had my OEM headers and cats changed out for FAB Motorsports 2016. Car ran fine for a while and then the CEL went off. Dealer said "exhaust bypass" code going off. They examined the valve and typical for OEM valve, needed replacement. FAB Motorsport valve put in. CEL still going off. Dealer realized the FAB Motorsport valve opens backwards so they turned valve around. Drove car and CEL light back on after about 100 miles. Again code was for "exhaust bypass". Dealer and I investigated and has already been stated, some have had trouble and some have not with the setup I now have. My limited understanding is that the aftermarket bypass valve does not communicate well with the rest of the car so that sensors inappropriately detect the valve position. Was told this "electrical box", presumably mentioned above, would fool the temp monitors and bypass the problem not setting off the CEL light. The box was placed on the car and the car ran about 500 miles before the CEL light went on again, code again for bypass exhaust. I have never had an issue with slow down lights. Sadly the car runs great. Even passed CA smog once the CEL light stayed off long enough for a quick visit for smog check. On testing the bypass fabspeed valve is working beautifully. So, from my very limited understanding, this seems like an electrical or sensor issue with stuff not communicating. I don't want to drive the car with the CEL light on all the time cuz that light has saved my bacon when it goes off and I have had other issues, like cracked radiators and water pump issues. I have asked the dealer mechanic to give me the part number for the box and I am waiting. Hopefully this additional information will help someone figure out my next steps cuz the car has sat for 6 months trying to figure this out. Thanks

 

The only sensor for bypass valve working or not is the TC/SDECU for that location. The bypass CEL your getting is as a result of the output of that SDECU. The motronics is looking for something voltage wise to make it happy and meet whatever criteria it has programmed to not throw a CEL.

I have tapped my sdecu output at that location and in my car with a correctly working BP valve the voltage when fully warmed up is .8v and when the BP valve opens when driving it climbs up to 1.4 - 1.8 v and returns to .8 it so on resuming cruising and valve closed.

I would have someone check the actual voltage of the output of the sdecu under above conditions and again make sure the components of the tc/sdecu are working and not faulty and creating the issue then if they are working as designed , revisit your BP valve function.

As I said, my BP valve CEL was as aresikt of a faulty thermocouple which when bench tested looked fine.

 

Thanks for the update, Erhst/SSE.
It's interesting to hear that the problem did not resurface for another 100 miles (after having the Fabspeed bypass valve re-oriented). Did the electronic box fail or did something else fail? I don't know if there is a general Motronics ECU OBD2 code for bypass valve problems or if there are separate codes for bypass valve failures, bypass valve solenoid failures and bypass cat failures.

I'm a little suprised to hear that your technician recommended an "electrical box" to fool the system. I'm trying to think of what properties an aftermarket valve would have which would trigger a message if it was working properly. It's just a valve which opens and closes and as it has been checked ok, then, as Bob says, we have to start looking at sensors and thermocouple ECUs (or possibly intermittent valve control problems).

Do you have an OBD2 code reader? Can you read and clear the codes in your Motronics ECU? These are always a good investment on cars of this age.

(EDIT: note that it is possible to interchange the thermocouple ECU (SDECUs) for faultfinding) If you can clear the codes, then swap over the thermocouple ECUs, this can be a good (minimal cost) first DIY faultfinding step). I think it's time to get rid of that electronics box.

 

Yes. The box shorted out. Was presumably working for a while. I suppose one option would be to try and repair it. I think all the sensors and temp readers have been tested and are working. One theory I have heard is that the OEM headers/CATS run hotter. This temperature increase sends a signal that is appropriate to the exhaust system and bypass valve position. My CATS apparently run cooler, so when the car is not driven hard the signal from those temp monitors sends a signal to the exhaust that the bypass valve is in the wrong position and the CEL goes off. Some cars don't have this issue, some do. Nonetheless, lots of people have altered their exhaust systems with 355s and at least a few have had issues so I suspect I am not alone and. I will have the mechanic check the TC/SDECU and the voltage. I will get more information on this "box" as well.

 

One additional interesting fact. THe CEL never goes on when car is moving and RPMs are up. THe only time the CEL goes on is when the car is idling, usually for an extended period, like at a long light or in traffic... if that helps.

 

as bob said above
If you have no bypass thermal couple ecu because you sed to have a box that bypassed that then that is why you are getting a cel.

As Bob said you need the tc ecu or your box fixed. The motronic as Bob said above needs to see an output.

Bob described it perfect above.

 

Interesting. So the CEL does extinguish. The Motronics ECU doesn't seem to hold the light on (to remind you there was a problem). Do you know if your OEM thermocouple ECU (SDECU) is still fitted to the car, but not plugged in? Did the add-on box completely replace the thermocouple ECU (and now talks to the ECU)? Is the new box also attached to the thermocouple? (It seems to be still picking up something as the light is going on and off)

It seems odd that you get the problem with the car idling. The valve should be closed and you would think there would be no temperature measurement by the thermocouple (except for general heat in the engine bay).