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

Yes but I believe Dr Bob also tested a tc and it worked n the bench but did not in the car do to a loose wire.

 

First time reading this thread. Not every post but a few posted by Jevs. Prior to this I have never inspect or seen a 355 TU. The big picture is clearer but a few areas remain opaque.

The first part is the TU testing. 12v is injected into the unit. Under ambience temperature the output voltage is .5v. When heat is applied to the TU, the temperature rises and the output voltage increases proportionally. This implies the TU acts as a variable resistor where the resistance lowers proportional to heat rising. Logical enough. I will verify this myself when times allows.

The second part creating a TU bypass. This is a little too hairy for my liking but then again all hairy thing scare me. The TU and ECU work in conjunction to prevent a catastrophe. It is a safety device to protect the engine. I read through you posts carefully to make sure I’m did not miss anything but one fact concerns me. The bypass generates a .5v regardless of the situation. I understand this useful to prevent nuisance SDL when engine is cold or under operating temperature. Logically, this implies the bypass unit does not impede the TU function so if and when the exhaust temp goes beyond the normal set point the SDL will trigger as intended. This is fine but something bothers me.

When a TU bypass is added, essentially it become a parallel circuit leading to the ECU. In a parallel circuit, current is additive and voltage will slightly less than the higher output. If 3v is required to trigger a SDL warning, the thermocouples would have to register higher than 3v output since the bypass circuit lowers the actual output of the TU. This can be disastrous since SDL won’t come on until exhaust temp reaches a much higher temp. Can you elaborate further on this point? Maybe I miss something.

FYI, I did not read ever post. [emoji851]

 

Jevs posts are very informing. From what I gathered, the ECU reads three different voltage setting. A minimum .5v. A mid ( ) that will cause the SDL to blink and a mac ( ) that will cause the SDL to be solid.

 

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It’s important the 355 community understands the SDL is a safety feature. Whenever we go to alter something like this, we need to make sure all areas are accounted for. The unintended consequences maybe more harmful than the nuisance SDL that comes randomly.


Above is a simple TU circuitry of the top my head with respect to OP I formations. Notice bypass circuit is connected parallel to the trigger set point circuits. We’re talking about a tenth of a volt matters. .5v will change the setting allowing temperatures to go higher before it triggers. Technically speaking the TU can be altered so it reads .5v all the time. The ECU will will trigger a SDL.

 

For easy reference, I have attached below a simple block diagram of the SDL system. One ting I have not established myself is the existence of the 5V bias voltage inside the ECU, on the line that receives the varying voltage signal from the SDECU but I have read here that it exists.
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Since your SDL is flashing whatever voltage you apply to Pins 14 & 15, it may be the case that this voltage does not actually reach the "Volt sensing chip" (so it gets the internal 5V bias) or the chip is faulty continuously sending an SDL flashing signal (calling it"Volt sensing chip" just as an assumption) . The fact that both SDL-s are flashing suggests that it is not a problem with the voltage from the SDECU reaching the chip (it is unlikely that both lines, from pin 14 and pin 15, are not reaching the chip due to bad contacts on the pins 14 and 15 or some internal loss of connectivity. So, it is more likely that you have a faulty volt sensing chip. I would start with inspecting the condition of the external connections at pins 14 & 15 then check the internal lines from the two pins to wherever they go (probably ending at a chip). If everything looks fine inside, then I would test with another ECU.
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Sorry, ignore the second picture (went in accidentally and it is also for the 2.7 Motronic).

 

If this is a 5.2 car there is only one SDL.

 

I have a 348 so not too familiar with the 355. If there is only one SDL, then a bad connection at the ECU either Pin 14 or Pin 15 is quite likely. If the connections inside the large ECU connector are good, then I would trace the lines from the two pins further on the PC board inside the ECU.

 

To clarify, for a 355:

2.7 has (2) SDL (Slow Down Lights) 1 for each bank.

2 SDL ECU's - 1 for each bank

5.2 has (1) SDL (Slow Down Light)

3 SDL ECU's - 1 for each bank + 1 for bypass valve.

 

The 348/355 community is well informed about their cars. The diagnose and solutions are spot on. The success rate however leave us more adventurous and accept everything at face value and never question anything. This is true with regard to the electrical system. A fix may rid one trouble item but may very well all it’s doing is masking the warning signs of cancer while the disease exists. I can’t imagine the things folks do to their car. Fortunately, the 355 engine is a solid working machine.


We all and believe it or not even I know what a SDL is. We know it’s intention. We know it’s purpose. We know it’s components but unless we have extensive experience with electrical circuitry, control and design, Its just enough informations to get us into a lot of trouble. What I’m getting at, before we go to alter, add, delete an electrical system in our cars, make sure we have a complete understanding of its inner working. If ypu did anything already, ask yourself the question, how do you know it works”?

 

Jevs, if you are still around, thanks for the bench test informations. The very thing I was looking for. Your bypass gadget on the other hand, I see a bug. You would need another line in the control circuit to maintain proper function. I won’t post that. Our purpose is to cure the disease not mask the warning systems.

 

Not sure if you got this from all these posts but Most people who were advocating for the sdl computer bypass were running straight pipes so meltdown was never a concern or would be.

If your car was running that rich the power loss would be easily noticable and you would slow,down anyway.

 

Find your problem Randy

 

Tazz, I assume your post is in reference to my comments. Let’s start with a new canvas so we can paint an accurate picture of the 355. At least the electrical part of it.


What is an SDL safety system? It’s an electronic safety shutdown that prevents the engine from operating above a defined temperature. It consists of a warning light, ECU and Thermocouples. The system constantly monitor exhaust temperatures with reference to three set points. It warns the driver of temp. abnormality and take action if necessary. At start up the TUs send out a signal to the ECU. At a specific voltage or higher but below a threshold to establish the components are working. If the temperature reaches a threshold, the ECU flashes a SDL telling the driver exhaust temp. has reached an unsafe zone. If the engine continues to operate in the unsafe zone, the ECU will shut it down. This is a summary of the informations I gathered from observation and members on this forum. We can pretty much conclude these are facts.


Now the law of probability states, the more you play with fire, the likely you will get burn. Altering, bypassing the SDL safety net only expose the engine to danger every time it runs. I’m sure some many of “us” spend a lots of time and probably money trying to bypass the system. If the objective is to get rid of the flashing light, would it be easier to just take the flashing light bulb out? And if going with a straight pipe eliminates the meltdown why bother with a bypass device at all. I’m willing to bet, everyone believes a bypass solves the problem. Let’s make this clear, a bypass does not solve the problem. it only rid the warning sign. The problem remains. It’s no different than unscrewing the SDL bulb. In fact a bypass has the potential to do more harm than unscrewing the bulb. Alternately, a $10 resistor in line with the ECU does the same thing. I suppose going with a straight pipe and unscrew the SDL bulb solves it for some. For rest of us, understand this a bypass DOES NOT solve the problem.Worse case scenario, unscrew the bulb. I hope the picture is clear.

 

The 355 community has to know one thing. A bypass device in any form does not solve the SDL problem. And if have no plan on adding a straight pipe, do not change, alter or delete your TU. It is safer to have nuisance SDL than than a meltdown.

 

Not yet Taz, but in my defense, I’ve been a little busy with “around the house/lawn” stuff. I want to go through your testing and reset suggestion. I will certainly post my outcome. One of the posts from m. stojanovic started me thinking about the circuit with in the Motronic. I may have the correct voltages entering the ECU @ pins 14-15, but where does it go from there? It may go to the circuit that monitors the over voltage first and then to the SDL...or any number of paths. But I like your idea to “force” the reset enough to cause the “chip?” to re-learn the parameters.

Thanks for remembering my problem and again, I do appreciate your time and help

 

+1

However, I think there would be very few people who would not be aware of this. Roth, if you look at the whole history of nuisance SD problems, you would see most cars/owners suffer because of unreliable OEM parts. Apart from people who dishonestly want to hide their SD problem so they can sell their car quickly, I think honest owners simply want an effective, reliable functioning system not a bypass system (unless they have straight pipes).

For those people with aftermarket high flow cats, are CELs or SD problems the biggest issue? Do high flow cats let through more temperature? If so, can they be desensitised enough to stop nuisance SD lights, yet still provide enough warning of cat meltdowns? I can't recall if someone was already looking at this. I have recently fitted high flow cats, but have had no problems so far. If I did, I would be checking the operation of OEM parts before I started looking for something to trick the system into thinking there was nothing wrong.

 

High flow cats are not the problem. They do not let "more temperature" through. In a functioning system high temperatures are generated by the cats due to some other malfunction which results in excess hydrocarbons in the exhaust. Straight test pipes will not generate an SDL problem, even with excess HCs in the exhaust, because there are no cats to burn off the excess HCs.

Flashing SDLs at cold startup is an indication of a problem with the SDL system, TC, TCU or ECU or related wiring.

Some people have reported problems with CELs when using high flow cats associates with cat inefficiency codes, but mostly such reports are from those running test pipes when the down stream O2 sensor needs to be fooled.

 

This morning I cleared my calendar (I’m retired LOL), and was bound and determined to get to the bottom of this SDL nightmare. Taking your OBD reset suggestion, I plugged in my OBD scanner, verified there were no CELs and/or pending CELs. Then, I completed another “re-clear Codes”...even though there wasn’t any thing there. Started the car and no SDL Went for a drive to heat up the cats and not a single flash. For the last 3-4 months, the SDL has been flashing, and regardless of what I do, it would never go away. I can’t begin to count the number of times I’ve reset the CEL and continue to have the SDL. But it seams you’ve figured out the key that unlocks the lock!

So, my sincerest thank you for sharing your suggestion and for your time and patience

Randy

 

G’day mate, I’m not so sure of your optimism.I like to think the community is aware of the bypass device mascaraed as a solution. One has to have a complete understating of electrical circuitry and design to know that. Having read many posts, I’m not there yet. Let me explain it in a way will hit everyone in the gut.


According to multiple studies, America tips the scale on the most obese country. Weight lost programs is a multi billion dollar industry and there’s room for everyone to weigh in. All it takes, a video of a fit girl on YouTube blogging about her daily routine of what she does and eat and all the fat girls would subscribe to the channel. The blogger could be lying and misleading. It doesn’t matter. The fat girls are desperate looking for gimmick to loose weight. We, 355 owners are the fat girls when it comes to the SDL issue. We are desperate for a solution. And like the fat girls starving to loose weight, we will spend great amount of time and money just so the flashing light doesn’t come on. I am not the fat girls.I am the nerd with the big glasses telling the fat girls cutting down you donut intake to half a dozen a day will not take your fat away. Of course they don’t want to hear it and probably accuse me of fat shaming.


Mate, you see where I’m going with this? IIf anyone really really really want a bypass device, I have a $10 resistor complete with drawings, instruction and specifications for a price. I will only sell one copy. Message me and make an offer. There is a minimum reserve.

 

Are you serious? You don't need to know anything more than if you use a by pass device you aren't monitoring cat temp and if you have cats this could result is a serious problem.

This is like saying you need to have a complete understanding of combustion to fill you tank with gas.

 

No........of course I’m serious. You think I enjoy taking time off work to come on here just to fat shame people. WoW, that thinking must be weighing heavily on your mind. You realize understanding how something works is not the same as a complete understanding of its inner working. I understand how a combustion engine works but I can’t tell if the timing is off by sight, sound or feel. A top notch mechanic can. That’s the difference.


So tell me Dr. K, what are the different parameters the SDL ECU look at when engine is running? Give an example of an electrical bypass circuit. It doesn’t have to be in Ladder Logic format. Just explain what it does. What won’t do. Oh never mind I’ve given too much details already.

 

Mods, how come my emojis don’t show in my posts. Hope is has nothing to do with fat shaming.

Understand I only fat shame to get someone into shape. Big love that’s all.

 

Here is the basic logic of the SDL system.


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As far as the main ECU is concerned the "threshold temperatures" are related to voltages that are output from the cat TCU (SDL ECU if you like). The cat TCUs are basically nothing more that thermocouple amplifiers with a gain of 100 and output limited to between 0.5 and 5.o volts. Yes they are more complicated than a simple amplifier because they need to have circuits for cold junction compensation and such. A bypass device simply sends the main ECU a voltage signal greater than 0.5V but less than the voltage associated with temperature threshold 1. There is no understanding of electronics needed to understand the operating principal and very little required to design a bypass device, several of which have been presented in this and other threads.

Should anyone use a bypass device if they have cats on their car. Emphatically NO. There would be a serious rick of fire should the cats over heat.

 

Slight revision.
Once the injectors are shut off and then turned on again the checks begin at the top.

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