355 Slow down light issue

Swap the two ECUs and see if the trouble moves to the other side. Ill bet it does. Then buy a new ECU.

 

Try to clean your TDC-sensors(Crank-sensors) and contacts thorougly. I had this problem a few weeks ago and i went through everything, but it didnt dissappear - until i cleaned and sealed the TDC-sensors. They collect dust and dirt, and will make your car run on just one bank if you have a problem with them.

Kimmo/Sweden.

 

By the way - Are you having the Ceck engine light also?

Kimmo.

 

Since you only have 1 slow down light you have a Motronic 5.2. It only says "slow down" but does not say which bank is causing the problem, like the 2.7 cars.

Disconnecting the cat ecu's and then reconnecting them does NOT clear the slow down light. The SDL only comes on when the Motronic gets an "over heating" signal from the cat ecu. So what you were doing was disconnecting and then reconnecting the cat temp ecu's, which did nothing at all. The cat ecu's have no memory storage. So what you plugged then back in you where just putting them back into the loop. The reason your car seems to running in limp mode is because it IS running in limp mode. The Motronic "thinks" the cat is over heating so it shuts down the back it is getting the signal from. If you want you run the car and not have it go into limp mode you need to disconnect the cat ecu's and leave them disconnected. This way the Motronic is not getting a signal from then telling it a cat is hot. But DO NOT do that just yet.

You said that you did not get a check engine light. To clear out the engine management control unit (Motronic) you either pull all the codes out and reset it, or disconnect the battery. But since you got no check engine lights this won't be necessary, yet.

So now what?

Here is what you do.

When the engine is stone cold.......

1) Disconnect only ONE cat ecu, start the car, and see if the SDL goes away along with the engine running in limp mode. The reason you need to only disconnect one is because you only have one light. So swapping the cat temp ecu's from side to side will not hep you determine what side is giving you the problem, the way it would on a Motronic 2.7 car that has one SLD for each bank. So...................you need to only unplug one at a time to see if the problem goes away.

2) If the SDL still comes on then plug in the cat ecu that was disconnected, and unplug the other side. And repeat the engine running test.

3) If the light goes away you have found the bad side, but you're not out of the woods yet. What you will need to do now is, swap the cat ecu's from one side to the other leaving the one that is "bad" unplugged on the opposite side. The reason you are doing this is to put the "good" cat ecu's on the side that was causing the slow down light to come on and putting the car in limp mode. What you are checking for now is to see if you have a bad thermocouple. That is the actual temperature probe that gets screwed into the cat. So by using the "good" cat ecu on the side that was shutting down the bank if the thermocouple is faulty you will still get a slow down light and the bank will still get shut down.

If the light does NOT come on you know the problem was the bad cat ecu on the other side that is STILL unplugged.

If the light comes on then you know that the thermocouple is bad because the cat ecu you are using is know to be good.

Now there ARE instances when the cat ecu's give a REAL overheating signal to the Motronic. But, the reason I want you to perform the test when the engine is cold is to completely rule out an overheated catalytic converter. Since the car has been sitting over night there is no way that you could have an over heated cat that is still stone cold. Capiche?

Give that a go and see what you find.

One more thing I forgot to mention. When you run the car with the cat ecu disconnected the Motronic will store a silent code, meaning that you will not get a check engine light for it, but it will still be stored in the Motronic ECU. The car will still run with one or both of the cat ecu's completely disconnected. So don't worry about that. The Motronic only stores the code to let you know that it wasn't getting a signal from the cat ecu/s. After you have done the test THEN you will want to reset the Motronic ECU by disconnecting the battery, or pulling the codes a resetting it.

Okay now go check things out.

 

Ernie's primer looks like a great diagnostic tool. One warning...there are three cat ECU's on a 5.2 Motronic, so be sure to identify them correctly. The left side ECU is on the left side of the engine compartment. The right cat ECU and the exhaust bypass valve ECU (looks same as CAT ECU, but used for different function) are on the low right side, behind the bumper. They are stacked on top of each other, and I forget which is which, but following the thermo probe wires back should clearly tell you which is which. Failure of the cat ECU for the exhaust bypass valve is usually accompanied by a "check engine" light with an exhaust bypass valve failure code.

I had an intermittent failure last summer and it was the right side cat ECU. New one from Ricambi America, and all was good. You could simply buy one and move it around to find the problem one. But that won't help you if you have two bad ones.

 

If you have a fouled plug it can cause the car to run rich, which in turn can get the cat over heated.

Hmmmmmm???

But what I don't get is why you are still getting a SDL when you had BOTH the cat ecu's completely disconnected???? Or did you? If not try running it with both disconnected and see what happens.

 

Both.

Disconnect the battery, let it relearn the warm up parameters, and then take it for a drive.

But what I'm still not getting is why you are getting a slow down light with BOTH the ecu's disconnected?????? Sounds like you could have a wiring problem if that's the case?

 

You are in Europe! Euro cars don't have the third cat ECU for the bypass valve.

 

Well, we just got the car out for a trip and the slow down light came on from engine startup, I have been reading this thread and have a couple of questions is there any significance to whether the light is flashing on and off or on steady. After startup, the light flashed on and off. Because I did not have time to research the issue, the car went back in the garage. Today, I have read as much as possible, and followed Ernie's procedures, with both ecu's unplugged I am getting the light on steady. What is the difference?

Thanks,
Jeff

 

Interesting.

I guess I should clear things up. First my "knowledge" comes from the 348 2.7 Motronic. So most of what I comment on about the 5.2 is really educated guesses. They are usually pretty close guesses, but guesses still.

Okay now having said that.

What I'm gonna guess is, I think that with the 5.2 they may have changed the programing to have the Motronic show the slow down light all the time when there is no signal from the cat ecu, as well as an over heating signal???? With the 2.7 this isn't the case. I have been running my 348 with the cat ecu's completely disconnected AND removed from the car. They are nowhere to be found in the engine bay. I get no slow down lights (the 2.7 has one for each bank), but I do get a silent engine code stored in the Motronic (4121). Meaning that the CEL does not illuminate, but the code gets stored. So that is why the guys with the 2.7 check things, the way we do, by disconnecting the cat ecu's. But I guess this isn't possible with the 5.2???

I would have suggested disconnecting the thermocouples, and leaving the cat ecu's plugged it, but then you would still have a possible bad ecu in the middle of the loop.

Hmmmmmm??????

Let me think.

Okay how about this?

If you have a friend with a 5.2 355, that has working cat ecu's, try unplugging his and see if that triggers the slow down light? Then try borrowing one of his cat ecu's and swap it out with one of yours to see if that gets rid of the SDL in your car? Don't put your cat ecu's in his car. Use the known good ecu to test things out in YOUR car. I'm trying to think of ways that you can find the source of the slow down light without having to hook it up to and SD2/3 computer = $$$. It's better to find the problem and ONLY replace that part, instead of just throwing parts at the problem until the light goes away. Not a good practice.

Sorry I couldn't be of more accurate help guys.

 

When moving the cat ECUs around try only swapping them around ......... I am thinking if you leave any connectors unpopulated you may get one of these errors.

 

But since the car only has one slow down light, what would swapping them accomplish? It would just trigger the light the same way would it not??

Jeff

 

Correct!

Which is why I "TRIED" to think of a way to test for a bad cat ecu without having to plug it into a code reader, or the SD2/3 machine at the dealer. But I guess that can't be done since it seems that the 5.2 will throw a SLD if it isn't getting a signal from the cat ecu.

I'm wondering if anyone that has the single MAF 5.2 Motronic, and working cat ecu's, can test out this theory of disconnecting one cat ecu to see if it gives you the slow down light?

 

The ECUs want to see something ....... so leaving any of them out of the electrical loop will absolutely give you an error and you will never find the culprit ...... try leaving them all plugged in 'electrically' then 'only remove one thermocouple' at a time from their respective bungs. This way you may be able to isolate which one is causing the slow down light. When and if you isolate the issue ...... move the problem cat ECU to by-pass position responsibilites and do not install the thermocouple probe ......... just cap the bung and zip-tie the probe off neatly to the side ....... this should keep the light off until you decide to replace the cat ECU.

 

Interesting idea Oz.

 

Okay,

I tried the thermocouple removal idea, removing one or both still results in the slow down light coming on (back to blinking) so now that seems to narrow it down to one or both of the ECU's being bad. I pulled the one from the left side found a small seperation in the plastic so I am sealing it up with epoxy. Next I will look at the right side, seal if necessary and reinstall. Then I will report back the progress. Anybody have a known good used ECU for sale? Are they the same for 2.7 and 5.2 cars?

 

I would buy new ones...not used. Not sure of the cut off point, but for the 5.2 355's, Ferrari changed the epoxy from black to green. The green ones are more durable than the older black ones. My car had frequent cat ECU replacements in it's history with the previous owner. Once they were all switched to the green versions, I have only had to replace one.

 

I think that is probably good advice, the car has 2 green ones and one black one. That is the one I epoxy sealed but no change.

 

I started having the same problem with my 355 (5.2). On a cold startup, the slow down light would come on right away and it was a solid light (not flashing). When it came on, the engine went into limp mode and one bank would shut down. There is no way that the cats can get hot that fast.

So I let the engine run for about 20 seconds, shut it off, opened the engine cover and touched the left and right exhaust manifold. In my case, the left manifold was cold. That had to be the bank that was shut down. I unplugged the left Cat ecu and went from a solid slow down light to a flashing, but at least the car was not in limp mode anymore. Not sure if is a ECU or a thermocouple issue, but I am ordering the ECU anyway...

 

BTW, from what I have heard, the green cat ECU's came out with the 360 (both use the same cat ECU). So all 355's probably came with the black ones. Green only if they were previously replaced.