Another 355 slow down issue

Hello all.

I've spent the last few days investigating a problem with a flashing slow down light on my '97 355 which is a European M5.2 car. I'm completely stuck as to what to do next, other than just hand it over to a dealer.

I replaced the cats with some new ones, non OEM, and took it out for a test drive. While out I noticed that the slow down light was flashing, so brought it home to investigate. Unfortunately I don't know whether the light was on as soon as I started the car because I had the steering wheel in a position where the airbag obscured the slowdown light so I only noticed it when I glanced down whilst turning a corner.

At this point you're all thinking CAT ECU or Thermocouple. But I've tested all of these and they all appear to be working properly.

When I reconnected the thermocouples to the new cats, the left hand side one was a bit tight so I had to work it in carefully. I wasn't careful enough though because having measured the millivolt reading I found it to be dead, so I replaced it with a new one expecting the light to go out but sadly it is still there. Both sides now give sensible millivolts across the thermocouples and I've converted these to a temperature using a page on the Fluke website, and I get sensible hot idle temps.

I already had a spare CAT ECU which is new, so I tried swapping that in on both sides to no avail. I only have one slow down light being a 5.2 car, so there's no hint of which bank is at fault.

Next I checked the output from both cat ecus at idle to verify they are sending a sensible output to the main ECU. Both are sending 0.5v (which they should do up to about 300 degrees celsius) and send 5v with the thermocouple disconnected, so they appear to be working correctly.

I've disconnected the 88 pin main ECU plug and carried out a continuity test between the two cat ECU connectors and their respective ECU pins. Both were fine. So I feel that the whole temperature sensing system is working correctly and it is pointing to something upstream of the sensors and ECUs. I've done an ECU relearn several times, and had the same result each time - light stays on. I've tried taking it for a drive but haven't strayed too far from home for obvious reasons, and it still stays on.

The light comes on from cold, so it isn't an overheating Cat. I'm not getting a check engine light so I don't think I have lambda problems. I do have a rough idle at the moment - exhaust smells a bit rich from cold if you get close to it and it misses a bit - but with some revs this goes away and the car drives fine so I think this is an independent problem.

Anybody got any further suggestions regarding the slow down light before I take it to the main dealer for a diagnosis?

 

Hi R, fancy bumping into you here! Glad you decided to post your problem here and I hope you find a solution soon.

Best!

 

At the risk of posting the obvious, I chased this same issue on my car for awhile. Maddening to start the car and before it even warms up the slow down light comes on.....gah.

In the end, the mechanic (I use the term loosely here) that did the engine out work on my car managed to reverse the electrical connections for my By-Pass & R/H side CAT ECU's.

I only discovered it after I got really frustrated and just jumped on the accelerator on an entrance ramp and the slow down light went out.....huh????

After a review of the wiring diagram I reversed the connections and all is well now for a few 1000 miles.....

May your issue be as simple as mine......?

 

To completely eliminate the catalyst ECU's as the problem, try connecting a 1.5v "fooler" signal in their place. Standard AA batteries will work fine for this.

The batteries need to be earthed on their negative side, then connect their positives to the 3rd terminal in the "non-thermocouple" harness connectors at the catalyst ECU's. One terminal brings +12 in, another brings earth, and the 3rd carries the 0.5-4.5v catalyst ECU signal back to the Motronic. You'll just need to probe the leads and figure out which is which.

If your "Slow Down" alarm was severe enough to generate a CEL, you'll probably need to reset your ECU's with an OBDII device or software.

 

Good point....

 

I've not had a check engine light at all, but I was wondering whether it just needs a reset with the SD1 to clear the error. However, I've read of plenty of other people who having cleared a fault had the light go out of its own accord.

Watson - the car is an EU one; they don't have a bypass thermocouple - only a left and a right. It's not possible to get them mixed up as the connectors don't reach each other and the ECUs are on opposite sides of the car.

 

Are EU cars OBDII compliant? Get a simple OBDII code reader...even without a CE light, there is probably a pending code stored, and that will tell you which side is acting up. If you have time, order an ELM 327 scanner online from Ebay for $12 or so. Cable connects via USB to your laptop, and you load some freeware it comes with, works great. They make bluetooth versions for a few dollars more.

I have one similar to this: http://www.ebay.com/itm/V1-4-ELM327-OBD2-OBDII-CAN-BUS-Auto-Diagnostic-USB-Interface-Code-Scanner-Reader-/200764652390?pt=LH_DefaultDomain_0&hash=item2ebe817b66

 

Hi R,

Some good feedback here I feel. Told you there's a wealth of Tech info here.

I’d definitely try bypassing the system with the 1.5v batteries. I know you already have tested both t/couple and ecu but ........ just to be sure to be sure. It’s only a few minutes work, couple of batteries, some insulation tape and speaker wire.

I presumed, given that you have the 5.2 Motronic that you had OBD connection possibility. Understanding now that you do not I do feel that you should try and add the functionality if you have the capability and instruction from others. It will be a useful feature to have way into the future and might also assist you with pin-pointing this particular issue.

Given that you know your cats aren't really overheating....... (You are sure of that aren't you? Have tested with IR heat gun or at least witnessed in darkness after a run?) ........ there’s no mad rush to solve the problem. I don't believe the flashing Slow Down (SD) is anything but a warning. I know it's distracting and annoying but I don't think it does anything to ignition or fueling so you can drive on and wait to see does it cease or get worse. If it gets worse and steps up to solid ON then it will be cutting fuel and something will have to be done.

Also, this all happened when the cats were replaced right? Re-trace the steps you took to carry out that job. Ask yourself what could possibly have been disturbed in the process and go back and check again.

Do please bypass both potentially faulty items with the battery trick and report back.

Best of luck.

 

I do have an OBD reader and have just been to pick it up today (it was in another car that was in storage) so will get it connected up tonight. I found the instructions last night that Eric355 had posted - it seems easy enough to enable it. It'll help investigate the idle issue as well I hope - although my money is on plug leads for that.

Will also do the battery test to eliminate that possibility.

I'm 100% sure my cats aren't overheating. The slow down light flashes as soon as I start the engine from stone cold. Having measured the temperature generated by the thermocouple I am also sure that it reaches a plateau at idle and after a run the cats do not glow red or generate excessive amounts of heat.

 

Greg, that is correct I noticed the problem after I replaced the cats, but the full story is that the car has just come out of storage having not been driven for over a year. It had a major service & cambelt change while it was in storage and I am unable to say whether the light has been flashing all along, and even whether it was flashing when I picked it up before I changed the cats, because position of the steering wheel (large airbag wheel) was obscuring the slow down light. I've lowered it a bit more now so I can see it ;-)

 

I've now tried the 1.5v battery test and the slow down light carried on flashing. This is not unexpected, given I tested the voltages coming from the cat ecu's and found them to be as defined by the Motronic manual.

I've also tried connecting my OBD reader to it, but had no joy. It just says it cannot establish a connection to the ECU. I used http://www.ferrarichat.com/forum/showpost.php?p=138424319&postcount=2 as the instructions. They seem pretty foolproof. It's probably the OBD reader - I don't think it's an ELM327 chipset.

So I'm still no further on sadly.

 

Well, since you've now eliminated the only two possible sources for the alarm, then the only other possibility is one of your main Motronics ECU's has a blown/burnt input for that alarm (AMHIK - ouch!)

I would start looking around for an ECU, perhaps initially try to beg/borrow one from another 355 owner?

 

To make sure the issue is the ecu hard wire the 1.5v signal right to the ecu by splicing closer to the connector. That takes wiring out of equation. Make sure you bend ecu pins connector pins for good temporary contact. You get joy then buy supremo gold connector kit. If no joy swap in another ecu to see if problem is gone. I'm pretty sure 355 5.2 moronics are not coded left right car specific like 360s but verify.

 

In my opening post I mentioned that I have verified continuity between the signal terminal on the plug which connects to the cat ecu and the appropriate pin on the main ECU connector. I did this for both sides and I did it by measuring the resistance between the two points. On both sides I got zero ohms, so I think the loom is fine.

I've also tweaked the contacts inside the main ECU connector to try and make sure a proper connection is being made for the two pins that receive the cat temp signal. At the moment I can't measure what is on the output pin for the slowdown light as firstly I don't have any idea which pin it is, and secondly I'm reluctant to break into the loom anyway. There is a breakout box available but it is mucho pounds.

Does anyone have any information about the slowdown light circuit or which pin on the ECU carries the output? I'm presuming that the ECU doesn't actually actively flash the light; it probably relies on a different circuit to do that inside the binnacle.

 

Saw your thread on my Google travels just now. Ouch indeed.

 

Sorry to hear you've had no success with this.

I guess a "loan" of someone's ECU would be a neat way to proceed but I have to say that I'd personally be a little reluctant to offer one of mine to someone to "play" with for fear of the same damage potentially being caused.

Maybe it's time for specialist diagnosis and possible ship it off for repair attempt? I know it's not something one can really ignore, bloody thing flashing away all the time.......

Bloody Ferraris!

 

There's a NOS 176224 ECU on eBay for around $1000. Don't know if that's the one you run in your UK cars or not?

Claudio might have something too.

 

well since the op is pretty good with a DVOM ever thought of trying to find out if the ECU 1 talks to ecu2 or if ecu1 and ecu2 report to the same SDlight via a spliced connection? Lets say you work backwards from the SDlight and find that each ECU can report to the light independantly then you could sever the connection on one ECU and prove you got a bad ECU by swaping ecus left right and noting sdlight or not since it is pretty unlikely it is both ECU's

 

Thanks for the tip; it's described as "-Valid from Ass. Nr. 26747 - Not for USA, CDN, AUS, SA and ZA - Valid for ABS BOSCH cars - Not for 355 F1 cars " which looks right - will try and verify my assembly number and see if there is a part number on the motronic later when I home later.

I'd love to swap ECUs with another owner to test, but wouldn't dare do it in case I popped theirs. That'd be double trouble!

fatbillybob I'm a bit confused by your question regarding ecu1 talking to ecu2. The car is a 5.2 vehicle so it has only one main ecu rather than a left and a right as in the 2.7. You weren't referring to the cat ecus were you?

Thanks all for your input - I know I keep saying "already tried that" or suggesting things aren't relevant but I've been looking at this for about a week before my first post and I've already seen a lot of your suggestions on other slowdown threads, so I've already tried a lot of the things that have been suggested.

I think it's bite the bullet time and ship it off to see what the dealer can make of it. although I'd still like to get an OBD reader on it but if the ECU is partly fried then that could explain why I can't get my existing one to connect either.

 

Have you tried to just unplug both cat ecu's? I drove my 355 several months like that - no slow down.

 

Yes, the light continues to flash. I think I read somewhere that works on a 2.7 car but not on a 5.2 car. Which was yours?

 

Hi Richard, what part of the UK are you in ? I have a spare exhaust ecu that you can borrow if it helps you out, I am on pistonheads if easier to contact via that route.
I was told by a respected dealer that the best thing to do with those ecu's is dissconnect them and pull the bulb from the dash, I was very suprised to hear this approach and questioned it to which the dealer said to me "you know when your car is not running right" and dont need a warning light to tell you, I am pretty mechanically minded and been around cars and bikes all my life. Now you seem to me to know one end of a car from another but I recon you have found your fault already in one of your previous posts when you stated cars is lumpy and smells of fuel, classic unburnt overfuel in cats syndrome no ?
Car is overfueling for whatever reason.

 

Hi Paul, I'm in Surrey.

Thanks for the offer of the spare cat ecu. I will get back in touch if I need it; it might be worth a try just to ultimately prove this is not the problem by swapping them both out for your spare one and mine, although every measurement and test I've carried out so far tells me the existing ones are fine so I'm still confused on that.

Tonight I managed to get a new (ELM327) OBD reader connected to download fault codes. I have live data showing but there are no faults listed which surprised me a bit if I'm honest. I expected some misfire freeze frame data or something given the rough idle. There is one pending fault which is a MAF sensor voltage, but when investigating the rough idle I disconnected the MAF so I'm pretty sure that was just me. The MAF reading itself is around 11-13g/s at idle which I think is correct. I haven't logged any data yet as I had a "visit" from a neighbour who came to tell me how nice my car sounds from her living room, so I had to shut it off.

Will report more on the lambdas and other readings in due course. Initial observations seemed to be that it isn't actually running rich, although I need to check my facts on the lambdas before I go any further with that. The fuel trims look ok too.

I'm still a bit confused although I have more information to look at now. I still think the rough idle and the slow down light are unrelated problems though as the slow down light is essentially an independent system. A faulty M5.2 ECU with more than one fault would explain it all I guess, as others have implied already.

With regard to the overfuelling thought, I can accept that if it smells a bit rich it might well be, but I'm having trouble aligning that with the symptoms. As I understand it, the cat will get very hot if there is unburnt fuel leaving the cylinders which is subsequently burning inside the already hot cat, but perhaps someone can correct me if I say the cat takes time to heat up in those circumstances. It isn't an instantaneous thing so if it were a genuine overheating cat the light wouldn't flash from stone cold, unless the ECU stores the state in flash memory somewhere. I don't think it does that because replacing faulty cat ecus does put out the light without any other form of reset other than a battery disconnect & relearn for other people. The first time I noticed the light I got under the car and put my hands as close to the cats as I dared. They weren't radiating massive amounts of heat. People normally report them glowing cherry red in those circumstances.

 

My understanding is that if your SD warning lights up within a minute of starting from cold it simply has to be faulty circuitry. That said, if you are dumping un-burnt fuel, it does not take a long time to get those babies cherry red. (Realistically the visual test needs to be done at night or in low ambient light). When I had a misfire (on one cylinder only) I'd be cherry red from cold within three miles of moderate driving.

I really don't think you have the typical SD problem on this occasion. You've done all the testing and outputs are correct.

 

Just a thought. Do you get the Sd light if the thermocouples are removed from the cats but still connected ? They are non-OEM cats correct? Perhaps there is intermittent shorting occuring with ground in the cat mount for the thermocouple.? The early trouble you had in mounting the thermocouple to the left cat makes me wonder.