F355 slow down light - how to diagnose fault?

F355 slow down light - how to diagnose fault?

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BMW A6

Original Poster:

1,276 posts

12 months

Thursday 21st November
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F355 5.2 Motronic; the cats are cold, as engine just started, yet the Slow Down Light (SDL) is permanently on and 1 cylinder back is shut down.

I have removed the 2 connectors into the cat ECU (I think 1 is the thermocouple and the other the 02 sensor?), and yet the SDL remains on.
Picture shows the left hand side module, but I have also disconnected the right hand side too.



Why is cat ECU still sending signal to shut down a cylinder bank when the 2 connectors into the cat ECU are disconnected?

Are there connections from the cat ECUs to another ECU, which is forcing 1 bank to shut down? If so, where are those?

Really don't know what to try to see what is faulty and on which bank?


Larry5.2

419 posts

56 months

Thursday 21st November
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If the 355 has an OBD2 port, I would get a reader and see if anything comes up.
Then replace all the spark plugs. If the fault persists, swap the ignition coils between banks. If the fault moves with the coils, it was one of them. If this doesnt work then it's injectors, fuel-pressure or ECU.... which is probably time to take it in....

In answer to your question, it may not be the lambda or temp sensors that are commanding the shutdown - other possibilities are misfire sensors, or self diagnosis on the injectors and coil drivers in the ECU. Unfortunately I'm not familiar with the 355 to know which of these it has.

Edited by Larry5.2 on Thursday 21st November 14:22

BMW A6

Original Poster:

1,276 posts

12 months

Thursday 21st November
quotequote all
Thank you Larry.
I was under impression that only cat ECUs would cause SDL?
Are you saying that it could also originate from faults in ignition system?

I will have to try and convert the 3 pin OBD plug into the 16 pin system, I guess?

Larry5.2

419 posts

56 months

Thursday 21st November
quotequote all
Sorry I'm guessing as I dont know your car - just commenting from knowledge I've gathered along the way.... All depends on the ECU and the strategies. If you've eliminated the Cat sensors, then it has to be something internal to the ECU. Some cars have misfire tubes in the exhaust so that the ECU can match a pressure pulse with an ignition command. Other ECUs sense the current through the coil to confirm spark and combustion. But do check the coils by swapping them....

Btw, shutdown is usually to prevent unburnt fuel from reaching a hot Cat. I'm not sure lambda sensors are the best tool for detecting unburnt fuel because they react the same way to lean mix and raw fuel. Thermocouples might trigger it.

If you google f355 shutdown, you might get some useful info, e,g,

https://www.ferrarilife.com/forums/pre-99-v8s-gt4/...


HardtopManual

1,260 posts

114 months

Thursday 21st November
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The light will remain lit with the cat ECUs disconnected, but the bank shouldn't go down. Turn car off, unplug the cat ECU on the affected bank (make sure it's the plug that goes to the Motronic, not the thermocouple - or I suppose you could unplug both), then turn car back on. You will still have a slow down light, but both banks should run.

BMW A6

Original Poster:

1,276 posts

12 months

Friday 22nd November
quotequote all
Thank you all for input.

I am still not sure what connects the cat ECUs to the motronic - is there a cable I can follow?
I think I can see the 02 sensor (2. in pic below) and thermocouple (1. in pic below) connecting into each cat ECU, but how does the ECU connect to next item to initiate cylinder bank shutdown?



Also, the 2 panels behind the seats do not have the standard 16 pin OBD port - they both are 3 pin. Are they both diagnostic ports doing the same thing?

Is there something I can purchase to plug into the car to reveal the faults? I have a 16-pin OBD tool which bluetooths car data nicely to my mobile phone on my other modern cars..........would be useful to do the same for the 1996 Ferrari.


F355GTS

3,513 posts

203 months

Friday 22nd November
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First thing that normally fails is the Cat ECU, the sealant fails and let's moisture in, used to be the black ones were the problem but the later green items as you have are also known to fail.

Easiest way to prove is swap it wit the one on the otherside and see if the problem moves.

If it's not the ECU then the Cat thermocoupler is the next most likely, you can buy a Ford one for about 10% of the Ferrari price although the cable is over length

You can fool the ecu by sending a voltage up the sensor wire, can't remember the value but it effectively tells the main ecu that everything is fine

phib

3,982 posts

207 months

Friday 22nd November
quotequote all
F355GTS said:
First thing that normally fails is the Cat ECU, the sealant fails and let's moisture in, used to be the black ones were the problem but the later green items as you have are also known to fail.

Easiest way to prove is swap it wit the one on the otherside and see if the problem moves.

If it's not the ECU then the Cat thermocoupler is the next most likely, you can buy a Ford one for about 10% of the Ferrari price although the cable is over length

You can fool the ecu by sending a voltage up the sensor wire, can't remember the value but it effectively tells the main ecu that everything is fine
This has always been the issue on mine after driving it in wet conditions.

Doesn't help they are behind the rear wheels !!

I took them off,dried them out and resealed them.

No more issues 10 years on !

Phib

650spider

670 posts

119 months

Sunday 24th November
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There are 3 of them if i remember correctly on the 5.2 motronic.

When i had that issue i bought 1 new one, removed them one at a time cleaning everything up, swapped them about and it did solve the problem...however as was posted earlier i didn't actually need the new one...cleaning up and reconnecting did the trick.

I seem to remember the green ones were around £160 each from Ferrari and sold it on Ebay when i sold the car.

HardtopManual

1,260 posts

114 months

Monday 25th November
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phib said:
I took them off,dried them out and resealed them.

No more issues 10 years on !

Phib
Same here - they even work to spec (which is detailed over on Ferrari Chat).

OP - the cat ECUs connect to the thermocouple and the motronic. They are just dumb signal amplifiers. Can't remember which way around the connections are, but the one that doesn't go to the thermocouple goes to the motronic :-)

BMW A6

Original Poster:

1,276 posts

12 months

Wednesday 27th November
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I've purchased a 3 pin to 16 pin OBD adaptor. Will plug it in to the connector behind the seats and see what it throws up.

davek_964

6,187 posts

123 months

Thursday 28th November
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If it is the Cat ECU - do some research when buying it. I needed one for my 348 years ago and assumed EuroSpares was cheaper than Maranello. After I bought it, I discovered that Maranello were £100 cheaper (think it was something like £250 vs £150 at the time).

Might not be the case now, but definitely check prices in a few places before you buy.

650spider

670 posts

119 months

Sunday 1st December
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davek_964 said:
If it is the Cat ECU - do some research when buying it. I needed one for my 348 years ago and assumed EuroSpares was cheaper than Maranello. After I bought it, I discovered that Maranello were £100 cheaper (think it was something like £250 vs £150 at the time).

Might not be the case now, but definitely check prices in a few places before you buy.
I actually found Ferrari were cheaper for lots of parts rather than Eurospares...in some cases the new part was cheaper than a Eurospares used part.

It probably helped that i used the same dealer for about 15 years so always recieved a good discount, but i always ask 'what's the max discount you can do on that' for anything i buy...its amazing how many people don't.