355 slow down light

Obviously some electrical problem. Had the same problem intermittently on my 328, so I cleaned the contacts and the problem went away. Good luck!

 

Scott,

Have you tried turning the main power switch off on the car, then back on, start the car and allow the ECU's to reset?

may work?, just a thought

 

Since your a new'b I'm not gonna bust your balls.

http://ferrarichat.com/forum/showthread.php?t=87702
http://ferrarichat.com/forum/showthread.php?t=86620
http://ferrarichat.com/forum/showthread.php?t=75458

But I will say, the search button is your friend.

In a nut shell one of three things are happening.

1) you have a bad cat ecu, which based on what you are describing I'm gonna guess is what's going on
2) you have a bad thermocoupling
3) your a/f ratio is running rich causing your cat to over heat.

 

Switch the Cat ECU's. If other 4-cylinder slow down light comes on, then you know it's a bad Cat ECU. I had this happen to me, and when driving, the 4 cylinders whose light came on shut down! I had to limp to the shoulder of the road and turn off car. Re-start, and the system re-set itself.......temporarily........until it would come on again. I swapped ECU's......the other slow down light came on for the opposite 4 cylinders......problem diagnosed and solved........new ECU.

Jeff

 

Are you sure? When the slow down light comes on it should say "slow down 1/4" or "slow down 5/8".

If you read in the threads I posted the proceedure is the same. Swap the exhaust ecu's from one side to the other. Then see if the light comes up on the other side. If it does then you know which ecu is the problem.

I'm not 100% sure on this but, on a 348 anytime you disconnect the power to the Motronic ecu's, (i.e. disconnecting the battery) you need to let the car relearn the warm up process. The '95 355s have the Motronic 2.7 as do the 348s, and anytime you disconnect the battery you have to go through the warm up process. So i'm almost possitive you need to do the same thing for the Motronic 5.2. If you don't the car will run funny, especially at idle and in stop and go traffic.

 

Well swap the ecu's and see if that gets rid of the light.

 

Pre-'96 F355's have 2 slow down lights, one for each bank.
When they switched to OBDII, only one slow down light was provided for both banks. Makes it a little tougher to diagnose without an SD1, unfortunately.

Before diagnosing, was ANYTHING done to the car regarding catalytic convertors, O2 sensors, exhaust mods?

Usually when the slow down light starts flashing as soon as the car is started, there is an open in the O2 sensor circuit or the thermocouple circuit. Swapping thermocouple ECU's won't do you any good, again, because of only 1 light. But you can check the connections and check the condition of the ECU's themselves. Are they cracked? Is the insulation intact, etc.?
Other than those physical inspections, I'm afraid you will have to have the car connected to an SD1 for proper diagnosis. It could be one of the ECU's, or a thermocouple (or even just a bad ground through the thermocouple). You just won't know without that tool. Just playing the swapping parts game can start to get very expensive.

Please post what you find as the remedy.

Best,
pino

 

Well that explains it.

 

Ernie's right. Also, a bad spark plug wire or a bad spark plug can cause fuel to be dumped directly into a cat without first being burned...a very bad thing.

Most suggestions so far in this thread presume that you have a faulty signal.

First off, for safety reasons, make sure that the signal isn't correct. For instance, are either of your cats glowing red when you start your car (or are they still glowing red after you shut her down)?

If you have a remote temp guage (e.g. laser or pyrometer), take a reading of your cats.

If the cats are fine, *then* start tracing out why you are getting a faulty signal.

I'm not sure if the OBDII systems trigger the slowdown light when you disconnect the exhaust ecu on either side, but that would be my first test (i.e. disconnect one ECU, check for Slow Down light upon start...if no Slow down light, then you've narrowed down which side has the signal problem).

 

Well then you have a bad cat ecu. The problem is, which one?

No the light won't come on if you unplug it, actually the opposite, it will shut the light off. But, it will send a silent code to the Motronic ecu stating that you don't have a signal coming from the cat. You shouldn't get a CEL nor a Slow Down light. It will get rid of the light and allow you to drive the car, but it will not solve the problem. At least that's how it works on the 2.7's.

If you know someone else with a 355 you can swap your cat ecu with his. Do one at a time to see which one gets rid of the light. Then give your buddy his ecu back, and go buy the one you need.

 

Swapping ECUs (or any electrical component) with a friend that has a perfectly good car...They would have to be a really good friend.

Although not too common, I have seen a short to ground and when the good electrical component was swapped...Well the new one went kaput, same as their friendship went kaput.

It was one of those "it won't hurt your car".

Just my .02

 

had similar problem with my "96" 355, and ultimately the cat ecu's needed replacing. remove cat ecu's and inspect the inside surface around the seams for cracks and shrinkage. There serems to be a fair amount of efficacy suggesting that miosture finds it way into cracked units.

PB

 

Well I guess the 348 brotherhood are better friends than the 355 sisterhood are.

Actually it shouldn't be a problem. The ecu cat only serves to talk to the motronic ecu. Besides what do you think they do at the dealer?

 

Oh I know that parts swapping is common. However, on high dollar low voltage electronic parts I have seen it cost too many freindships.

When one person is chasing a Gremlin and then the nice accomodating person develops any sort of problem that wasn't there berfore (and probably not even related) well....It can cause a lot of bad feelings.

A new problem is then related to: "I did swap my bla bla bla last, and now my turn signals don't work".

Anyway, to each their own.

 

Hmmm - if the light stays off when you remove both, then is it not the case that if you put them back in turn (running car inbetween) then the light will come on when you connect the dead one?

 

The 355 has 3 cat sensors, when the slow down light is blinking, a code has been set in the main engine ecu, and will not go away untill cleared with a scanner (I use a generic obd 2 scanner from snap-on). Each sensor sets a seperate code. I don't remember the numbers off hand, something like p1485, p1489, p1490 or something.
Clear the codes with a scanner, with the engine running, and chances are it won't return unless there is a real problem with one of the three sensors or its associated module.
If you have access to a scanner, you can get codes, clear them, unplug each module one at a time and retrieve codes each time to figure out which code goes with what module.

Or you can pay me to do it.

happy hunting

E

 

Just sent you a PM, might have something useful for you.