"Slow Down" warnings
I’m posting this in Tech Talk as it may be of interest to more owners than just 550’s.
As some may be aware, FBB from the other site & I are working together on a little project to eliminate the bogus “Slow Down” alarms and trips. We’re doing this on our 550’s but if successful, the solution should probably work across all/most other models.
For those who have forgotten, in a nutshell, the “Slow Down” indication is there to warn the driver of an overly hot catalytic converter. If it begins flashing, you should obviously slow down. If it comes on solid for more than half a second, then the Motronic ECU’s will kill the injectors on the offending engine bank in order to protect the catalytic converter and/or the engine & car from going up in smoke. So you’ll have only half your engine running, and at that point it really is time to shut it down, put it on a flatbed and bring it to a specialist to get it sorted. Unless of course the alarms are bogus……. But how to know if they’re bogus or real?
Ferrari obviously didn’t have a lot of confidence in their own system because in the WSM, the 1st flashing alarm point is stated to be 1720 + 86 degrees Fahrenheit, and the 2nd alarm point is given to be 1778 + 86 degrees Fahrenheit again. Never mind that’s a range of 192 degrees on each point, and the two ranges have significant overlap!
According to my esteemed colleague FBB, this system has been a real PITA since it was brought out, and has been the cause of numerous bogus tows, and limp home episodes for many owners. I’ll take his word for it, as his Ferrari ownership experience is both deeper and wider than mine and crosses over several models. So, since both FBB and I are both tinkerers and DIY’ers, we decided to try and sort this issue once & for all.
FBB initially came up with a solution that involved 1) bypassing the two SDECU’s (Slow Down ECU’s) and injecting a constant 1.5v signal into the Motronics ECU’s to keep them happy, and 2) installing a set of analog gauges to monitor the cats’ temperatures directly. This was a very cool idea IMO, but it obviously puts the onus on the owner to actually monitor these gauges and take the appropriate executive action when necessary to prevent the cats and car from catching on fire. FBB realised this wasn’t an optimum solution and started looking into ways of keeping the original functionality, but making it more reliable, along with keeping the analog gauges. A belt & suspenders solution, as it were, and that’s when I became involved.
I had a hunch this system might be a fairly standard thermocouple & transmitter setup, ie something that takes the very nonlinear and very low level (millivolt) output of a thermocouple, linearises it, and “amplifies” it up to a 0-5v output.
So I decided to run a few tests to confirm my hunch. To do this, I needed to be able to simulate a thermocouple, and be able to run this simulated thermocouple through the range of temperatures it’s liable to see living inside the cats (ambient up to around 2000 F). This takes a thermocouple calibration box, or simulator box, something every self-respecting EE or ex-EE should have in his toolbox.
So I rigged up my simulator box and “mapped” this system, and learned that it was indeed an industry standard thermocouple/transmitter setup. Here’s the graph of the output:
Obviously, Ferrari weren't interested in the range of temperatures below 500 degrees or above 1925, as the system flatlines in those regions.
Now before anyone decides to take this data as gospel, or decides to double check it and finds their readings are different, let me state that I’ve mapped this system three different times, on three different days, and come up with three different sets of data. So yes, I've found the readings do vary, and as much as 20 degrees or so, hence Ferrari’s stated range of “+ 86 degrees Fahrenheit”.
Now what was fairly stable was the level of voltage out of the SDECU’s into the Motronics that would trip the 1st and 2nd alarm points. Those points were 3.75v and 3.91v, however on one day those points would correspond to 1650 & 1725 degrees, and on another they would be 1640 and 1710, etc.
The next step is to substitute some “real” thermocouple transmitters for the Ferrari ones, and confirm they’re more stable. I picked up a couple of Moore Industries transmitters which have been used in the process industries for years and are known for their reliability and stability. So we’ll see how they perform.
30 September update:
The Moore's didn't work - they were current output type only. So I picked up some universal transmitters and set one up on the bench this morning and the results were encouraging. In the pic below, the blue curve is the OEM Ferrari SDECU, and the red curve is the new transmitter.
Next step is to test the transmitter at different levels of input power to ensure it's stable, then I'll put it in my car for a bit, and finally will use it to help diagnose another guy's SDECU problems. Then we'll field test it for a few months to see if it's actually an improvement over the Ferrari ECU's and if so, we'll turn it loose to the community.
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