Slow Down Light DIY Fix on a 1989 328 GTS

Great write-up, James...many thanks.

I’ve been plagued with false slow-down warnings for months, sometimes when the car was stone-cold.

So, how hard was it to locate and remove the cat probe and O2 sensor? (I’m currently traveling so I can’t just go out to my car and have a look). Also, do you have the Bosch part number?

Thanks again,
Alan
N.J.

 

I've removed the cat completely.
My slow down light still comes on now and then intermittently during every drive.

 

The thermister probe is just a piece of metal that increases resistance as it heats up, and that change is read by the Cat ECU. Cleaning the probe can help, as can cleaning the connection on the other end to the ECU. I uses a brass wire brush on the themister, and a bit of deoxit on the electrical connectors, and all has worked fine for many years to solve a flicker problem along time ago. Should be independent of the O2 sensor. The thermister itself shouldn't wear out, but it is in a harsh environment, and the wire attached to it could internally oxidize over the years, and alter the resistance readings and give a wrong signal.

Of course the problem with all this is that sometimes the probe is reading accurately that some unburned fuel is getting into the cat, typically from worn plugs, extenders, dist cap, rotor or coil, all of which do wear and fail and that is what the slow down light is detecting. If the car is idling and accelerating without hesitation, it is probably just the sensor. But it is a good time to inspect and/or reflect on the the last time the various ignition items were replaced.

 

Apparently, the Slow Down Light system uses themocouples not thermistors. Here is a thread which discusses the circuitry:
https://www.ferrarichat.com/forum/threads/interest-in-slow-down-light-cat-ecu-substitute.498691/

Unfortunately, JMAZ never brought his units to market.

 

Probably 20 years ago I had a similar problem with the slow down light illuminating on my 1987 GTS. The cure for mine was a new oxygen sensor. The heat sensor is a thermocouple.

 

Ah yes, I stand corrected. The thermocouple being heated generates a voltage in the sensor and is detected in the ECU. Works at high temperatures, resistance based thermisters are typical for lower temperatures. In any event, cleaning the probe and electrical contacts does help resolve issues.

 

James:

That was very useful...many thanks!

I look forward to getting this fixed once and for all.

Alan

 

This is fine if the problem is with the thermocouple, etc. But typically the problem is with the electronics. My '89 328 - bypass pipe and with the thermocouple wire-tied to the chassis - will still illuminate the light at random intervals, indicating a temp problem when the actual temp in the area where the thermocouple is located is less than 100F. I often wonder what happened to the
guy who posted here that he was developing a replacement electronic uint and then just disappeared.

Pretty much like the guy from Fabspeed who claimed they were developing a muffler system that would produce 30 more HP! Maybe they (and the Slowdown computer guy) live in some alternate universe where claims actually equal performance!

 

It can't be that hard to get something done with a thermocouple. Here is an off the rack solution....the exhaust probe and digital guage, you could probably retrofit the gauge to the OEM probe, and mount it under the dash where you can't see it, but you can set an alarm temp which will give an audible warning. Probably better than the flickering light on a dash panel anyways. All for $40. I bet if someone sent an old Ferrari ECU to the company and asked them to replicate it they could probably come up with a new OEM type unit.

http://www.thermomart.com/index.php?route=product/product&product_id=226

 

I corresponded a fair bit with Jim (jmaz) when he was developing the modules. He started out just wanting to replace his own broken one on his 328. I sent him my 308 QV modules and thermocouples for him to compare to his 328 modules. If I recall correctly, he is/was a professor of physics at the University of Colorado.

I think his plans may have gotten a little too grand; he was hoping to commercialize the modules. He had designed plenty of stuff but really hadn't manufactured anything before. The project stalled.

It's sad that he didn't post the schematic because many of us could probably just build them ourselves. The circuitry probably isn't that complex.

​

The modules really don't do much: they turn on a light bulb at a trigger voltage from the thermocouple. I think Jim had actually designed his to start blinking at a trigger temperature and blink faster and faster until the light stayed on solid at some higher temperature. He also had some other ideas.

It would be REALLY nice to have a small module that drove both Slow Down lights AND that was located in the trunk rather than behind the passenger footrest (its there on the 308 (dumb!); the 328 may have it in the back: I'm not sure). The difficulty is finding thermocouples of appropriate physical dimensions (probe length, retainer nut thread size/pitch, etc.). It would be just great to put the modules in the same trunk box with the spark boxes and run plain wire up to the Slow Down lights rather than having to run the thermocouple wire through the central console tunnel every time a thermocouple goes bad. … and apparently it is usually the thermocouple that goes bad; not the modules.​

 

It's been some years since I removed the cat BECAUSE the slow down light was malfunctioning. I would have preferred keeping the cat in place. As I recall, I tested the thermocouple and it was OK. Nowadays, I guess it would be pretty easy to put some sort of temp sensor on the cat and have it trigger the dash light at whatever temp. I might consider that.

 

Has your light ever gone on since removing the cat?

 

Absolutely! It might come on at idle, it might come on at cruise - totally random. Might blink, might be steady. It has never stayed on more than maybe 20-30 seconds. Again, this is with no cat and the thermocouple wire-tied up against the car body.

 

Same as random illumination as mine other then the fact that mine never blinks.
Good to know.

 

Follow up:

I did as James suggested and removed the thermocouple from the catalytic converter and polished it with 1000-grit sandpaper until it was nice and shiny. The whole job took just a few minutes. I then took my GTB for a spirited run today and the slow-down light never illuminated (it would have certainly done so before today).

I didn’t replace the O2 sensor because I’m a one-thing-at-a-time kind of guy.

Let’s hope this permanently resolves my problem...time and miles will tell.

Thanks to the Forum for all of your assistance!

Alan
N.J.