My first 328 slow down light while cruising! :(

My light would flicker intermittently and sometimes remain on. Rather than immediately purchasing a new control unit, I tested by driving until the light came on. I pulled off in a rest spot and checked the exterior temperature with an optical means. The exterior was about 700 F which seemed ok to me. I have seen others at 1200 F where they glowed. I drove the car for several months with the light flickering or sometimes being on once I had convinced myself the cat was not too hot.

Just for s---s and grins I replaced the O2 sensor. After a few outings the light never came back on other than the initial test when you first start the car. That was about a year ago. I never really got to the bottom of the problem but it went away. The O2 sensor may have been the problem or it may have been a coincidence. Your results may vary....Universal O2 sensors are cheap. The one I replaced was the original.

 

I have had the same problem. Everything checked out OK - turns out it was an issue with the wiring in the ECU.

 

GUYS ---

The "slow down" warning system has nothing to do with the O2 sensor circuit or the main ECU's (MicroPlex or Jetronic). There is a thermocouple mounted in the case of the catalytic converter (2, on the 308). If the cat reaches a high threshold temp (somewhere around 900-1000 F, IIRC) a relay / reference t-couple box (located behind the passenger foot well on the 308 --- might be in the boot on the 328, I can't recall) turns the warning light on.

 

If you lose a bank of ignition AND the warning light system is working correctly, the warning light will come "on", but it will also come "on" if the thermocouple loses continuity of if the warning light ECU is flaky

Since you report no change in performance, it's more likely that the warning light ECU is just flaky. One thing you should do IMO is watch the self-test behavior at start-up very carefully -- if that doesn't work perfectly every time (and no light coming "on" at all looks OK, but is really a failure), that's another sign that you can't trust the system.

The O2 sensor can (indirectly) cause the warning light to come "on", if it fails in such a way that the injection ECU (wrongly) "thinks" it needs to add more fuel (as the excess fuel burns in the cat raising the outlet gas temperature).

 

Interesting,
So, if my 328 does not have the slow down light coming on at start up every time, then the ECU might be flakey? I have had my light pop on on while driving, but not very often. However, I have noticed that the start up procedure is not always perfect when I turn the key on.

Is there a way to fix the ECU or is a replacement the only way? I know I have cleaned the connectors there, and the ECU looks like a epoxy thing. I think the dealer wanted something like $600 for it.

If they can't be fixed, is there any really thought on getting a replacement before they go away forever?

Thanks in advance
Bob

 

So glad I have a Euro car..

 

Cat temperature is affected by the A/F ratio which is affected by the O2 sensor. That's why I changed it. I have another vehicle whose mileage suddenly dropped from low 30's to mid 20's over a trip that I frequently take. I tested its O2 sensor with a voltmeter and propane torch. It was sluggish to react and voltage output was low compared to a new one. So I replaced it and, bingo, fuel mileage returned to low 30's. If your fuel mileage drops significantly and you are not leaking fuel and are driving in the same manner, your mixture is too rich and your cat will be hotter. That's why I changed the sensor on the 328.

 

The light in my Mondial 3.2 came on pretty much as you described. I put in a Stebro exhaust and a cat-bypass pipe and it has never come on since. (Note that the car also runs a hell of a lot better.) Maybe your cat is getting clogged up and about to fail.
Just my 2 cents!

 

Clean the connectors of the temperature cat probe, and clean the connectors at the cat "black box", then put some DeOxit on the contacts.

http://www.all-spec.com/brands/Caig%20Laboratories/?gclid=CJ3PrLWn3KECFV195QodDD-vIwe

Take the probe out of the cat, and use a brass wire brush to clean it, gently, its expensive. Inspect the braided wire to ensure there is no cracks or deterioration, its gets real hot by the cat and the wire can fail.

If the car is running fine otherwise, take it for an emmissions test to make sure there is not some eg. problem with a spark plug extender or distributor cap that may be undetectable in driving, but dumping abit of fuel when its shouldn't. The test would show if you are running rich. Might also help in determining if the cat is plugging up.

If all is ok, and you still have the light, then its probably a black box failure, or maybe a failed temp probe. Since both are expensive, doing the other things first looks like a useful approach.

From your description, My bet is the contact cleaning solves the problem.

 

Certainly that self-test function is flaky if it isn't coming "on" for a second or two at each start-up. Doesn't mean for sure that the temperature sensing function of the ECU is OK or not, but it can't be taken as a good sign of overall health IMO. The price has gotten a lot more unreasonable over the last few years, and no report of anyone ever having successfully "fixed" one (because of the potting)

 

And this will be your life with the 328 from now on until you replace or remove the CCU. You'll think it was a one time thing and then out of nowhere, usually when you are doing something special (long road trip, track day, etc.) it'll pop on again and have you second guessing. Life with a faulty Cat Control Unit. It sucks. I have a test pipe now, and my Slow Down light was on almost the whole time that I was tearing ass around the track Monday. Funny thing is that it seems to work fine in the winter, but as soon as warm weather came it started flaking out again. I have a thread sitting dormant right now in this section discussing my intention of repairing it, or reverse engineering it. I've just got to perform some tests that Steve has suggested to determine the effect it's having (if any) on the injection ECU.

 

+1!!!

Drove all day from the south of France back home. 800 miles with a Giallo Challenge Stradale in my rearview mirror. We never dropped below 110 mph. Don't want to think about how hot that exhaust was