Slow Down Light

As a first step, you might confirm the temperature of your cats using an infrared thermometer. The trigger temperature for the Slow Down lights are posted here in another thread (somewhere) and those temperatures are high. The thermometer readings will tell you if you have a real high temperature problem or if it is just that your Slow Down system is misbehaving.

Engine temperature is not affected by cat problems and is not an indicator of cat operation.

The normal operation of the Slow Down lights is to begin blinking at one threshold temperature then to stay lit at a higher temperature threshold.

 

What Brian said!

The system on my '89 328 started randomly doing that sort of thing not long after I purchased it in '08 and soon after, the light wouldn't illuminate for a few seconds when the ignition was first turned on, as it is supposed to do. It would still randomly light up on the road. It still randomly illuminates even though there is no longer a Cat and the thermocouple is zipped tied up out of the way against the frame. I'm a big believer that if the slow down system is not working, the CAT has to be removed/a straight pipe installed due to fire danger in the event of an ignition problem/cat overheat. Given a choice, I'd rather have the cat/working slow down light, but I don't.

Someone was developing a new module for the slowdown light which sounded promising but that was over a year ago and nothing seems to have come of it.

 

Same here, my 1989 328 has a slow down light that from time to time can have a mind of it's own so the cats came out. Feel so much better with that set up and as a bonus the car breathes better and sounds great.

 

My '87 328GTS slow down light began coming on years ago, perhaps 10 years ago. In my case a new O2 sensor cured the problem. The problem has never recurred. When the problem was occurring I checked the temperature of the CC after a drive down the interstate and a quick pull off to a rest stop. The infrared thermometer indicated a little over 700 F meaning the internal temperature would be higher but how much higher I do not know. Unfortunately, I have never re-measured the outside temperature now that the light does not come on other than for the few seconds after starting the engine. So it could have all been a coincidence, I don't know.

 

I have replaced my oxygen sensor too will little to no improvement.

 

Yeah, me too. O2 sensor change had no effect on the SD light.

 

Put mine down to a faulty electrical module.

 

I have a random/intermittent Slow Down lamp on my '85 US 308. The thermocouple is clean, the wires are intact visually, and are connected properly at the ECU.

I regularly temp gun my cat after driving. After spirited canyon climbing in warm Los Angeles weather, I will see immediate external temp at 800-900 F (400-500 C) with the motor still idling. Also don't know what it means about internal temps, but assume they're not double(?).

There are some posts on this forum that support the thermocouple being K-type, but it's a completely custom shape. Don't know any recourse but (a) buying a new one for $700, (b) buying a new cat ECU, or (c) going cat-less and dealing with that 1 day every 2 years.


Beau

 

I had the slowdown on three separate instances:
1st - mechanic did a tune-up. I test drove her around the corner. No power. Mechanic said it was fouled plugs. I left the car at the shop. Turns out he re-assembled one cap incorrectly so one bank wasn't firing.

2nd, oil got past the cam seal & coated the distributor cap. I didn't notice the power loss. After sitting for a while, she fired up fine until the oil covered the connections again.
Took a little to find. A new cam seal & cap wipe fixed the issue

3rd - caught in a drenching down pour leaving a mechanic's garage. One bank kicked out. I nudged her back 2 miles to the shop. Molten aluminum was dripping from the heat shield. Water had gotten into one of the coil caps. A quick wipe-down and new wire cap fixed the issue.

 

Try the real easy things first, remover the thermocouple in the exhaust, a 12mm wrench should be all it takes, and give it a bit of a clean up with a toothbrush or small brass wire brush. Inspect the wire for any fraying or damage, sometimes it rubs against things. Find the other end where it connects to the cat ecu unit, and put a dab of Deoxit on the connector to ensure a no resistance connection. Might be all that you need. The thermocouple is a really simple resistor type sensor, the cat ecu measures the change in resistance with changes in heat. It can give off confusing signals if the tip/probe is dirty, the wire frayed, or the connection is adding its own resistance.

Helps to know the basics of what this sensor is doing. The sensor detects heat in the cat, as a result of unburned fuel being dumped into the cat, usually as a result of an ignition problem within the one of the two banks of cylinders. Hence why it has nothing to do with whether the engine itself is running hot. The 328 engine is really more like two four cylinder engines with independent systems, so the car can run reasonably well on only one bank. Because it can run like this, you might drive and not actually notice anything, so the sensor is there to detect the raw fuel dump from the inactive cylinder or bank and risk of fire in the cat that creates. A gas fire in the cat can burn so hot it can in the extreme set the whole car on fire. So that's why the engineers put in a sensor, as the outcome of running the car on one bank (usually from a coil failure) or less then all cylinders (dist cap, rotor, wires or plug problems) is not trivial. That said, usually, you will feel a bit of stumble, or less acceleration or something if one or more cylinders are not firing properly. From what you describe, it is more likely just the thermocouple or its connector needing a cleanup. It could be the cat ecu failed, but that would be a much less frequent cause of an intermittent blink. Of course, "more likely" still means a blink or light going on is scary as you do not want your car to go on fire. But the simple probe clean up may solve so start with that.

As a second task, I would visually inspect ignition items, look at the plug extenders for pinhole shorting, make sure the wires are seated properly and such and if you want check the plugs for black soot fouling (signs of running too rich) that might give some clues as to if all is running as it should.

Beyond that you are dealing with failed/failing coils, the dist caps/rotor/wires and such. Because these items are much pricier than in "normal" cars, they are often not replaced as frequently or preventatively as you might in a more typical vehicle. So reflect on how long ago these items were replaced. Once you are inspecting dist caps, the labour involved means you should probably just renew the items.

 

Great insight !! Thanks

 

This might be referring to the work I've been doing, re. developing a cat ECU replacement. Things have been moving along slowly as I've tried to fit the development in with other projects, but a couple of my microcontroller-based units have been undergoing testing in a Mondial for several months, and the replacement I made for my own '89 328 has been working fine for a year or so.

Several folks have expressed interest, so I need to make a decision soon as to whether it's worthwhile for me to make some of these.

 

Where did you get the ECU? Everybody says they are not available.