What does the exhaust thermocouple do?

It tells you when your catalytic is overheating.

 

It's also used on the K-Jet with Lambda models to "tell" the injection ECU that the cat is up to some minimum temperature, and therefore, the O2 sensor must also be up to a reasonable minimum temperature for reliable operation, and the O2 signal can be used for closed loop operation. Have you read Section 3 of your OM?

 

That thermocouple is what triggers the Slow Down light when, as noted above, the cat is overheating. Cats will overheat if un burnt fuel is dumped into them and ignites there due to the heat. This can and does happen with a poorly running engine. If the light comes on, the cat isn't the issue, it is the symptom. Look upstream.

Now, it is not entirely unheard of for a seller who doesn't want to be bothered fixing the car before selling to disconnect it, so that no Slow Down lights come on during a test drive. This might work, but it also comes at the risk of having the car go up in flames like a Roman candle, with you in it.

Always check to make sure that when you turn on the ignition, all of the lights come on and cycle off, including the Slow Down light. If it doesn't come on, first double check the bulb, but if it still doesn't, make sure that pesky thermocouple is connected.

 

It's worth being aware that failure of the slowdown light system electronics is a common problem and there is no solution because there is no replacement unit of any kind. IMO, the only thing you can do with a failed slow-down warning system is to remove the Cat and put a straight pipe in. That way, if there is an engine problem that causes fuel to do be dumped into the exhaust, there is no danger of a car fire.

 

Recently saw a QV at my local shop where the slow down light was ignored/not understood and this resulted in a small fire, melting a lot of expensive body work.

 

It is a good idea to keep this warning system operational given how smoothly these engines can run even with a major misfire.

I replaced one of my thermocouples. In a 308 doing so is a pain because the wire cannot be spliced so must be threaded from the cat around the engine, through the firewall, through the central console to the passenger footwell.

The following is a thread that may be of interest:
http://www.ferrarichat.com/forum/308-328-sponsored-yellow-compass-group/498691-interest-slow-down-light-cat-ecu-substitute.html

 

I have a US 328 with a straight pipe replacing the catalytic converter. I have the O2 sensor mounted in the straight pipe, but the Thermocouple is not physically connected. Will this combination negatively effect the O2 operation? And if so, how can this be corrected, i.e. installing the Thermocouple into the pipe, clamping the Thermocouple to the outside of the pipe, or perhaps adjusting the Air/fuel mixture using the mixture screw located on the fuel distributor? Thanks, Paul

 

I believe with "the Thermocouple is not physically connected", your K-Jet with Lambda system will always run in the open-loop mode (so never use the O2 sensor signal and operate more like a K-Jet without Lambda system). When you say "not connected" does that mean that the thermocouple leads are not connected to the slow-down warning light ECU or the slow-down warning light ECU has been disconnected? Is the self-test at key "on" working?

I don't know if the thermocouple is mounted in the straight pipe exit (and is connected to the slow-down warning light ECU and that ECU is working) if it would get hot enough to ever have the slow-down warning light ECU send the proper signal to the injection ECU that tells it: "OK, I'm hot enough now, but not overheated, so start using the O2 signal to operate in closed-loop mode". You could confirm/deny this by measuring the DC voltage on the single signal wire (while still plugged in) from the O2 sensor relative to ground at warm idle -- if it is in closed-loop mode this DC voltage would be wandering "drunkenly" from 0.1~0.2V (lean) to 0.8~0.9V every couple/few seconds (whereas, if in open-loop mode at warm idle, it would just be a more constant value). Are you having any negative "symptoms" -- like popping during hard deceleration, or stumbling at the end of hard deceleration? Without a cat, the mixture can be set fairly rich and this might mask any negative symptoms.

 

Thanks Steve, you confirmed my suspicions. My 328 surges at idle when warm and set to 1K rpms. At 1200, it's fine. I think my next step is to work with the mixture screw to slightly richen up the mixture.

 

With everything set to spec a 200 or so RPM surge was considered normal. It can be watched on a duty cycle meter. It was a side effect of the slow reacting O2 sensor and control unit in use in that period on the CIS/Lambda.