My slow down light illuminated solid red at all times on my USA version 308 GT4. Heres my fix. On passenger side remove 4 foot panel screws to access thermocouple control unit. Pull panel back to expose thermocouple units. Using my multi meter I checked the functionality of the thermocouple by testing for resistance (you should see about 8.0 ohms). I observed no resistance, so I determined that my illuminated slow down light was caused by a bad thermocouple mounted in the catalytic converter. See video link below if you need help in testing for resistance. Using a 10mm open ended wrench I removed the thermocouple from (1-4) lower cat. I decided to cut the metal braided wire at 16 inches from the cat. I then measured the demissions of the actual probe and found a replacement from a company called Blaze Technical services (www.BLAZEPROBES.com). The part number for my probe was E082020-0002 and cost was $40.00. I was not sure that their compression fitting would fit my custom cat, so I measured and ordered additional fittings from( www.mcmaster.com). You will need sleeve for 1/8 tube compression (part number 8239K271) and nut for 4mm tube OD compression fitting (part number 5269K771). After receiving the probe and fittings I was able to splice the new probe into the existing steel factory braided wire. View attachment 3006937 View attachment 3006938
This is absolutely super documentation. Thank you! I am one of many who need to find a solution to a non-functional thermocouple. Thank you again! Does the splice affect the temperature when the Slow Down light is triggered? A repeated concern in prior threads is that the wire is integral to the thermocouple and that splicing changes the performance of the thermocouple. I am no expert so only worry that this might be true. FChat member “JMAS” has measured the voltages for the threshold temperatures for the Slow Down light system. They are the temperatures where the Slow Down Light module starts the light blinking and the temperature when the light begins to stay lit. Here is one of his threads: interest in slow down light / cat ECU substitute? There is room in the trunk where the spark boxes are to move the Slow Down light modules there. I believe the 328 already has them located there instead of the passenger footwell in the 308. It would be great to relocate them there and just run plain wire to the light bulbs. Are the leads on the thermocouples that you used long enough to reach the corner of the trunk? FChat members “lm2504me” and “kcabpilot” are also both very familiar with general thermocouple performance.
You can order a full length wire to eliminate any issues with splicing. From my research, it’s an acceptable practice to splice thermocouple wiring and achieve reliable readings.
I have how replaced the catalytic converter thermocouples in my 1983 308 QV. I can add to what has already said above by confirming that the factory thermocouples are plain K-type with a 1/8” diameter probe and a 154-inch lead. The fitting to hold the thermocouple in the catalytic converter is an SAE 5/16" fine thread (5/16-24) female fitting which attaches to male fitting on the cat. The flats on the fitting need a 3/8 inch (9.5 mm) wrench (a 10mm wrench is too big). I purchased the Blaze Technical Services thermocouples that have the 154-inch lead. This eliminates the need to splice. This configuration is not listed on their website so you have to phone or email them to order them. The Part Number with the 154-inch lead is 071223KU-154. Rather than buying new compression fittings from McMaster-Carr, I just reused the ones on the existing thermocouples, although I did use the new small compression rings that came with the Blaze thermocouples. The hardest part of this project is threading the lead around the engine and through the center console to the passenger footwell. I had to remove my passenger seat and much of my console to get the lead through. I attached a string to the old thermocouple lead and pulled it back out to the engine compartment, attached the new thermocouple lead to the string and pulled the string back again to the front. It jammed in a few places within the console and I had to futz around under the HVAC sliders. The yellow wire goes on the smaller silver screw (positive terminal) and the red wire goes on the larger brass screw (negative terminal). In my prior post, I had included a link regarding “JMAZ’s” interest in building new modules. He has sold his 328 and has abandoned the module project. Before he lost interest, he did determine that the “Slow Down” lights begin to blink at 1,600 degrees Fahrenheit (880 degrees Celsius) and illuminate continuously at 1,700 degrees Fahrenheit (930 degrees Celsius). He also confirmed that the thermocouples were a plain K-type.
One of my thermocouples has very little clearance to an air injection pipe. I inquired to Blaze as to whether a thermocouple can be bent. Their reply was: “They can be bent, I would recommend bending them with at least a ½” radius. You don’t want to have a sharp 90° bend and would recommend bending it along the same plane obviously.”
It's ok to splice tc's, but best to keep both joints the same temperature when in use for minimal error. I built a 48 channel piece of test gear that drove all connections where the tc wire connected to copper instrumentation, to same temperature using 1/8 thick copper bars that thermally connected all connections by conduction, but electrically insulated them from the copper bar and whose temperature was measured with a high accuracy thermistor. Tc's are relative temperature beasts so the measurement site needs a reference temperature or cjc for cold junction compensation. So solder generally ok. Check out law of intermediate metals discussed in Omega temperature catalog or try omega.com and go to temperature catalog. I worked aerospace thermal analysis and testing for many years and I always bought my tc's from Omega, usually copper constantan (type-T) with ptfe (teflon) insulation, for general work. High temp tests like electrical component temps subject to a molten solder wave wanted to be type K with welded beads. Big thermal shock wanted thin wire to react quickly to temperature change. Omega's technical literature is top notch including tc voltage vs temperature tables. Lower temps fine with simple cross and almost any solder. Keep the bead small. We normally used 30 awg. with lead tin electrical solder. Bill
