Good morning FChat. I have a 95 F355 with a very odd issue. When the engine is running, the 1-4 CEL will illuminate and then the 1-4 Slow Down light will flash and the relay for secondary air and aux RH fan will trip. Both CEL & SD lights will then go out. The cycle repeats every 6.6 seconds. The secondary air system is functioning as it should. As well, the aux fan is also working as it should. The relay trip is extremely fast and the secondary air and fan is not effected. I have found no wiring issues in the circuits for the secondary air or aux fan system. I have found the wire from ECU to the 1-4 SD light has no continuity from the ECU to the 24 pin connector in the engine compartment. There is no issue from the 24 pin to the dash. The 5-8 side has full continuity and no issues. The issue is not in the ECUs. They have been run in another car without issue. I am currently looking to replace the bad wiring harness on the 1-4 bank and will reevaluate after that problem is addressed.
Intermittent fault would indicate bad earth / broken wire. The fact its regular in occurrence and timing would point to a sensor or similar?
As mentioned in PM, there is a diode in that circuit. Make sure your multimeter probes are set up for the right polarity. Forward biased diode is said to be 600 ohms (not zero). Should be "OL" in the opposite direction.
Yes, I have tested the circuit and will rerun some test this morning to confirm my findings. The odd part is the LH circuit reads the same in both directions and that side has no issue. The RH circuit has an open condition from the ECU to the 24 pin connector that is right next to the ECU. (rear ECU car).
Please post your findings when fixed. Also, when you say the fan and secondary air relays "trip" I assume you mean they energize and the fan and secondary air pump both come on?
Also worth asking, did you look into the SDECU system and confirm that it may be the cause? Also you say that you suspect a circuit problem with the 1-4 slow down light, but the light flashes when the issue is happening? If the RH fan and the secondary air pump are coming on could it be that the ECU logic is purposefully turning them on (by grounding the relays) because of a perceived cat overtemp coming from the SDECU?
The relays "trip" so briefly that the fan and secondary air have no time to react. I removed the cover from two old replays and there is a very quick blue flash as they are energized and released. I had the dash out and the relay clicks every second with no warning lights plugged in. I cam also remove the bulbs for CEL & SD and the relays trip every second then as well. I will add more when I know more.
Here's someone with the same issue: https://www.ferrarichat.com/forum/threads/355-check-engine-and-slow-lights-cycling.148543/ ...with a solution here: https://www.ferrarichat.com/forum/posts/148828445/ I don't know if this helps or hinders. Perhaps lots of faults can cause this issue. Does the LH circuit show close to zero ohms in both directions? If this is the case, I wonder if the RH side should be the same. Anyway, open circuit is definitely wrong. Unfortunately, as said in PM, we are not 100% sure that the diagram we are using is correct. The fact that the L/R sides are different does raise a red flag, though.
Disregard. This has been resolved. Summary: The WSM engine diagrams (1994 2.7) seemed to be different from actual wiring on a 1995 car. However, what we didn't know was that the interface plugs have the same strange numbering system as the door connectors. The small pins are numbered first. Image Unavailable, Please Login
