I have pulled an S5 NA 13B from a convertible which had an automatic, I put a 5 speed behind it and installed it in a Jensen Healey, using the stock S5 ECU. It fires up and runs great, until you try to rev it way up. Then the tach (a VDO) quits working. Since the tach output comes from the trailing coil/igniter I have to assume the ECU is killing the trailing ignition. It also shuts down the fuel pump ground, thus killing the pump. Since the OE pump relay runs on that weird resistor setup that works all the time when the ignition is on (I assume that's to run the pump at lower speed, thus lower pressure, thus reduced power and I have bypassed it), I am assuming that when this happens the ECU is going into 'limp mode'. I bypassed the ECU ground for the pump relay so now when this happens I lose the tach but the engine still runs.

If I shut it down and wait about 2 minutes and then restart the engine, the tach works again until the next time I try to buzz the engine way up.

I *think* the ECU will go into limp if it does not 'see' the electronic OMP. I am using the OMP and it's working, I started it on premix and watched the lines, you could see the air bubbles move through the lines until they were completely filled with oil. So I have to assume the ECU 'sees' the OMP.

I have an FSM, it is nearly useless for stuff like this. So my question: what else, other than the OMP, could make the ECU go into 'limp' mode?

 

simple answer, nothing.

BUT to be sure this is limp mode you can try to keep turning the key off and on REALLY fast every 3 or 4 seconds, or about the time it should take to redline in 2ND gear.. i drove a car with a working but electroniclly faulty OMP, and limp mode can be 'delt' with.

 

Not entirely true about limp mode. A failed AFM can cause it too, but it just happens to be far less likely. Those electric OMPs always fail. Here's a partial list of engine codes. Note the fail-safe operation modes.

Code - Input Device - Fail-safe operation mode

• 01 - Ignition Coil (Trailing) - Trailing ignition pulse cut
• 02 - Crank Angle Sensor (Ne) - Fuel Injection and ignition cut
• 03 - Crank Angle Sensor (G) - Fuel Injection and ignition cut
• 08 - Airflow Meter (AFM) - Basic Fuel Injection and fixed timing (Cripple mode)
• 09 - Water Thermosensor - Coolant temp fixed at 176F
• 10 - Intake Air Thermosensor (AFM) - Intake air fixed at 68F
• 11 - Intake Air Thermosensor (Engine) - Intake air fixed at 68F
• 12 - Throttle Sensor (TPS) Full Range - Fixed at 20% open
• 13 - Pressure Sensor (intake manifold pressure) - Fixed at 29.9 inHg
• 14 - Atmospheric Pressure Sensor (Inside of ECU) - Fixed at 29.9 inHg
• 15 - Oxygen Sensor - Feedback cancelled
• 17 - Feedback System (poss O2 sensor) - Feedback cancelled
• 18 - Throttle Sensor (TPS) Narrow Range - Fixed at full-open
• 20 - Metering Oil Pump (MOP) position sensor - Basic Fuel Inj and fixed timing
• 27 - MOP (General) - Basic Fuel Injection and fixed timing
• 37 - MOP (General) - Basic Fuel Injection and fixed timing

http://www.johnr.com/cpucodes.html

So, go pull some codes and figure out what it is for sure. If it is the OMP, the OMP driver in the ECU will probably burn out on you too. That's usually the way it goes.