Fail Safe on an NA 2011 Corvette Z06

[Steve@HPE]

I have a 2011 Z06 with slightly higher than stock compression (12.0:1), but that higher compression required the timing to be retarded quite a bit. So, I'm installing an HFS-3 in the hopes that the timing can be advanced.

I've got everything installed except the fail safe signal. Since this is a naturally aspirated engine, my thought for the fail safe is to use the HFS-3 change over relay to essentially cut the IAT signal to the ECU. This should produce 0v to the PCM for the IAT signal, which the ECU correlates to 300+ degrees Fahrenheit. We could then adjust the ECU calibration such that the appropriate amount of timing is pulled for the high IAT reading.

To do this I was planning to cut the IAT signal wire (tan in color) coming from pin E of the MAF/IAT connector; then connect one segment to the white wire of the HFS-3 ECU cable and the other segment to the brown wire of the HFS-3 ECU cable.

Will this work as expected? Will the ECU receive the uninterrupted IAT signal while the engine cranks on start up?

[Richard L]

This may not work because most ECUs have an internal pull up resistor tied to the IAT sensor. If the circuit goes open circuited, the pin will to up towards 5V instead of OV. This will make the ECU think the IAT is very cold.

You need to ground the black wire of the grey harness upon failsafe activation. Make sure you don't link the DR1 or the dummy resistor will get very hot and eventually melt the controller box. White wire t-splice into the tan wire, no need to cut the wire.

Instead of ground the black wire, why not place a 47-100 ohm resistor inline so you can actually simulate a known temperature (hot). Some ECU may throw a CEL if the reading is out of range.

[Steve@HPE]

Originally Posted by Richard L

This may not work because most ECUs have an internal pull up resistor tied to the IAT sensor. If the circuit goes open circuited, the pin will to up towards 5V instead of OV. This will make the ECU think the IAT is very cold.

You need to ground the black wire of the grey harness upon failsafe activation. Make sure you don't link the DR1 or the dummy resistor will get very hot and eventually melt the controller box. White wire t-splice into the tan wire, no need to cut the wire.

Instead of ground the black wire, why not place a 47-100 ohm resistor inline so you can actually simulate a known temperature (hot). Some ECU may throw a CEL if the reading is out of range.

Thank you for the response Richard! I see no way around cutting the tan IAT signal wire however. Attached is a logical diagram I created based on your suggestions. I see this being viable, although determining the correct resistor value may be a bit of a challenge. Thoughts?

[Richard L]

Here is a diagram that may work well. I suggest trying a 47ohm resistor first

[Steve@HPE]

Originally Posted by Richard L

Here is a diagram that may work well. I suggest trying a 47ohm resistor first

Thank you Richard. I'll give your method a shot since it does not break the direct connection between the ECU and the IAT sensor.

Is there a way I can force the HPF-3 into failsafe mode so I can measure voltage delta?

Thanks again.

Steve

Just reading along and thought I'd throw a few answers out.

To force the failsafe, just turn the gauge off, with the DHB jumper linked(default position).

From the first post regarding the failsafe at start-up, the failsafe should switch pretty quick at power up, later versions are near instant, in depends on the version and build date of your unit.

Good luck.

[Dust]

Steve, is this the way the Nitrous LS1 guys do it? It would be something to look into, as I am sure that they have figured most of this out.