knock sensor

[whitebroncoii87]

my knock sensor broke off at the part sticking out.. instead of replacing it, i was thinking is there a way to bypass it so that it always sends back a success signal back to the computer? will shorting the two signal wires together do anything? what voltages is the sensor supposed to send back anyway?

 

[RonD]

What engine and year?


Knock sensor is usually there so you can run 87 octane fuel.

Pre-ignition, knocking/pinging, can cause engine damage pretty fast, melted pistons and burnt valves, which is why "they" add knock sensors.
"They" set a more advanced spark timing to get better emissions and performance, but that will also cause engine damage if lower octane fuel starts to knock.
When knock is detected computer retards spark timing.

 

[whitebroncoii87]

1987 2.9, can I bypass it by shorting the two wires? If not what does the computer do when the sensor doesn't respond?

 

[AndyB.]

Your best bet is to replace, it. Next best is to leave it disconnected. Worst is to jumper it together. I think jumpering it together would send back a constant knock signal. If I remember from testing these, when there is a knock it closes the circuit (produces a small voltage). On some cars, when first started, the timing will advance until a knock is sensed to assess the grade of fuel, then adjusts the timing curve accordingly. I don't know if the 2.9 is like this. My fear would be that by not sensing a knock it would send the computer into open loop operation.

I'm guessing the part is an E3TZ-12A699-A, but I'm not sure. You might have a hard time finding a new one.

 

[RonD]

According to this page: http://www.auto-diagnostics.info/ford_eec_iv

Pin 23 is the knock sensor and it lists "3volts running"
Which to me would mean computer is sending it 5volts, Pin 26 "reference voltage"

I would test that, see if one of the wires has 5volts with key on.

If so then you could add a resistor between the wires to get pin 23 down to 3volts.
I think you would need a resistor to ground as well to drain some of the power so resistor doesn't heat up.
It's called a voltage divider circuit.

 

[whitebroncoii87]

Knock sensor is usually there so you can run 87 octane fuel.

I think i can give eec's pin23 a steady 3v with a voltage divider.. but i only use 87 octane regular gas, so do you mean i should have it in there if i use 87 octane?

 

[RonD]

You need to have a working knock sensor not a bypass to prevent knocking with 87 octane, but it really depends on how aggressive the spark timing is in the computer software, TFI module sets base spark timing, the computer uses the SPOUT connection to the TFI module to advance the spark base on engine parameters, i.e. rpm, MAP pressure and throttle position.

EGR valve/system will also prevent knocking, it's primary purpose is to lower combustion temps and so reduce NOx levels as cylinders heat up under a load, and since knocking is the air/fuel mix self-igniting from the heat of compression, this lowering of combustion temps helps reduce overall cylinder temps so reduces knocking.
Octane is a heat rating, the lower the octane the lower the self-ignition temp.

You can always test it with the bypass and see, if you get pinging/knocking under load then take your foot off the gas pedal and drive it easier.
Use blended 87 octane, 10% ethanol blend tends to knock less.