I just swapped to a 730 ecm and everthing is working good exept 5 seconds after the engine starts i get a code 43. I have the proper, new, knock sensor but still the code. My service manual says that pin f9 for the knock sensor is supposed to have 9.2 volts with the key on, it only reads a continous 4.88 volts. anybody got an idea?

 

Not sure how far along with the knock system swap over you are. So I'll first outline what needs to be done.

The knock module on the firewall needs to be removed and bypassed.

Then either the knock sensor needs to be changed or paralleled with a 3.9 K resistor.

You mentioned changing the knock sensor. Measure the resistance from the sensor connector to ground (disconnect the wire first). The original MAF sensor will be about 100K ohms. The correct SD sensor will be about 3.9 K ohms. You may have been sold the wrong sensor.

On the SD system the voltage at the knock sensor should be around 2.5 volts. That is what the ECM is looking for to know that the sensor is connected. The 4.88 volts says either the it is the wrong sensor, or it is not grounded to the block, or the original module has not been bypassed.

Does all of this makes sense?

RBob.

 

Here is everthing i have done. I removed the old spark control, jumped pins c and e( the black and white wires ) bought a knock sensor for a 1991 z28 305 tpi, then wired the black wire from pin e on the spark harness to pin f9 on the ecm, I'm shure that its correctly wired. but what i dont understand is my service manual says thats pin f9 is supposed to have 9.2 volts with the key on and 9.3 with the engine running. also the computer is from a 91 z28 305 tpi. My diagram shows a dark blue wire going from the ecm to the knock sensor with nothing in between. Is this right? and also it measure 4.85 at the knock sensor. havent checked resitance yet.

 

Pin F9 is the correct knock sensor input to the ECM.

Jumping pins C & E on the ESC module connector is correct.

ESC module connector pin C to ECM is correct (you posted E, but at this point they are both the same).

The voltage stated by the manual of 9.2 volts is incorrect. A properly working system will be in the range of 2.5 volts. The knock sensor resistance range is 3300 to 4500 ohms. This can be checked at the ESC modue connector. Don't even need to go under the car (unless it checks out wrong, and the sensor then needs to be checked).

RBob.

 

Thanks. but one more question. If the manual is wrong which it probably is why do i get 4.88 volts instead of the 2.5 range. i tested it on two computers so i guess theres nothing wrong with the ecm. Either way i'm getting the wrong voltage. If the knock sensor has a resistor in it that would knock it down a little but shouldn't the line voltage be about 2.5?

 

The ECM applies 5 volts to the sensor. The 3900 ohms of sensor resistance pulls the ECM voltage down to 2.5 volts. The ECM measures the voltage and knows that there is an issue with the sensor or wiring.

RBob.

 

It pulls the voltage down inside the sensor to 2.5 correct? So my ecm voltage of 4.88 is correct? That being true, why do you think i have a code 43? maybe the connecter is bad- maybe it dosent make a good connection?

 

No, the sensor pulls the voltage at the ECM down to 2.5 volts.

It is a voltage divider setup. The 5 volts at the ECM sensor pin F9 is supplied through a 3.9K resistor. With no sensor it is at 5 volts. Once the proper sensor is connected it adds 3.9K ohms of resistance to ground. Now the voltage on pin F9 is divided in half, for 2.5 volts.

The ECM actually measures the voltage on pin F9, and sees 5 volts. The ECM then knows that the knock sensor is either bad or not connected.

Goofy ASCII art:
RBob.

 

Thanks i got it now, i flared the pin out inside of the connecter so it would make a better connection. One question though- when the knock sensor detects knock it changes the resistanc and thats how the ecm knows?