Automotive failsafe designs. Looking for flaws, issues or anything I missed.

Thread Starter


Joined Apr 26, 2020
The first fail safe:

Lean air fuel boost cut.

I will try to keep this short and to the point and only put in the information I think is necessary, sorry If I add too much.

This idea was sparked by LeadFootin (all about circuit member) who reminded me the narrow band 02 sensor signal that the Lc1 wideband simulates and I am using as a narrow band sensor at the ECU is programmable.

The wideband controller has two auxiliary outputs 1 is 0-1V programmable and 2 is 0-5V programmable. It also has a stereo jack connection that certain gauges can connect to.

Both the outputs allow the owner to program the voltage out signal across the air fuel range, changing these outputs does not change the readings the controller puts out only the aux signals.

The car is turbocharged and runs 3 stages (settings) of boost stage one is the wastegate spring adjustment, stage two is a manual pneumatic dial, stage one and two are switched with a 12 V solenoid valve, closed = 1 wastegate, opened = 2 manual dial. The 3rd stage of boost is also controller by an adjustable solenoid valve Closed = 2, opened = 3. The 3rd stage of boost runs with water/methanol injection, the w/m kit shuts down stage 3 boost if tank level is low or if tank pressure is low, it also has hall sensors on the injection solenoid valve and status lights. I have not found a way off monitoring line flow during operation without adding a programmable controller. What I do have are air/fuel ratio readings on the wideband gauge, because with water/methanol injection the air to fuel ratios should be and are 1part less air to 1 part fuel (12.0:1 should be 11.0:1 with w/m), the operation of the kit can be visually monitored in a backwards way through the gauge.

Although better than nothing it's still not a good warning system.

My Idea is to use a normally opened pressure switch set above stage 2 boost and run the 0-5V auxiliary wideband signal to the pressure switch, the wideband would be set to deliver 0V below X air fuel value and 5V above X air fuel value. The pressure switch output would be used to shut off the stage 3 boost solenoid when boost is above stage 2 pressure and air fuel ratios are above X.

The wideband controller would act as a air fuel comaparator, the aux out signal has select-able sensor reaction times to aid in cleaning up the signal, the longer the reaction time the more the results are averaged out.

Basic diagram.
View attachment 264607

My first concern is this set up just cycling on and of due to signal delays or something else I mist and anything I haven't though of.

I also wonder if the normal operating output signal for the auxiliary 2 could be monitored using a comparator while the aux 2 signal still controls the display on a gauge without affecting it's readings due to current draw of the comparator.


The second failsafe :

Engine knock boost cut.

This is an attempt to copy the factory knock control failsafe or bring it back.

The car is running the factory ECU and the factory knock control. The system is set up to pull up to 6* timing and lower boost by deactivating a solenoid at the turbo. Unfortunately the same solenoid is used by the ECU and deactivate at low and high rpms in every gear. Because the solenoid is operated by a single 12V+ from the ECU the solenoid is no longer used (It and others have been modified and re purposed as stage 1 and stage 3 boost solenoids).

The knock system works as follows:

It consist of a knock control unit that has four pins, it receives a 12+ signal and a ground to power it. 1 wire goes to the knock sensor and one to the ECU

The knock sensor has two wires one is ground and the other is a 3.3-5V signal generated by the knock control unit (this voltage is fixed but varies from car to car) when knock is detected and the knock sensor vibrates it generates resistance and the signal of 3.3 to 5V generated by the controller drops between 1.3 and 2.6V and the signal going to the ECU does as well and the ECU pulls timing. (I am not sure if the knock controller conditions the signal but it shouldn't mater).

Unnecessary but the research, foot work I have done so far and the reason for this project:

Over the year I have looked into piggyback knock control systems, audible knock detection devices and stand alone engine management knock control.

Most or all require dialing in the bandwidth frequency of knock for a specific engine. The frequency for the engine I am running is not available or I can't find it, no specs are available on the factory knock sensor or controller...

Last week I looked at knock control again hoping the devices had improved and the prices had gone down. Although my search was disappointing it dawn on me, why not use a second factory knock sensor and controller to monitor knock. I would have the same 3.3-5V signal the ecu gets, I just had to figure out how to monitor a voltage drop and use it as a digital signal. I will be honest, armed with very little electronics knowledge and guessing I gave up and asked on (I was confident I could eventually come up with a solution but also knew there would probably be hundreds of better/more accurate/more reliable solutions I have never heard about or can imagine) .

A comparator was suggested, a comparator with hysteresis was suggested and a schmitt-trigger with IC was suggested. So I looked into all 3. The comparator Vin and Vref had me confused being labelled +- but I think I finally understand a comparator.

From my understanding a comparator compares two voltages one from a varying signal (Vin) and one from a control or controlled signal (Vref). The voltage range is different with different models. It is powered (when DC) by a +dc current and a ground current. The dc power current differs with different models.

When Vin is above Vref voltage the + power voltage is at the Vout. When the Vin is below the Vref the ground power signal is at the Vout.

A comparator shares the same electronic symbol as an OP amp and an op amp is sometimes used as a comparator but has slower reaction time, a comparator is not used as an Op amp.

It is like a relay that's N.O. and N.C., that delivers both + and - signals between states and is activated by voltage changes instead of on off signals.

There are inverted comparators, duel Vout comparators, fixed, variable, programmable...

Hysteresis is a way of stabilizing the Vref signal to make sure the comparator only switches over when the Vin is actually higher or lower.

Schmitt trigger with IC is like a comparator with Hysteresis built in and more features.

Comparators reaction time are rated in ns and μs (had to look nano and micro up) and are more than fast enough to pick up an knock event, on a four cylinder 4 stroke motor at 6250 rpm my math comes to one spark event or power stroke every 0.0048 seconds, I would asume knock events to be even shorter.

I used some electronic symbols but for the most part I try to draw wiring diagrams anyone can follow (car guy) and not electronic diagrams.

The basic diagrams (Missing some details).

Knock Control3.drawio.png

Question 1 (A). Vin. Does Input bias current mean current draw or variation or accuracy (the ADCMP341 for example has an input bias current typical 0.01 nA Max 5 nA I am trying to figure out if the current draw from the comparator Vin would affect the factory ECU signal and if I need to add a second dedicated sensor and controller or if I can just use the signal going to the ECU without affecting it? Looked at a few comparators within the V range I think I need, trying to get an average range of current draw from the Vin monitored signal with plans to look up how if at all that would effect the signal voltage.

(B) I need the comparator Vout signal to be ground above 2.7V Vin and +Vcc below 2.7 Vin. Does an inverting comparator act just like a non-inverting comparator, I have read in my research that inverting is actually an Op-Amp. Can the +Vcc and -Vee be reversed to change Vout from + to - and still power the comparator or is it polarity sensitive. Would it operate the same if Vin and Vref where reversed, If Vin was fixed and Vref was a varying voltage signal? On a comparator with Vout a/1 and Vout b/2 does the second output signal just reverse the first output signal?

Question 2. Would the varying 11.8-13.7V generated by the car as a power supply for the comparator effect it's operation?

Question 3 (A). Would the Vout from the comparator require any protection from the relay or LED, I have read that on some comparators voltage feedback from devices can damage them.

(B) I can't find a datasheet for this relay (BU509TD) would the possibly very short signal generated by the knock controller and comparator be too short to trigger a relay? This relay is unique as it gets it's positive coil signal from pin 30 through the timer and pin 86 is only a trigger to start the timer. Unlike normal relays it can only be used to control positive current not ground.

Question 4. Similar to question 2, I know the varying voltage generated by the charge system will effect the voltage put out by the variable rate resistor, what is the best way to maintain a steady lower voltage from the varying supply current?

Question 5. Hysteresis. From the circuit diagrams I have looked at Hysteresis is set up using a resistor between the Vout and Vref. I can't understand the circuit diagram (or circuit diagrams), I have searched for the answer to my confusion but only find the reasons for resistors and why they are used...

In the following example a comparator without and with Hysteresis are shown.
Hysteresis 2.JPG

The first thing that confuses me is R3 100K and Ry 100K, It looks like the 5v signal used for Vref Uses a resistor to regulate the voltage going to the comparator but also looks like that voltage is connected to a second resistor that connects to ground, this is what confuses me, I have read on this forum that it's sets the input impedance, prevents input signal from floating, prevents charge accumulation, I under stand current needs a return path to function. But from a practical sense it doesn't make sense, I can't take a positive 12V automotive signal and run it through two resistors in series and touch it to a grounding point without shorting to ground?
Using a voltage divider calculator the voltage going to Vref in the above diagram would be 2.5V. So the point the Rh 576K resistor ties to has 2.5V would that not allow and 0.37 volts at the Vout connection witch would also short back to ground if a comparators Vout is at 0V ground?

In the following diagram they show the operation of a comparator with and without Hysteresis.

They mention T on the chart is -3.9 micro seconds. This leads me to understand the green lines change of states was moved to the right of the chart for visual reasons. Which of the two would change states first, with or without Hysteresis.?
Are the fluctuations of the red line the results of fluctuating Vin signals due to noise or the comparators reaction to the Vin voltage at the threshold voltage ?

The timed relay I plan to use will reset the timer with every signal pulse to the trigger pin 86 so a fluctuating Vout signal would not be an issue. There is limited information available for the knock control system on this car, it's 34year old technology. With most knock control systems the ECU counts the knock pulses and reacts accordingly. The reason I question adding Hysteresis is because I want to be able to see every instance of knock flash on the knock LED, would hysteresis cause the comparator to stay in one state during rapid Vin fluctuation?