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How to Electronically Modify Your Car, Part 13

Wrapping-up our brilliant DIY series

by Julian Edgar

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At a glance...

  • Analysing car systems
  • Thinking through the required modification
  • Using the workshop manual backwards!
  • Series conclusion
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This article was first published in 2009.

Last week in Part 12 of series we looked at using interceptors to modify digital and analog signals. This week, in the last in this series, we look at the best overall approach to DIY electronic modification of cars. 

Systems Approach

If you are to understand the best approach to modifying electronic car systems, you require at least some understanding of how the specific car system works.  

For example, if you are modifying an engine management system, you must know that the system uses input signals (eg airflow meter, coolant temperature, knock sensor, etc) and has outputs (eg fuel injector opening time, ignition timing).  Furthermore, you must understand what the ECU uses the input signals to determine (eg airflow meter – engine load, coolant temp sensor – temperature of the engine coolant, knock sensor – whether or not detonation is occurring). 

It then logically follows that if the airflow meter signal is altered, the ECU will calculate that the engine load is different from what is actually occurring, or if the injector drive signal is intercepted and altered to extend the injector opening times, that the air/fuel ratio will be richened as more fuel flows for the same amount of air. 

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The best way of understanding a system is to have a good workshop manual - and in nearly all cases that's the factory workshop publication. 

In pretty well all cars but grey market imports, a full workshop manual will be available. Even if it costs a few hundred dollars, we'd recommend that you buy it. (And if you can't buy it, see if you can beg or borrow one to do a major photocopying exercise.) Many workshop manuals are also now available cheaply on CD – but make sure it is a full manual, not just a short list of fault diagnoses. 

Most - although it must be admitted, not all - factory workshop manuals have detailed coverage of the electronic systems in a car, including the basics on how things work as well as how to diagnose faults, wiring colour codes, etc. 

Thinking Through the Modification

So you're armed with the workshop manual, complete with circuit diagrams, wiring colour codes and some coverage of how things work. What you need to do now is to mentally work through the best modification approach.

The questions to answer are:

  • What modification outcome do you want? 
  • What parameter do you need to alter to achieve this outcome? 
  • To alter this parameter is it easiest to make modifications on the input or output side of the ECU?

These are the fundamental questions that must be answered if you're to make an electronic modification that works - on that's on anything from a factory boost controller to an auto trans control system to the engine management system.

Doing It

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Let's say that the outcome that you desire is leaner full-load mixtures. This means that the parameter that you want to alter is the amount of fuel that the injectors flow at high loads. Next question: do you make the modification on the input or output side of the ECU? 

If you make the modification on the output side of the ECU, you're going to need to alter the duty cycle with which the injectors are driven - that is, lessen the full-load duty cycle so that they squirt less fuel.

Alternatively, you can make the modification on the input side of the ECU, for example changing the signal coming from the airflow meter so that the ECU thinks that less air is flowing into the engine than actually is. This in turn will lean the air/fuel ratio as the ECU will have the injectors on for less time (for the same amount of air flowing into the engine) than was previously the case.

So how do you decide whether input or output modification is best? It's a case of consulting the workshop manual and looking at the airflow meter (or MAP sensor) signal characteristics. In nearly all cases you will find that coming out of the airflow meter is a variable voltage signal. Variable voltage signals are a lot easier to modify than variable duty cycle signals, so in this case the approach is clear - you attempt to modify the input signal to the ECU.

To show how the same three decisions need to be made with any car electronic system (the questions again: the desired outcome, the parameter to alter, and whether it's done on the input or output side of the ECU), let's now take a completely different example. 

The outcome that you want is heavier power steering in a car with electronically-controlled variable assist steering. So the parameter that you want to alter is the behaviour of the power steering pressure control valve - but do you try to modify the electronic inputs or outputs to achieve this?

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The workshop manual shows you that the system is very simple - there's a single input of vehicle speed and a single output of a variable duty cycle that goes to the power steering assist solenoid. Neither the input or output is a constantly varying voltage (ie both signals are pulsed) but you look at the diagrams in the workshop manual and realise that the speed input also connects to the cruise control ECU and engine management ECU. Therefore, if you attempt to modify the speed input you might also upset these other systems - in this case it might be better to attempt to modify the variable duty cycle output signal.

What about turbo cars? A particular turbo car uses a MAP sensor which, when the boost is wound up, causes the ECU to trigger a fuel-cut. The outcome that you want is to disable the fuel cut - basically, you don't want it to occur. The parameter that triggers this is the MAP sensor output. Modifying the output signal of the ECU is near-impossible (doing this mod would require that you kept the injectors pulsing correctly after the ECU has stopped working them!) so you look instead at the input side of things. In this case it's the MAP sensor that is measuring the over-boost condition, so you need to modify the ECU input signal from this sensor so that a ceiling isn't exceeded.

Here's another example. The problem that your high-boost turbo car has is detonation - the outcome that you want is more stable combustion, and the parameter that you want is to alter is the ignition timing at high loads. To alter ignition timing on the output side is very hard - again it's a pulsed signal - and on the input side it's scarcely less easy, because again the main timing signal (eg from a crank angle sensor) is pulsed. But there's another input signal that can be altered to tweak timing - the intake air temp. This sensor uses a variable voltage output, so it's easy to alter. Again, the mod can be switched in and out with a load switch - a boost pressure switch would be ideal in this case. 

Even if this approach proves not to be effective – and a full ignition timing interceptor is needed - it will have cost very little time and money to first try the modification of the intake air temp sensor.

Directions to Take the Signal

Once you have identified the signal (either input or output) that you need to modify, the next step is to work out what direction you want to take it. This sounds simple - but in some cases it isn't.

Let's take the airflow meter signal modification. If you desire leaner mixtures, you want the ECU to think that there is less air flowing into the engine than there actually is, so that it will inject less fuel with the same actual mass of air flowing in.

Now, does that mean that you want the signal voltage from the airflow meter to be higher or lower than normal? If you immediately said "lower!" think about why you said that - it's because you assume that a higher voltage from the airflow meter represents a higher amount of air flowing into the engine. In other words - voltage goes up with load. In this case you're very likely right, but - by direct measurement or by consulting the workshop manual - you need to be sure of this relationship.

Remember that some sensors may also be initially complete unknowns – eg the g-sensor in an ABS or four wheel drive torque split control. Without knowing how that sensor is behaving, you cannot make effective modifications to its system.

Working the Manual Backwards

As car systems get more complex, it can be difficult working out what parameter to actually change. You know what performance outcome you want - but how do you get there? One approach is to read the workshop manual backwards - looking at what the manual describes as fault symptoms, but what you see as a good performance outcome.

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For example, an electronically controlled auto trans might have listed a fault condition termed "harsh engagement (1st > 2nd)", with a variety of possible causes for this condition then listed. Have a look at what is also shown as causing "harsh engagement" for the 2nd > 3rd, 3rd > 4th and 4th > 5th gear changes - that is, all of the up-changes. In the case of the manual that I have open in front of me now, the only common cause of these conditions is the solenoid modulator valve.

From this you can deduce that modifying the signal that goes to the solenoid modulator valve can be used to firm up the shifts. Having come to this decision, you can look at the diagnosis tables to see what other conditions will occur if this signal is modified. In this case the table also shows that it's likely that the car will also then have a harsh engagement when changing from Neutral > Drive, and Neutral > Reverse. If you didn't want the latter conditions to occur, you could switch the modification in only when the throttle is open past the idle position.

Even very complex systems can be examined with this 'backwards diagnostics' approach - another excellent reason to have the complete workshop manual available.

Unique Modification

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A good example of the modification approach covered here was when I modified the regen braking on a Toyota Prius hybrid. As far as I know, this mod was a world first. 

Regen braking occurs when the car’s electric motor becomes a generator, pushing juice back into the high voltage battery and so slowing the car. I wanted to make regen work more strongly.

  • System

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The system that integrates regen and hydraulic braking on the Prius is complex. (Click on the workshop manual diagram to enlarge it.) In this car, the ABS ECU handles regen braking as well as ABS functions, sending a signal to the hybrid ECU to tell the hybrid ECU how much regen to impose. But how does the ABS ECU know what to do? 

Rather than measuring brake pedal travel (which could vary with pad wear, etc), the system uses pressure measuring sensors to detect master cylinder pressure. The higher the master cylinder pressure, the harder the driver is pushing on the brake pedal. 

If the driver is pushing only gently, the piston displacement will be small and so the hydraulic brakes will be only gently applied. In this situation, the ECU knows that the driver wants only gentle deceleration and so instructs the hybrid ECU to apply only a small amount of regen. However, as master cylinder pressure increases, so does the amount of regen that can automatically be applied. 

(In fact, there are four pressure sensors in the braking system and two pressure switches – but it’s the master cylinder pressure sensor that is most important.) 

  • Modifying the System

    So if the amount of regen that occurs is largely dictated by the output of the master cylinder pressure sensor, what about intercepting and altering this signal? That way, the ABS ECU would think that there was more master cylinder pressure than was actually occurring, so resulting in more regen being applied. Since the actual hydraulic pressure going to the brakes would be unchanged, there’d be a greater proportion of regen braking in the mix.

    The measured voltage output of the pressure sensor ranges from about 0.4 – 3 volts, rising with increasing pressure. So if a small voltage could be added to this signal, the ECU should respond with more regen braking. But if this was done, would it detect a fault condition? The workshop manual states that a fault will be detected if the voltage from the sensor is outside of the range of 0.14 – 4.4V, or if the voltage output of the sensor is outside a certain ratio to its nominally 5V supply voltage. Further, the latter is checked when the brake switch is off (ie brake pedal is lifted). 

    In other words, the voltage needs to be within a certain range and in some cases this is checked with the brake pedal not being used. 

  • The Circuit

    Click for larger image

    Leaving out a lot of the ECU connections, here’s what the master cylinder pressure measuring system looks like. From top, there’s the voltage signal from the sensor, the regulated 5V supply to it, the input from the brake light switch (12V when the brakes are on), and the earth connection. (Note that for the following circuit, it doesn’t matter which regulated 5V supply and earth connections on the ECU are used – there are several.) 

    Click for larger image

    As indicated above, what we want to do is to lift the voltage output of the master cylinder pressure sensor, especially at low sensor output levels. This is easily achieved with a single 100 kilo-ohm pot. If the pot is wired between the output of the sensor and a regulated 5V, and the wiper of the pot is then connected to the ECU, the voltage that the ECU sees can be varied from a constant 5V (not wanted!) right through to the standard signal. If the wiper is adjusted so that it’s just a little way towards the 5V end, a small voltage will be added to the signal. Note that to allow for the required fine adjustments, a multi-turn pot should be used.

    But what about the way the ECU checks the sensor output voltage when the brakes are off? In that case, it might spot that the output voltage of the sensor always appears to be a bit high. 

    Click for larger image

    The easy way around this is to add a relay that bypasses the pot whenever the brake pedal is released. This can be achieved with a low current SPDT 12V relay. As shown here, whenever 12V is available on the brake light circuit, the relay opens, sending the signal through the pot. But when the brake lights are off, the relay closes and so the sensor input voltage to the ECU is effectively standard. As a result, the system works as standard until the brake light switch comes on, whereupon whatever adjustment has been set on the pot immediately comes into effect.

  • The Results

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    This graph of the input and output voltages at different braking efforts shows what happens. When the brake pedal has not been pushed, the input and output voltages are the same. When the brake pedal has just been tripped, the output voltage rises. The amount that the output is greater than the input progressively reduces as braking effort increases, until at heavy braking load the signal is back to standard. 

    Click for larger image

    Logging the input signal from the sensor and the modified output signal to the ECU shows that over a 6 minute hilly urban drive, the average value of the sensor signal was 0.507V and the average value of the modified output was 0.562V. However, as the lower trace shows, the modified output of the sensor has a lot more ‘area under the curve’, a better indication of the changed feel on the road.

    In short, the modification worked very well.

  • Series Conclusion

    Over the thirteen parts of this series we’ve come a long way – from starting with a circuit featuring literally just a light bulb, battery and switch – to electronically modifying the braking system in one of the most complex cars in the world. And it’s worth pointing out again that the Prius regen braking mod used only a relay and a pot – very simple components indeed.

    Electronic modification of the vast majority of cars produced over the last 20 years is absolutely possible for the amateur – that’s modification of engine management, auto transmission control, power steering control, and so on. In fact, name the system and you can make changes. 

    Electronic car modification tends to be talked about in hushed tones, as being able to be achieved only by absolute geniuses or by workshops with many tens of thousands of dollars worth of equipment. But as I have shown in this series, neither is the case...

    The parts in this series:

    Part 1 - background and tools

    Part 2 - understanding electrical circuits.

    Part 3 - volts, amps and ohms

    Part 4 - using a multimeter

    Part 5 - modifying car systems with resistors and pots

    Part 6 - shifting input signals using pots

    Part 7 - using relays

    Part 8 - using pre-built electronic modules

    Part 9 - building electronic kits

    Part 10 - understanding analog and digital signals

    Part 11 - measuring analog and digital signals

    Part 12 - intercepting analog and digital signals

    Part 13 - the best approaches to modifying car electronics – and the series conclusion

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