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.
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.
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:
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.
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?
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.
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.
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.
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...