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DIY Modification of Car Electronic Systems - Part 1

Simple and cheap stuff.

by Julian Edgar

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Despite cars being stuffed full of electronic systems for decades - and consequently sophisticated and interesting cars now being available for nearly nothing - the basic, DIY electronic modification of car systems seems to be pretty well unheard of. So what about the really simple - but potentially very effective - electronic mods that can be made for nearly nothing? Like, I am talking a components cost of a few dollars, and a required electronics knowledge which is pretty low as well. But do these mods actually exist? And can normal people do them - you really don't need to be an electronics engineer? Read on...

But first a warning. The mods that we'll be covering in this story are easy and effective - but they're not always the best way of doing things. They're perfect on budget cars, where spending $1000 on an interceptor or new chip is just a ridiculous prospect. And personally I am also happy to use some of these techniques on expensive cars, including ones that I have owned in the past and own now - but others would look askance at my doing so. You pick.

The Workshop Manual

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As far as we're concerned, to achieve success in any of these basic electronic mods you need to have a good workshop manual - and in nearly all cases that's the factory workshop publication. We don't have a lot of patience with people who delve into their car's wiring harness, basically not knowing any of the colour codes, plug pin-outs or the functionality of the systems. Sure in some cases it can be done - but we're not interested in doing things that way.

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.) 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, check trouble codes, etc. In the case of a grey market import, the minimum information that you should have is a circuit diagram for the car's wiring. Again, even if you have to pay for it, it's worth it.

So if about now you're thinking - "Nah, I don't need the manual or a circuit diagram", well, stop reading! Your chances of achieving success are near zero.

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 approach.

The questions to answer are:

  1. What modification outcome do you want?
  2. What parameter do you need to alter to achieve this outcome?
  3. 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 it 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.

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And the full-load only part of the requirements? That's easy - you just switch your modification in and out on the basis of throttle position, for example with a microswitch like the one pictured.

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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. (The ECU output signal to the injectors is universal in all cars - it's a pulsed, variable duty cycle signal.) 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 (see breakout box for more), 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. So 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 damn-near impossible (doing this mod would require that you kept the injectors pulsing correctly after the ECU has stopped working with 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.

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Here's a final 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.

Usually, it is better to modify a carefully chosen input signal rather than tweak an output signal.

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. Think that one through until it's clear in your mind!

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.

Not all sensors behave in this way...

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.

Next week: turning theory into practice

Signal Types

There are only two basic types of signals used in cars.

The first is an analog voltage. With this type of signal, a voltage goes steplessly up or down. For example, an airflow meter might have a voltage output of 2.5 volts at low engine loads and 4 volts at high engine loads. All the 'in between' loads have 'in between' voltages.

The second type is a signal that goes on and off rapidly - a frequency signal. For example, a crankshaft position sensor might send out a pulse every time a tooth on a crankshaft-mounted cog passes it. The faster the cog is turning, the faster the frequency of pulses coming out of the sensor. The ECU can work out engine speed by looking at how fast the pulses are being generated.

Frequency signals can also vary in what is called duty cycle - not only might the pulsing change in speed but the 'on' versus 'off' proportions of the signal might also vary. An injector works on this type of signal, where at low loads the injectors might be pulsed-on for only 10 per cent of the available time every rotation of the crankshaft. This 'on' time is called duty cycle.

With one exception that we will cover later, frequency signals (both input and output) are hard to change with very simple electronics, whereas voltage signals are easy - either by altering the value of the sensor resistance or by creating a voltage offset.

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