Scan Tool Facts

autoxray_1.JPG (12594 bytes)In days of yore, Scan Tools were looked upon as the dominion of factory trained technicians: a kingdom where mortals could never tread, certainly not weekend wrench warriors.

A parallel was the first oscilloscope based engine analyzer. Your dealer would roll out an immense cart with a large TV screen used to display the intensity of fire being routed to all eight of the spark plugs at the same time. Dealers had them first, then the independent shops and finally dedicated DIY (Do It Yourself) types began to buy them.

Scan Tools have gone through a similar process starting with the expensive Tech 1 device so beloved by GM dealerships in the early 1980s. Then came the (even more expensive) Tech 2 used by GM on the 1996 C4 Corvette and all C5s.  Eventually this technology evolved (albeit without all the capabilities of the Tech 2) into designs like the AutoXray EZLink Scan Tool sold elsewhere on this website.

Now, anyone with $199.95 can tap into the C4’s computer just like the big boys.

Whence Cometh Automobile Computers?

In the 1970s, members of the California Air Resource Board (CARB) were looking for a solution to the L.A. smog problem.

Eventually they said: “Since cars cause the majority of the problem, why not force the manufacturers to more tightly control exhaust emissions?” They developed standards and said to the US Environmental Protection Agency (EPA): “Are you in for the whole country?” And the EPA said, “You bet”.

Since then, emission standards and the sophistication of the automobile computer have become progressively more complex forcing first a standard called On Board Diagnostics-I (the OBD-I system used in the 1982 through 1995 Corvettes), and then OBD-II, a much more capable system adopted for 1996 and later Corvettes.

Of Ones and Zeros

By 1996, the Corvette was a rolling computer farm and coping with the digital world required an approach to troubleshooting that defeated many professionals and virtually all shade tree mechanics. It wasn’t that they were dumb, they were just lacking in training and proper tools. A whole new mindset was required to troubleshoot these Corvettes.

Computers use digital logic, meaning the information they store and process has only two possible states: ground or some standardized voltage level such as 5 volts. Anything in between ground and that level is neither fish nor fowl. These states were called “logic low” or “logic high” and eventually a 0 or a 1 was used to express the state of the information, a system called binary notation. Without getting into the theory, using just these two states and stringing them together, in serial fashion, you can express any number you wish. A serial stream of +5 and ground states of 10101 (+5, ground, +5, ground and +5) can equal the number 21 to a computer for instance.

Don’t worry about it though. Only computer geeks like the webmaster of this site get excited about this sort of thing. Besides, you don’t need to know binary notation to cope with your Corvette’s computer.

The computers lurking in your 1982 C3 and later Corvette takes analog inputs (inputs that vary in voltage or resistance) and digitizes them into ones or zeros before doing anything else. They then make decisions by comparing those incoming binary codes to binary codes held in either a PROM (Programmable, Read Only Memory) or an EEPROM (Electrically Erasable Programmable Read Only Memory)

scantool_4.JPG (13885 bytes)For example, if the digitized Oxygen sensor output departs from the desired value held in memory, the computer will shorten or lengthen the “on time” of the fuel injectors in response. Whatever can be done to arrive at the magic 14:7 air/fuel ratio is done which holds down air pollution. scantool_5.JPG (13258 bytes)








Everything an automobile computer does is handled in a similar fashion: (1) look at a sensor, (2) compare what is happening in real life to what is desired and (3) make an adjustment via a switch, coil, motor or variable resistor of some sort. Problem is, all of this is happening extremely fast--so fast it appears to be instantaneous.

Computers are sort of like dogs in that they chase their tail.  They run through a list of instructions (called the instruction set)--monitoring sensors, handling a problem or two--endlessly doing the same things over and over again very fast. Far to fast for a meter or test light to track.

The content of the instruction set is known though and since the computer tattles on itself through a special connector under the dash called the ALDL (Assembly Line Data Link), you can see what is happening within the computer provided you have a magic decoder ring.

Enter the Scan Tool

Once configured for the appropriate Corvette model year and engine, the Scan Tool will show any trouble codes plus monitor the raw data coming from the sensors while the car sits there idling. Or, you can select the capture mode, set out in your pride and joy and record the parameters being processed by the computer plus see the actions taken as a result of those inputs.

Have a stumble or hesitation as you are zinging along the highways and byways? Just press a button and mark the occurrence for later playback back in your driveway or garage. If you have an intermittent, this is definitely the ticket to ride.

Another use for the tool is the wiggle test. Since many problems are caused by bad connections, you can place the tool in the capture mode with the engine running and then shake harnesses or connectors. You may set a trouble code or cause the loss of a signal as you shake suspected wires and connectors. If you do, the Scan Tool will rat on the culprit faster than a politician’s mistress.

Panacea or just a tool?

Powerful though the Scan Tool may be, it is still just a tool. Sometimes it will lead you directly to the problem (poor connection, bad sensor, etc.), but often you have to interpret the computer’s inputs and outputs via the Scan Tool to arrive at a solution.

An example: your Vette starts and then dies.

You scan for trouble codes and don’t find any. You wiggle wires and don’t find any bad connections either. So you do a capture while you start the car. The Scan Tool shows that the fuel pump sense voltage is zero and you now know why the car is dying: no fuel is available beyond the initial two second pressurization cycle from when you first turn the ignition key.

scantool_2.JPG (10500 bytes)But why no fuel?

A look at the service manual shows that the oil pressure-sending unit will shut off the fuel pump if the oil pressure goes below 4 PSI but that sensor isn’t monitored directly by the computer on this model Corvette so direct information isn’t available to the Scan Tool.

What are the possibilities then?

The oil pump has failed, there is an obstruction in the oil lines/galleries or the oil pressure sensor itself has bit the dust. You change the fuel pump relay just for grins (because it is easy to do) but the problem remains.

You remove the oil pressure sensor, screw in a pressure gauge and there is plenty of oil pressure during the brief period when the engine is running so, right away you know the sensor is kaput or the connection is dirty or the wiring harness is bad.

You can see the connector isn't dirty and the harness does not respond to a wiggle test. So you change the sensor and immediately the problem disappears. New sensor = No more problem.

scantool_3.JPG (13699 bytes)Did the Scan Tool lead you to the sensor directly?

No but it allowed you to see things the way the computer did. In essence, it showed you the whole picture at one time and in a way that allowed you to see the cause (the sensor inputs) and the effect (the computer’s commands) in plain English.

Wouldn’t a voltmeter have done the same thing in this case?

Yes, but since you looked at the whole picture with the Scan Tool, it saved you a lot of time and frustration. In a 1984-1995 C4, you scroll through at least 40 parameters on the screen instead of checking them at the sensor or output device one by one.

Plus you see both sides of the equation: the inputs the computer received and the commands it issued. This is the real value of a Scan Tool and why even a casual DIY owner may wish to consider one.

If you think the AutoXray Scan Tool is something you want to add to your garage, it is available for $35.00 off list price ($164.95), at the C4Store