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What to Look for When Considering the Purchase of an ODB-I Scanner

Technically Speaking: Old school … OBD-I Scanner. Scanner Choices for OBD-I

James Berry Aug 30, 2016 0 Comment(s)
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Q: I have read all three “Rough Running” articles and have a question. Part 3 specifically addresses 1982-1995 (OBD-I) EFI problems and talks about diagnosing fuel injection problems using a scanner. I have looked up the scanners pictured with the article and they all seem to be OBD-II scanners.

Are these scanners also capable of displaying Integrator and Block Learn data from an OBD-I Corvette? None of the data I have found on them indicates that they are capable of being used on vehicles equipped with OBD-I.

Are there scanners that can display Integrator and Block Learn data from an OBD-I car? If so, can you provide make/model information on them? I have a 1985 Vette that has been sitting idle for several years and I suspect that the injectors need to be cleaned. I’d like to use a scanner to evaluate the system as part of bringing the car back to life.


A: Thanks for the question, Wayne, and yes, you are correct, the scanners pictured with the article are OBD-II scanners. However, most of today’s high-end scanners, such as the Snap-On Modis pictured in the article, come with all of the OBD-I software already installed in the scanner and are ready to use on your OBD-I equipped vehicle. We interviewed David Lazear with Snap-On and he stated that most all of today’s Snap-On scanners were OBD-I compatible.

Adapter Cable 2/5

You will need to purchase the adapter needed for your vehicle application, which are inexpensive. There is also a kit available for most scanners that cover most manufacturer vehicles that are OBD-I equipped.

Adapters 3/5

When we talk about OBD-I General Motors vehicles we mean vehicles made from approximately 1981 to 1995. These vehicles used two different types of ports that you would access: a 12-pin port or a 5-pin port, for early production vehicles. Make sure you purchase the correct adapter for your vehicle.

Some OBD-I vehicle communication adapters require a 12V power source be provided to establish proper communication between the vehicle and the scan tool. This is achieved by plugging the Lighter Power Cable or the Battery Power Cable into the silver power port on the side of the OBD-I adapter.

After reading your question I went online to see what scanners were available for OBD-I applications. There are several good choices depending on your budget. Just be careful not to purchase a code reader; these units normally are not able to display data.

There are some good used professional scanners on the market at this time because shops are upgrading their diagnostic tools to keep up with the technology in today’s modern vehicles.

Obd Used Scanner Adapters 4/5

If you purchase a used scanner make sure the adapters come with the unit that you may need for your application. Some scanners also have cartridges and keys so you will want to determine if the scanner you are thinking about purchasing needs cartridges or keys and if the cartridges provided will have the software needed for your application. It’s usually best to pay a little more for a complete unit that has updated cartridges and the keys.

There is also another option for OBD-I diagnostics according to David Lazear; you can use your personal computer (PC). There are basically three things you need to record diagnostic data.

• A PC computer, preferably a laptop for freedom of movement, to collect, log and analyze the OBD-I data.

• An Assembly Line Data Link (ALDL) cable to interface your PC computer to the ALDL OBD-I diagnostic port on your car.

• A software program to capture the OBD data. There are several free and pay programs available online. Just be sure there is software available for your vehicle before you purchase a laptop and/or the ALDL cable.

OBD-I Reading Data

An OBD-II scanner can report up to 15,000 possible data parameters, while OBD-I is only capable of approximately 300 data parameters. Those 300 parameters are more than enough to diagnose a problem with your vehicle.

Wayne, these scanners should be capable of reading a data stream and display Integrator and Block Learn data on most OBD-I vehicles. We are also going to review how to determine Integrator and Block Learn data in the Field Service Mode section using a jumper wire, even though this is somewhat primitive.

Retrieving Fault Codes Using a Jumper Wire

If you don’t have a code reader or a scanner at all there are some diagnostics that can be performed with just a jumper wire on OBD-I equipped Corvettes. The system has the ability to recover Diagnostic Trouble Codes (DTC) for several onboard systems, but for this segment we will only cover the engine controller fault codes and Field Service Mode.

Obd Corvette Aldl Jumper 5/5

Code recovery for the OBD-I Corvette (1984-’93) is entered using the following procedure:

Place a jumper wire between pins “A” and “B” on the diagnostic connector. This connector is located under the dash, just to the right of the steering column.

Turn the ignition to the On position but don’t start the engine.

Depending on the model year, either the Check Engine Light or the Service Engine Soon light will begin to flash.

As the code display sequence begins, you will see a single flash followed by a pause and then two more flashes. This is the indication for the number “12”. The number 12 will always be the first fault code you will receive. This fault code simply tells you that the engine controller is not receiving any pulses from the distributor, which is normal since the vehicle is not running.

Code 12 will flash three times and then any other fault codes will flash (also three times). When all codes have been displayed or if there are no stored codes, the number 12 will again flash in a continuous loop.

For example, a throttle position sensor fault code 21 would be represented by:

12: flash (pause), flash, flash, (long pause)
12: flash (pause), flash, flash, (long pause)
12: flash (pause), flash, flash, (long pause)
21: flash, flash, (pause) flash (long pause)
21: flash, flash, (pause), flash (long pause)
21: flash, flash, (pause), flash (long pause)
12: flash (pause), flash, flash, (long pause)
12: flash (pause), flash, flash, (long pause)
12: flash (pause), flash, flash (long pause)

The ECM code display will repeat in a loop until you turn off the ignition switch or remove the jumper wire.

Field Service Mode

Field Service Mode on OBD-I equipped vehicles can be entered by jumping terminals “A” and “B” on the ALDL while the engine is running. This will not harm the engine controller. When the vehicle is in Field Service Mode no operational parameters will be changed. In other words; the vehicle should run the same while in Field Service Mode as it would when it is not in Field Service Mode.

When the vehicle is in Field Service Mode you can observe the engine controller loop status (whether the vehicle is in Open or Closed Loop). You can also monitor the fuel offsets (the signal from the oxygen sensor to the engine controller, relaying a rich or lean condition). While in Field Service Mode this information will be relayed through the MIL lamp (Malfunction Indicator Lamp) for diagnostic purposes.

How To Interpret The Flashing MIL Lamp

If the engine controller is in Open Loop mode the MIL lamp will flash extremely rapidly, about 2.5 times per second. If the engine controller is in Closed Loop mode it will flash about once per second.

When the engine is first started the system goes into Open Loop operation. In Open Loop operation the engine controller will ignore the signal from most of the engine sensors and calculate the air/fuel ratio based on inputs from the coolant and MAP sensors, along with a pre-programmed memory.

The system will stay in Open Loop until the following conditions are met:

• The O2 sensor has varying voltage output, showing that it is hot enough to operate properly, approximately 600 degrees.

• The coolant sensor is above a specified temperature, approximately 104 degrees.

• A specific amount of time has elapsed after starting the engine.

When these conditions are met the system will go into Closed Loop operation. In Closed Loop the engine controller will calculate the air/fuel ratio based on a variety of sensors. This allows the air/fuel ratio to try and stay as close to 14.7:1 as possible.

When in Closed Loop mode, if the MIL light is off more than it is on, this indicates that the engine is running lean. If the MIL light is on more than it is off, this indicates that the engine is running rich. This is what the oxygen sensor is seeing.

If the MIL light is flashing off more than it is on the engine controller will then command the Block Learn/Integrator to enrich the fuel mixture above the 128 count baseline. If the MIL light is on more than it is off this indicates that the Block Learn/Integrator is commanding the engine controller to lean the fuel mixture below the 128 count baseline.

The Block Learn Multiplier value can range from 0-255. A value of 128 is ideal because it is the center point between 0 and 255. To maintain the correct air/fuel mixture (14.7:1) the engine controller continually adjusts the Integrator or Block Learn Multiplier.

You cannot read the Block Learn/Integrator values while in Field Service Mode but you can get an idea of what the engine controller is observing. Since the engine controller continually adjusts the Integrator or Block Learn Multiplier you should see the MIL light on and off roughly an equal amount of time.

Since the engine controller is continually adjusting to try and obtain the perfect air/fuel ratio on a good running vehicle, the Block Learn Multiplier will typically range from 105-150. The Integrator values will also typically range from 105-150.

If the values for the Block Learn Multiplier are higher than 128 the engine is running lean (the engine is not getting enough fuel).

Remember, all this is based on what the oxygen sensors are telling the engine controller. Other problems must be eliminated before cleaning or replacing an injector. Other fuel related problems can include vacuum leaks, weak fuel pump, restricted fuel filter, leaky fuel pressure regulator and so on can cause the Block Learn Multiplier to adjust the pulse width (injector on time).

Wayne, Field Service Mode was very helpful when these systems first came out and once you start using Field Service Mode it won’t take you long to figure out the flashing MIL light. I must admit the old way works but the use of a scanner makes this a lot easier and more precise. I hope this helps you in diagnosing problems and finding the right scanner for your vehicle.




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