A Diagnostic Tool

The diagnostic connectors on the firewall enable you to monitor the major components of the computer-controlled emission system, that is, if you have the necessary diagnostic tool. Here is a picture of one that I cobbled together using parts from my many junk boxes. The only things I had to buy were the Molex connectors. The 0-1 volt meter shows the actual output voltage of the O2 sensor, and the vertical column of LEDs shows the state (on or off) of the signals available at the diagnostic connectors on the firewall. The cables (telephone cord) are long enough to reach through the side window to the dash inside the Jeep. This way I could watch what was going on as I was driving and catch a malfunction "in the act."

A simpler, smaller, and cheaper diagnostic tool

The two signals that provide the most useful information are AØ and BØ, which actuate the stepper motor on the BBD. The behavior of these two can tell you a lot:

Are both the Thermal-Electric Switch and the Coolant Temperature Switch working?
Is the MCU taking control?
If it is - when?
Is the O2 Sensor working?
Is the BBD Stepper Motor working?

A small tool to monitor the AØ and BØ signals can be built in an hour for almost nothing. Here's all that's required:

Two LEDs (any appealing color)
Two 470 ohm resistors (½ watt)
A small project box to put them in.
Some wire
A soldering iron and solder

Here's a diagram showing how it's wired. There's nothing to it. Three wires come out - AØ, BØ, and ground. Hook AØ and BØ to the appropriate wires at the harness connector under the dash and the ground wire to any good ground. Which LED is AØ or BØ doesn't matter as long as you remember which is which. I recommend three different wire colors to avoid confusion.

I decided to tap into the harness right at the MCU, since it's only a foot away from the spot under the dash where I decided to semi-permanently mount the tool, and there seemed to be no point in going through the firewall to get to the diagnostic connectors on the other side.

Here is what the finished project might look like. I already had the LEDs and resistors, and bought the rest at my local Radio Shack for less than five dollars.

If you don't have a box and/or would rather just cobble it together, you can punch holes in a piece of cardboard, push the LED leads through, and wire the resistors on the back side, using liberal amounts of insulating tape to hold it together.

Here is what the tool will show under normal operating conditions.

As the MCU energizes the stepper motor on the BBD, moving the metering pins forward and backward in an effort to correct the mixture in response to the voltage from the O2 sensor, the pattern will look something like this. The pattern provides clues as to what's going on with the computer and with the carburetor. If it's stuck at one end for instance, you can tell immediately if it's too rich or too lean, which gives you valuable information for finding the problem. If it displays something that looks similar to what is shown here then the whole system is probably working as intended.
If you look down the carburetor while watching the LEDs, you will understand immediately what's going on.

Taking it a step further.

If you are not intimidated by this little project, or even if you are, it can be expanded to include all the signals available at the D2 diagnostic connector by simply adding a resistor, LED, and wire for each one. You could skip pins 4 and 7, and pin 1 too if you don't have the PCV shutoff solenoid (you shouldn't have this).

From a performance standpoint, and in rough order of importance, the key signals are:

Pins 10 and 12
- The temperature switch in the air cleaner must be open, and the one in the intake manifold water jacket must be closed before the computer can kick in and take control of the carburetor.

Pins 11 and 14
- The BBD stepper motor AØ and BØ signals should both flash rapidly for a couple of seconds at startup as the computer initializes the stepper motor, and then they will stop. They will not start flashing again until the engine warms up, or until 15 seconds elapse if the engine is already warm. They will not flash at all if the stepper motor is unplugged, or if there is a problem with the stepper motor connector or wiring.

Pins 9 and 15
- Performance will suffer if the 10"- 4" Vacuum Switches are not working correctly.

The rest of the signals I would consider useful and/or interesting, but optional.

Bear in mind that in these simple circuits some of the LEDs will be normally on, and will go off when actuated, while others will be normally off and will light when actuated. This can be confusing at first, but you will quickly learn what to look for.
I compensated for this in the diagnostic box at the top of this page by using CMOS inverters to drive some of the LEDs, but that's considerably more complicated, requires a lot more parts, and is time-consuming.