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
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:
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 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.
- 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.