I took the following straight from the D.I.Y. article by Steve Brotherton. You don't need a scope or an impulse counter. All you need is an LED wired between pin #3 of the diagnostic connector and ground. Mushedroom, if you need further help consider emailing me or even calling me.

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This process of reading codes for these early systems works up to 1994 for all systems, and later on some systems. Basically, it will work up to OBD II systems, and applies to all systems, not just engine management systems. The code retrieval method is very simple. The communications take place over a single line on the appropriate diagnostic connector. On the car I used (a 1991 300TE), the connector (X11/4) is located next to the battery, next to the right side hood hinge.

The KE controller is accessed on data terminal #3. With the key on, engine off, self-diagnostics is activated by grounding the #3 pin for two to four seconds. Once this takes place, the control unit transmits the codes by itself, grounding the #3 terminal. (Remember that all systems work the same; all you need is a directory of which pins to activate and the code table to interpret.)

The impulse counter makes life easy by counting up some high numbers. In the scope pattern in Figure 7, I connected the scope negative lead on the battery positive and the positive lead in the #3 pin.

I attached a jumper ground for three seconds and the control unit grounded the pin seven times, which could easily be counted. This was done again and code 13 was read, again and code 14 was read, and again and code 27 was read. If you are using the scope to read this, the time scale must be appropriate to get the whole thing, as every further activation moves on to the next code. They are listed one at a time until they are repeated. This can be done over and over. Each time the pin is grounded for two to four seconds, the next code is communicated by the control unit, grounding out the code number in "blinks." An LED can also be used to see a "blink" for counting. Once all codes are read, they can be erased by holding the jumper to the appropriate pin for six to eight seconds. After this is done, another activation with a two- to four-second ground will read the next code, and a further six- to eight-second ground pulse will erase it. This is continued until all codes have been read and erased. This will be shown by the final code of "one blink."