Volvo Repair: retrieving codes for engine light, blink 1 2, posit

Question
I have a '90 740 GL wagon automatic, non-turbo. My engine light comes on periodically (usually after driving on the highway). I have located the  small black box on the driver side with the red led but I'm not sure how to get the codes in order to determine the problem.  please help! THANK YOU!!!  

Answer
Hello Dave, Well here goes,take the probe wire and put it in posit.# 2.With the key ON,press the button for 3 secs.the led will blink one or more times,there is a pause between the blinking sequences.as in 'blink    blinl/blink   blink = 1.2.1 Now repeat the step untill you see the first code repeat,the the comp.knows that all the codes have been rear and now can be erraced. Press the button fot 15/18 secs,release and when the led lite up press the button again for 15/18 secs.wait 3/5 secs. and press the button again 3 secs. and you should 'see' the  blink    blink    blink = 1.1.1= confirm codes erraced.Let me know! Thank you and good hunting! PS.the ignition # is #6 repeat the same procedure there( #2 is the fuel sys.)  PKD