I have a 1988 GMC suburban 1500with TBI 350, 700R4 transmission, 235NP transfer case. When I stop at a lights or parked, any thing idle my service engine light comes on, when I drive it service light goes out.

 

Check the battery voltage at idle with a meter. Should be at least over 13 volts. Alternator may be getting worn out.
If memory serves me a '88 was a paper clip in the connector to read the check engine light codes by counting the blinks and pauses between blinks. That was a long time ago since I had my Blazer.