Not too long ago I replaced the ECM om my '89 Trooper with the 2.8 engine. Prior to replacement I could not retreive trouble codes, and the EGR system was not working. The new ECM cured both problems. I'll paste in the instructions for getting trouble codes. Can't remember where I got this, but it works well. If you lift out the two trays between the front seats, you'll find the ECM, and there should be two wires there to connect for the test.

******************

THROTTLE BODY FUEL INJECTION

The ECM test connection is located under the center console.

A built-in, self diagnostic system will identify problems most likely to occur in the throttle body fuel injection
system. When a problem is detected, the diagnostic system will illuminate the "Check Engine" lamp in the
instrument panel.

When the engine is started, the Check Engine lamp should turn oft. If the lamp remains lit, the self diagnostic
has detected a problem. To determine where the problem exists, ground the "Test" terminal of the connector with
the ignition On, engine Oft and observe diagnostic trouble code display. Each DTC will flash three times in
numerical order (lowest to highest). Each diagnostic trouble code will indicate a fault in a particular circuit.

MULTI-PORT FUEL INJECTION

Amigo, Pickup, Rodeo & Trooper II

On Amigo, Pickup and Rodeo (except 1993-95 models W/3.2L engine), connect the single wire diagnostic leads
located behind the left hand kick panel together. On Trooper models to 1991, the single wire diagnostic
connectors are located under the center console. On 1992-95 Trooper & 1993-95 Rodeo models W/3.2L engine,
connect a jumper wire between data link connector terminals 1 and 3. On 1992-95 Trooper models the data link
connector is located behind the lower front of the instrument panel. On 1993-95 Rodeo models W/3.2Lengine,
the data link is located under the left hand side of the instrument panel. On all models, turn the ignition switch to
the On position. At this point, the "Check Engine" lamp should flash.

********************************

That doesn't look too good on my screen, but the information is there. Somewhere I have pictures of the ECM and associated wires, if you need. Remember, the 2.8 engine is a GMC engine, just as found in any number of GMC vehicles.

HTH,
Ken