Check engine light

[unclefuf]

Just purchased at auction,1994 k2500 5 speed. Driving home, the cruise disengaged and check engine light came on. Disconnected batteries and reset, but I haven’t driven it again yet. Any ideas? Thanks

[DmaxMaverick]

You'll need to pull the code(s) that commanded the SES lamp. It'll tell you where it hurts. Being that it's a 1994 model (OBD-I), all you need is a paper clip and a little patience. 1996+ require a code reader/scanner.

[unclefuf]

Yes I saw that my code reader won’t work, what’s the paper clip trick?

[DmaxMaverick]

https://www.thedieselpage.com/members/trblcode.htm

(in case you can't access the above)
Engine off. Key out.

Bend a paper clip so the ends are about 1/8" apart and about parallel.

Have paper and writing utensil handy.

Jumper terminals A and B in the ALDL port with the paper clip. These should be the top row, two right-end terminals (assuming the ALDL hasn't been relocated, it should be oriented with the flat tabs/flange at the bottom). Attach a pic of your ALDL if you're unsure if it's oriented correctly.

Key on (no start).

Watch the SES lamp. It will flash (count them), with short, long, and longer pauses. A short pause is between the single digit number count. The long pause is the break between digits within the 2 digit DTC. The longer pause breaks between the 2 digit number DTC's. There may be none, or several.

A flashing 12 (1--1-1....1--1-1....1--1-1....) is the diagnostic system self-check code. It only means the system is working correctly, and any numbers to follow (other than 12) are actual trouble codes (write them down, they will repeat 3 times each). If there are actual trouble codes present, it may or may not begin with 12, beginning with actual codes. I've seen it both ways on different vehicles, not sure why.

A DTC 13 (ESS circuit fault) will look like: 1--1-1-1....1--1-1-1....1--1-1-1....

When the codes repeat, that will be all the codes stored.

When the codes (or 12) repeat in sets again, there are no codes present.

Key off.

Remove paper clip.

Done.

If you can't access the above link to the Member's area on The Diesel Page, report back here with what you find. If you plan on keeping the truck, please consider a subscriber membership to The Diesel Page (https://www.thedieselpage.com/form.htm). There's a whole 'nother world of information dedicated to GM Diesels, from the beginning to current, not available here on the Forums. The printed books are second to none, too.