I just bought a 1986 Chevy S-10 Blazer. The "service engine soon" light comes on when driving. My service manual tells me there's an "ACLR module" from which I can jump A to B to receive a code isolating the problem.
The book tells me on one page that it's located on the left side of the firewall under the hood, and on another page that it's under the dash. Neither one shows a picture of the box.
I'm not sure where you came up with "ACLR Module". The correct terminology is "ALCL Connector" (Assembly Line Communication Link) which is located under the driver's side of the instrument panel. If it's not plainly visible, someone must have unbolted it and left it hanging under the dash somewhere.
The ALCL is a twelve-cavity connector labeled A through M while leaving out the "I". With the ignition "on" go ahead and jump terminal A to B (a paper clip will work just fine). If the ECM (Electronic Control Module) is functional, then the "Service Engine Soon" light on the instrument panel will flash a "Code 12" three times (Flash-Pause-Flash-Flash x 3). The trouble codes which are stored in memory will then present themselves in the same manner going from the lower to higher numbers. Write the codes down and find the appropriate diagnostic chart in whatever service manual you're working with.
Always start with the lowest numbered trouble codes first. The first repair may correct the higher codes at the same time. This is the old-fashioned way. A quality Scan Tool will plug into the ALCL and immediately give you the stored trouble codes along with a multitude of additional information to aid in diagnostics.