How do you set the timing on a 1985 GMC S15 2.8 v6?

There is a special procedure.

Because doing this wrong will possibly seriously harm your truck I recommend getting a manual from your local Library or buying one.

The diagram of what connector and what connections to short is critical.

Other than that hitch, a timing light and a wrench.

There is a way around this and is more accurate. You need:

• A scan tool that will read what the computer is adjusting the timing too.
• A timing light that has a accurate dial to adjust the flash the timing light makes to top dead centre.

These two items allow you to look at the scanner, dial the correct number into the the timing light then adjust the distributor to 0 and you have a accurate easy setting. The plus is you don't have to fight to keep the engine running like the short the terminals method.