Following.

I have one of these in storage that needs a little help too.

Have you checked the switch stacks that keep track of the runners? A lot of Gottlieb switch stack screws loosen up over time and cause weird behavior. Tightening helped some of my issues. Off the top of my head there are some relevant relays towards the front right inside the cabinet.

The good news is this game follows the rules of baseball so it’s easier to determine if it’s working right.

The base runner circuit in Grand Slam and the other 70s Gottlieb baseball games relies on some clever circuitry explained at:
https://www.funwithpinball.com/learn/gottlieb-grand-slam-base-runner-circuit-animation

Any time the Score Motor runs, like when the ball drains, each base goes through a process:
- the base runner is checked to see if it should score a run
- the base runner is cleared
- the base checks to see if a new base runner arrived at the base

In your case it sounds like the first two steps are happening but the last step isn't. The relevant part of the schematic is:
Grand Slam bases relays (resized).jpg
Do you get base runners by hitting targets, or only by landing in the kickout holes? If only the kickout hole switches work to put runners on base I would suspect the wipers on the Hit Unit stepper (in the dashed box).

/Mark

I just went through one of these a couple months ago and got it working.

I’m drawing a blank now and only had snippets of the schematics to work with but I remember watching a video of Ron from Joe’s classic arcades working on a Gottlieb Home Run with the same base runner circuit which helped me understand what was happening.

Upon closer inspection the events I listed earlier happen when a target or rollover earns a hit, but not when the ball lands in a kicker hole. The kicker hole runs the Score Motor and puts a man on base but it's as if he parachuted in without getting a hit.

Does your Hit Unit stepper advance when the ball lands in the kicker holes? It shouldn't but it would explain how the bases are getting cleared if it does.

I suspect that the Hit Unit runs, and your bases clear, any time the Score Motor runs. It shouldn't do that. You could test that theory by giving the Score Motor a nudge out of its home position.

It may be that one of the switches in the red box is stuck closed somehow (mangled, bent solder tabs, touching wires, etc.):
Grand Slam Add Hit Unit (resized).jpg
It might also be that the E/Single relay fires when the ball lands in a kickout hole.