check engine light Q's

[WOTandSDWYS]

ok, at one time I had 2 240's, one was a coupe SE, and the other was my current fastback, both DOHC

I wanted to know if the digital speedo cluster would be plug and play between the two. After I figured out that I could go 130mph just by revving it in neutral, I changed the clutster back.

After I put the original cluster back in, I noticed that the check engine light would not come on with the key in the on position, like it's supposed to.

all the other lights like the e-brake and battery light come on, but not the check engine. I've checked the bulb and it is not blown, and I've checked every fuse in the car, and they are good.

anyone know of anything I've overlooked or something you've delt with before?

thanks.