Need help with OBDI code 41!

[CAChevy]

Hey Gang. The saga continues with this truck. It's a 1984 CA emissions equipped with a new GM 350 crate.

Finally got it firing on all eight cylinders and the miserable CCC Quadrajet is as tuned as I can get it with a dwell meter and a vacuum gauge.

I took the truck out today and ran it. Other than being disappointed in the "get-up-and-go" that this truck is lacking, it seemed to run pretty good. However, I did notice slight pinging under semi-hard acceleration. Base timing is set at 8 deg BTDC, 87 octane gas. I may try to set it back to the recommended 6 BTDC and see if that helps.

I got it home and for giggles put it in diagnostic mode to see if there were any codes. The check engine light wasn't on but I pulled a code 41.

41
EST circuit
No distributor reference pulses to the ECM at specified engine vacuum.

I have no idea where to start. The ignition module is new, the MAP sensor is new. I don't know if the original parts were bad or not, I just replaced almost everything under the hood during the rebuild.

I did accidentally momentarily ground the tach lead of the distributor when I was setting my idle. Just a quick spark. Don't know if I messed something up in doing that. The lead is still providing a tach signal.