Bad LM causing CEL flash on knock problems?

My NYer died at my second job on Saturday night. I pulled the codes and got 27 and 42. I thought it would be the PM, but it ended up being a problem with the LM. One of the first things I noticed when coming home was that it wouldn't cause the CEL to come on and stay on after I hit 2-3PSI. That was a constant problem with the last LM, and I frankly thought it had to have been a problem with the cal.

Knowing that others have had problems with their CEL not working at all, or sometimes in strange ways, I'm wondering if there is a possible fault with the board itself which is causing these problems? Perhaps some bad caps or diodes?

I'll be watching my CEL very closely as I go to work over the next few days. It would always show a little knock at about 10-11" of vac even though I've pulled about 4* of timing in that area.

The Turbo Z I just sold had a bad connection on the back of the cluster, where the ribbon going to the CE and warning lights on the side attached to the board on the back.

I don't think this one is a problem in the cluster. Two days of driving now with no knock in the normal spot. That could be due to weather, but I would think that high temps and rain would bring it out more than a super cold morning with no humidity.

Try disconnecting the knock sensor (temporarily) just to verify if the sensor is at fault only . If it goes away then look for a bad sensor or if the sensor is pick up a real knock or is a mechanical noise (ie alternator bushing /loose mount or bad wist pins........) being produced

OK. I've driven the car over the last few weeks with varying weather conditions and I've only seen the CEL on knock flash for a brief instant. I've recently switched to 87 octane too, so this leads me to believe that the other board was always faulty, or that the circuitry was somehow more sensitive on that unit.

I haven't had problems with hot starts since I've made the change either. That was a problem before, but is now a thing of the past.

The only problem I've had is when the car wanted to cut out when it was very rainy for a few days. I'm thinking this is due to moisture in either the injector section of the harness under the drip try, or under the cap. It cleared up once the car got up to operating temp for a while. That's was the only time I had the problem, so I'm almost certain that it was moisture causing the issue.

I have seen quite a few instances of LM's going bad. Many run but have odd issues, similar to yours. Hairy Kyle's car (screename on here) was doing pretty much the same thing when I was building a cal for it, was having timing or cutout issues. He swapped to a smec and I made a cal with the same settings and no issues. LM had been suspected but that verified it. Could have changed the LM to another but he had the smec stuff laying around.

the issue with moisture could be the plug wires

I had a 79 f150 with lucus plug wires (lucus god of darkness)

when wet or foggy my buddy I bought it from couldn't start the truck

I backed out of my garage one day and rinsed off the truck with the garden hose - didn't put any water under the hood

I went to move the truck again and it would not start
I start looking for why and I found it
the rubber boots on the plug wires at the cap - were full of water , sucked right out of thin air

truck then started - I then resprayed the truck , again it would not start
again the boots were full of water

FULL ..I'd pull them off and a BIG dribble of water would run down the cap tower as the boot came off

Thanks for the additional suggestions guys. I plan on looking at the LM to see if I can find the source of the trouble. I will be converting the injector harness to a Deutsch connector at the end and I'm rebuilding the sub harness there.