I have a 1985 Camaro that I put a new engine, ECM ('730) and wiring harness in.

Last week, I went to my brother's shop and hooked up his Snap-On Scanner. It worked fine, had no problem collecting data.

Yesterday, though, I kept getting a No Communication error. I triple-checked all connections and verified that the Scanner was working fine by hooking it up to another vehicle. No matter what I did, I couldn't get it to communicate.

I checked all of the included fuses, none are blown. Because it's a new wiring harness with its own fuse block, I don't think that any fuse in the main fuse box could control it, could it?

I'm not sure if this means the ECM is bad (it is a rebuilt unit I purchased through Ebay)? If so, does that suggest other issues with the ECM as well or could it just be the communication?

Have you got the SES light installed? ( the new harness has provision for it )
Jumper the ALDL terminals as you would to check the codes and see if light flashes a it should
None of the fuses on that harness relate to ECM comms

I did not install the light yet. I was focused on getting the car driveable because it's stuck in my brother's shop and taking up a lift (and has been for a couple of months). Was hoping to get it so I could pull it in & out as needed and then fix items like the light.

I can still do that, but with all of the issues I'm having trying to get the car running (most likely related to timing issues), I figured I would see if the lack of communication could mean the ECM is bad or something.

You must have the ses light hooked up and a functioning bulb installed. It loops power to the aldl connector.

That's interesting - how on earth did it work the first time, then? I certainly didn't have the bulb hooked up before, and I know it was working because it was giving me my TPS voltage (and it was the same as I was measuring with a volt meter). I was also reading some other voltages, my temperature sensor, etc.

Is it possible that one of the main fuses in the car - the ones under the dash - blew and stopped power from going to something?

Not sure if this matters, but the connector for my brother's Scanner is powered - you have to hook it up to the battery to get power.

I figured this out yesterday - turns out that the problem was me. Since my car is a '85 but I had the computer out of a '90, I didn't have a correct VIN to enter into the scanner. I picked the generic 5.7L TBI that was listed, but apparently there are multiple generic entries and if you don't pick the one that transmits data at the correct rate, you get a communication issue. As soon as I selected the right engine, the scanner started working just fine.