GM TBI on a 1966 283 Engine - Getting a Code 42

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At one stop light the IAC clicked up several ticks and I could hear the sucking sound and noticed the increased RPM. Once again, I think the ECM is trying to compensate for a below set-point idle speed so the IAC overcompensates. The A/C was on at the time and that old A6 style compressor really puts a drag on the engine. Combine that with the variation I see in the idle speed screw setting and it's easy to imagine the ECM is doing some wacky stuff.

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Do you have a vehicle speed sensor? IAC control on the 7747 can be a PITA, especially without a VSS.

I don't have a VSS. The kit I purchased has the VSS feature disabled but I'm told by the seller that's not a significant issue. As I struggle to get this thing working I wonder if installing a VSS might be a good idea. I can purchase a 2000 pulse per mile VSS from Jags That Run and wire it into the ECM. The guy that sold me the TBI kit is willing to provide a new chip that utilizes the VSS inputs. Buying and installing the VSS is on my mind and I may go that route at some point.

Here's another thing to consider...

In speaking to the TBI guy he noticed my voltage is low from time to time. Looking at the recorded data the voltage reading is in the mid 13V range much of the time 13.3 - 13.6 for example. However, once in a while it dips down, sometimes quite a bit. Scanning down the voltage column in the Excel spreadsheet I see it dipped down to 10V, 6V and even 0V. In some ways this doesn't make sense to me. How could the voltage be that low? Or is it some sort of spike? If so, what would cause this type of spike? When the voltage reading in the data was 10, 6, or even 0 the CEL did not come on. Only near the end of the 20+ minute drive did the CEL come on and the voltage was 12.4

The battery is brand new. It does seem a little on the anemic side but the vehicle never fails to start. Do I need to add an external voltage gauge to keep an eye on this? Or can I trust the data being recorded by the 1320 Electronics scan tool? Should I be looking at troubleshooting my alternator and voltage regulator? Both items have been installed for years but seem to work. Would a new alternator and regulator solve the voltage spike problem or should I be looking at something else? My alternator is the type that has a large wire for the 12V attached to a stud on the back and a smaller 2-pin connector on the back. Should I consider a 1-wire alternator? Other?

Is this voltage thing something I need to be concerned about or is this another wild goose chase?

[QUOTE=nvestys Should I consider a 1-wire alternator? Other?

Is this voltage thing something I need to be concerned about or is this another wild goose chase?[/QUOTE]

Don't believe you should consider a one wire. Mad electric has some good tech info at " http://www.madelectrical.com/electrical-tech.shtml ".

Good luck!
Bob

Thanks for the reply. I replaced my alternator with another of the same type - external regulator. After replacing both the alternator and regulator with parts I purchased from Summit the voltage is very steady in the approximately 13.5+ at idle and 2000 RPM. I'll keep an eye on the voltage as I record data with the scan tool but it's bound to be better than before. I can already tell the difference in the brightness and consistency of the headlight beams.

Assuming I've eliminated the voltage question... I'm back to why does it continue to produce a code 42??? I'm getting better at repeating the code 42 condition so maybe that will ultimately help track down this gremlin!

Please keep the comments and suggestions coming.

When you replaced the ICM did you clean the interface and use a good die-electric paste?
You say your ECM is on the floor? Is it on dry carpet? Can the case be grounding out to the body intermittently?