I'm talking about an 85 350 TPI Vette, all stock, high miles. Getting it tuned up I throw my Diacom on it and get weird readings.

- No trouble codes
- ECM sees all the sensors and they appear to be giving "sane" readings.
- After a short warmup the ECM flips into "Learn mode: on."

In short, everything APPEARS normal except- it says spark advance is 0* under any and all conditions. Park, neutral, drive, idle speed, revved up, loaded against the converter, etc. Always 0* Also, the ECM is calling for the cooling fan to be on all the time even though the coolant temp is far too low for that.

Now here's the topper- if you put a timing light on the engine it actually has ~22* advance at idle. Rev it up and it stays locked at ~22* under any and all conditions. Put the jumper in the ALDL, however, and the actual timing drops down to 6*- exactly factory spec. So, the ECM IS DEFINITELY ADDING TIMING although my Diacom always says 0*. And it's a CONSTANT 22* actual reading- it won't change higher or lower if you rev it up, load the engine against the converter, etc. LOCKED at 22*. We also tried the old "blip the TPS sensor" test to try to get it to advance the timing a little further and slightly increase the idle speed but it contiunes to idle completely unaffected by this (yes, the ECM IS seeing the change in TPS voltage when we do this but no change in ECM-indicated or actual timing).

It's almost like the ECM is in some kind of diagnostic mode but it isn't. We tried it in all the various diagnostic and non-diagnostic modes Diacom offers for that ECM and there is no difference.

How can the ECM think 0*is the proper amount of advance without throwing a trouble code or show that it is in some kind of diagnostic mode?? And how can there be a FLAT ACTUAL amount of advance being thrown in by the ECM even though it says 0*

The whole thing doesn't add up. I'm stumped. As always, any help/suggestions you guys could give would be much appreciated.

 

I've seen this very thing discussed before. You may want to check the archives a bit. You're right, your ECM is in a different mode. Wonder what the idle speed is, and what's the setpoint?

 

I'd be more than happy to check the archives but I don't know what to search for. Suggestions welcome.

Idle speed I don't recall but it's normal- 700 or so. It's hitting the idle speed it wants to.

 

If the fans are on all the time it usually means something went wrong with your chip program-if it's a UV increase the erase time, either way, try programming it again and double check your programmer settings- address etc. If this is the first time it's happened to you, you've been lucky!

 

If you ask me, either the 85 ECM diagnostic routine sticks or Diacom stinks. I have emailed them many a time with these exact questions and never got a response. Every person I know who has run Diacom on an 85 Vette will never get a spark advance reading. So why the heck do they put that field on there? Just a hoax to make it look like a lot more data can be cgotten than in reality.

So in short, Diacom will NEVER show you spark advance on your setup for the 85. Actual timing will change however, as you noted with a timing light.

The solution for me was to update to speed density. Also viable is to upgrade to the 86-89 ECM. Both would require repinning the ECM and a chip change.

That's the way the ball bounces. Just wanted you to know a lot of us have been there before. -Matt-

 

Don't get me going on Diacom. Even on SD there is some information that is being communicated to Diacom, but Diacom choses not to display AND make no revisions/upgrades to their software to accomodate it. A good example is that with my SD car, I would like to see if my TCC is being commanded to be locked or un-locked. My review of the code shows the TCC switch is in the ALDL stream, but Diacom doesn't feel it is important to report this. Oh well.

But I would also recommend an "upgrade" in the ECM on the simple fact that there is a LOT MORE information available on the $8D SD 7730 or $6E 1989 MAF 165 ECMs, thus you can "tune" better.

 

Do the WinALDL guys know about this? Seems like the right place to introduce an alternative. They did a great job on some of the other stuff...

 

You guys get the gold star for the day. The ECM is advancing the timing just fine in actuality. Diacom just isn't looking at the right bits in the datastream to get the actual number from the ECM for whatever reason. We finally came to the same conclusion and disregarded the 0* reading, using a timing light and our past espereince with TPI motors to judge if it was doing "sane" things with the timing.

Were not able to reporduce the same "locked timing" when we went back the next day. Not sure why.

It had a marginal idle and sluggish performance which was why we were putting the Diacom on it in the first place. That was eventually traced down to a fried coil, primarily, which then trashed everything in the distributor (massive arcing, carbon tracking). A new module, coil, cap, rotor, plugs, and wires and it PURRS! Cleaning literally 1/4" of carbon buildup out of the area behind the throttle plates didin't hurt either. Another 205HP TPI Vette hits the streets for it's second lease on life!

So eventually NOTHING was found to be wrong with the ECM or the sensors, although we are replacing the original 1985-vintage O2 just on general principle later this week.