The timing advance is controlled by the PCM via timing tables which specify X degrees of advance for rpm vs. MAP value, and another cross referenced table I can't remember at the moment. This is a fact.

But I am here to tell you that the ICM has an effect, too, or at least CAN have an effect. Here's why. Last month I replaced the ICM in my '95. Against my better judgement, I bought a parts store brand, Wells. Mistake. But initially, the results were encouraging. I gained a solid 2mpg in avg mpg readout, same driving style, same approximate weather conditions. But then came the problem. Some surging, and a weak feeling beginning in midrange. Logged some driving with Datamaster recording via my laptop and cable. Ton of knock counts, but she was pulling timing per spec to keep in the safety zone. Bad gas? Wasn't sure. Plugged in my LT4 knock module (plugs into socket underside of PCM, small two bolt cover over it). No 2nd Datamaster log ran, but didn't need it-- same drivability issues, surging and weaker than usual @ midrange+. But avg MPG holding the same 2mpg improvement.

Printed out the datamaster screens (same ones I posted here last week), removed the Wells ICM, took it back to the parts store. Showed them the printouts, they agreed to refund.

Re-installed the previously removed GM OEM ICM: MPG drops by 2, back to what it was before the Wells ICM. But, the surging is gone, midrange+ is strong again.

Same tank of fuel for all of the above.

So this is strange indeed. But it has all the earmarks of the Wells ICM adding X degrees to timing advance above and beyond what the PCM was commanding from it's tables: MPG improvement, which would be expected from moderate timing advance increase, at the expense of light to moderate knock under load. Same thing I used to get when playing with the timing old school style back in the day on my old school carbed SBC's.

Although the above demonstrates a certain level of folley on my part, for even considering a parts store brand for a system critical ignition component to begin with, much less installing it, it raises some questions as well.

 

You're not alone (No, I didn't buy a Wells module). I was having "issues" with my 85 and had narrowed it down to a timing problem. I checked the timing and set it to 6* BTDC, then the problems really started to come out, pinging, knock counts through the roof, stalling, hard starting..the list goes on. So, I replaced the 20 old factory module with a new AC Delco, major improvements but still a little pinging. Back at it with the timing light... Get this..The timing was 20+ * BTDC! Reset the timing to 6* again and all is well.
So, Yes, the ignition module itself can affect your timing (base anyway).

 

I think that is it, it seems to have effected base timing somehow. Who would have thought?

 

Interesting. I may have to add one of these to my next order from Superior just because of my car's age and mileage.

 

Well, that's sort of the reason I replaced mine, every component of my ignition system had been replaced within the last about 12,000 miles, with the ICM the only remaining factory original piece.

But I was an idiot to try a parts store brand.

Luckily I had enough of the white thermal compound paste to re-install the original ICM with.

Superior's going to be recieving two orders for an LT1 ICM this week, Corvette Kid NC, because I'll be ordering one up as well.

By the way, the ICM is *the* easiest part of the LT1 ignition system to change out: All you have to do is put a nutdriver on those two small bolts holding it to the heat sink (and the heat sink to the coil mounting plate on the head), and the module and heat sink come free in your hand ...Five minute job.

 

I replaced mine with a NAPA unit and the check engine light came on.
Installed the factory module and check engine light went off.
I replace my parts when I think they will need it ,not when they fail.
Beware to those that have a module failure and use aftermarket.
5 minute ha ha what about the bolt underneath?

 

All I had to do to remove the module and heat sink was remove the two small 7/32" bolts that hold the module to the heat sink. Removing them also frees the heat sink from the backplate it is mounted to, and both the heat sink and module are then free in your hand-- just have to be careful not to drop either of the bolts as you're removing them.

The bolt underneath, you must mean the one that secures the power steering pump fluid reservoir? No need to remove that one unless you're going after the ignition coil.

I agree about the after market module situation-- you would think that they would all be the same spec, since it is supposed to be the same Thick-Film technology used to produce the GM style modules, but apparently the 3rd party brands are not doing something right. In the case of Wells, I truly think they thought they could give the customer some added fuel economy by slightly adding to the base timing, but that technique falls flat on its face when applied to an already aggressively timed platform like the LT1/LT4, etc... just causes light knock and makes the PCM retard timing, as I found out.

 

I can see it affecting spark but not timing. The ICM is removed when you install a deltec and replaced with the DELTEC box that controls high and low res signals and it still works fine. I guess if it created a weaker spark and the High res signal was affected it could cause the ECM to advance timing who knows that is a weird problem because the ICM has no direct feed back to the ECM, the ECM actualy sends the signal to the ICM.

 

I know, I'm not sure how it effected timing. The PCM works off of set tables with degrees of spark advance referenced for a given RPM and MAP value, for example. Here's the post where I caught the spark retard happening by scanning with Datamaster while driving, the post features some screen captures I made from the Datamaster playback:


http://forums.corvetteforum.com/show...03&forum_id=48


Most tell-tale is the last screen cap where it shows 44% TPS% (part throttle), 26mph and a high MAP value because I was intentionally loading the engine but not enough for her to command a downshift yet. It shows two knock events in that screen cap, and timing being stepped down in response.

I haven't re-scanned via Datamster yet with the old GM ICM back in, but I can tell you the car is running ***** up since switching back to the old GM ICM, so I know the knock is gone.

 

The aftermarket module probably isn't dwelling as long as the original. So maybe it isn't adding advance as much as not losing as much as the stock piece, if that makes any sense. Hence the poor midrange feeling, it's just not making spark as hot as the original. Still that dosen't explain the mileage difference.

 

Hey there - this is a little off of your topic but I'm curious about the ICM. I'm leading toward mine failing on me but not for sure. I've been dealing with an intermittent no start. It seems to be heat related, but it's hard to reproduce. The car has always run fine and never died on me. But sometimes I will turn it off and it just won't re-start. If you let it sit a while and cool down, it will start right up just fine. The first time this happened I suspected fuel pump, had the car flatbedded home, and no sooner than we unloaded it from the truck it started right up. Talk about a kick in the nuts. Then it was throwing code 16. Sign of opti death (brand new gm opti last fall). But the last time it happened I had no codes at all. But for sure no spark. Ran out of time going through all of the diagnostics before it cooled off enough and restarted.

Does this sound like an ICM that is beginning to fail? If so, sounds like I should stick with the Delco unit.

 

Looking at the data it is not the ICM pulling timing it is perfectly normal knock retard, you have an incrimenting knock count so the ECM is pulling timing just like it's suposed to. Now my best guess is that the new ICM may have a problem with not providing a hot enough spark but that is just a guess.

 

Agreed. PCM was retarding timing in response to knock, but the $64,000 question is since the ICM was the only variable that changed (same tank of gas, etc), what was it about that Wells ICM that caused the knock to start happening. Whatever the answer is, we may not find out, but at least we can all agree to stick with GM ICM's!

 

I would be interested to see the spark for boththe Wells and the GM ICM's on a scope. It would tell the tale I have a feeling, my gut says the wells delivered a spark that is either not as clean or not as long. I think your information is great info to stick into the memory bank incase it ever comes up again.