KDirk

An unusual and unexpected fix - ICM and transmission issue

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For a while my fuel economy on the good 88 has been less than it should have been even with my tendency to go heavy on the pedal. In addition I have had some harsh shifts and also a tendency for the transmission to "hunt" gears and not always activate the TCC between about 45 and 55 MPH. I suspected a transmission problem but never found anything wrong nor could I find a vacuum leak or other explanation for all of this.

Well, earlier this week the car started to run terrible. Lack of power and acceleration, rpms jumping around at idle and wanting to stall at stop signs and red lights. Checked the MAF, IAC, TPS, O2 sensor via diagnostics and the other usual suspects no trouble found. Finally decided to swap the ICM. Knew it wasn't a collapsed cat as that has been previously addressed.

I should point out that I installed a brand new Delco ICM and used coils on this car shortly after I acquired it a few years ago. Has always had the harsh shift and stumble between 45-55 since I owned it. Anyway, a replacement ICM not only solved the poor running engine but smoothed out the shifts, elminated the periodic stumbling and shuddering at highway speed and has markedly improved fuel economy already. Can tell it is running closer to spec and not rich as it was before the recent ICM swap (exhaust smell was noticeably rich). So, in the end, all these problems I had are attributable to the ICM. Appears it was faulty from new and suddenly got much worse forcing me to diagnose and change it. Since I previously suspected a transmission problem I never addressed it, expecting the tranny to let go and would just replace it then.

Worth adding that, as usual, no codes were set by the faulty ICM so diagnosing is strictly seat of the pants style with no help from the computer. I wanted to post this in the hope it may save someone else the aggravation of suspecting a marginal or failing transmission and a loss of proper fuel economy as I endured for a long time. I had no reason to suspect the ICM until the symptoms suddenly and inexplicably got much worse.

The frustrating thing is that had I suspected the ignition module I would have replaced it much sooner and avoided the mildly irritating symptoms it was exhibiting all this time. I'm glad it is fixed now, the car drives and shifts better than at anytime in my ownership so far. The inability of the on board diagnostics to recognize a failure of the ICM is a major shortcoming though. I do find it odd how much it affected the operation of the transmission, I'd not expected an ICM replacement to smooth that out.

In any case, another odd case for the files (seems like I get these a lot).

KDirk

Edited by KDirk (see edit history)

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Thanks for posting....... lets see if anyone else fixes similar problems by changing the ICM (ignition control module)

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Yet another case of "bad ignition can act like anything". I used to run about a 2/3 ratio of good vs bad Delco ignitions from recycling centers but have been working more on the camping trailer recently. Haven't used my ignition analyzer in years.

 

Do know that 2016 will be different just not sure how yet.

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Padgett says: "Bad ignition can act like anything". Now there is a statement that should be framed and placed in the front hall as a reminder to all. What really gets me about this is that until the recent deterioration in performance, the engine ran fine (plenty of power, great acceleration, smooth idle) for years on this defective ICM but screwed up the transmission shifting properly; which is to say that the major evident symptom seemed to be a problem with the transmission.

The average fuel economy was not great but I do have more city than highway miles typically and the instant fuel economy seemed to look ok under normal driving conditions. I also tend to go a bit heavy on acceleration so kept passing it off as a combination of my driving habits and typical routes that were keeping the MPGs lower than I thought they should be. We also have nothing but 10% ethanol gas here and that is a contributing factor in poor fuel economy. These things all worked together to cause me to write off the lower MPGs as normal and yet I could tell it ran rich just by the exhaust odor.

The harsh shifts is one issue that had me irritatated and stumped all this time. I had been all through the vacuum system, replaced the vacuum modulator on the transmission, done a transmission seevice and the pan and magnet was as clean as it could be (a minimal amount of metal particulate but no large pieces are anything else of concern at the time) and since it is a 4T440 no electronic control to be concerned with outside the TCC solenoid which checked out good under testing. And now it turns out it was the ICM causing the problem. I never would have pegged that as the cause absent the other symptoms that recently appeared. That was the only thing that brought this to light in the end.

I guess I would suggest an unofficial policy of automatically suspecting a bad ICM for owners of 3800 equipped vehicles of this era who are having drivability and performance issues that exhibit without setting codes or giving off any other obvious tells. Seems like a lot of aggravation and wild goose chase troubleshooting can be avoided by casting a skeptical eye at the ICM. If I had done so sooner, I would have eliminated my only complaint with this particular car 3-4 years sooner.

At least now I will have the this knowledge for any future instance. I also cannot stress enough that a new part is not neccessarily a good part. I discounted the ICM as an issue becuase it was a brand new Delco module I installed when I converted from the Magnavox iginiton on this car. Clearly, I assumed too much and I even know better from other past experiences. Sometimes my own expertise is my downfall as I overthink a problem and miss the true underlying cause. So this is also matter of my own accumulated knowledge pointing me away from the real problem by belieiving it to be elsewhere. Don't know how to overcome that tendency but surprise outcomes like this are a wakeup call, no doubt.

KDirk

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What is your theory on how the ICM was causing the harsh shifts? There really isn't an electrical connection between the ICM and the transmission shifting that I know of - other than the feedback the ECM gets from the internal switches in the transmission that tells the ECM when the transmission shifts and what gear the transmission is in. To my knowledge there is no electrical control of the transmission other than the converter lockup solenoid.

 

I know the PCM on some transmissions, like the one in my V6 Equinox, retard the timing between shifts. It does it for smoother shifts and to protect the transaxle for longevity. If you really put your foot in it you can actually feel the engine power drop when the shifts occur.  I wonder if the Reatta ECM sends a signal to the ICM to do the same thing? That would explain why the Reatta transmission has the switches inside it to send information to the ECM?

 

I don't doubt the ICM caused the harsh shifts. I'm just wondering how and why so maybe we can come up with a way of troubleshooting a problem like this instead of having to stumble across it like you did. My transmission shifts harsh at times and I just assumed it was because of an aging transmission. Maybe I have the same problem you did.

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Ronnie,

That is the key question I've been mulling over. Right now I don't have a solid working theory on why a failed ICM impacted the way the transmission shifted. Since the transmission is primarily hydraulic and vacuum control I'm not sure how the interaction of the ECM, ICM and transmission (which is rather minimal with respect to the transmission to the two modules) created these symptoms. The TCC is under ECM control so maybe that is the start and end of it right there, since the problem was most pronounced at speeds where TCC lockup would occur. I am theorizing that the ECM was not getting proper feedback from the ICM which caused the TCC to either not lockup or rapidly engage and disengage causing the violent shudder that would happen between 45-55 mph. I characterize this as feeling like it was hunting for the right gear but maybe that isn't the best way to describe it. It was annoying whatever it was.

Then again maybe it was the ignition screwing up with poor spark or bad timing. Giving it gas and getting outside the range where it would shudder would cause it to smooth out again. So, perhaps it was never a transmission problem at all. Sure felt like one though. If it was just a miss in the ignition system it sure had a strange way of manifesting itself.

I guess I need to sit down with the FSM and read up on the details of how shift points and timing are determined in the 4T440 and how the ignition system plays into that. I have never been a transmission expert, I view automatics as black boxes with magic inside due to their complexities and internal parts counts. I have a basic understanding of how they work but know that there is a lot I don't know. Until I have a chance to see exactly how this all works together a system, I can't say anything more that isn't just supposition and wild ass guesses.

If someone here has a solid theory I'm game to hear it. The crux of the whole thing is that I never would have expected the ICM to cause these symptoms nor did I think replacing it would correct them. That's part of what makes it so maddening - it just doesn't make sense to me based on my knowledge of how this setup works.

For comparison, my first car was a 1984 Ford Escort (you can stop laughing now) and the auto transmission on that car could have it's shift points adjusted by moving a slide block that was secured to the throttle linkage and in turn to a linkage running to the transmission. Move it up or down on the linkage and tighten the bolt down and voila, you moved the shift points. This was used to mechanically tweak and correct for early or late shifts. Wasn't pretty but it worked [well enough] for such a simple and unglamorous system.

The Reatta by comparison is an entriely different beast. To this point I've had no reason to be an expert on this aspect of the cars operation. Now I guess I need to get educated on it since I don't like being unable to discuss technical things without the confidence of knowing what I'm actually talking about.

KDirk

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Ronnie had a good point. While the tranny does report to the ECM which gear is currently engaged and obviously the ECM controls the TCC, there is no feedback loop to the ECM of what the tranny is ABOUT to do (i.e. shift) as the tranny, as noted, is hydraulic and vacuum controlled.  There could be a table in the ECM of engine/speed/load parameters vs tranny gear selection whereby the engine adjusts itself in anticipation of the actual shift and then once it occurs as indicated by the tranny gear signal, returns to normal operation.  If such a table exists, just about anything that messed with the engine output would negatively impact the tranny shifting pattern.  

 

I saw something similar when I discovered that I had inadvertently pulled the rigid vacuum line from the manifold to the vapor canister loose at some point when working in that area (the rubber hose between the rigid line and the canister was shot as well).  The engine ran fine, and the shifting seemed okay.  It was AFTER the issue was corrected it that I noticed how much smoother the tranny shifted, particularly between 1st and 2nd.  Granted I realize that the vacuum modulator is on the same manifold port so that vacuum leak was playing hell with the vacuum modulator, but again, until I fixed the vacuum issue, I did NOT even realize I had a problem.  

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We had nice weather so I took my Reatta for a drive this afternoon. After driving for a while to get everything warmed up and working normally I disconnected the electrical plug from the transmission to see if anything electrical would effect how the transmission shifts. As far as I could tell there was no difference in how the transmission operated other than the converter not locking. Shifting was the same as far as my seat of the pants test could determine.

 

I did get a warning on the CRT periodically telling me there was an electrical problem detected. When I hit the return button to clear it the message would return after a couple of minutes. The SES light was on also. I expected that to happen.

 

When I checked for codes I had: E026, E027, E028, E029, E031, E039. The 26, and 39 were set due to the TCC circuit not having continuity. The 27,28 and 29 were probably set because the ECM could not detect the transmission switches changing as speed increased. When in diagnostic mode the CRT showed the transmission to be in 4th gear all the time so I guess the ECM knew that wasn't right.

 

The code that surprised me was the E031, (park/neutral switch). That switch is mounted external of the transmission. I guess the ECM sets that code if it sees the p/n switch in park while I was sitting still and the CRT was indicating 4th gear at the same time.

 

Once I reconnected the connector on transmission I cleared the codes and went for a drive again. No codes were stored when I checked again.

 

I don't know if any of this information is useful or relevant in trying to understand how the ICM effects the transmission but I thought I would throw it out in case someone could make some sense of it.

Edited by Ronnie (see edit history)

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Really have not a clue why the ICM would afftect shifting in a non-E trans. Do know that drivability issues that do not set a code are usually ignition related. Only thought is that somehow the manifold vaccuum being affected as mentioned above and so is the trans vaccuum modulator.

Since errors can be so diverse I just keep a spare set around to swap since is easy. I also generally carry a spare in my cars since anything you have a spare of won't fail.

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I wonder how things are working out for this guy?

Several times in as many years I've recommended a person change the ICM for driveability issues

only to have the thread dropped.

This is a fix Padgett brought to the Forums' attention over a decade ago.

I think he expressed it as a jerking or hunting when in closed loop, engine at operating temp., at or near 55mph while accelerating or under load as when passing a vehicle or going up a grade.

I've had this happen to me on at least two occasions where changing out the ICM absolutely fixed the problem.

Now if the engine seems to be bogging down or something similar then maybe it's some other cause but, if it starts that herky- jerky thing or what I would call "bucking" then I would suspect the ICM which is why I suggested it when I saw the video posted by bkcamaro.

 

John F.

Edited by Machiner 55 (see edit history)

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John,

That is almost exactly the symptom I was getting (prior to the larger scale failure that led to my replacing the ICM) but it really felt like an issue with the way the transmission was shifting, especially since it was so consistent in the speed and under the conditions at which it occurred. There was also a much smaller "hiccup" when coasting down (under 30 MPH) as the trans shifted down. This latter symptom still seems to be present but doesn't always exhibit itself and I have to assume this is a transmission issue as it feels like a hard shift.

The more this gets discussed, the more it seems my identifying this as a transmission problem was erroneous and that it was entirely caused by a miss (for lack of a better term) induced by the ICM. I'll say I find it difficult to believe the problem I was having could have nothing to do with the transmission but now I'm leaning that way based on the further discussion that has taken place since my original post.

Anyone have a technical explanation for how/why the ICM exhibits this particular symptom? Is it a spark timing issue causing a cut out or stumble or am I off base thinking that?

This is a malfunction that has obviously kept others guessing before until the ICM was evetually hit upon as the underlying cause. I'm slightly embarassd that it fooled me too, especially for so long a period of time. At least now I will have the immediate inclination to suspect a marginal ICM in the furure if one of my other cars exhibit similar symptoms.

KDirk

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"There was also a much smaller "hiccup" when coasting down (under 30 MPH) as the trans shifted down. This latter symptom still seems to be present but doesn't always exhibit itself and I have to assume this is a transmission issue as it feels like a hard shift."

 

I might be able to offer some help with preventing that from happening. I had the same problem and here was the fix:

Try adjusting the kick down cable (TV cable)  that runs between the transmission and the throttle body. Press in on the adjustment button and push the cable housing toward the front of the car. Tthat will put a little extra slack in the cable that connects to the bellcrank on the throttle. Don't allow  it to move much. A little be goes a long way to effect shift points. Moving the cable forward will cause the downshift to occur at a lower speed but it will also cause the up-shift to occur sooner which may make acceleration seem a little more sluggish. Before you start you should mark the position of the cable so you can go back to where you started if needed. Hope that works for you! I stopped the clunk on downshift for me.

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Ronnie,

Thanks for that info, I'll try that and see what happens. If it smooths out then the car will be running 100% perfect for the first time in my ownership. Really, it is subtle enough (and somewhat infrequent) that it doesn't bother me but if it can be corrected, why not?

I don't want to retard the acceleration though, one of the notable things about the 88 models is the amount of pep off the line being notably better than the three later years due to the cam profile being a bit more tame from 89 on. I'm not gunning it, but I notice the 88 accelerates more like the 4.9 in my Deville where the 91's - even with their increased HP - are missing a bit of that juice on acceleration in the 0-30 range. Which is not to say the 91's are a slouch, I just like the feel of the acceleration on the 88's a bit more. It's a small thing, but I notice it when switching between cars.

KDirk

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Kevin, I see ICM was replaced and fixed problems, are we to assume that you reused the same old used coils?

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Should mention that for me the Reatta has "enough" accelleration but I rarely use it all. To be antisocial requires the Crossfire (nail it in first out of a light. Halfway through the intersection it hits the rev limiter but a hard shift just engages the traction control. Usually I just start in 2nd.) Is not really a 6 speed, more like a 5 speed (5th is 1:1) with a granny low.

Secondary ignition issues can feel like almost anything and a weak spark is liable to start missing on an upshift. Can also have one plug that won't fire above or below a certain RPM (have had both happen).

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Kevin, I see ICM was replaced and fixed problems, are we to assume that you reused the same old used coils?

 When one goes to the salvage yard be it You Pick or Full Service you get the coils with the ICM. I just had the same problem with a Delco upgrade. I had an original equipment Magnavox in inventory and installed that while I went to get another Delco. Picked it up [tested] from a Full Service yard for $31.00 which is better then going to my local You Pick and paying $25.00 for one I have to pull and test.

 You throw away the ICM and keep the coils for a later failure...

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I had a set of used coils (all three good) that were on the used ICM I swapped in. The three coils on the known bad ICM are in waiting now. I intend to test them when time permits and if any or all are bad they will also get chucked. I did buy a set of three Borg Warner coils the other night so I can complete a Delco swap on my scrub 88 which still has the Magnavox setup on it but is also showing signs of failure in unsteady idle and wanting to stall periodically. Seems these are hitting me at once, and people wonder why I own multiple cars.

KDirk

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night and day differance on my engine crank/starting time when I converted to the series 2 im and cp. but my original im and cp where probably on their last leg, to be fair

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Unsteady idle and stall is usually a dirty IAC for me. Magnavox ignition is not that bad, just the Delco is better/less expensive/available.

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I'm sure I'm not the first to do this...but I  used my original coil pack bracket.  I never even removed the bracket from the engine.

I lined up the new coil pack and module where it seemed to make the most sense, and drilled a hole in the alum bracket and used a lock washer and fiber locking nut on it and it has been fine for about 4000 miles now.

 

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Edited by tb3 (see edit history)
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