Drive train hiccup...does AT need work?

[heygibb]

My 90 coupe is still running fairly well, other than a no start condition I had a few weeks back. I'll address that in another post. But I've noticed a hiccup while underway that concerns me.

At 51-53mph, my Reatta seems to skip and chug in the power train. I’m not sure I can clearly describe the symptom but it is more a feel than a noise. It’s like the transmission is between gears and it feels like it’s free from any power influence. If I give it a little gas, it reacts in a staccato fashion, not smooth, before it accelerates normally again.

I found a very informative video on how the inner components of an AT interact, perform and fail.

https://www.youtube.com/watch?v=szu-0IqMygA

At around 4:20, he explains a common failure in ATs that involve the clutch pack and friction rings controlling higher gear engagement. There is a stack of thin clutches that react to transmission oil pressure to engage particular gears. The hesitation I am experiencing seems to relate to having degraded friction plates. Maybe they don't grab on a constant basis anymore. Am I on the right track here?

If so, is it a condition I can live with for awhile or should I anticipate total failure, at some point?

Thanks

[Ronnie]

Here is a problem similar to what you describe and the fix: When you  reach about 50 mph under light acceleration the transmission is in 4th gear and the torque converter locks up. If you try to accelerate a little more and the engine hesitates the problem is likely to be caused by a misfire due to a weak ignition system. It is often mistaken as a transmission problem. To see if this is your problem, the next time it does it keep your foot constant on the accelerator while it is hesitating and then lightly put your left foot on the brake pedal. Not enough to apply the brakes but just enough to activate the brake light switch. That will unlock the torque converter. If the cars seems to smooth out and run better you like likely have the ignition problem I described above.

[waltmail]

Yup. 50- 55 is one of the inputs for the T C lockup to occur. Large drop in rpm when that happens.

Edited July 11, 2018 by waltmail (see edit history)

[heygibb]

Awesome. I hope your diagnosis is correct. I will try it next time out.

I replaced ICM, coils, crank position sensor and cam position sensor in Dec/2015. ..all new.

I replaced wires and plugs ~60k miles ago.

I'll check back after a trial run.

Thanks

[heygibb]

It looks like it's the ignition system. While depressing brake slightly at the trouble point and steady gas pedal, it cleared.

I'll do a plug change first. So gap at .060 w/ Rapid fire #14 plugs? I have Bosch now, I believe. 

I have three separate coils, not the old Magnavox all in one design.

 

Asa, I didn't get the drop in rpms on deceleration.

Is this the modulator you speak of?

 

I've never changed it, ever. What brand/design is best suited for replacement? There seems to be two designs. One is like the old and one isn't.

Looking at the business end, it looks like it plugs into position. Is that correct? If so, I guess I pry it out?

 

       

 

 

[DAVES89]

look carefully as there is a 10mm bolt that holds a forked bracket. this set up that holds the modulator in place. The smaller one is adjustable and after market. The bigger one is like the original one.

[heygibb]

Thanks, Dave

[heygibb]

Has anyone tried the Iridium plugs? If this graph is accurate, they seem to outperform other designs in every way.

[Ronnie]

I would stick with the Rapid Fire plugs and save the extra money for a new set of spark plug wires. Old spark plug wires breaking down are the most common parts that causes a low speed misfire from my experience. Most any modern, name brand, spark plug is good for about 100,000 miles. I don't see any of the things on that chart being too relevant when you're installing the spark plugs in a low RPM, low horsepower, engine like the 3800. Just my 2 cents....

[Ronnie]

10 hours ago, heygibb said:

What brand/design is best suited for replacement?

 

Either design will work. The small one is adjustable by sticking a small screwdriver inside the hole where the hose goes on. I guess that could be an advantage or a disadvantage depending on who's doing the adjusting. In my experience the larger factory modulator valve is more reliable and lasts longer but it's also more expensive. Usually the modulator fails by the diaphragm inside breaking. You can check for that by removing the hose and inspecting it for transmission fluid inside. If the diaphragm breaks it can cause really hard shifts.

[heygibb]

1 hour ago, Ronnie said:

I would stick with the Rapid Fire plugs and save the extra money for a new set of spark plug wires. Old spark plug wires breaking down are the most common parts that causes a low speed misfire from my experience. Most any modern, name brand, spark plug is good for about 100,000 miles. I don't see any of the things on that chart being too relevant when you're installing the spark plugs in a low RPM, low horsepower, engine like the 3800. Just my 2 cents....

Good to know. I'll do the Rapidfire and change modulator, then check results. Wires may be next.

Here is link to rebate for plugs I found at RockAuto in case anyone else is in the market.

https://www.rockauto.com/lang/en/promoCat.html#acdelco

[heygibb]

All I can say is 'wow'. You Reatta wizards know this car. After plug change, the hesitation/chug disappeared. This is only after one test drive with the Rapidfire brand.

My new modulator should get here by Wednesday. Should I change it out? I still have OEM mod installed. I see no ATF in hose or at port...completely dry.

I'm all for preventive maintenance but also believe in the 'if it ain't broke, don't fix it' credo.

 

[2seater]

I would leave the modulator alone if the car is now working to your satisfaction. One change at a time and then verify the fix for a while. You can always change it later. If the plugs alone cured the problem, how did the ones removed look? 

[heygibb]

All were clean. They were the Bosch Platinum with the funky electrode design. This is what they all looked like w/ 60k miles of use. One test drive isn't necessarily all conclusive. I'll drive the car a bit more and see how she goes w/out the mod change. Suits me if I can shelve it for awhile.

[padgett]

Looks like a JCW fire injector.

[2seater]

10 hours ago, heygibb said:

All were clean. They were the Bosch Platinum with the funky electrode design. This is what they all looked like w/ 60k miles of use. One test drive isn't necessarily all conclusive. I'll drive the car a bit more and see how she goes w/out the mod change. Suits me if I can shelve it for awhile.

Well there's your problem, the spark was confused

[heygibb]

???

[heygibb]

I decided to remove and inspect my OEM vacuum modulator and see if my smaller after market replacement unit fit as it should. After all, 28 years is a long time for a component to work as designed. It turns out, like Ronnie suggested, that the diaphragm had been compromised. While I saw no sign of leakage on the vac line fitting, I did see fluid on the internal end of the mod. After installing the new mod, my test drive was beyond expectations. Idling is smoother and acceleration is very clean at the trouble spot, as well as at other shift points. I couldn’t be happier w./ this fix. There is no telling how long that modulator has been bad.

I don’t know how I missed any previous discussion of the vacuum modulator but this repair makes all the difference in the world on the torque converter lockup issue. It’s never been on my radar before now. This has been one of the more satisfying fixes this board has provided. Many thanks.

Here are a few pics on what is involved to change it out.

[2seater]

Recheck the transaxle fluid level again. I am sure that was part of your previous investigation but I had a blown diaphragm allow the engine to ingest a pint of fluid in a short time. Just a double check but it sounds like you caught yours early. Glad to hear it's happy now.

[AZVET]

I just changed mine to the same style and the acceleration has improved quite a bit.  I was able to chirp the tires from a standing start today for the first time!

 

[heygibb]

Fluid level is good, if you mean ATF level. 

Re chirping tires...never done it either. I'm all for cheap thrills...maybe I'll try it. 

[AZVET]

5 minutes ago, heygibb said:

Fluid level is good, if you mean ATF level. 

Re chirping tires...never done it either. I'm all for cheap thrills...maybe I'll try it. 

To be fully transparent; we have had some dust storms here in Arizona so there is a possibility there was some loose sand on the pavement at the time.  It was still a surprise though.

[heygibb]

haha...we have sand here, too. I'll keep your advice in mind.

[Ronnie]

1 hour ago, Asa Bergman said:

The modulator isn't just responsible for shift firmness and they get overlooked too often

 

What else does the modulator do?

[heygibb]

4 hours ago, Asa Bergman said:

 

Well, I'm glad someone listened to my advice! I always tell people to replace the modulator but people always pop up the say not to. The modulator isn't just responsible for shift firmness and they get overlooked too often. Anyone still using the original giant can should replace it by now. I'm glad you've seen improvements.

It's funny I've never been aware of that part, considering so many other parts I"ve replaced, some multiple times, in the past 28 yrs. Mine was overdue.

But like almost anything you change when fixing an issue, sometimes there are unwelcome side effects . Yesterday, I took a 90 mile road trip and noticed a new performance change. I'm not sure if it rises to the level of being an issue but it is noticeable. For the entire length of time I've had my car, one of the things I liked about deceleration from higher speeds, was how it would seem to coast "forever", at a uniform rate, if I didn't brake. Kind of like a bench grinder with good bearings, continuing to run for a minute or more after I shut down the machine,  It was always very smooth and drawn out. I've never had a vehicle do that before and I liked it. Now, I'm guessing the modulator is engaging in this process where it didn't before, because of the tight seal of the new part. From high speed and no braking, every two seconds or so, it feels like the transmission is downshifting and then releasing, to encourage the car to slow down. I noticed it right away and don't care for it. It is subtle and not clunky by any means, but I miss the old elongated rolling stop effect. 

[Ronnie]

1 hour ago, heygibb said:

It's funny I've never been aware of that part, considering so many other parts I"ve replaced,

 

The transmission vacuum modulator has been discussed here many times. You must have somehow missed it.    I doubt the modulator was the reason you felt the car was coasting better before you changed it. The engine RPM could have been staying higher than normal when you let off the gas due to raw air and transmission fluid being pulled into the engine due to the diaphragm inside the modulator being broken. The function of the modulator is to make transmission shifts firmer or softer depending on the load on the engine (based on engine vacuum).

[heygibb]

21 hours ago, Ronnie said:

 

The transmission vacuum modulator has been discussed here many times. You must have somehow missed it.    I doubt the modulator was the reason you felt the car was coasting better before you changed it. The engine RPM could have been staying higher than normal when you let off the gas due to raw air and transmission fluid being pulled into the engine due to the diaphragm inside the modulator being broken. The function of the modulator is to make transmission shifts firmer or softer depending on the load on the engine (based on engine vacuum).

Yes, stupid comment by me. Spending hours and hours on this (and your) site doesn't translate into having picked up on every repair.

Shifts are definitely firmer. There has to be a connection to the mod change though, otherwise, why the noticeable difference in deceleration now? Your explanation covers the connection.

[heygibb]

11 hours ago, Asa Bergman said:

 

Do you notice the RPM's fluctuating when this happens? I'm aware of what you mean by the Reatta seeming to coast for awhile, the ECM opens the idle air valve to crudely maintain your speed if it doesn't receive a signal from the brake pedal switch. Not the same as cruise but the car doesn't immediately start to slow like a carbureted engine at idle. If the brake switch is faulty and the circuit is stuck open it'll constantly enable DFCO in the ECM and as well your TCC circuit will never close(thus no lockup). Likely your cruise control won't engage either if this is the case.

I did not check RPMs during decel but I will.

You hit the nail on the head. I tried my cruise control on my trip and it did not engage. I need more training to understand some of your acronyms. No clue what DFCO is,  but from your diagnosis, I must have a faulty brake switch? If so, is this it? Kudos to ROJ.

http://www.reattaowner.com/roj/component/content/article/index.php?option=com_content&view=article&catid=84:brakes-electrical&id=163:abs-pressure-switch-replacement

http://oemcats.com/oem-parts/25533700.html

Edited July 23, 2018 by heygibb
more info (see edit history)

[Ronnie]

3 hours ago, Asa Bergman said:

Modulator pressure connects to the internal pressure regulator to modify main line pressure. TH350 also does this, it's pretty much standard auto trans design. Incorrect line pressure from a faulty modulator causes the entire trans to not work as intended because these are 'dumb' hydraulic valve bodies.

 

In other words the modulator does what I said. It controls the firmness of the shift depending on load (torque demand) of the engine.

[heygibb]

18 hours ago, Asa Bergman said:

 

Yikes, not that brake switch! That's the brake PRESSURE switch for the Teves unit, don't order that one. Lol. I forgot the Reatta is an overly complex car.... The brake PEDAL switch, the one that controls your brake lights. It's attached to the brake pedal. There's two. The bottom one is for the brake lights, and the top one  controls the TCC and ECM circuit. Watch the RPMs next time. Usually if I'm going ~60 and I tap the pedal to turn on DFCO the RPM's will drop down to around 1000 while if I just take my foot off the pedal they'll sit around 1700.

 

The brake pedal switches adjust by pushing in the brake pedal by hand and then gently pushing the switch in until you hear a few clicks. The switches are threaded but don't thread into it, there's a little tab that just holds onto the threading. After you push the switch in a few clicks you just let the pedal back down and it'll adjust itself. I'm not sure how the 90-91 diagnostic mode works but if you can watch ECM input EI71 it should read High when the pedal isn't pressed and low when it's pressed. Could also switch the wire from the other switch temporarily(if it fits) and watch the brake light. Anyways, if the adjustment makes no change you could try replacing it, the switch IS part of the TCC circuit so if it's worn out it'll cause funny problems. I'm not saying this is your problem but brake switches are overlooked and much much cheaper (~$8)to replace in hopes rather than taking apart your trans in despair.

 

DFCO is Deceleration Fuel Cut Off. When you are fully off the throttle the car normally keeps the engine going at a steady pace and uses the IAC(Idle Air Control) valve to try to crudely maintain your speed or at least slow your deceleration. IF you however are off the throttle and you push the pedal the top brake PEDAL switch triggers an input to the ECM(engine control module) that says "Hey! I wanna slow down!". What the engine then does is limits the firing of the injectors or completely turns them off. This helps slows the vehicle on flat ground or can help maintain speed going downhill. You can take advantage of this feature on mountains. On the highway I take there's a three mile stretch going down a mountain, I tap the brake pedal to turn on DFCO and the car maintains 50-55 MPH and uses only trace amounts of fuel for that three miles and not using the brakes. If I don't tap the pedal the Reatta will barrel down that mountain at 70mph. You can watch this happen by watching the injector pulse width data on the 88-89 Reatta CRT, I don't know if your 1990 one can view this data as well but it's not important.

Just wanted to acknowledge this post. Several things there for me to do, that I have not had the time to do.

re the switch... I'm glad you cleared up the switch(es) we are talking about. ? I did a quick search for brake switch and that is what came up, so thanks for the followup. (Of course, that lead to another issue of acquiring a discontinued part for the time when mine eventually fails.) Anyway, I removed the plastic shroud covering the electronics under the driver's side to take a look at them. Not easy to work on but I see their position and how they are installed, if I have to pull one or both.

The rain is keeping me from roadtesting for the time being, but I'll check the RPMs and probably the ECM diagnostic readout when I can.

Edited July 25, 2018 by heygibb
add pics (see edit history)