45 to 50mph Issue, Feels Like TC is locking and Unlocking

[Dashmaster]

I have been driving my car regularly and been running great. Yesterday after a stop and getting back onto the highway car pulled the hill at speed fine. When letting off the throttle to slow down or maintain speed I am getting big pull down like TC is locking. Usually it just unlocks on grades when lifting off the throttle. At this time also the Cruise warning came on, the Cruise warning has been doing this for while randomly. Going the rest of the way home, Flat road speed 50 mph, car accelerates smoothly and shifts thru all gears up to speed fine, let up slightly on the throttle to maintain speed, this lockup or issue  cuts in and out, looking at gauges I see about 300 rpm drop. The TPS is original and the Cruise needs to be replaced and both can affect TC lockup. I did not get any codes other than the Cruise. Do you all have any other thoughts on this?

 

Thanks, Wayne

[padgett]

The diagnostic screen available on an 88 CRT has an indicator for TCC locked/unlocked. I'd monitor that while trying to make it happen.

 

If slipping when being commanded I'd try a bottle of Seafoam Transtune in the transmission, have seen that cure sticky shuttles before.

Edited June 16, 2016 by padgett (see edit history)

[Dashmaster]

Thanks for the input, I will have to see where I can display the icon. I am not sure its the TC but feels like it. I did do Filter and fluid change about 2K miles ago on the trans. I have not had any shifting or slippage from this transmission since I have had the car on the road.  I did not have time to look to see if a vacuum  port broke off the Cruise Servo, I know the lines are good I replaced bad lines last year. Cruise Servo Ordered because the warning was getting on my nerves. Its been the only thing setting any warning on the car. Been running so good I figured I was good to go to Allentown, Now have something new to figure out.

[KDirk]

If you determine the TCC is operating normally, consider the ICM as a possible suspect. I had a problem somewhat similar to yours in my good 88 and had long ascibed it to a tramsmission issue, turned out it went away entirely when I changed the ignition control module after it failed outright. 

[Dashmaster]

I have my old Magnavox ICM with coils I can swap in and test if needed. The ICM I have on the Car is a 91 setup I got from Junk yard with new coils on it. Thanks for the input. Since it has not set any codes other than Cruise, I have been trying to think of anything that does not set a code that could cause the condition. I will give it a good going over this weekend and see what I can find. Car was fine until that last stop.

[Ronnie]

I had a similar problem as the one you describe with my 1999 Chevy S10 Blazer. The converter would kick in and out and the cruise would kick off. The problem turned out to be a bad brake light switch. It took me several months to figure out the problem. Besides turning on the brake lights, the switch has a set of contacts in it that tells the ECM you have hit the brake, which will unlock the converter and kick off the cruise control. When I replaced the brake light switch the problems went away.

[KDirk]

Both an ICM fault and the brake switch issue Ronnie mentioned would be no codes set malfnlunctions. So, there is that. If you have a know good spare ICM and coils on hand, it's easy enough to swap and test the theory.  

[Dashmaster]

Thank You Ronnie for that suggestion. Traffic in the afternoon for me is a bear, on the brakes allot, stop an go at the start and up and down 2 mountain ranges on I 70 with people who can't control speed well. So the switch could have gone bad so I will check all these things and report what was my Fix. Thank You all for the suggestions. 

[Dashmaster]

I Installed the new Cruise Servo, Mine was bad still have warning on it. Test Drove with Diagnostic on and watched the TCC light. When the problem started again it was not the TCC locking and unlocking.

Swapped in my old good Magnavox ICM and coils. Problem is the same.

It will do it in 3 or 4th gear TCC locked or unlocked when lifting off throttle  and or coasting, RPM drop is about 400rpm. It is not throwing any codes.

Any more ideas??

[Dashmaster]

I will have to find the thread I saw recently that mentioned the correct laptop cable and file to use for this. I put my Delco ICM back on and did test the TPS and it is in spec maybe  a little low on the high side. .4 - 4.0 were my readings. This also only happens in Closed loop.

[Dashmaster]

I just decided to clean the MAF and Throttle body. I did this about a year ago and have put about 2000 miles on it since then. before do this I was playing is the

diagnostic checking all the Cruise functions and switches. I just started the car and for the first time today Cruise light is out, I will have to test drive the car to see if cleaning the MAF made any difference for my main problem. Also I have been doing gas mileage calculations and the car is averaging between 27.5 and 29.8 MPG. So the car is running good.

[Dashmaster]

Problem still exists. During test drive I had diagnostic service mode up for ECM data, When problem happens only BLM fuel did drop a little in sync with condition. IAC did not seem to change with surge. At 50 mph and I take my foot off to let it coast the problem starts,  TPS is at .4 which is the lowest reading with foot off throttle,  with sight throttle raising the TPS to .44  the problem goes away. But this might just be because I am adding a little gas. This one has got me stumped right now.

Cruise light is back on.

[padgett]

"At 50 mph and I take my foot off to let it coast the problem starts,  TPS is at .4 which is the lowest reading with foot off throttle,"

 

Well when you take your foot off the gas at 50, the TCC should immediately unlock. I'd readjust the TPS to .37-.38 at idle and see if the issue changes.

[rogold99]

Could a bad transmission modulator be causing bad shifts, and a vacuum leak bad enough to throw off the cruise control? Just a guess

 

[Dashmaster]

After sleeping on it and I have said the Car was running great before. No Idle issues or anything other then Cruise warning. The Problem started on Wednesday, I just recalled the I got gas and filled the tank at a station I don't use much but they sell lots of gas, I have gotten gas there before but it was not the summer blend. So right now I am suspecting gas quality. I added some Startron to it and plan to run it around and re-fill to see if it changes. The gas I got was at a Sheets, I usually get gas at Sam's club. 

 

Also just for reference,  OX, IAC, EGR were installed last year, TPS is smooth .4 to 4.0, I have never removed it. Coil and ICM are good, Plug wires are about 2 years old NAPA brand, Rapid Fire Plugs new last year. All of these parts have about 2000 miles on them.

[Dashmaster]

Drove today, Still have issue, I have also watched different parameters for ECM data. Only one that seem to change out of spec is the OX sensor will 0 out and restart count up. IAC and TPS seem normal. I also noticed that is does this also at low speed light throttle 25 mph. Its like engine staring to stumble. My fuel pump is only 2 years old but I will check pressures, I will also recheck plugs to see if I see any fouling. I am running Rapid Fire plugs gaped at 60 because I am using Delco

ICM. I chacked fuel regulator to see if there was any fuel on the vacuum port and there was none. 

 

This thread is a lot like the Shudder Thread.

 

 

[Dashmaster]

OX is less then 2 years old and less then 3K miles on it. My exhaust it new from down pipe to muffler. I Can check TPS with Meter. EGR and IAC were also replaced anout the same time as OX sensor.

[Dashmaster]

I checked the TPS it seemed ok but I had a new Delco one so I installed it. No Change, I set new TPS to.42. Checked Fuel pressure, it is 40 psi key on, running 34psi and when you hit the throttle jumps back up to about 39. This pump seems weak,its only 3 year old with about 4K miles on it.  Reatta Store has the Delco for my 88 and the Bosch, which one would you choose?

 

I am not sure if the Delco branded parts you get now are as good as they once were.

[Dashmaster]

After looking at factory specs the reading I am getting are in spec. I am running out of idea's on this problem. Specs I have found list idle at 40-47 psi and 35-40

running.

[Ronnie]

On 6/18/2016 at 6:59 PM, Dashmaster said:

At 50 mph and I take my foot off to let it coast the problem starts,

 

The fuel pump numbers you posted are not off enough to warrant replacing the fuel pump in effort to correct the problem you are experiencing in my opinion. Have you performed all the fuel pump tests posted here: Fuel Pressure Testing

[Dashmaster]

About the only old part I have on this car is the fuel regulator, I did have to change it last year due to hesitation  and found fuel on vacuum line. The part I put on was off the original fuel rail.I did not do pump load test,On and running.

[Dashmaster]

I am going to try 2 more things. Replace the OX sensor since I have seen it crash and reset count, CRT displays 0 -75 count, when problem occurs it stops count up and goes to 0, Its a Bosch sensor which I am not a fan of for reliability. AC going in. I will also

replace the wires and plugs.The wires are NAPA  brand which I have had issues with on other cars. Since I am going to pull the plugs for inspection I might as well change them.

[padgett]

I prefer to set the TPS at .37-.39v at idle and over 4 at WOT.

 

Might also monitor the Instantaneous fuel (companion to BLM) and see if it jumps when it shudders.

[Dashmaster]

I just installed a new OX sensor and will give it a test drive later. Interesting is that the one I pulled out was a Bosch that I got from the Reatta store, The new was in ACdelco box but don't think its ACdelco from what I have read. Sensors are the same in every respect. Its same height as the Bosch which I know is taller. I will give it a try, I have the new wires and plugs ready to install but will be later today.

[Guest Flacko9091]

Did you try different fuel by chance.  I did notice my Reattas run better on fuel with less ethanol.  Try and find a station in your area that sells ethanol free gas.  Just a thought.  And you have replaced the cruise servo correct?  Just making sure you don't have a vac leak in the old unit if it's still hooked up.

Edited June 24, 2016 by Flacko9091 (see edit history)

[Dashmaster]

I have installed the new OX sensor and New Plugs and Wires. I did find some small issues with plugs but nothing major they could have caused a slight miss or

hesitation. Problem is still there. I have removed the EGR and cleaned it (less than 1 years old anyway) re-adjusted the TPS and set to 3.8.  I have looked over

everything and do not see anything out of place. Vacuum lines have been checked.  I have not driven it yet sine the EGR and TPS changes above. I have not put fuel in it yet so this might have something to do with it. It feels like something is turning on and off when it happens but its not the TCC. Still open to any other suggestions.

[Dashmaster]

Yes I can raise it and run it, Since engine is running good and I have found no faults I am now looking at the trans. I am going to

replace vacuum modulator since it is old, I did check it for fluid leakage and line was clear. I am going to hookup DVM to monitor the TC lockup and see what the voltage reading are and I can compare this to the ECM light, If I see anything on the meter for lockup that the ECM is not showing I plan to disconnect the lockup and test it manually.

 

I did drop the trans pan this spring to replace the gasket and filter. Its has about 2K miles on it since this work.

[padgett]

You can monitor the TCC command in the diagnostic screen.

[Dashmaster]

I have been monitoring the ECM Data screen light for the TC lockup, I have not monitored the command but think it will pattern with the TC light but I will compare. This problem occurs after TC lockup in 3rd or 4th gear cruising, When you lift off the throttle to slow down, TC unlocks like it should then something engages in cycles that slows the car down, RPMS drop 200 to 400 RPM. Give it throttle TC locks and accelerates normally, let up, cycle will start again.  Nothing in the ECM data is out of normal during this

cycle.

My test with meter will allow me to see actual

voltages in the lockup circuit, There is a possibility that its engaging but not by command from the ECM thus not setting the light.  

 

The slow down is like any car that in 4th gears stays locked up when you lift the throttle, or a manual transmission car. My 97 F250 7.3 Diesel is like this. I can drive the truck on the highway almost entirely by the throttle from 45 mph and up TC is locked.

 

I have wired a manual TC lockup before on 91 Chevy 700R4, I used the to tow a trailer with, made a huge difference with power and trans temps pulling hills and adding better engine braking from being locked. The programming on the 91 would unlock on hills and unlock when off throttle.

Edited June 28, 2016 by Dashmaster (see edit history)

[2seater]

This sounds like the normal Deceleration Fuel Cut Off but it may be activating at an unusual speed? I do not remember offhand what the speeds are for this to activate, but am pretty sure there are limits programmed in. If I get a chance I will check the parameters in the chip files I have. Does the injector pulse width drop off somewhat in sync with this feeling?

[2seater]

I looked at the DFCO parameters in a 1990 chip program and I must admit some of it is beyond my present understanding. Perhaps Padgett could better define, but from what I saw, this system should not operate below about 145* engine temp. It also shows a 1275 rpm enable and 1000 rpm disable and a 1.6 second delay before activating, plus just 1.6% TPS should deactivate. If the BLM changes at the same time, it is likely changing to a different cell to read in the BLM chart.

[padgett]

Is really a misnomer DFCO (deceleration fuel cut off) is really an IAC command to keep the throttle plate from slamming shut and kicking up the emissions briefly. Please explain "cycling" - do you mean the revs move up and down ? Any surging at idle ?

Edited June 29, 2016 by padgett (see edit history)

[2seater]

Padgett, Thanks for looking at this. I know very little about the actual effects of some of the programming in the chips. I have been trying to learn a bit about this for my different turbo installs so I hesitate to state any conclusions because almost all of the files I am working with have been modified to some extent by someone else. The reason I mentioned this is I have noticed some similar effects during some of the recording/driving regimes I have been doing to observe the operation of the various chips. The O2 will sometimes drop to zero on coast, cross counts also momentarily go to zero, sometimes there is a definite feeling of engine braking when lifting off throttle, even though TCC has been programmed to operate only above 55 mph.

[Dashmaster]

Installed Modulator and did not solve problem but it shifts better than before.  But a couple of times when parking after a run it will idle really bad for about a minute, I have checked ECM when is happens. Its very random but may be linked. The only part I have not changed with a new part is the Fuel  Pressure Regulator. The one on the car is the original, When I got the car running I swapped the rail with injectors and regulator from a junk yard. I did clean and test all injectors then. Car ran on that setup last year but had hesitation issue and found fuel in the regulator and swapped the original back on. Since the fuel regulator is vacuum run and at idle vacuum drops, this could be causing both issues. New regulator is supposed to arrive Friday. I hope this is the fix. 

[Dashmaster]

Here is the list of things tried so far, I did make the car run better I was staring to get low speed miss\hesitation. The Main Issue still exists!!!!

All Grounds are Clean

ICM, Swapped with old Magnavox Original: No Change

Plugs and Wires: No Change

OX sensor swap, No Change

TPS Swap, No change

IAC, cleaned it was one year old anyway: No change

MAF, Cleaned: No change

Trans Vacuum Modulator: No Change  (better shifts though)

Fuel Regulator: No Change

EGR, Cleaned, was 1 year old, tested ok: No Change, (even test drove with it unhooked)

TCC monitor with meter: No Change  (TC is working properly)

Cleaned Fuel Injectors: No Change

Check Vacuum Lines, all Ok: No Change

Unhooked ABS: No Change

 

TC engages in 3rd at about 40mph, and 4th gear at about 50mph.

 

Engine runs smooth, accelerates well and is smooth and stable at idle. I have been in the ECM Data screen and do not see any parameters that directly cycle with this problem.

BLM is around 125 to 130. Injection PW 4.0 to 4.4, I have not seen this cycle with the problems.

Something commanding fuel shutoff could be an issue. The cycle that it does it varies, could be every other second or sometime longer, The pulse when RPMs drop also varies some. The RPM drop is 400 or more sometimes.

The problem always can be tested in 4th or 3rd gear, In throttle TC locked, Lift Throttle and the TC unlocks then the problem will start, accelerate or brake it stops.

I really at my wits end in this one.

[Dashmaster]

I just put the car on stands and tested, At 24 mph on speedometer, 2nd gear the problem shows up. I had ECM data up on BLM and no major changes. It acts just link either fuel or ignition is being turned off then back on.  I then hooked up my Fuel pressure gauge and its now reading 40 psi at start and drops to 35 psi running. The gauge held steady at 35 psi when the problem happens.

 

I am not sure what the fuel regulator does with fuel pressure on the rail. When I tested rail pressure last week it was just barley in spec 40 psi.

Since I did not see pressure drop even that its low, it does not seem fuel is being cutoff. The fuel pump in the car is about 3 years old and was a Delphi pump.

Any thought on something that could shutdown ignition in cycles?

[Ronnie]

2 hours ago, Dashmaster said:

The RPM drop is 400 or more sometimes.

The problem always can be tested in 4th or 3rd gear, In throttle TC locked, Lift Throttle and the TC unlocks then the problem will start, accelerate or brake it stops.

 

I don't really understand what the problem is. Could you explain it a little better?

 

When you let off the gas the TC should unlock and the RPM should drop. What is happening that is not suppose to when you let off the gas? Is the engine surging? What is the RPM when this happens?

[Dashmaster]

TC unlocks as it should, verified with meter and ECM light, the car should coast and roll and does for a little bit, then something pulls the engine down then stops this repeats not at exact times interval. My Crank Sensor is 3 years old but I don't suspect  that because it starts fast, runs smooth. My fuel pressure is showing lower than the other day, Not sure if you are supposed to pull off the vacuum line on the regulator for reading. I have not in the past. The car accelerates very well as it is now.

In my last test, Car raised, got speed up to 24 mph, Started acting up, it like you would be flipping on and off a switch. The Fuel Gage was steady during this 35-36 psi.

 

[Ronnie]

Have you drove it with the MAF unplugged to see if it still does it? You will get a check engine light but you can still drive it.

 

Does it do it when the engine is stone cold before it warns up in the morning?

[Dashmaster]

Car has always been in loop, since it warms up so quick its hard to test when cold. I have not tested car with MAF unplugged , Air flow reading at idle is about 6.5 to 7. I did clean the MAF the other day, But it was not very dirty because I did it last year.