4L60E TC Lockup

Good afternoon everyone,

I've got a funny issue with my '95 caprice wagon I am hoping to get some insight on. There have been a few trips recently where the torque converter would not lock up. This case was where I was on the freeway running 65+ on relatively level ground. I would run up to 75 and back out of the throttle a little in hopes that it would choose to lock up, but to no avail. Other trips it will lock up int 3rd and OD normally.

Any thoughts on a possible cause? Does the PCM monitory engine or trans temp and prevent lockup until a target is reached? Maybe a bad sensor would contribute?


Thanks,

Andrew

Your brake pedal has a disconnect switch on it. You may need to adjust it. Check with a VOM before any adjustment.

I will check out the switch on the brake pedal. Is it just the signal from the stop lamp switch, or is it a completely separate sensor? Also, other than reduced fuel economy, what is the danger of running high speeds in OD with the converter unlocked? I seem to remember reading somewhere that towing in OD is bad because the trans will hunt in and out of lockup and the fluid will heat up due to the fact that flow through the cooler is reduced in OD (by design). Am I misremembering?

Thanks,

Andrew

What does the owner's manual for your car say about towing? How many miles on the car? Original transmission?

The trans fluid is circulated through the cooler when the TC is locked.

Thanks again for the quick replies gentlemen. The car has 140K on it. I've only put 3k on it myself, so I know very little about it's history, but the trans definitely looks like it has been in there awhile. I changed the shift valve for the '96+ style last fall so it definitely is a 1995 or older transmission.

Regarding the owners manual, I doubt there has been a copy of the owner's manual in this car for the past decade, but I will try to look up towing info online.

One last question - is there a temperature input to the ECM for trans lockup or not? Like it won't go lock the converter until the coolant or trans fluid reaches a certain temp?

I should have a chance to look at the brake pedal switch this weekend and will report back.

Thanks,

Andrew

96 roadmaster owners manual link:
https://my.cadillac.com/content/dam/gmownercenter/gmna/dynamic/manuals/1996/buick/roadmaster/1996_buick_roadmaster_owners.pdf

Yes, there is a temp sensor on the pressure switch assembly inside the transmission.  The PCM reads that, though I THINK TCC lockup is actually only dependent upon engine temp, not trans temp.

If I were you, I'd get an interface cable, download eehack to your laptop and start looking at data to see if the PCM is even commanding lockup. eehack is free; and you can also buy a premade cable for $60 here that's a fair deal.  

If money's real tight, you can buy just the interface adapter and hardwire it to the back of your ALDL port.  Something like this: USB to TTL cable. I'm not endorsing that PARTICULAR one but something like it.   Read here for more info.

I have not driven the wagon a lot lately, but it appears that my TC not locking up issue has gone from an occasional thing to a constant one.  I got motivated to nose around the break pedal switches and discovered that there were at least two down there with a total of 3 or 4 plugs coming off of them.  I pulled the one in the photo, which appears to control the brake lights with the two pin connector and something else with the three pin connector - is this the TCC switch?  If so, how do I test it, i.e. what signal should I see across which pins with the plunger up & down?



Thanks,

Andrew

Check your FSM, or Haynes manual for the schematic.

Hello again everyone! It took a while, but I did get and ALDL cable and nosed around the PCM using Scan9495.

I haven't had a chance to pull any data while driving, but did notice the following:

1) TCC brake switch shows to be behaving as expected, triggering a 'brake on, TCC off' signal.

2) Transmission range detection appears to be non-functional. i.e. is shows being in park regardless of what position I put the shifter in.

3) ECT showed 46 deg F while trans and IAT showed 57 deg F. This was parked in a 57 deg F garage.\

Hopefully I can get some over the road data soon - both #1 and #2 above have me very curious....

Also connected to the PCM using EEHack and saw similar data to what I listed above. No DTC show using EEhack or Scan9495. I would think if the vehicle were in motion when the shift selector indicated park that the PCM would be very upset!? We will see.

EEhack has a "mode 4" section which allows you to do all sorts of GM diagnostics. There should be a way to force TCC lock on it. See if you can do that while driving along in 4th and see if the RPM drops. If not, the TCC regulator valve in the valvebody is likely worn out. I had a '96 recently with absolutely no lockup; I replaced that valve with the Sonnax 77754-03K valve. You'll need the reamer to install it, and valvebody gaskets as you'll be removing the valvebody. If you have a local trans shop that like Sonnax, they might be able to do this for you for the same price as buying the reamer. While you're in there, replace the AFL filter screen in the separator plate and check the 2nd gear checkball in case it's embedded into the plate.

Sherlock, thanks for the input! I was able to take the wagon out today and drive with Scan9495 running. Sorry I am not using EEHack, I find the Scan9495 to be a little more intuitive to use.

Early in the day I had driven over 10 freeway miles and the converter didn't lock up the entire trip.

The car was warmed up when I hooked up the cable up with ECT around 185. The software did recognize the gear position with the engine running, so that does not appear to be an issue.

I did notice that the PCM was not indicating closed loop mode, so I forced closed loop and went for a spin. Everything worked flawlessly, TCC engaged as expected and disengaged when I tapped the brake pedal, then re engaged. I parked the car for about 10 minutes, and went for another drive. The car went into closed loop on it's own and everything worked perfectly again. Engine temp was in the 190's trans temp in the 170's, air temperature 40 degrees.

My best guess is that something is preventing the car from going into closed loop and this preventing the converter from locking up. I will need to do a little more digging on what requirements need to be met to reach closed loop. Maybe take a couple more drives from a cold state and see if the car goes into closed loop on it's own.

"My best guess is that something is preventing the car from going into closed loop"

A flaky O2 sensor will prevent closed loop operation. Could also be affecting your fuel mileage.

Hey guys, I got some more drive time in on the wagon today and I think I am getting close to a solution.

On two trips the car went into closed loop on it's own, normally, but ECT on the freeway was 105 F and 130F in town. I believe the ECT needs to hit 140F for torque converter lockup to be commanded, so I am guessing I've got a stuck open thermostat or a problem with the temperature sender.

Thermostat was new last year and temp gauge shows a little above 1/4, which is where it always has been. Also, from a cold start the ECT is lagging 10 degrees behind ambient, so I am leaning toward a bad temp sending unit.

Let me know your thoughts!

Thanks,

Andrew

Got another update on my TCC issue. I was able to use an IR thermometer on the water pump and confirm that it was reaching 180 degrees while the ECT was showing 135. Based on this, I am picking up a new ECT sensor tonight and will swap it out next opportunity. I will let you know the result.

Thanks,

Andrew

Just for grins I unplugged the ECT sensor and plugged it back in to see if that made a difference. So far things are working flawlessly again. I will probably drive it a few more times to see if this fix is real before I install a new ECT sensor.

Either this forum had gotten extremely quiet, or everyone completely lost interest in my TCC issue.  If anyone is still interested, I think I have it licked - I unplugged the ECT sensor and plugged it back in and things have been working perfectly?  Pre start temp for ECT, IAT, trans, and outside air all match within a few degrees, and the engine runs 180 - 190 degrees going down the road and transitions into and out of lock up when expected.

Thanks again for everyone's help!  Gotta love a repair that requires no new parts and minimal labor!

Now I am on to tracking down the cause of my moderately high idle and how IAC, TPS and other sensor inputs factor in! Having a scantool is great but also a slippery slope since I can track all the data I could ever hope for now.

Andrew

Maybe there was some corrosion in the connector or on the sensor? Removing and reinstalling the plug cleaned it up a bit.
Nice that it is working properly now anyway!

If you have a scan tool you can see Commanded idle speed and Actual idle speed. If the Commanded is higher than normal, you have a sensor problem. Another cause can be an alternator problem that gives a low charging voltage.

If the Commanded is low, but Actual is high, you have an IAC, vacuum leak, dirty throttle body bore, etc. type of problem. If the throttle body heat has been bypassed, sludge will build up more quickly.

Rev Bob wrote:

If the Commanded is low, but Actual is high, you have an IAC, vacuum leak, dirty throttle body bore, etc. type of problem. If the throttle body heat has been bypassed, sludge will build up more quickly.

If you install a catch can in the PCV system, it will minimize the build up in the TB, and the intake manifold. I put one in my 454, and the oil does not go into the intake. I did install a drain tube with a petcock on it to make draining it easy. You can get some nice ones on ebay for about $15.

Yes, I am thinking that unplugging and replugging the ECT sensor cleaned off some corrosion and brought the resistance down. The chart on shbox.com shows increasing resistance = lower indicated temperature http://shbox.com/1/4th_gen_tech2.html#ect , so that checks out.

Regarding the idle, it is much improved now that the engine doesn't think it its at 100 deg when it is actually at 190. It still is running over 700 RPM in gear when the target idle is 662. IAC is also very low, (like 1-10) so I will be hunting for vacuum leaks soon.

Thanks for all the feedback!

Andrew