All,

I picked up an 86 convertible as a project over the summer and am down to working out a few minor issues after rebuilding the L98. Car is essentially stock with an automatic. Runs extremely well, no codes, everything good.

Here's the deal....

The torque converter is locking up as I believe it should be. It does not lock before the engine reaches 140 degrees and it seems lock above 45+ mph.

It's just not unlocking as it should, but only in one condition.

When stopping the car, it unlocks without issue. Also, if I am at speed and the TCC is locked up, I can slightly depress the brake pedal and it will unlock, allowing the engine to rev another 2-300 rpms.

If I am travelling at highway speeds, and press the accelerator, the car bogs. Pressing the brake with my other foot unlocks the TCC and the car feels much better. Release the brake and the TCC locks up again almost immediately.

From what I have found, the ECM should unlock the TCC under these conditions if the throttle is more than 25%. In my car, I can feel the TCC unlock when I press the accelerator, but I have to press it far, and when the TCC unlocks, the tranny downshifts into 3rd immediately. That should happen way past 25% of throttle (75% I think)

It seems that something is not allowing, or signaling the TCC to unlock at 25% throttle.

My first thought was the TCC lockup servo, but since the tranny seems to be working as designed in almost every other scenario, it doesn't seem like it is sticking.

I've seen some diagrams of the 700r4/4l60 that have a 4th gear pressure switch but haven't found much info on what that does.

Has anyone faced this issue and resolved it?

TIA

 

Are you sure the problem is the torque converter and not a fuel issue?

 

Pretty sure. As part of engine rebuild I replaced fuel pump and changed to a brand new set of Bosch III injectors from FIC.

Car has no hesitation issue otherwise.

 

Make sure the tv cable is adjusted properly. When I had a 700r4, I locked up the converter using a switch on the console. I used the lockup as a fifth gear on the freeway. It eliminated the witchcraft going through the ecm.

 

Thank you all for the information. Over the last weekend, I replaced the exhaust from the cat-back (could not stand the drone anymore).

While under there, I tightened the rear C-Beam bolts. I also adjusted the TV cable using the factory procedure. The car drives well (and does not drone).

The lugging at cruise under throttle is still there but seems better (might be placebo effect).

My next thought is to go back in and see if the front bolts on the C Beam are loose as well. To do that, I need to drop the front of the exhaust.

The car has 94K miles, and I HATE having to keep dropping the exhaust, so I figured I would take the opportunity to drop the driveshaft and replace the universals on it as well. There is a bit of a clunk when I drop it into gear, so I am not discounting the fact that I might be chasing a mechanical vibration since the TCC behavior is close to being normal and there does not seem to be any issue with it disengaging when directed to do so (brake pedal).

I figure making sure the C Beam is tight and the universals in the driveshaft are good can't hurt.

 

Just wanted to give an update as I hate it when threads don't have some kind of conclusion.

OK, so I dropped the driveshaft to tighten the front C Beam bolts. They were withing spec, so that probably was not going to be the cause of the issue.

While in there, I replaced the driveshaft universal joints (and then dropped the half shafts and replaced those universal joints too).

Got to looking and thinking and the only other major source of the problem could be the torque converter itself. With the driveshaft and exhaust out, I was literally less than 10 bolts away from having the tranny out. So yeah, I dropped the tranny. I have a four post lift and a buddy loaned me a tranny jack so it wasn't bad. By the way, you folks who do this on jackstands are owed some respect. It sucked several feet up in the air

I replaced the torque converter with a new one (same stall speed). Yet some more while-i'm-at-its consisted of new front and rear, tv cable and dipstick seals for the tarnny.

After reinstall, the car definitely drives much better. There is still some shudder, but it's greatly reduced and just feels like the engine is lugging. It's just turning too darn slow at 65...

So in the final analysis, I think I maybe just had a buildup of several worn components, some of which have now been replaced.

Thank to all who commented and helped.

 

Thanks for the reply. It is good when someone gets back with results rather than leaving things hanging. In addition to offering additional opportunities for thoughts it might help someone else down the road.

 

So does it still not disengage when you give it throttle?

 

It will disengage, but it does it right before it downshifts. I have not been able to get it to simply disengage the lockup without downshifting into 3rd.

If there's something I should be looking for that you know of, I'm happy to look.

 

It shouldn't take that much throttle to disengage the converter. Did you check your throttle position sensor?

 

Yes. After I put the motor back in, I set the idle position and the per the factory spec. It's set dead on at .54 volts. At full throttle it's over 4.5 volts as well.

 

OK, a final update on this one as I have made quite a few improvements.

Since my last post, I discovered a vacuum leak in one of the lower intake runner gaskets, after replacing that gasket the engine has smoothed out and vibrations are reduced.

But the problem, while smaller, was still there.

I did some testing and just plain old thinking and started to troubleshoot the last remaining issue as a cylinder miss that was being felt due to the TCC being locked up.

I tested the ignition coil with my digital volt meter and it tested fine. But since I had an MSD coil in my 78, I swapped the MSD coil into the 86 and low and behold the issue is gone. For good measure I replaced the ignition module with a new ACDelco unit.

Test driving the car last night showed that the issue is resolved.

In totality, I think I had the following going on:
* vacuum leak in lower intake manifold gasket
* weak HEI coil
* Aging U-joints

The Torque Converter probably was worn, but I think the TCC was fine, but oh well....

Moral of the story is check your basics I guess.

 

IDK why you'd want it to. A disengaged converter turns HP into heat. When I want more tq from my 6 speed, I don't slip the clutch. I think you converter is functioning as it should.

Glad that you fixed the misfire....that would have been annoying, for sure.