TCC Lock w/saw-tooth tach input [7060 or 7427]

Hey guys I have been struggling to get my TCC to lock up consistently and need your help. The issue is at steady speed once the TCC is set to engage in the tune, it locks, then unlocks sort of a hysteresis deal. It will randomly hold for X secs and then it always unlocks Y secs later. It will not hold for longer then 5-10 secs max. I've datalogged the heck outta this issue and I think the issue is that my RPM / Tach input looks sort of a saw-tooth pattern (200-300 rpm swing), which is why I think it unlocks/locks ("Negative slip prevented TCC Lock").

I've gone back and forth with TCC problem and Engine misfires as my working theory and I'm now at the end of my rope and need your help.


The setup:

-Carb'd BBC with MSD 8365 HEI
-Using 7060 PCM to control 4L80E: Hooking MSD conditioned tach output (gray wire) to feed 7060 PCM + factory tach (purple/white wire for trans control {CKT 430, PCM Input D14, EST Reference} and white {tach lead} wire for dash tach is hooked to gray MSD tach wire). The 7060 controls the shift points perfectly.
-Carb TPS adapter.
-4l80E with PTC Torque Converter



Attempted fixes:

-New Plug wires, Distro, Plugs (Plugs look good)
-Changed curve with different HEI springs
-Locked Timing
-New upgraded Triple Disc Torque Converter
-HD2 Transgo shift kit installed
-Burned new chip to have TCC engaged at 60, 70 and 80 MPH w/no change. Also upped TCC Max/Min duty Cycle to 98% min and 99% max (I even hacked the XDF to add Max Duty Cycle in 7060).
-Tried to hook gray MSD tach wire up ONLY to purple/white 7060 PCM input to see if maybe sharing the output was the issue.
-Verified Distro End play is w/in spec: .012

Temp fixes, these have fixed the issue for a very short time:

-Changed to "large" silver springs so the advance comes on late (3500RPM is full timing) & changed plugs.
-Locked timing- this worked until I added an efan (????)


The truck runs and drives fine, so I'm thinking there may be an issue with the tach output of the MSD (?). The weird part is my tach gauge looks completely fine crusing down the road, but the 7060 PCM somehow pics up a 200-300 delta in the RPMs at steady cruise, this is really weird since both are hooked to the same source (??). Maybe the dash tach doesn't have the same resolution as the PCM?

My question for you guys is in the 7060 and/or 7427 PCMs is there a way to "Force lock" the TCC so it's only inputs for lockup logic would be MPH & Brake pedal (and disregard the erratic / saw tooth RPM)? Sort of like a manual switch to lock TCC based of MPH. I realize this is a bandaid-type fix, but I've thrown enough $ at this already with no results. I really want to steer clear of the physical TCC Lockup switch, which introduces human error like forgetting to switch it off when coming to a stop.

I know this is sort of an out-of-the-box question, but if anyone has any inputs they are greatly appreciated!

Thanks!

I'm not very experienced with the tuning of these at all yet but just reading the post seeing that you said the locked timing had it working until you added an e-fan, just thinking but is it a possibility that the current draw from the e-fan activating is altering the input voltage to your msd distributor causing the output voltage to change for the same rpm level sending false information to the ECM? Just a thought but maybe try disconnecting the fan power supply and see if the problem still exists?

What are you using for VSS? Fairly sure it is required for TC lockup... I'm not to sure why you're feeding the PCM a tach signal? You need to run a 8 pin ICM for the PCM to read RPM correctly (High & Low reference). Hopefully you are running a GM HEI because then you can just swap in a 7 or 8 pin module into your distributor. If not you are going to have to externally mount one and wire it up to the pickup coil in your distributor. Here's a good read: http://www.megamanual.com/ms2/GM_7pinHEI.htm

The carb I can sort of understand, but I see no reason to not be controlling timing with the PCM. Much easier to tune rather than messing around with centre plates, weights, springs and a vac can.

Thanks for the reply's guys. I ended up taking the distro apart a few nights ago to investigate and see if there was a mechanical problem. Sure enough the nut on the lockout shaft backed out a bit causing the distributor shaft to move somewhat independently of the rest of the mechanism (only a few degrees of movement). I tightened it back up and took a test drive and everything seemed to work ok now. Apparently there was just enough slop in the hole to cause the tach output to go wonky.

From what I'm told anything >150 rpms of movement at steady cruise can cause the TCC to go haywire.

Tayto- Yes I'm running a GM style HEI distributor locked out at 36*. Running the stock 8 pin ICM is a great idea. You mentioned controlling the timing with the stock PCM/distro. Is there a way to accomplish this with the TBI (and IAC, Injectors, Knock sensor, EGR) all missing in action? I figured the PCM would either go into limp mode or set the timing to who knows what.

I would say MAP, CTS, VSS, TPS, KS (if you want knock detection) and a small cap HEI or the 8-pin ICM for your large cap HEI. Here's a good read, different ECM but concept should be similar... http://www.thirdgen.org/forums/diy-p...ming-only.html

You basically would be running a system similar to the CCC used in the 80s, but with out an enrichment solenoid in the q-jet. You would have a ton more resolution to control things, not to mention controlling the 4L80E.