Greetings! I have a 92 Chevy 4x4 with an 87 GTA 5.7 TPI engine in it. I’m using a 730 ECM.

Last week I swapped out my 5-speed for a freshly rebuilt 700R4. I switched from my customized AXXD MT bin to SAUJP and made the appropriate changes to suit my needs. I added the necessary wiring for TCC, 4th switch, and the P/D/N switch to the ECM. The truck runs good and the new transmission works well too, but I’m noticing some lockup strangeness.

When accelerating down the street my lockup engages in 3rd gear like I want. When the trans shifts into 4th, the lockup opens for a split second then locks again, causing a brief flair which is quite annoying and probably added wear on my new torque converter.

At first I suspected a pressure problem in the transmission causing the TCC to let go when it engages the band for 4th . But looking closely at the status bits in my Datamaster logs, I can see the ECM unlocking the TCC for about 12-15 records, starting at same record or record after the 4th gear bit changes, then locking again.

Is it normal for the ECM to unlock the torque converter for a second after a 3-4 shift? Is there a way I can disable this? I’ve never owned a 700R4 before, but I’ve driven plenty of 4-speed autos with lockup and have never felt one unlock the TCC after a 3-4 shift, then locking again.

Attached are my current TCC settings in my SAUJP. These are similar to a half-ton 700R4 truck.

Any ideas? I’d attach an exported chunk of my log to show you exactly what’s happening, but Datamaster doesn’t have an option to add status bits to the export (that I know of).

 

To disable the brief unlock function set the value at address $319 to 0. That is the re-lock delay. Setting it to 0 bypasses the unlock feature.

I thought that the unlock on shift was for a 4-3 downshift (not upshift). Although I didn't look into the code enough to know for certain. You may need to invert the 4th gear switch polarity. This is normally an option bit in the calibration.

RBob.

 

Ahh! Thank you! You’re right! I didn’t catch it until you said something, but sitting in park with the engine idling, the 4th gear bit (2) in the he FMDBYTE1 register is 1. When the trans shifts into 4th, the bit goes to 0. So it looks like it's backwards. I guess the trucks get a NC switch in the valve body but the cards got NO...

Anyone have an idea where to change the calibration to support the different polarity 4th switch? I’d like to keep the 4-3 downshift unlock feature if possible.

Edit: I think I found it, Switch 16 BIT 2, but I cant test till I get home and reburn the memcal. I'd kill for an Ostridge....

 

it is normal for the TCC to unlock on the up shift.

mine unlocks just as it starts the shift into 4th gear & locks back up after the shift is completed i believe they did it for a smoother shift into O/D. if it doesn't unlock there is a bit of jerking on the upshift.
i would think its probably a little easier on the trans for the TCC to unlock for a shift.