[Diy_efi] Using GM '747 to control TCC lockup only

[Logan Lingle]

David M. Ingebretsen wrote:
> My dilemma is that I have a 700R4 automatic in the thing and like that the
> GM ECU ('747) controls the TCC lockup. I will lose that feature in the Mopar
> unit.
>
> Any ideas about the feasibility of using the '747 to just control the TCC
> lockup? 
>   
Hi David..   I own a transmission shop and deal with the 700R4/4L60 
lockup issue constantly.

You can do this cheap and do it yourself.. No problem.

It's easy..   Ditch the GM ECM completely and convert over to whatever 
you want.

First, you'll need a cruise control type brake light switch. 

Now with a normal brake switch, it's normally open and the contacts 
close when you hit the brake, turning on your brake lights. The Cruise 
style (and it can come from anything, I recently used a Lexus switch in 
a 57 Chevy, just because it fit well) has a normally closed side as 
well..   It works in the opposite manner. When you hit the brakes, this 
normally closed side opens and breaks the circuit. The reason for this 
is that in 4th gear (Can also be 3rd in some units) and locked up, if 
you hit the brake you need to release the TCC Clutch or you will cause a 
bind up. This clutch isn't designed to be abused and you don't want to 
bind it. I know some people/after market kits omit this step, but it's 
wrong and a professional install will include this feature.

Second, run fused power (Solenoid is 100% duty, should draw about half 
an amp) through the brake switch and a suitable conductor into the plug 
on the trans and (Typically - Internal wiring differs, but you can 
modify the circuit to suit your needs..  It's real simple) into the + 
(It's diode protected) side of the solenoid. From there, come out of the 
- side and to the 4th gear pressure switch.

And that's it.

Now when your 4th gear PSI switch sees pressure, it will close and, 
provided you aren't depressing the brake pedal, the circuit closes, 
backs up pressure in the imput shaft and turns on the TCC Clutch.

As you slow down from speed, governor pressure will drop, shifting you 
from 4th to 3rd..   which will take pressure off the switch, and turn 
off the TCC.

Likewise, when you hit the brake, you will turn off the TCC.

You can do more, depending. on the particular valve body you're using. 
Some of them have a thermal switch wired inline which will inhibit TCC 
operation below about 130 degrees..   Some of them have a PSI switch for 
third also..   Some of them will have a pass switch, which gives a 
little cleaner 4-3 kickdown. Some people install a manifold vacum switch 
also, to help with clean kick downs..

But, regardless..  That's the right way to get lockup in 4th with no ECM 
control. It works and it's the correct way to do this.

If you have trouble locating the PSI switch, lemme know and I'll try to 
find a diagram for you.

PS: Trans Fluid is non-conductive, so you don't have to go to allot of 
effort wiring the thing.

Regards & good luck.