Smokin 5000HP+ Posting Maniac
Join Date: Feb 2009
Location: Saudi Arabia
TH700R4 Swaps__Something Else That Needs To Be Done
After driving around with my TH700R4 for the past 2-1/2 years I finally added the finishing touch this morning. The torque converter's clutch is SUPPOSED to disengage whenever the brake pedal is depressed. But mine didn't.
Prior to installing my 3.70's my lockup wasn't any problem as it automatically unlocked whenever it downshifted into 3rd gear. But after installing my 3.70's I noticed my engine started "bucking" and drivetrain started "thumping" when I slowed for a signal or stop sign and it was still in 4th gear. Hmm.
After pondering the situation for a while I came to the conclusion I needed to find a way to disconnect my lockup when I was braking as that's how it's supposed to work. The early C3's use a 2-terminal brake light switch but when cruise control became an option in the mid '70's the 2-terminal switch was replaced by a 4-terminal switch; the additional two terminals being used to disconnect the cruise control during braking.
So I installed a 4-terminal brake light switch this morning; using the front portion to light my brake lights and the rear portion to disconnect my lockup. The entire job only required a new $15 4-terminal brake light switch and about 1-1/2 hours of my time. As the IGN and BAT terminals of my fuse block were already loaded with other things I got my power from the 20-amp circuit breaker on the firewall.
So when swapping in a TH700R4 I suggest adding the later 4-terminal brake light switch so your lockup will work like it's supposed to work.