Hello, I've been searching around on the site for a few days and have looked at hundreds of threads, but have not found specifically a solution to what my transmission is doing other than some broken links that might have been helpful. Forgive me if it's here and I missed it. I have a 1989 Firebird with the 305 TBI and 700R4 and I've had the car about a month now. I've owned several of these cars of different years and I've never had this problem before. I'm usually pretty good at working on these cars, but this one has me stumped, mostly because I've never ran into this before. When driving at any speed, it feels like someone is tapping my brake pedal peroidically (best way to describe it). I think it's the torque converter going in and out of lockup, but I'm not sure what's causing it yet.

Three things I know for sure. It does this at ANY speed and randomly, but sometimes it doesn't do it at all.
If it starts to do it and I put the car in cruise control, it stops and will not do it anymore until I turn off the cruise.
If it starts to do it and I lightly ride the brake pedal it will quit as well until I let off the brake.

The car shifts fine and drives just fine, just has that little quirk that I'd like to figure out. I've had a couple other T/As in the past with 700R4 and they didn't do this. Any ideas or links to a thread I missed about it? Thanks so much!

 

https://www.thirdgen.org/forums/tran...-problems.html

Given the TCC is controlled by going to ground at the ECM ;
could be a simple as a random ground fault on the control wire back to the ECM ( inc the wire inside the trans pan ) giving intermittent lockup outside of when trans should lock

On the other hand TCC could be engaged as it is supposed to and a fault with the wiring to / from the trans is causing it to randomly unlock
Hitting the pedal cuts the power to the TCC so it will unlock regardless of what wiring cond / faults

 

Does it have this issue more when cold, hot, or anytime it wants? Mine had the issue that the TCC would randomly unlock whenever it wanted when the trans was up to operating temps. I went through all of the tests and everything tested good, electrically. I did buy a solenoid for the TCC lockup but never installed it before I yanked the 700 for my T-5 (riding at 2900 or so at 70 just kills gas mileage on long trips).

 

It does it anytime it wants and sometimes doesn't do it for a few days, or does it every day. The thing I really don't understand is when it starts to do it, I can put on the cruise control and it stops and won't do it anymore, but the second I turn off the cruise, it comes back shortly. Is the cruise also connected to the TCC? I didn't think it was and that's why I'm baffled.

 

You see the green thing on the top of the TB lever? That's the throttle valve cable. Assuming the cruise keeps the speed constant and at a constant throttle opening, the throttle valve (TV) isn't moving. The purpose of the throttle valve is to provide boost pressure to one side of the shift valves in the transmission to hold said valve in a certain position until the pressure on the other side of the valve, from the output-driven governor, matches or exceeds this pressure, causing the valve to move and a shift to occur. So, as long as the cruise keeps the TV valve from moving too much too suddenly, it may or may not keep the TCC valve in the proper position.

 

Could also be the vavuum valve on the brake pedal switch being out of alignment. I dropped my steering column not knowing about the vacuum switch alignment and dropping the steering column moved the switch and it would no longer allow lockup.

Easy to drop the hush panel and look at it.

 

That vacuum switch is for the cruise control... There is NOTHING vacuum driven on a 700R4/4L60 (it's not a TH400 or TH440-T4/4T60). It's primarily hydraulic with an electrically controlled TCC lockup solenoid valve. The other electric parts in the trans (not ours, as I've seen) are gear pressure switches that tell the ECM what gear the trans is in.

The only other switch in the TCC system is on the brake pedal, yes, but there isn't a vacuum hose connected to it...

 

No vac valve on F-Bodies , only a elec switch on the brake pedal to unlock the convertor

 

The vacuum switch is mounted to the brake switch so that the lock up is terminated when the brake pedal is pressed on the 700R4 trans.

Not being adversarial, but I found out about it on a post here, not sure where..... wasn't aware of it myself until I ran into the problem and found that post.

 

Ok, that makes sense. Never thought of it like that. So, in effect, cruise control is affecting the TCC even though it's not directly connected. Maybe my foot just sucks.lol I've had this exact setup in other T/As and didn't have this problem though. Every time I buy one of these cars, I buy the shop service manual (the real books, not the autozone specials) and have taught myself how to diagnose and fix just about anything on them, but this one has me stumped for now.

I've done the usual electrical tests and all checks out. The brake switch is adjusted correctly as far as I can tell. I am going to study it a little closer this weekend. It was 22 degrees when I checked it the other day...I might have missed something.
Some other known things: The transmission does not seem to be shifting gears during this at all ie: 4-3, 3-4 and in fact when it starts to do this, the RPM gauge doesn't even quiver and holds steady. It's also doing it "fast"...in out in out in out at about 1 second intervals. Out..1 onethousand...IN...1 onethousand...Out...etc. Just like someone tapping the brakes. Today, it didn't do it at all on the way to work this morning or on the way back. I'm thinking I may just have to "live" with it until whatever is causing it breaks or gets worse to the point I can narrow it down. I don't just go replacing parts to find a solution, I try to narrow it down to the root cause whenever possible. Haven't been able to narrow this down yet though. I appreciate the responses.