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Need Help Understanding 700R4 Converter Lock Up

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Need Help Understanding 700R4 Converter Lock Up

Old 01-11-2019, 09:50 AM
  #1  
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Need Help Understanding 700R4 Converter Lock Up

OK, so I'm totally confused. I'm wanting to swap from a T5 to a 700R4 in my '89 Formula.

I've owned my car since it was new, but it's a manual. All of my experience with and understanding of automatics is non-overdrive units. I don't understand how the lockup function works.

I've been talking to different manufacturers, and all I've done is become more confused.

The guy from TCI says they use a vacuum-actuated lockup. The guy from B&M says theirs works off the VSS: it locks up at 35 mph, unlocks under 35 mph. The guy working on my car says the only thing electronic in the converter lockup is the brake-pedal switch. Touch the brakes, and it unlocks. He says the transmission lockup is internal, and works off of line pressure.

I thought in automatic Thirdgens, the ECM was involved in controlling the lockup.

Can anyone educate me?
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Old 01-11-2019, 01:37 PM
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Re: Need Help Understanding 700R4 Converter Lock Up

Lockup is a converter clutch, just like the clutch in a regular 5-speed manual. It comes on and takes the fluid out of the equation so the engine-trans is 1:1 just like a manual.

It is strictly for economy, unless you specifically buy a converter for lockup at WOT (3-plate disc).
The converter is the most important part in the entire car when there is an automatic trans.
A good quality converter is easily $900+
Do not use B&M or TCI converters, they are not "good".

Yank converters are my favorite FWIW

Alright, lockup details nitty gritty
For starters, I always prefer to have ultimate control over lockup from inside the car, so I always install a manual switch. Whether 700R4 or 4l80e etc... It helps to have a switch that has the final say.
The proper way to use lockup is to allow the computer to decide when to lockup the converter. Relying on MPH or vacuum based is a band-aid approach, not ideal.
You NEVER want it locked at WOT Like I said earlier, unless you have a converter designed for WOT lockup, because it will ruin the converter/transmission (the clutch will slip just like the manual transmission style, heat and wear occurs, metal goes everywhere, everything dies)

If your factory computer controls the lockup converter then great. Otherwise I think vacuum control is... acceptable... I suppose. I've never used it but I assume it would be fine because it will unlock anytime you get near WOT (less than, say, 7" of vacuum or 5" of vacuum probably). The issue I potentially see however is a down shift occuring with a locked converter, i.e. if you are cruising in 4th, and step down slightly, say to 10" of vacuum, the converter may still be locked when the trans decides to downshift. This would be very bad. You want the converter slippage for downshifts (NOT locked up) so whatever you decide to do, make sure that during downshifts the converter isn't locked.

You want lockup for CRUISE situations. It is an ECONOMY feature. It is also a COOLING feature. The lockup clutch takes the fluid out of the equation which normally creates excess heat. In other words, whenever the converter slips, it generates some extra heat, and this is normal for all automatic transmissions. However, when you cruise on the highway, the 700R4 needs to get rid of that slip- it needs to be locked. Otherwise the constant slipping will gradually overheat the fluid (or try to) and this is also wasted energy that costs $$ (fuel is wasted) so you DESIRE lockup for a cruise situation else you waste that fuel and generate that heat.

So why the manual switch...
sometimes the computer control is annoying. Say you cruise on the highway and downshift 3rd, well guess what- the converter locks in 3rd also. If you want to race somebody you have to unlock the converter.
Some computers have a delay, it will 'hang' for a moment and the clutch will slip before unlocking. Other transmissions are worse at this and if they are modded (shift kits and stuff) it can be bad for the trans to have that delay.
So that is one reason I prefer the switch. It allows you to command lockup "OFF" no matter what the other devices in the car tell it to do, so you can be assured when racing it doesn't accident try to lock or lock partially (sometimes they try to do that). Its just an extra step to ensure you dont lock during WOT or right before going WOT. How annoying the moment before you step into the throttle to feel the clunk of lockup then, then a delay, and then release. it doesn't jive well with the mechanical parts.

Another method (not recommended but....) Is you can use JUST the switch for lockup, i.e. only locks when you flip the switch and never any other time.
I did this for a while but if you ever forget to flip the switch on the highway you can toast the trans. Or if you forget to turn it off before a downshift. yeah it gets annoying.

your best bet if no computer control is available is to use a vacuum switch that only activates in overdrive, and include a manual over-ride for instances where you don't want it to lock at all, and ensure through testing that it unlocks before any downshifting.

more questions? I recommend calling DANA and Probuilt transmissions (700r4l60e.com) he is and always has been for 30+ years (it might be 40+ years) the 700R4 guru and happily explains everything, every detail, with anybody even if you don't buy anything from him.

Last edited by Kingtal0n; 01-11-2019 at 01:47 PM.
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Old 01-11-2019, 02:51 PM
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Re: Need Help Understanding 700R4 Converter Lock Up

Originally Posted by Kingtal0n View Post
Lockup is a converter clutch, just like the clutch in a regular 5-speed .... happily explains everything, every detail, with anybody even if you don't buy anything from him.
Wow, thank you so much for taking so much time to dumb things down enough for me to understand.

Is there a specific vacuum switch you'd recommend?
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