1988C4

I have a 1988 coupe with automatic transmission. I would like to install a switch in series with the TCC/CRUISE BRAKE SWITCH (purple wire) to prevent the TC lockup (and subsequent "bog down") when driving around town, saving the lock up function only for highway driving. Will this have any adverse effect on my transmission or its reliability? I heard that operating the transmission in OD at highway speeds could result in overheating if it is not "locked up. If this is the case, could I wire the TCC/CRUISE BRAKE SWITCH in series with the OVERDRIVE ENABLE SWITCH to provide TC lockup only when the shift selector is in OD?

Joeyvette

Why not just drive with the car in "D" vs OD?

0ski_dwn_it

People often ask me about this when I do chips. SInce you can control the lockup points with respect to MPH.

I typically try to talk them out of this since it was advised by my tranny maker, pro-built, not to mess with them for these reasons.

1. When you hold a lockout a lockup converter they generate much more heat than they would if they were locked. This added heat is the #1 killer of a trans.

2. A lockup converter is not as efficient when not locked.

3. Gas mileage will suffer as a result of not being locked.

Some confuse the effiency of a non-lockup with the effieciency of a lockup. A non-lockup designed converter is much more effiecient that a lockup conveter unlocked.

The effieciency of the two in comparison in their designed states are a topic of many debates. Who is right? No-one knows. :cheers:

BookSmart

I just wish it was less touchy. WIth the new exhaust the drone at 40 isn't bad in lock-up.

My beef is that you let off the gas in town it will un-lock when you get on it. Then lock back when you get to your speed. Like say you are going 40 you let off the gas at 41 touch the gas at 39 it will un-lock and lock again when you apply slight pressure at the desired speed 40.


Yah I sometimes drive in 3rd if under 35. 3rd lock-up is not bad. And save the 4th for 40+ as long as it is a steady pace.

VetNutJim

WARNING: THIS REPLY IS LOOOOOONNNNNGGGGG!!

I drove my 84 for four years with no lockup. A lot of around town and a lot of highway driving.
The transmission never complained. My temps never went over normal.
I finally got around to fixing the TCC control circuit. (There were a LOTTTTT of other more pressing things to fix on it first)
It drove GREAT with NO TCC lockup. Good throttle response and felt really good.
Now it locks up WAYYYY to early. Locks and unlocks when I don't want it to and is generally a pest and a pain to drive. I've been seriously considering putting the manual defeat in series with the brake interupt switch as you are thinking of.
BTW for the 84 GM issued an updated PROM to raise the TCC lockup speed.
I ordered one from Jeff Kopp BUT I got the same one that I had originally.
Put it in there and NO CHANGE. Thus the switch modification idea was born.

Here's the real deal from someone that IS NOT a transmission man but I have THREE qualifications for my response here.

1) I HAVE GIVEN THIS AN ENORMOUS AMOUNT OF THOUGHT
2) I have run mine for a few years with no lockup
3) I stayed in a Holiday Inn Express last night :p: :p: :lol: :lol:

With all that said think of this:

The transmission torque converter WILL slip more without locking the clutch.
HOWEVER.
It WILL NOT slip much because of the stall speed.

The stall speed of the converter determines the RPM where the rotor and the stator transfer maximum torque to each other.
IT ALSO WHERE THEY ARE BOTH TURNING ALMOST THE EXACT SAME SPEED.
On most converters thats between 1500 and 2200 RPM.
In around town driving what RPM do you see on your tach? About 1500 to 2200 RPM, right. Thus VERY LITTLE slippage is happening in the converter because you are at or above stall speed almost ALL the time.

Most of the time you are just "maintaining" speed. Not accelerating or decellerating. That places the TC right in the range of MINIMUM slippage.
Minimum slippage = minimum heat buildup.

Ok let's take highway driving. At 70 mph what RPM do you turn. Most of us are running about 2000-2300 RPM. THose with 2.59 rear gears are turning 2000 RPM. Guess what? We are at or above the stock stall speed again so there is VERY little slippage in the converter.
How much you ask.
Before they invented torque converter clutches the "standard" amount quoted by "those in the know" was THREE PERCENT. Yep 3%.

I don't think todays torque converters are any more efficient than those of the seventies so I do believe the three percent is a pretty accurate number to work with.

Ok so let's analyze what that "slipping" torque converter might do to our transmission life.

Let's pick a mileage number out of our hat that we think may be the normal life of our 700R4. How about 100,000 miles?
Ok lets say the torque converter slipped 3 % for the ENTIRE life of the transmission.
Let's do the simple math,

100,000 * 3 % = 3,000
Ok now instead of getting that rebuild at 100,000 we have to get it at 97,000 miles. That's not a big deal is it?

That is ASSUMING the 3% is applied to the WHOLE transmission and not just the torque converter. In reality the detrimental effect is probably FAR smaller than 3 % but I have no scientific figures to support this, just good common sense. The CLUTCH PACKS aren't slipping 3 %. IT'S THE CONVERTER.
There may be a slight amount of increased heat buildup but it WILL BE SLIGHT and will be more likely in highway driving than town driving.

That three percent will also apply to your fuel mileage.
DO the math again,

Pick a number, say 22 MPG. Subtract 3 %
That will give you 21.34 MPG with no lockup whatsoever.
My observation is my mileage got much LESS around town after I repaired the TCC control. Why?
With the TCC locked at 25 MPH the engine is almost always in a "lugging" situation. What does that do? The mileage goes to pot and so does my engine life.

The fact of the matter is the TCC is not locked up for most of the transmissions life anyway. In almost all around town stop and start driving
it's not anywhere NEAR the lockup parameters anyway.
And the way the General has them programmed to lockup is a pure NUSAINCE.
The CONTINOUS LOCKING AND UNLOCKING IS GENERATING MORE HEAT THAN JUST BEING UNLOCKED ALL THE TIME.

I'm trying to program a PROM for my 84 with the updated TCC control speeds. If I don't get it done pretty soon, I'll be going to the maunal interupt switch.

So make up your own mind. I make these observations based on my own experiences. Your "mileage" may vary.



[Modified by VetNutJim, 12:00 PM 5/15/2003]

VetNutJim

Well, I got to say more boys and girls.

I have reprogrammed my 91 TCC to engage at 52 MPH instead of 42 MPH.

Original paramaters were lockup at 42 in low and high gears.
Unlock at 41 in low and high gears.
I bumped both up by +10 mph in the PROM.
It drives a LOT better now. FOr ME anyway.

BTW, I have a 2800 rpm stall converter in the 91.

FD2BLK

If you prevent your trany from locking up your converter it will cause your trany to heat up. I have a temp gauge on my trany and have had a toggle switch on my lock-up. I had to run this way for while when I swithed to carb before I installed a kit for automatic lock up in 4th. If you install the switch I sugest you don't run the trany in (D) run in 3rd.

MrNuke

I've had my TC Lockup changed for the past 5+years. It doesn't lockup until just over 41mph.Stock was like 25mph.. Now THAT was annoying.
I get great gas mileage, and the tranny is still working pretty good (knock on wood) after 105K hard driven/racing miles..

I dredd the time when I will have to have it replaced by a rebuilt one as 95% of the time they last very little time as someone always screws something up, or uses cheap parts, etc.. etc...

VetNutJim

SWITCHED TO CARB??????

GET OUTTA TOWN!!!!!

just joking. Read above post. Think about it. :blueangel: