Here's a great article on how to keep your lockup converter working without a computer:

Wiring A Lockup-Converter Pressure

Written by: Mike

Peek under a hot rod today and it's
not uncommon to find an automatic overdrive resting between the frame rails.
These transmissions are affordable, durable, and easy to install. With an
overdrive transmission and 500 hp, fuel economy exceeding 20 mpg can be a
reality. But, before you install that new TH2004R or TH700R4 tranny in your
early hot rod, there are a few things you should know about how to make them
work. Both the 2004R and 700R4 use a lockup-style torque converter that
requires 12 volts and a pressure switch to lock up in Fourth gear. This
eliminates the need for a manual switch. When retrofitted into early muscle
cars, these trannys require the pressure switch to be changed and a few wires
rerouted. Changing the switch and wiring can be done in the car without a
problem. If the switch is not changed and wired properly, the converter will
not lock up in fourth gear and possible trans and converter damage may occur,
not counting your gas mileage will suffer.

Normally open/pressure closed switch - GM

Single-pin TH400 kick-down
connector switch - GM #24205373

5 or
6 female spade connectors ?

Normally open
brake cancel switch - GM #25524845 (or junkyard donor

Low-vacuum switch - GM

Low-vacuum switch wiring
harness - Junkyard donor truck or 2 slim female spade

#18 gauge wire

Vacuum hose ?
TH700R4 filter & gasket - Advance Auto Parts #FK171
(PROFormance Filter Kit)

Dexron II
transmission fluid (approx. 5 Qts.)

Prices I paid for these parts as of 10/24/2000:
(The GM parts listed are wholesale prices).


GM 24205373
GM 25524845


In the
original CHP article that I read, they never mentioned the low-vacuum switch or
brake cancel switch. You need the low-vac switch so the converter will unlock
in low vacuum situations, such as going up hill and heavy throttle. The brake
cancel switch unlocks the TCC when the brakes are applied. Trucks that came
stock with 700R4 trannys came with a low-vacuum switch and a brake cancel
switch. The stock low-vac switch didn't let the TCC unlock soon enough with
this mod. I checked the vacuum on the stock switch and it kicked in at 3.5",
meaning the vacuum has to get real low before it will unlock. The vacuum switch
I recommend works real well as it kicks in at 7.5", which will let it unlock
sooner. The brake cancel switch on 700R4 equipped trucks also disconnects the
cruise control (if equipped) when the brakes are applied.

Are you are doing this mod for one
of the reasons I did, to get rid of some of the wiring and vacuum hoses that
came on our trucks. When I removed the EGR bleed solenoid and related wiring, I
had to rewire my 700R4. This is one of the reasons I did this. If your truck
came with a 700R4 stock, the only parts you will need to buy are the normally
open/closed pressure switch PN GM8627332, single pin TH400 kick-down connector
switch PN GM24205373, and the low-vacuum switch PN

lock2.jpg The factory installed a
couple of different types of pressure switches depending on the application.
You will want to install a normally open/pressure closed switch (PN GM8627332).
After you have drained the fluid and removed the pan, you will also need to
remove the filter. The pressure switch is located on the righthand side of the
trans. When you install the switch, be careful to not over-tighten

lock1.jpg The next step is to replace
the original four-pin connector on the driver side of the trans with a
single-pin TH400 kick-down connector switch (PN

lock3.jpglowvac3.jpg The last step is wiring.
Before you change any wiring, find the solenoid (arrow, photo A) and the two
wires connected to it. You will need to install a female spade connector on the
end of the black wire and run that wire to the new pressure switch (photo B).
The red wire from the solenoid will be run to the new single-pin connector
(photo C). This red wire will also require a female spade connector. With the
wiring completed inside the transmission, install a new filter, the pan/gasket,
and fluid.

Now to the outside wiring. I recommend running an
accessory hot wire (hot only when the key is on), through a normally open brake
cancel switch. This switch will allow current flow, only when it is depressed.
The brake pedal depresses the switch, normally, and releases the switch when the
brake is applied. This switch works exactly opposite the brake light switch,
whereas the brake light switch allows current, only when the brake pedal is
depressed, the cancel switch interrupts current when the brake pedal is
depressed, thereby unlocking the torque converter clutch, such as in a panic
stop. The cancel switch is mounted on a bracket under the dash. The brake
pedal arm makes contact with the switch when it is all the way out. Just so you
will know, most of the time the brake cancel switch is also a cancel switch for
the cruise control, if you have cruise. This is why it is called a cancel
switch, it cancels power when the pedal is pressed. You need to make sure that
when the brake pedal is all the way out there is power going through, press the
pedal and the power is interrupted.

From the brake cancel switch, power is routed
through the low-vacuum switch (PN GM14014519, photo D). You see the two
terminals that you need to hook to. These are slim so this is the reason I
recommend to use a junkyard donor for the plug (black arrow, photo D). If you
can't find a plug, you can either use some slim female spade connectors or
solder the wires on. Mount the low-vac switch on the firewall beside the master
cylinder, right by the plastic sleeve with all the vacuum hoses going through
it. Then connect to the new single-pin connector at the transmission (arrow,
photo C).

For vacuum going to the low-vacuum switch you will
need to use a manifold vacuum source. On my truck I have a TVS that controls
vacuum going to the switch. It will not let any vacuum go to the switch until
the engine coolant temp reaches 170 degrees. From the TVS or manifold vacuum
source run a hose to the low-vacuum switch (white arrow, photo D). At the
switch I have a delay valve at the hose end where it connects to the switch.
This helps to delay vacuum loss at the switch. You don't really have to use the
delay valve, but I already had it so why not?

Now you can go and enjoy the


Since writing this article, I've had a lot of
inquiries about it. The photos in the article are of a 2004R tranny. They both
use the same principle to lock in 4th. They look different so it is confusing
to some. These photos were taken from a Chevy High Performance magazine
article, not by me, well except for the last one, photo D, which I did take.
Also, there are many different variations of valve body's with several different
types of pressure switches and wiring schemes. Early 700R4's locked in 3rd
also. This is the reason they have so many pressure switches. My 89 model has
only one. The 4/3 pressure switch. If you have the early type, you may have as
many as 4 pressure switches, 1 beside the solenoid, and 3 at the rear. The
pressure switch which needs to be changed is the center, rear one.

Now again, with so many different variations of
these things, you may also have a single wire solenoid. If this is the case,
instead of the GM #8627332 pressure switch listed above, which is what is needed
for the two wire solenoid, you will need GM #8643710 pressure switch. It's
around $4.00 at the GM dealer. To wire the single wire solenoid, go from the
TH400 connector to the 8643710 pressure switch, then to the solenoid. I think
this will work on any of them. I'm not 100% positive. I'm no tranny expert,
but I believe this to be true. My 700R4 came from a 89 Chevy 1/2 ton pickup.
It only had one pressure switch. It came with a two wire solenoid also. I
changed my solenoid to a one wire type, GM #8654123, and used the GM #8643710
pressure switch. Either way works the same. There is no difference in the way
it locks. I just happened to have a new one wire solenoid and was changing the
fluid and decided to put it in. It didn't change a thing from the two wire one
in the way it locks in 4th. If you do this mod, you change to the new pressure
switch, depending on which type of solenoid you have, and leave the other
switches in the valve body. Remove the old wiring, but save everything in case
you want or need to put it back like it was.

Now, this is only a belief of mine. I have
absolutely no proof, other than what I have seen. Early 700's locked in 3rd
also. I know for sure this to be the case. Why, I don't know, but they did. I
firmly believe this is one of the reasons early 700's died so quickly. People
use these trucks to haul and tow. Not knowing any better, people would leave it
in OD when doing this. So what happens is, it locks and unlocks, over and over
again. This will kill them in hurry. It causes them to overheat, which is the
number one killer of 700R4's. So, then they heard to leave it in drive (D) to
keep it from doing this. This won't let it go into 4th of course, but the older
ones still locked in 3rd (drive). So, in my mind, they are still locking and
unlocking. Maybe not as much as being in OD, but I think they will still do
it. I may be wrong, but so what, I've been wrong before. GM changed this
sometime down the road. Not sure when exactly. My 89 isn't this

Again, just to let everyone know. I am no
professional mechanic or tranny mechanic. I believe all things said to be
true. They are to the best of my knowledge. I hope it all works for