Adding “Transmission
Perform” Selector to your LT1’s Automatic Transmission
Vince Geglia
Trifecta Performance, Inc. (www.trifectaperformance.com)
10/25/2006
Credits: Some
of the information for this article has been derived from public information
available on the internet. Other pieces
of information were derived through my own experimentation. If you contributed to the information on the
internet relating to this procedure and haven’t been properly recognized here,
please accept my apologies and let me know so I can add you to the credits.
Disclaimer:
This is a technical article explaining a process for adding a feature to
a car that did not come with it from the factory. It requires a fair amount of electronics
know-how. It should not be attempted by
anybody unfamiliar with such procedures.
The authors cannot be held liable for any problems or damages otherwise
arising from attempting to implement this feature. The authors have taken every measure possible
to ensure the accuracy of the information presented here, but cannot be held
responsible for any inaccurate information presented. In other words, proceed at your own risk.
Introduction
Have you ever noticed the LT1
Pontiac Trans Am cars with an automatic transmission also have a mysterious “Transmission
Perform” button in the center console? For
those that haven’t seen or heard about what the switch does, it essentially
transforms the driving experience by firming up the shifts from the transmission
and making it downshift more aggressively.
The car feels much more sporty and responsive. Sadly, GM only installed this feature in the 1994-1995
Pontiac Trans Am LT1 cars equipped with an automatic transmission, not the
Camaro, Caprice/Impala, Fleetwood, or the Buick Roadmaster.
The good news is that if you
own a 1994 or 1995 Chevrolet Camaro, Corvette, Caprice/Impala, Buick
Roadmaster, Cadillac Fleetwood, or any vehicle utilizing this PCM, you can add
this feature to your vehicle by adding a few wires, a switch and an LED.
This article talks about how
this feature works, and what must be done to enable it in vehicles that did not
have it from the factory.
Adding “Transmission
Perform” mode to your car
It is possible to add the
sport mode switch to ANY 1994 or 1995 LT1 with a 4L60E automatic transmission
that is still under the control of the PCM.
Starting in 1996, GM apparently did away with this feature when they
switched to the OBD2 PCM. However, if
you have a 1996 or 1997 LT1 with an OBD1 PCM from 1994 or 1995, you can still
enable this switch.
There are two steps to adding
this cool sport mode feature. The first
step is to wire the switch and indicator light to the PCM,
and the second step is to reprogram your PCM to enable the feature.
NOTE: It turns
out that if you have a late 1994 or 1995 Camaro Z28, you can skip the second
step, because the Camaro Z28 appears to share the same PCM programming as the late
1994 or 1995 Pontiac Trans Am. If you
have any other vehicle, however, you must have your PCM programmed to enable
this feature UNLESS you are using a PCM from a late 1994 or 1995 Camaro Z28 or
Wiring the switch and
indicator light
The sport mode switch and
indicator light need to be wired into the PCM.
If you want to enable sport mode, and you have a Camaro Z28, the process
is a bit more straightforward then if you have any other LT1 vehicle. This is because the Camaro Z28 shares the
wiring harness with the Pontiac Trans Am, and the appropriate wires were added
in the Camaro Z28 from the PCM, even though they weren’t ever hooked up to
anything.
The sport mode switch needs
to be a “normally open momentary contact switch”. You can find these at most auto parts stores
or Radio Shack. The indicator light
should be an LED with an appropriate resistor (usually 470 ohms will do fine).
If you have a Camaro Z28 (and don’t want to mess with
the PCM wiring):
NOTE: I haven’t
actually tried this procedure, as I don’t own a 1994 or 1995 Camaro Z28, nor
have I ever. These instructions are
based off information found on the internet.
If you decide to follow these instructions, it is highly recommended
that the wiring be checked with a multi-meter.
Make sure the light blue wire is attached to pin C13 at the PCM, and the
dark blue wire is attached to pin A15 at the PCM. See the section below for more information regarding
how to find the wiring at the PCM.
Behind the passenger kick
panel are two connectors – a white one and a blue one. In the blue connector, at pin position A, is
a light blue wire. This is the signal to
the PCM to enable sport mode. Connect
one side of your switch to this wire, and the other side to a vehicle ground.
In the white connector, at
pin position H, there should be a dark blue wire. Connect the ground side of your indicator
light to this dark blue wire, and the power side to a “key-on” power source.
If you don’t have a Camaro Z28 (or do have a Camaro
Z28 but want to wire it directly to the PCM):
The switch itself is wired to
pin C13 on the PCM. This is pin number
13 on the “gray” connector (looks clear to me).
Which pin is pin 13 you ask? If
you look in the connector on the PCM side you can see the pins are
numbered. If you are holding the PCM,
looking at it, and the “gray” connector is the upper-leftmost connector, pin 13
is on the top row, 4th from the right. One side of your switch should be wired to
pin C13, the other, a vehicle ground.
The indicator light is wired
to pin A15 on the PCM. This is pin
number 15 on the “red” connector. Again,
if you are holding the PCM with the “gray” connector in the upper-left, the red
connector will be below it. Pin 15 is on
the bottom row, 2nd from the left.
If you are using an LED, make sure you get the polarity correct. The ground side of the LED should go to pin
A15 on the PCM, and the power side of the LED should go to a “key-on” power
source.
The following picture
illustrates the connector placement on the PCM.
The following picture is a close
up of the “gray” and “red” connectors.
If you do not have a Camaro
Z28, you will find there are no wires coming out of the PCM connectors at
either C13 or A15. This also means there
are no pins in the connector. You will
need to find some connector pins, crimp them onto a wire, and install the pins
in the connector. I went to the junk
yard, and cut out a couple pins (with a few inches of wiring attached) out of
the PCM connector for another car. It is
possible to disassemble the connector in your car and add the pins you
need. If you need help with this, send us
email (info@trifectaperformance.com). I also have a bunch of spare pins with wiring
for sale if you would rather just purchase them from me. Below is a picture of the spare pins I have,
with the wires attached.
Programming the PCM to
enable “Transmission Perform”
NOTE: The cars
that did not have sport mode from the factory may or may not have valid data in
the performance mode tables. It is
highly recommended that a competent PCM programmer review the programming in your
PCM before you attempt to use sport mode.
Invalid data could cause unpredictable results.
NOTE: If you have a late 1994 or 1995 Camaro Z28 you
will not need programming to enable sport mode unless you have changed your PCM
programming for things like different gears or tire sizes.
Unless you’re a PCM
programmer, you’ll need to have your PCM professionally programmed to enable
this feature. This is a service Trifecta
Performance can provide and you can contact us via email at info@trifectaperformance.com.
If you are a PCM programmer,
there are a few details you need to be aware of:
For B-body PCM calibrations, there
were two known revisions used in production:
“C”, and “E”. Only 1995’s
revision “E” supports sport mode.
Revision “C”, which was found in 1994 cars, does not support sport
mode. However, it is possible to use
revision “E” code in 1994 B-body cars.
For F-body PCM calibrations,
there were three known revisions used in production: “B”, “C”, and “E”. 1994 cars can be found with either revision
“B” or revision “C”. 1995 cars had
revision “E”. Revision “C” and revision
“E” both supported sport mode, but revision “B” did not.
There are “normal” and
“performance” versions of the following tables in the PCM:
Up/Down Shift Points (adjusts shift points)
Line Pressure Offset Vs. %TPS Vs Gear (adjust shift
firmness)
TCC Mode Engage MPH Vs. Gear
Vs. %TPS (adjusts TCC lockup)
TCC Mode Release MPH Vs. Gear
Vs. %TPS (adjusts TCC unlock)
Both the “normal” and
“performance” versions of these tables must be filled out with valid data.
Testing the
“Transmission Perform” switch
With the ignition key in the
RUN position (either with the engine running, or not), pressing the sport mode
button will cause the indicator light to light up. Pressing it again will turn it off. If, after pressing the button, the indicator
light does not come on, either the PCM program does not support sport mode, or
the wiring is incorrect.
If the indicator light is on,
the PCM is using the performance mode tables!
How it all works (for
those who want ALL the details)
Stored within the programming
of the PCM are “data tables”. Depending
on the year and make/model of your vehicle, there can be upwards of 100
different tables in the PCM. A good
number of them define the behavior of your automatic transmission such as what
speed to shift gears at, how much line pressure should be applied during the
shift (affects firmness), when to downshift, and many others. Below is “normal shift speed table” from a
1995 Pontiac Trans Am LT1:
|
|
Shift speed in MPH |
|||||
|
%TPS |
1st to 2nd |
2nd to 3rd |
3rd to 4th |
2nd to 1st |
3rd to 2nd |
4th to 3rd |
|
0% |
12 |
20 |
28 |
10 |
18 |
26 |
|
6.3% |
12 |
20 |
30 |
11 |
18 |
26 |
|
12.5% |
12 |
22 |
33 |
11 |
19 |
26 |
|
18.8% |
14 |
26 |
40 |
11 |
19 |
26 |
|
25.0% |
19 |
35 |
48 |
12 |
19 |
27 |
|
31.3% |
23 |
44 |
62 |
12 |
20 |
30 |
|
37.5% |
27 |
52 |
70 |
12 |
22 |
35 |
|
43.8% |
30 |
57 |
78 |
12 |
23 |
40 |
|
50.0% |
33 |
63 |
89 |
12 |
26 |
45 |
|
56.3% |
35 |
67 |
95 |
12 |
32 |
50 |
|
62.5% |
38 |
70 |
103 |
12 |
36 |
56 |
|
68.8% |
41 |
76 |
130 |
12 |
43 |
65 |
|
75.0% |
42 |
80 |
160 |
12 |
47 |
78 |
|
81.3% |
42 |
80 |
160 |
12 |
47 |
152 |
|
87.5% |
42 |
85 |
160 |
12 |
242 |
152 |
|
93.8% |
49 |
93 |
160 |
29 |
255 |
152 |
|
100% |
49 |
93 |
160 |
29 |
88 |
152 |
Let’s take a look at what
this table is telling us. First off,
both upshifts and downshifts are represented here. Take a look at the first column, 1st
to 2nd. This column tells the
PCM when to shift from 1st gear to 2nd gear based on the
vehicle speed and the throttle position.
The topmost cell (12) means that if the vehicle is in 1st gear,
and the throttle position is between 0% and 6.3%, the transmission should be
shifted if the speed exceeds 12MPH.
You gear heads are probably
now looking at the shift speed at 100% throttle position (wide open
throttle). In this car, it will shift
from 1st to 2nd gear at 49MPH, if the pedal is to the
metal. It will shift from 2nd
to 3rd at 93MPH, and from 3rd to 4th at 160MPH
(WOW this car needs some gears!).
The downshift columns work
similarly, but there’s an important difference.
In the upshift columns, an upshift occurs if the car’s speed goes above
the value in the cell. In the downshift
columns, a downshift occurs if the car’s speed either is below, or falls below
the value in the cell. As such you can
see how “kickdown” works. Kickdown is when
the transmission shifts to a lower gear because the accelerator was pressed to
the floor. If you are in 2nd
gear, the transmission will kickdown to 1st
gear as long as the car isn’t going faster then 29MPH (100% vs. 2nd
to 1st). With this table,
kickdown effectively happens at throttle positions above 93%.
An aside: in reality, if you were going, say 28MPH, and
the gear selector was in Drive or Overdrive, you’d most likely be in 3rd
gear. If you put the accelerator all the
way to the floor, the transmission would actually “double kickdown”. It would first downshift from 3rd
to 2nd (because 28MPH is less then 88MPH), then it would downshift
again from 2nd to 1st (because 28MPH is less then
29MPH). This happens so quickly it would
seem like one kickdown.
So, you can see almost all of
the transmission’s shifting behavior is dictated by this single data table.
There is a second copy of
this table in the PCM, with different values, and a different name. It’s called the “performance shift speed
table”. Here’s what it looks like from
the same 1995 Trans Am:
|
|
Shift speed in MPH |
|||||
|
%TPS |
1st to 2nd |
2nd to 3rd |
3rd to 4th |
2nd to 1st |
3rd to 2nd |
4th to 3rd |
|
0% |
12 |
20 |
27 |
10 |
10 |
23 |
|
6.3% |
12 |
20 |
27 |
11 |
18 |
24 |
|
12.5% |
12 |
22 |
28 |
11 |
19 |
26 |
|
18.8% |
14 |
26 |
38 |
11 |
19 |
26 |
|
25.0% |
19 |
35 |
48 |
12 |
19 |
27 |
|
31.3% |
24 |
48 |
62 |
12 |
20 |
30 |
|
37.5% |
31 |
56 |
70 |
12 |
22 |
35 |
|
43.8% |
35 |
61 |
150 |
12 |
27 |
43 |
|
50.0% |
38 |
67 |
150 |
12 |
31 |
52 |
|
56.3% |
40 |
71 |
150 |
12 |
38 |
142 |
|
62.5% |
42 |
75 |
150 |
12 |
51 |
142 |
|
68.8% |
42 |
80 |
150 |
12 |
63 |
142 |
|
75.0% |
42 |
84 |
||||