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| ECM EPROM Data for 1989 Pontiac Turbo Trans Am | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
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Scott Mueller
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| At the request of several members of the CARS forum on Compuserve, I have written this file which describes several of the changes that I have made to the ECM PROM from a 1989 Pontiac 20th Anniversary Trans Am which has a Buick 3.8 Liter (231 ci) SFI Turbo V6. In If you have any questions, you can leave me a message on CARS, CIS EMAIL, by standard mail, or phone.
The ECM used in this vehicle includes a 4K x 8bit EPROM for storing both the program and the data tables used to operate the ECM. The chip used is a 2732 or 2732A. I recommend 200ns (or faster) chips, although slower ones might work. The data tables occupy approximately the first 2K, while the program occupies the remaining 2K of code. The data tables include fuel injector and spark advance maps, an EGR map, as well as the items listed in this file. I have not changed fuel, spark, or EGR curves as I feel that these are already optimum from the factory, and this system uses a MAF (Mass AirFlow sensor) and Oxygen sensor to continuously adjust the fuel injector pulse width, as well as a knock sensor to monitor spark advance. In my experiences, excessive spark advance only caused the Knock sensor to retard the timing, and the vehicle to surge, especially during the 1-2 shift. I have made a chart showing the particular bytes that I have changed in this vehicle's ECM PROM, with columns showing the offset of the byte in the PROM code (in both decimal and hexadecimal), the function of the byte or bytes, the original Hex value, the Decimal equivalent, and the meaning of the number. In some cases a translation formula is used to derive the meaning from the original number. I will add comments throughout the chart. I cannot be sure that these locations will be the same for other ECM programs, but the patterns and values should be similar.
The above table shows the stock and modified TCC lock and unlock points in MPH. These are minimum values at low throttle, as by adding throttle you can also cause the TCC to disengage. In stock form, the TCC locks at 23 MPH in 3rd gear, and will unlock if you slow to 22 MPH. As you can see, I changed these to lock at 45 MPH, and unlock at 44 MPH in 3rd gear with low throttle. I have also changed the 4th gear points as you can see. The stock values give the car a very sluggish feeling around town, as when the TCC is locked, the engine RPMs are reduced by several hundred. With my new settings, the Torque converter remains "loose" under 45 MPH, which gives me torque multiplication and greater response at these lower speeds.
The previous figure controls the minimum temperature that the TCC is allowed to function. Since I run a 160 xF thermostat, I reduced this value by a couple of degrees.
The previous table controls the amount of throttle required to unlock the TCC at different speeds. The offset into the chart is the relative vehicle speed, while the byte values in the table are the relative throttle position required for unlocking the TCC. Some of the chip vendors have taken all of these values and changed them to FFh, which would effectively lock the TCC all of the time in 3rd and 4th gear, provided you were above the minimum lockup speeds listed earlier. Normally during anything near full throttle, the TCC will remain unlocked as it is not designed to take the full engine torque. I recommend leaving these values stock, as by locking the TCC at full throttle (as many of the chip vendors do), you will rapidly destroy the TCC. My own testing showed that locking the TCC at full throttle is worth about 2 MPH and maybe .05 to .1 in the quarter mile, which in my opinion, is not worth sacrificing the TCC.
The previous values control electric fan operation. In this particular vehicle, there are two fans, one is controlled by the ECM (the left one), and the other is controlled by a thermostatic switch. The ECM controlled fan is factory set to turn on at 206.6 xF, and off at 199.4 xF. This works well with the stock 180 xF thermostat. Since I have changed the thermostat to a 160x unit, I have altered the ECM controlled fan operation points to turn on at 177.8x and off at 172.4x F. Setting the fan on or off temperature any lower results in overlap with the thermostat operation, resulting in the fan running continuously. The second (thermostatically controlled) fan turns on at 203x F and is not under ECM control. GM does not make any sensors which will activate at a lower temperature, and this is adequate. The last byte here controls the MPH where the fan is turned off. As you can see, I left this stock. A note for any of you with standard V8 Camaros or Firebirds, your stock thermostat is 195x F, and your ECM fan comes on at 223x F! Additionally, the second (non-ECM controlled) fan uses a thermostatic switch that turns on at 238x F! This is above the boiling point of pure water! Of course a normal mix of water and antifreeze boils at a higher point, but this is still way too hot for normal engine operations in my opinion.
These previous values are very interesting, as they are the speed governor on the vehicle. As you can see, my Firebird already had the governor set to the maximum allowable value, so no changes were needed here! This is because the Trans Am comes with Z rated radial tires, which are rated beyond the maximum speed capability of the vehicle. My Buick Grand National had H rated tires (130 MPH) and the fuel cutoff was set for 125 MPH, and restore set for 122 MPH. Thus, if you hit 125 MPH, the fuel would stop, and when you coasted down to 122 MPH, the engine would resume running. I have used this to develop a "valet" chip, which limits the vehicle speed to anything I wish. In my valet chip, I have set the fuel cutoff for 36 MPH, and fuel restore at 35 MPH. This is ideal for when you have to leave the vehicle anywhere for service!
The previous values control maximum turbocharger boost in this vehicle. I have not fully determined the meaning of the offset into the table, but I do know that it involves which gear the vehicle is in. Boost is allowed to peak in first and second gear, and is reduced in third and fourth. It is hard to calculate accurately a given amount of boost from the hex. number, as this is really a pulse width value for a boost bleeder solenoid, which only increases boost from the stock wastegate setting. Thus the PSI figures I have given are approximate, and based on estimation, some testing, and the stock wastegate setting. In my "valet" chip, all of these bytes are 00h, which limits boost to the wastegate setting (about 10-11 PSI on this vehicle). In my modified chip, I have raised all of these values by an even 10h, preserving the "curve". I now also use an adjustable wastegate which allows me to vary the base setting (and thus the maximum as well) by simply turning a screw.
These previous bytes control the PROM ID which can be seen using a scan tool. A dealer can use this to check to see which PROM is installed in a vehicle. Most of the aftermarket chip vendors will place their own number here. I have an ID of 3 here as this is my 3rd revision. If you did not want a dealer to be able to tell whether you had a modified chip installed, then simply use the stock values. I have included stock ID values for three vehicles that I have worked on.
This last item is very important. It controls whether the chip is checksummed at ECM powerup. If you modify any bytes in the chip and do not change this byte to AAh (no checksum), then the vehicle will run in "limp home" mode in what is called the calpack or memcal chip. The check engine light will flash continuously and a trouble code of 51 (ECM PROM failure) will be set. This byte is actually an element of program code and is not a data value. Finally, here is a list of some of the acronyms used throughout this file:
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