Ad Radar
Car Craft
Click here to find out more!

Transmission Computer: Controllers - In Control

By , Photography by

No Magic Box

One of the first things we discovered when we began testing controllers is that we had unintentionally combined a very high-torque engine with a loose converter and a used 4L80E transmission. We began questioning the first couple of controllers when even with maximum setting on the trans line pressure, we couldn't get the rear tires to bark on the gear changes. After we talked with the controller manufacturers, one point became very clear: The controller is not a magic fix for a weak transmission. HGM Automotive, the builders of the CompuShift II, told us it designs its box to be used with a new transmission and assumes the trans has been modified for performance use. So when we were not able to hit the rear tires as hard as we felt appropriate for a performance application, it was obvious that while our transmission was in decent shape, it was not capable of what you might call harsh shifts without some help in terms of a mechanical shift improver kit or even a rebuild with new clutches. If you are contemplating the use of a previously owned transmission, remember that even the best controller cannot compensate for a weak or damaged transmission. This is less of a disclaimer as it is an acknowledgement of the physical capabilities of the transmission.

While all the controllers offered the option of using a paddle shifter or up and down buttons to shift, we opted to merely place the TCI floor shifter in Drive and modify the controllers to get the transmission to shift at the correct rpm for each gear change. We thought most car crafters would prefer this method. While we didn't test the paddle-shifter options, there's no doubt that each of the controllers would be fully capable of making those electronic gear changes.

Tuning Tricks

There are several ways to tune these electronic transmissions. Though the process may seem confusing at first, once you know what each step represents, understanding the effects of these inputs will make you a better tuner. For example, the maximum engine rpm input is not a rev limiter. It is actually the maximum shift rpm default at which the transmission will always upshift. But maximum rpm is the point at which the transmission begins the shift process. For example, PCS told us that depending on the condition of your transmission, it could take anywhere from 400 to 800 rpm to complete the shift. So if you indicate a 5,500 maximum engine rpm in the controller, the engine could reach as high as 6,300 or more before the shift is completed. If you don't want the engine to rev past 5,500, you need to experiment with a lower maximum engine rpm limit.

We should also discuss line pressure. The CompuShift II, for example, offers a standard called PRB or pressure boost. Most of the other controllers indicate a similar parameter expressed as a percentage of available hydraulic pressure. The CompuShift II will command pressures between zero and 100 percent of available pressure from the transmission. Absolute pressure is what the trans is capable of producing and is measured from a pressure gauge connected to a tap on the transmission case.

If your transmission is equipped with a lockup torque converter, there are several tuning areas available for torque converter clutch (TCC) lockup. Maximum TCC lockup is usually expressed as the percentage of TPS or throttle opening. Most converters will not lock up past 50 percent TPS to prevent damage to the lockup clutch. There are separate options for minimum TCC lockup, as well. For larger engines like ours, it's best to set the lockup minimum at zero percent to prevent the clutch from oscillating at lower throttle openings. If the percentage is set too high, a very light throttle opening may not achieve TCC lockup at highway cruise speeds. This will also retain lockup under deceleration. Most controllers also feature a brake-light switch that unlocks the converter anytime the brakes are applied.

Enjoyed this Post? Subscribe to our RSS Feed, or use your favorite social media to recommend us to friends and colleagues!

I have a 93 chevy c1500 with a carbed 355 and 4L60E. The trans has a 2400 stall and custom shift kit, and I recently installed the EZ-TCU on it. I ran the stock TBI engine with the rebuilt trans for a while and it worked great with the stock computer control, but as soon as I installed the TCI unit, the 2-3 shift hesitates under 3/4 to full throttle. I have adjusted the shift firmness/agressiveness, high and low rpm shift points, max shift rpm (set at 5000 for allowance to go up to 5800 like advised in the article, and any other adjustment I can find. Regardless of how I adjust the settings, it always hesitates going into 3rd and allows the truck to wind up to about 6200. I have replaced the control box with a new unit through TCI, and I still have the same problem. Can you guys at CC recommend a course of action for me?

Car Craft