XVI. Driveline

C. Electronically-Controlled Automatics

Yeah, I know. You're not particularly thrilled with the trend toward electronically-controlled automobiles. After all, you're MECHANICALLY inclined, right? You never imagined you'd be needing the skills of a computer technician. First, it was just ignition, which wasn't too bad. But then EFI and feedback carbs started to show up in big numbers along with computers that handle all the goings on under the hood from emission controls to the subject of this section, torque converter lockup. Now, we're getting smart suspensions, computerized anti-skid braking, digital dashboards, and chip-controlled shift points. Will it ever end?

Probably not. As Boss Kettering, the inventor of point and coil ignition and founder of Delco said many years ago, "Nothing is constant but change." There's no doubt that the transition from the mechanical age to the computer era is going to be difficult for everybody who works on cars. But, like it or not, it's happening, and if you're going to remain competent, you've got to bite the bullet and learn a lot of new stuff. Fortunately, the system that controls the clutch in GM torque converters is easier to understand than some other gee-whiz automotive wonders I can think of.

A torque converter is a wonderful thing. It allows a car to sit still while idling in gear, and, by a seemingly magical process, puts out more twisting power than is put into it.

In spite of its good qualities, it has one serious drawback: It slips, and that wastes fuel. With the automakers frantically trying to get mpg up, this characteristic couldn't be tolerated. Chrysler moved first. In '78, it introduced a converter with an internal clutch that locked up solid at cruising speed. But it was controlled hydraulically. The next logical step in the evolution of a more efficient transmission was a switch to electronic means of deciding when the clutch should be engaged, and the most prominent example of this is GM's TCC (Torque Converter Clutch) operated by the C-3 (Computer Command Control) system.

How do you translate a computer-generated electrical signal into a mechanical act? Easy, with a solenoid-operated valve. When the brain sends voltage, the solenoid
moves a check ball in a fluid line that routes apply pressure to the clutch servo.

This simple component is easily accessible with the trans in the car. On transaxles, just remove the side cover. On regular automatics, pull the pan.

The ECM (Electronic Control Module) determines the best time to lock up the clutch on the basis of data it receives from various sensors and switches:

     First, there's the VSS (Vehicle Speed Sensor) -- mph must be above a certain point before the TCC can be applied.
     Since the engine should be warm, CTS (Coolant Temperature Sensor) input is considered too.
     The clutch is supposed to disengage whenever the car is accelerating or decelerating at a particular rate, and the computer gets this info from the TPS (Throttle
     Position Sensor).
     On some transmissions, a third or forth gear switch lets the ECM know what gear is currently engaged so it can vary the conditions under which it applies or
     releases the clutch, but the trans doesn't have to be in high for the clutch to engage (if you see three or four wires coming out of the TCC connector, the transmission has gear select switches).

Other controls don't go though the computer. One is a normally open third gear switch in series on the battery side of the TCC solenoid. It prevents lockup until third is engaged. Another, found on certain transmissions, is a 4-3 (four speed) or 3-2 (three speed) pulse switch that opens the TCC solenoid circuit momentarily during a
downshift. Finally, there's the brake switch. When the driver puts his foot on the stop pedal, the switch opens, which breaks the circuit to the TCC.

As far as fuel efficiency is concerned, this system works great. But many motorists are annoyed or dismayed by the way it feels. They'll think their transmission is
constantly shifting up and down as conditions lock and unlock the clutch, and they'll swear there's something wrong. Others will say their engines are misfiring. So, the first step in troubleshooting a complaint is to ascertain whether or not a problem does indeed exist. Take the car for a test drive and really concentrate on what's happening, then read the owner's manual or try another similar GM car to refresh your memory. Maybe the TCC is working as it's supposed to.

Right here I'd better caution you that this is an area where the old saying about a little bit of knowledge being a dangerous thing definitely applies. You might think it's a very clever idea to cure that annoying "hunting" condition by just disconnecting the TCC wire so the clutch never engages. It'll cost a little in mileage, but the trans will act just like a pre-fuel crisis unit, right? Unfortunately, no. On the 200 4R and the 700 R4, doing this can cause fourth gear to burn out and ruin the front pump. And it'll increase fluid temperature to the danger point on all models. Mileage will drop more than you'd expect too -- clutch-equipped converters slip a lot in the unlocked mode. GM has issued bulletins warning us never to run a car with the TCC disabled.

What about splicing a toggle switch into the wire that sends voltage to the TCC solenoid? No good either. You're risking all the same problems mentioned above whenever the switch is off.

Actual (as opposed to perceived) problems you may run into are a clutch that won't disengage -- of course, that'll stall the engine. If the clutch never locks up, the only thing you might notice is a considerable drop in fuel mileage (in fact, you may even prefer the way the car drives). If you're a little more sensitive to your vehicle, and it has a tach, you might also be worried when you see that the engine is turning 300 to 500 more rpm than it used to maintain the same speed. And you may be heading for a megadollar trans failure because of the overheating and lubrication problems that occur when that clutch doesn't work.

Before you start blaming the electronics for the trouble, make some preliminary checks, including fluid level, linkage adjustment, and the condition of vacuum lines. Since the engineers have given you self-diagnostics, you might as well use them. Find the ALDL (Assembly Line Diagnostic Link) under the dash (or on the odd duck such as the late lamented Fiero, in the console), turn the ignition on but don't start the engine, then connect terminal"B" of the connector to terminal "A" or a good ground. This will activate the self-criticism mode.

Code 12 (flash-pause-flash-flash) will be signalled by the "Check Engine" light three times, then any troubles the computer has recorded will be flashed out. A 24, which fingers the VSS circuit, is the trouble code most directly related to TCC function. But others suggest problems that can affect the system too: 14 or 15 for the CTP, 21 or 22 for the TPS, and 33 or 34 for the MAP sensor.

Of course, if you have one of those neat aftermarket C-3 scanners, you can read the trouble codes without having to count flashes, get a direct verdict on the VSS circuit, and compare how many rpm and mph relate to clutch on/off status.

When presented with a code 24, you should consult the logic-tree chart in the factory manual for the car in question, but the procedure for an average A-Car is typical:

   1.Make sure the speedometer is working.
   2.Raise the drive wheels, put the trans in Neutral, turn the ignition on, then connect a voltmeter between ECM terminal #16 and ground.
   3.Rotate a drive wheel by hand. The meter reading should alternate between three and six volts.
   4.If voltage varies, check the Park/Neutral circuit, then the TPS setting. If they're both okay, the ECM connection or the ECM itself is bad.
   5.If there was no voltage variation while you turned the wheel, disconnect the VSS and measure the voltage between harness terminals "A" and "B," then "B" and "C."
   6.If both readings are over six volts, check the VSS connections. If they're okay, replace the VSS.
   7.If either of the voltage readings are low, you've found a poor connection, an open wire, or a faulty ECM.

Another thing worth doing before you come to any rash conclusions is a test of transmission fluid pressure. Tee a gauge into the return line from the cooler, attach a tach, disconnect the hose from the vacuum switch or sensor, raise the drive wheels, warm up the engine, run it in Drive until it shifts into high, hold 2,000 rpm, and check the gauge reading against specs. If there's not enough pressure, nothing's going to work properly even if the TCC components and controls are in fine shape.

If nothing's wrong so far, it's time to look up the TCC system diagnosis chart in the proper manual. While I'm not going to squeeze the whole boring step-by-step into this section, I'll tell you how to start the sequence so you can get an idea of what's involved. First, warm up the engine and raise the drive wheels. Connect a test light from terminal "F" in the ALDL to ground, start the engine, put the trans in gear, and accelerate to 35 mph. The light will go on if the TCC is being sent a signal. Hold the throttle position and touch the brake. If the brake switch is okay, the light will go out. From there on, you'll have to follow the chart, which, I'm happy to say, is pretty easy.