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Don't Be Shocked!Posted 3/15/1997By Kevin Caple
A few years ago, some of the shock absorber companies came up with the new idea of an adjustable shock absorber. By turning a selector on the shock, you could change the size of the orifice to increase or decrease the dampening rate of the shock. You could have a firm ride for high speed or high performance driving. Great idea, right? The only problem: did anyone ever crawl under their car to change the shock settings to match their driving? Enter GM's Computer Command Ride System. This smart suspension system incorporates an adjustable shock or strut at each wheel that is constantly being adjusted to match driving conditions. This is accomplished by a small computer that uses vehicle speed, lateral acceleration, and dive and lift of the car's body to instantly adjust the struts to obtain the most control and comfort for any given driving condition. The computer has a set program for making ride adjustments. For example, at 10 to 40 mph if there is no dive or lift signals (hard accelerate or hard braking) or no lateral acceleration (hard cornering), the system will select a soft or comfort ride setting. Then at 40 to 55 mph it will switch to a normal ride firmness and with hard acceleration, braking or cornering, it will set a firm ride for maximum control. Now that we know what it does, how does it do it? Remember on those old adjustable shocks there was a pointer that you would move to the ridge setting you desired? You had to look at the shock to see where it was so you knew which way to make adjustments to increase or decrease the dampening. The computer system needs this same information, so in each strut there is a series of "pads" the computer sends a voltage to (see diagram) and, depending on which pad the pointer is on, will pull the voltage down to a given value so the computer knows exactly where the strut is set. There are also spacer pads placed between the ride set pads. This gives the computer a voltage signal to look at so it can tell if the strut is being adjusted when commanded and how far it must move to get to the desired setting. Starting to sound complicated? Thank goodness the engineers built in a "smart self-diagnostics" system that monitors opens, shorts or even a stuck actuator motor, as well as any problems with the lift/dive and lateral acceleration circuits. Best of all, they made the self-diagnostics to match what we are used to seeing on all other GM cars; simply jumping pins D2 to D16 at the ride computer will start a light on the computer flashing codes, first a code 12 three times followed by any other codes in the system (each three times), then a code 12 repeated again. Almost too easy, isn't it? As you've probably guessed by now, the areas of most concern on this system are the connections and the actuator motors that turn the orifice adjuster in the struts themselves. The rest of the system relies on solid-state circuitry with built-in, over-current protection that has proven to be very reliable.
Code 26 ... Not Again!Posted 3/15/1997By Les Bentley
Mr. Smith just drove in the lot with his '89 Buick Park Avenue. He said that the Service Engine Soon (SES) light is coming on. Not again! This is the third time in the last 10 days he has come in complaining about that darn light. And each time, the code stored has been 26. You begin flashing out trouble codes. And yes, there it is -- a code 26, quad driver error. You have checked everything. You even installed a new injector and EGR valve, and the problem just won't go away. The complaint can never be duplicated and there is no other driveability problem except for the light coming on. Worse yet, the car belongs to the shop owner's father-in-law! Driving the car around the block revealed nothing and the scan tool was no help either. So you try looking up any service bulletins that apply. No bulletins are found for 1989, but a 1988 bulletin describes a quad driver as a device or driver inside the computer that can monitor and control anywhere from one solenoid up to four different solenoids. This can be in one external device or four different ones. It also explains that the computer monitors for correct operation by monitoring the voltage at the computer connector for each particular device; case in point: ECM disconnected and ignition key on, engine off. If you check voltage at terminals BC2, BC3 and BD2, which operate the EGR solenoids, they should all show 12 volts. This would also hold true for any device operated by the ECM on the other three quad drivers. After determining that all circuits show battery voltage, you reconnect the ECM and road test the car. Still, the light does not come on and again the scan tool shows nothing wrong. In desperation, you break down and call the customer and ask him under what driving conditions the SES light comes on. He explains that the light usually comes on after about 10 miles of driving and remains on until he slows way down or comes to a complete stop. You also ask if any other lights come on at this time. He answers no. Great! You finally have the information to duplicate the complaint. While road testing the car you finally get the SES light to come on and to store a code 26. Taking a snap shot on the scan tool still doesn't show anything abnormal. Reading the bulletin again, you find that when the ECM turns on a device, (grounds the circuit) the voltage should be near zero. If this is true, then monitoring the circuit with a voltmeter should duplicate what the computer is seeing when the code is stored. The bulletin also indicates that Quad Driver 2 will not set a code 26. If this is true, test those circuits last. This leaves you Quad Driver 1, which controls the canister purge solenoid, the SES light and low speed coolant fan; Quad Driver 3, which controls the hot light and TCC solenoid; and Quad Driver 4, which controls all three EGR solenoids. While looking for technical service bulletins that pertain to code 26, you find one about poor connections at the computer. Good point. If the computer is supposed to be seeing 12 volts at a device that it has not grounded and if there is a poor connection at the computer, it could set the code. All connections are tight and clean. This must indicate that something is happening to the circuit once it has been activated for a while. Start with the quad driver that has the least functions - Quad Driver 3. "Hot" is not coming on when the SES is so that shouldn't be a problem, but what about the TCC? With a voltmeter, backprobe terminal YC10 on the computer. Of course, at this point, the car is just idling and the voltmeter shows charging system voltage. A strange thing begins to happen during a test drive. When the computer first grounds the TCC solenoid, the voltage on the meter drops to 0.3 volts. But the longer the TCC is engaged, the higher the voltage gets until the SES light comes on, and 1.0 volt is showing on the meter. Is it a bad ECM or a shorted TCC solenoid? The solenoid tests fine, with no excessive current flow. This indicates that the driver inside the ECM breaks down over time and is unable to hold the TCC solenoid to ground. When the computer sees the voltage level on the circuit increase, it knows the TCC should be on, so it sets a code 26. Replacing the ECM fixed the problem. Now, somebody is going to say that a TCC problem should have set a code 39. Not in this case. There was no open TCC circuit or inoperative circuit inside the ECM. It was just that the quad driver was unable to hold ground level and started releasing the ground -- not enough to disable the TCC, but enough to turn on the SES light and store that nasty code. I hope this helps the next time you see a code 26. Remember to verify the customer's complaint and find out what is happening when the code sets. Then try to duplicate the complaint. Be sure to test the component when it is failing -- not when it's working properly. If you have any questions, just give me a call!
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Copyright (c) 1996-2008. Automotive Service Association. All rights reserved.
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