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Vehicle: 1989 C-10 Pick Up, GM Code 5Posted 8/10/1999By Jim Watson
Problem: The brake light came on intermittently for the customer, the RWAL system set a Code 5 (The definition of Code 5 is "excessive actuation of the dump valve during an anti-lock stop."). The problem cannot be reproduced. The technician installed an Isolation/Dump valve and a new ECU. All the wiring appeared to be in good shape and voltage drop on the powers and grounds showed all was OK. Here's what normal vehicle operation would be: The ECU sees vehicle speed with no application to the brakes. It also sees that the brakes have been stepped on, causing the voltage to drop low at pin F. Not wanting the rear wheels to lock up, the ECU watches the vehicle speed input at pin D for a decrease in frequency that drops so fast it thinks the rear wheels are about to lock up. If the ECU sees the frequency drop too fast (a lock-up about to happen), it immediately blocks any more fluid/pressure from getting to the rear brakes. To do this, the ECU modulates power to the isolation valve blocking fluid/pressure to rear brakes. Continuing to watch vehicle speed, the ECU decides if that helped, it will release the Isolation Valve and continue to watch vehicle speed. However, if the speed signal input did not recover, the ECU modulates the dump valve, reducing the pressure in the rear system until the vehicle speed input indicates the wheels are turning again and out of a lock-up situation. That's all fine when everything is working correctly. Now, what about that Code 5? Back to the definition: excessive actuation of the dump valve. The ECU says, I see the brake pedal stepped on, so I know you want to stop ... I see my speed signal input frequency drop rapidly enough that I know we are in a lock-up situation ... I modulate the isolation valve to block pressure to the rear brakes and do not see a recovery in my speed signal input frequency so I modulate the dump valve in an effort to reduce pressure but that does not help ... so I continue to modulate it with no improvement ... I have been keeping track of the number of times that I modulate the dump valve and according to my program something should have improved, but it did not. So I am setting a Code 5 - SO THERE! The key is that the RWAL ECU and isolation/dump valve are working properly, and doing their best to control an assumed lock-up condition. They respond correctly to the inputs they are supposed to watch, namely the brake switch and the speed signal input. Looking at the two inputs, if the brake switch is not opened the ECU does not even begin thinking about a lock-up condition. So that leaves the speed signal input. If it drops out with the brake switch open, the ECU has no choice but to believe that in fact the rear brakes are locked up. The one piece of the puzzle that the customer never mentioned is that every time the brake light came on, the speedometer read 0 MPH. The reality is, the ECU was responding correctly all the time! Further diagnosis led to a failed DRAC not giving the IPC, PCM or ECU the correct information about the true condition of the vehicle speed. Sometimes your customers are more aware of a light flashing at them instead of a gauge that might drop out. Don't forget to ask about any other symptoms that might seem totally unrelated.
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