I have been studying the Teves MKIV anti-lock braking system for our Thunderbirds. I believe I have figured out how to bleed air from the hydraulic control unit without using any special tools. I have studied the wiring harness connector along with a teves repair manual I found on a BMW forum. A special thanks to Mike8675309 cause without his leg work I don’t believe I could have found all the information I needed.
The ABS system is pretty simple in design. There are 8 valves in the hydraulic control unit (2 for each wheel) that open and close to control hydraulic pressure to the wheel cylinders. For each wheel there is an inlet valve and an outlet valve. During normal braking, the inlet valves allow fluid pressure to reach the wheel cylinders and will remain open as long as the system is passive (no ABS needed). The outlet valves bleed off pressure from the lines and will remain closed while the system is passive. At each wheel is a toothed wheel and a magnetic switch. When the wheels are turning the magnetic switches send pulses to the Control Unit which is interpreted as wheel speed, using the time between pulses (pulse width). As the wheel slows down the time between the pulses gets longer. The CU uses this information as well as inputs from other sensors to determine if ABS is needed.
If ABS is needed, the outlet valve open to bleed fluid pressure back to the brake fluid reservoir until the CM determines the wheel is rotating. To maintain hydraulic pressure in the system while the inlet/outlet valves are opening and closing, the CM will turn on a hydraulic pump that is part of the HCU. Near the pump is a small transformer (pump sensor) that will send voltage to the CU when the pump is rotating. By using pressure reduction, the ABS control module can control wheel lock-up.
With most ECM’s voltage spikes are silent killers to electronic components and so they are designed to control ground. The control unit can turn on and turn off components by applying ground to one side of the component (load). The voltage on the component is always there. The teves MKIV brake control module works the same way.
In one of Mike’s earlier post, he was concerned about varying current to the control valve and exceeding the valves load limit. Mike I apologize if I miss-quoted you, I couldn’t find your post to refer too. This is a valid concerned but does not apply here. Because the voltage is constant to the coil, and the CU only applies ground, current remains constant. (I=E/R) Either the valves are open or they are closed. No in-between. Knowing what the valves do, grounds are applied differently between the valves. When the system is passive, grounds are applied to the outlet valves to keep them closed. The outlet valves will fail close upon loss of power. The inlet valves have no grounds applied and thus will remain un-phased on loss of power. They will also fail open. During an ABS event the grounds are reversed, grounds are applied to the inlet valves allowing the coils to energize and the valves are closed. Grounds are removed from the outlet valves allowing spring pressure to open the valve.
Knowing that the valves are nearly identical in construction and operate basically the same way, voltage can be applied nearly indefinitely to the coils without damage. There is slightly higher resistance on the inlet coil, 6-8 ohms vs. 3-5 ohms on the outlet.
There were questions that I had to answer before I felt comfortable with bleeding the system without the special tool.
1. Current and voltage to the control valves: I felt that the control module module may contol voltage and/or current to the control valves but the teaves operation manual clearly states the voltage to the control valves is 12VDC and the control module control the valves by applying ground. Since the voltage is constant, current is controlled by the load (valve) and not the control module.
2. A positive displacement pump will be damaged if it dead-heads. Not much information on the pump but I feel 99.9% sure its a positive displacement pump. That means that when bleeding at least one recirculate valve must be open to prevent pump damage if the pump is on for more than 20 seconds. I would avoid using the bleed screw except to bleed the individual wheel lines and would still have one recirc valve open when bleeding lines.
To bleed the HCU, the pump needs to run to circulate brake fluid from the HCU back to the reservoir with the outlet valves opened and the inlet valves closed. Refer to these illustrations:
If I can remember right, the pump needs to run for 20 seconds with the valves closed so the pump can build up pressure. Next, the pump needs to run 20 seconds (?) with the outlet valves open to push air bubbles to the reservoir. Repeat until no air bubbles in the reservoir.
Disconnect the wire harness from the brake control module.
Close the inlet valves. Apply ground by shorting pins 1, 20,38,54,55.
The outlet valves should already be closed.
To start the pump, disconnect the wire harness from the brake control module. Turn the ignition on. Apply ground to the main and pump relay by shorting pins 1, 15, and 34. This should energize the relay and the pump should start. Run for 20 seconds.
Stop the pump by removing ground from pin 15 and 34.
Open the outlet valves by applying ground (pin 1) to pins 2, 21,36, and 18.
You can manipulate any valve you want by applying the grounds to the right pins.
You can view the Teaves operation manual
here