by Rich Hirsch
We were working a rally near Salem, MO (about 100 miles southwest of St. Louis), traversing a gravel road, when the ANTILOCK and BRAKE warning lights came on. There have been numerous reports and indications that ABS brakes are troublesome and expensive to repair, but we have had no problems with them on the `87 Milano since we bought the car. This was about to change. Checking the Owner's manual revealed the following insights:
"Caution - Should the ABS trouble indicator, ANTILOCK warning light light up at the same time as the low-brake-fluid/worn-brake-pad indicator BRAKE on the Alfa Romeo Control, stop the car immediately without depressing the brake pedal violently and call for the nearest Alfa Romeo Service Dealer."
This advice, considering our location (also, it was Saturday) was not easily heeded. I tried to stop and there seemed to be no impairment of the brakes. Checking under the hood, things looked normal, brake fluid level in the reservoir was fine There was however a strong acrid smell which was later traced to the accumulator pump. We got back into the car and thought. Well, we'll carefully drive it home, which should be possible, since there was no loss of brake fluid and ABS systems are designed so that when the ABS system fails, only the antilock features disappear, not the ability to stop the car. Boy was I wrong!
One more good stop was all I could manage. From that point on, to stop the car, I needed to really stand on the pedal. I also needed lots of pavement. I've driven cars before that have had master cylinder failures, where there was virtually no braking power whatsoever. This was different. The pressure exerted by my foot was transmitted to the brake pads, but with very limited efficiency. later, I found out from the Workshop Manual, that if just the ANTILOCK warning light comes on, the car still has power-assisted brakes. If both the ANTILOCK and BRAKE lights come on (as they did in my case), the car is without power-assisted brakes, a very dangerous condition. It's ironic that ABS brakes, arguably the most important safety innovation of the `80s, which is supposed to make stopping safer, actually put us in peril.
I want to briefly describe how ABS brakes work in general, and then discuss the way they're implemented in the Alfa. The concept behind ABS brakes is to stop the car in the least distance possible while allowing the car to be maneuvered. This is done by keeping the wheels from locking, since a car with locked wheels cannot be steered. In theory a car with antilock brakes stops in a shorter distance, since a rotating wheel has a slightly higher coefficient of friction than a locked wheel. In the real world however, this apparently doesn't make much difference and whether or not wheels are locked, a car will stop in about the same distance. It's just that with antilock brakes, the direction of a car may be controlled during braking to possibly avoid an accident.
1 Front brake caliper 10 Brake master cylinder and hydraulic operating 2 Front impulse emitting wheel cylinder 3 Front impulse pickup 11 Filter 4 Min/max pressure switch 12 Rear impulse emitting wheel 5 Electropump unit 13 Rear brake caliper 6 Accumulator 14 Rear impulse pickup 7 ABS MARK II system warning light 15 Brake pedal (antilock system failure) 16 Antilock system control unit 8 Brake fluid minimum level and brake pad wear 17 Brake pressure adjusting valve warning light on AR CONTROL 18 Braking pressure modulating electrovalve unit 9 Brake fluid tank
Figure 1. Alfa's ABS brake system
ABS systems use a combination of electronic and hydraulic systems to modulate the brakes individually to prevent them from locking. Antilock systems basically consist of wheelspeed sensors, a hydraulic control unit (HCU), and an electronic control unit (ECU) - see Figure 1. During braking the wheelspeed sensors measure the rotational speed of the wheels and forward this information to the ECU. If the ECU determines that one wheel is decelerating at a much greater rate than another (indicating that the brake is about to lock), the ECU directs solenoid valves in the HCU to open to release hydraulic pressure to the brake until the rotational speed of the errant wheel matches the others.
Because of the complexity of the system, when the engine is started, a self-test sequence occurs. If the check is successful, the ANTILOCK and BRAKE lights will extinguish. If some component fails to check out, one or both lights will remain lit. One or both warning lights may also come on while driving, also indicating a problem. If just the ANTILOCK warning light comes on, some ABS component has failed, but the car still has power-assisted brakes. If both the ANTILOCK and BRAKE lights come on, the car is without power-assisted brakes, which is very dangerous. If just the BRAKE warning light comes on, this may be caused by low brake fluid or worn brake pads.
The ABS system Alfa uses is made by the German company, Alfred Teeves (ATE) and is of the 3-channel variety. This means that hydraulic pressure is supplied to both rear brakes as if there were only one, although wheel speed is measured at all four wheels.
One of the more interesting aspects of the ATE ABS system is how power-assist is provided to the brakes. Conventional braking systems usually use engine vacuum to increase braking capability. For years Alfa used such a system to provide reliable, effective brakes. But instead of using vacuum, the ATE ABS system uses an electric pump to pressurize brake fluid to provide power assist.
The pressurized brake fluid is stored at ca. 180 bar (2600 psi!) in a reservoir known as the accumulator. From the accumulator the high pressure fluid is supplied to the hydraulic operating cylinder in the master cylinder assembly. When the brakes are applied, a lever connected to the brake pedal lets in varying amounts of the high pressure fluid to assist in stopping the car.
As mentioned earlier, Alfa's ABS system consists of three hydraulic brake circuits. Each of the circuits is controlled by two solenoid valves - a load valve, normally open, and a drain valve, normally closed. When the ECU detects one wheel decelerating at a rate much greater than the others, the ECU closes the load valve and opens the drain valve for that wheel to bleed off hydraulic pressure and prevent lockup. Once the imbalance is corrected and the wheel is decelerating at roughly the same rate as the others, pressure must be reapplied to the brake caliper to help stop the car. This is accomplished by closing the drain valve and opening the load valve. But we have a problem here. If we have to depressurize and pressurize a brake caliper repeatedly, the brake pedal would eventually reach the floor. That is, unless there is some mechanism for raising the pedal after pressurization. There is, and this results in the odd and somewhat unsettling sensation of having the brake pedal pulsate beneath your foot.
To make a long story short [it's much too late for that, ed.], the electric pump used to pressurize brake fluid and provide power assist had failed. To test the pump, press the brake pedal down hard at least 20 times with the ignition off to bleed off pressure. Then turn on the ignition. You should hear (or feel) the pump run until pressure builds up to the specified level. The pump should then turn off. If the pump doesn't run, check the supply to the pump at the power connector (see Figure 2). It should be 12 volts with the ignition on. If it isn't, the electropump fuse may be blown. If the fuse is open, it might have blown when the electropump failed, so don't expect to just pop in a new fuse and drive away. (You can't exactly pop in a new fuse anyway, since the relays and fuses for the ABS system are behind the "regular" fuse/relay panel in the driver's compartment and fairly inaccessible.)
The electric pump may also be tested by briefly connecting it directly to the 12 volt battery. Use a volt/ohmmeter at the power connector of the electropump to determine which terminal is ground and which is positive. Then, using jumperwires, carefully connect the electropump momentarily to the battery. Important! For this test the plumbing should not be disconnected, since high pressure brake fluid is corrosive and dangerous.
Remember! When working on the ABS system it's important to discharge brake system pressure by pressing the brake pedal down hard at least 20 times with the ignition off.
After replacing the electric pump, it is necessary to bleed the brakes. The factory procedure for accomplishing this will be published next month.
When I took apart the electric pump, I discovered that the motor windings were fried. The motor directly drives the pump head (see Figure 2). This is weird because the motor shaft has to spin within a seal and yet the seal must keep the hydraulic fluid out of the motor bearing and windings. A pretty tough job at 2600 psi and elevated temperatures! ATE has provided a weep hole so that in case fluid leaks past the seal, it may drain out harmlessly. On the other hand, if brake fluid makes its way alon the shaft into the motor bearing and windings, it could lead to serious problems.
I don't know of a better way to design this thing (or even that a better design is possible), but considering the adverse conditions under which the electropump operates, I'm amazed it lasted as long as it did (five years).
The cost of a new electric pump is close to $1200. I was shocked, to say the least. It comes in a nice sealed metallic pouch, but otherwise, just looking at it, it doesn't seem to be worth that much. About every part is put together by crimping, rather than with bolts. And if you think that the pump is expensive, the master cylinder for the ABS system goes for about $2000. Apparently there are no rebuild kits for when the rubber parts wear out, as they invariably do.
Consequently, if your ear has ABS brakes, it should pay to maintain the ABS system to make sure the components last as long as possible. This means renewing the brake fluid every year.
The problem arises of what to do for an older car of limited value that requires replacement of one or more expensive ABS components. One could replace the ABS system with conventional brakes, but that's another article. One I'd rather see someone else write.