In every case we can think of (with all the variations out there, there may be a rare exception), any foundation-brake failure that would cause the red "brake" warning lamp to glow also will illuminate the ABS telltale and cause the ABS to quit modulating brake-fluid pressure to the individual brakes. But if the failure in the basic braking system isn't too severe, the car may still be able to stop.
It's interesting to note that in some cases, an ABS problem can also be a foundation-braking system problem. Take the Teves Mark II system, as used in several GM and Ford cars from the mid-1980s to early '90s. This integrated setup uses its accumulator not just for reactivating any brake whose hydraulic pressure was cut to prevent wheel lockup; pressure from the accumulator also applies the rear brakes and provides driver assist.
Mixed-up systems
This 1990 Oldsmobile 98 Touring Sedan has the Mark II system, and the intertwined nature of its ABS and foundation-braking operation brought on a condition that had more than one technician scratching his head.
The car would stop, almost uneventfully, according to its owner. During any but very light pedal applications, the driver would hear a warning chime and see both the "brake" light and "antilock" light menacingly lit up. But all warnings would discontinue when he would release the pedal. This situation, obviously, required thinking through before reaching for any tools.
The glowing red "brake" indicator light and clanging chime indicated a basic braking system problem. The also-glowing amber ABS light additionally warned that the system would not work to combat wheel lockup, should such action be needed.It's worth considering that although this sequence of illumination indicated the ABS wouldn't function, it did leave open the possibility that the ABS was OK and would work if the problem with the basic brakes were repaired. Another factor worth noting was that although the car always seemed to stop fine, it had only been subjected to the kind of driving you might expect from a mature individual operating a meticulously maintained, old Olds. Conservative driving, in other words.
Looking for information
No diagnostic trouble codes (DTCs) were stored, but GM's shop manual indicated any system failures related to maintaining appropriate accumulator pressure would not set codes. The pump that pressurizes the accumulator seemed to be working; it was audible, but it didn't seem to be running excessively. Boost seemed sufficient for everyday stopping duties.This seemed to indicate a problem with accumulator pressure: It had to be at least sufficient to not let either warning lamp light up except during braking. But was the fact that pressure ran out a result of a failed accumulator diaphragm or depleted nitrogen charge not pushing brake fluid back into the hydraulic system? Another cause could be the sensor/switch on the "wet" side of the diaphragm signaling the pump to shut off too early.
The recommended – and best – method to check accumulator pressure is to install a specialized T-fitting adapter between the accumulator and the HCU casting and connect a pressure gauge and read it while trying out the system. If pressure is below spec, the sensor/switch is a possible culprit; it could be telling the pump to quit too soon. Pressure at or near spec in a static check would indicate a weak accumulator charge, a signal to look for a noticeable pressure drop under use.
All that was academic when we started looking at the Olds; the required adapter wasn't available. But we did come up with a hypothesis: The pump could pressurize the accumulator, but if only a small amount of brake fluid could be pushed back (from either the accumulator or the pump), the system might be able to accomplish an easy stop or two. And if that pressure was coming primarily from the pump, which wasn't designed to provide constant operating pressure during stops, there wouldn't be much pressure left after those one or two easy stops.
In a situation like this, where you don't have every tool at your fingertips but you have a theory, a specific, functional test may, at least, be able to confirm or disprove your hypothesis.In this case, the functional test was to try several moderate-to-hard stops in rapid succession. Sure enough, boost was OK for the first two stops but the pedal became successively harder — and the lights and chime didn't shut off as readily — after them. The adequate initial boost indicated the pump and sensor/switch were getting the pressure up to or near spec (enough to prevent the red light coming on when no boost was called for), but the drop-off in assist indicated the accumulator wasn't maintaining adequate pressure. The cause had to be a failed accumulator.
It probably would have required breaking the old accumulator open to determine the exact cause of the failure. New ones aren't easy to find. Fortunately, a local salvage yard had a similar-vintage, Teves Mark II-equipped Pontiac SSEi that hadn't been junked because of brake problems.
