Stop to Match the Go

All About C4 Antilock Brakes and Traction Control

By Hib Halverson

As I write this, the NASCAR Sprint Cup Series' annual pilgrimage to New York's famed road course, Watkins Glen International, is on TV. Road racing a Cup car–3400 lbs, 850-hp, small tires and no ground effects–has got to be the most fun you can have except for maybe sex and stuff that's illegal.

Jeff Gordon (#24 DuPont Chevrolet) and Tony Stewart, (#20 Home Depot Chevrolet) are running 2nd and 4th. Each has three wins at the Glen and Gordon has three more at NASCAR's other road race venue, Sonoma, California. Obviously, they're very talented drivers, but what connection do they have with the subject at-hand? Great car control under braking. Be it a Cup car or your Corvette, the tires which provide traction and they do that best when there's some slip between them and the road. Under braking, this is when the tire rolls slower than the car is moving but is not locked-up. Engineers call this "negative slip."

            Braking to near wheel lock-up (aka "threshold braking") is a requirement if you want to be a road racer. The problem is: to be consistently good at threshold braking requires significant driving skill, experience and very good feel for the car. At the level Gordon and Stewart race, such qualities are instinctive. For the rest of us; there are antilock brakes.

Some History

Between 1986 and 1992, two automotive breakthroughs were four-wheel antilock braking systems (ABS) and traction control systems (TCS) as standard equipment on high-volume cars. The C4 Corvette was at the front of both trends. ABS was perfected by the aircraft industry. An airliner sliding off a slick runway into the weeds is, at best, disconcerting. It's bad PR for the airline and damages a multi-million-dollar piece of equipment–not to mention the possibility of injuring passengers. Half a century ago, as all-weather air travel became more common, pilots, engineers, along with, no doubt, airline risk management folks, desired brakes which could be applied, full-force, but, on slick runways, would automatically modulate to resist wheel lock-up and loss of control. In the 1950s, the British firm, Dunlop, developed "Maxaret", the first, practical, antilock brakes for airplanes. Today, just about anything in the air the size of a Cessna Citation or larger has antilock brakes. In the early-'60s, Maxaret, an electro-mechanical system, was adapted to the Fergeson P99, an obscure, British Formula 1 race car and, later, the Jensen FF, a just-a-bit-less-obscure, late-'60s, British road car.

In the U.S., Kelsey-Hayes developed an analog-computer-controlled, rear wheel ABS, known as "Sure-Track" which was optional on 1969 Lincoln Continental Mark IIIs and 1970 Oldsmobile Toronados. In the mid-'70s, K-H experimented with a Sure-Track off-shoot which was doubled-up to make a crude four-wheel ABS. Chevrolet had enough interest in this research to have K-H fit the system to a C3. After evaluation, this system was deemed not ready for prime time. Meanwhile in Germany, Robert Bosch Corporation developed a digitally-controlled system. Its second generation, "ABS-2", the first digital system on a production car, was an option on 1978 Mercedes-Benz S-Class and BMW 7-Series sedans.

            By the early '80s, a more developed ABS-2 was available and, around the same time, Bosch and Chevrolet were negotiating the use of Bosch fuel injection parts on Corvette and Camaro for model year 1985 (MY85). During those meetings, Bosch pitched ABS-2 to the Bow-Tie guys.

Paul King, Chevy's Engineering Director at the time, eventually decided all '86 Corvettes would have ABS, making them the first cars in North America to have four-wheel, antilock brakes and one of only two brands in the world at the time to have them as standard equipment.

            It really was an elegant idea: a digital computer doing "threshold braking" in a faster-acting and more consistent manner than can 99.5% of most Corvette drivers. And, in the front, when necessary to preserve steering control, doing it on a "per-wheel" basis, a feat no driver–even those named Gordon or Stewart–can accomplish. It was a risky decision for Chevrolet because of the expense. According to retired, Corvette Chief Engineer Dave McLellan's excellent book, Corvette from the Inside, a full 10% of the cost of an '86 was in its antilock brakes. ABS-2 was used until MY89. From MY90 to '94 on the base car and from '90 to '95 on ZR-1, there was an improved variation, known as "ABS-2S". The base car for MY95 and all '96es had an even further developed, Bosch "ABS-5". The system of interest to second generation ZR-1 DIYs is, of course, ABS-2S.

The Components

The ZR-1 antilock brake system has two main hardware devices: the "electronic brake control module" (EBCM) and the brake pressure modulator (BPM), both in the compartment behind the driver seat. For ABS-2 and -2S, the EBCM and the BPM are separate but, for ABS-5, they're a single unit.

The EBCM (some say "ECU", "ABS controller" or "brake controller") is the brain of the system. It's a computer which controls the BPM based on its programming and information about vehicle dynamics parameters. There are two controllers:  '90-'91 and '92-/'95. From '92-up, it's correctly known as an "electronic brake and traction control module" (EBTCM) because of the addition of traction control capability.

 The BPM (sometimes called a "modulator valve" or a "hydraulic unit") is both the heart and the muscle of the system. Its key parts are solenoid-operated valves which control brake pressure at the left front, right front and both rear wheels; a high-pressure pump, which returns brake fluid to the master cylinder when necessary; an accumulator; some dampers, microfilters and various ball check valves. There were two BPMs: '90-'91, '92-'95. The major difference between them is the latter's ability to control the rear brakes independently when operating in TCS mode. 

          The ABS needs wheel speed data and,

 because its operation must be near instantaneous, that data must be high-resolution so only a short period of wheel speed data acquisition is required for the EBCM to determine if ABS intervention is necessary. Each wheel hub has a 47-tooth ring and a magnetic-inductive sensor in each suspension knuckle. There is a tiny gap between the teeth and the sensor's coil. The rings rotate at wheel speed and each time a tooth passes through the sensor's magnetic field, it generates a pulse. The frequency of this AC voltage is proportional to wheel speed. Since it's a high-resolution "47x" signal, changes in wheel speed can be detected very quickly and precisely.

            ZR-1 ABS systems use a sensor which actually measures lat rather than just turning on at .5g. When the module receives the switch's signal or data indicating high lat, it alters antilock strategy to better suit operation during high-g turns. The system advises the driver when it's active with an "Antilock On" or "ABS Active" light. If it experiences a problem, an "Antilock" or "Service ABS" light (some call it the "service light") comes on.

How does all this Work?

ABS is transparent to what engineers call the "base brake" system until it's needed. It's, also, "fail safe" so, if there's a problem, it shuts off, leaving the base brakes unaffected. It's a three-channel system, meaning it outputs three control signals: one for the left front brake, one for the right front and a third for the two rears. At the rear, it's a "select-low" system meaning the wheel with the least traction determines the level of brake control.

            When you turn the key on, ABS tests its electronics and, once you drive at least 3.6 mph in either direction, it tests its hydraulic functions. This verifies proper ECBM function and operates the BPM's solenoids and pump momentarily. The driver usually hears the pump run and may feel its momentary operation if his/her foot is on the brake. ABS goes active when one or more wheel sensors indicate a rate of change in wheel speed characteristic of impending wheel lock-up, that is: the wheel(s) are slowing so quickly that lock-up is inevitable. If this is happening at the left front. The ECBM applies a 2.5 amp current to the BPM solenoid valve controlling brake pressure to the left front caliper. The valve moves to the "pressure hold" position, blocking flow and keeping fluid pressure from going higher, regardless of how much harder the driver pushes on the brake pedal.

The ECBM continues to watch wheel speed. If the trend towards lockup doesn't abate, the controller doubles the current flowing to the solenoid. With five amps across it, the valve moves to the "pressure decrease position" and, even if the driver is pushing harder, brake pressure drops and lock-up is no longer eminent. At this point, the pressure decrease stops the wheel's impending lock-up. Once the ECBM detects that, the solenoid is released and brake pressure builds, again, however, if the wheel starts to lock a second time, the controller will, again, command either the pressure hold or decrease modes. This cycle continues until lock-up abates and it can happen up to 10 times per second.

When solenoids operate in the pressure decrease mode, if brake fluid released from the base brake system is not replenished, the pedal will bottom. The ABS pump solves that problem by pumping fluid back into the base brake system. In the very short time the pump is coming up to speed, fluid is temporarily stored in an accumulator. This pump makes a buzzing/grinding noise and causes a vibration the driver feels through the brake pedal which serves as tactile feedback that ABS is active.

Traction Control

Bosch brought what the Germans call "acceleration slip regulation" (ASR), to production in the 1987 S-class and 7-series. GM was first to use it in North America on the 1990 Cadillac Allante. It became standard on Corvette for 1992. The Traction Control System (TCS)–some still call it ASR–has the same goal as ABS: controlling tire slip, however, it addresses the other end of the scale, when drive torque exceeds available traction and the rear wheels spin, "break-loose", "burn rubber" or, as engineers say, "exhibit positive slip."

            Traction control drove change in some existing ABS components and the addition of new devices to the system. During TCS operation, the EBTCM has capability to retard the engine's spark timing, reduce its throttle opening or apply the rear brakes independently of the driver. The engine control module (ECM) and the EBTCM are linked so TCS can gather engine speed and throttle position data, enable spark retard and cut-off cruise control.           

A new piece of equipment, called either a "throttle relaxer" or an "adjuster assembly," was added under the hood to enable the EBTCM to control the engine's throttle body via additional throttle cables. When TCS applies the rear brakes, it needs a supply of brake fluid, so '92-'95 ZR-1 systems have a "prime pipe" which brings fluid into the BPM from the master brake cylinder. TCS detects positive slip at the rear wheels using the same wheel sensors as ABS, along with engine rpm information. If the rear wheels start to spin up, it has a three-step strategy. First, the EBTCM commands the ECM to retard spark but, at no time, will the spark come later than top-dead-center. While spark retard was implemented for MY92, the '93-'95 ZR-1s had that function disabled.

           If spark retard doesn't control slip, the EBTCM reduces throttle opening. If the wheel spin still is not controlled and vehicle speed is below 50 mph, the EBTCM will turn on the BPM's pump to apply the rear brakes. It can do that with a differential basis, if necessary, when the system detects a yaw component in combination with wheelspin. 

When Trouble Strikes

When ABS diagnostics discover a problem, the system is disabled leaving the base brakes unaffected, information about the fault is encoded in the controller's memory, the "Service ABS" warning light comes on and you're in for an ABS diagnostics session.

First, you need service data. No aftermarket repair manual available to consumers covers ABS/TCS adequately. You need a GM Service Manual for the model year of your Corvette and, if it's a '90 or '91, you, also, need the Section 8A Supplement which contains part of the ABS troubleshooting data. They're available from Corvette Central. Also of value are two GM training publications Bosch 2U/2S Antilock Brake System (PN 15005.0B-4) and Corvette ABS/ASR Operation and Diagnosis (PN 13003.02-1)

Both are out-of-print but might turn-up on eBay Motors or at swap meets. If you see them; bid on or buy them.

            Turn the key to "run" (some techs say, "key-up") but don't start the engine. If the service light comes on, but goes off 2 seconds later, the problem is intermittent. If the light flickers briefly, you may or may not have a problem. The Service Manual has information on either of those scenarios. If the service light stays on, you definitely have trouble and what's next depends on the model year of the car.

On-Board Diagnostics

            GM required Bosch to build ABS with diagnostics similar to that of the car's engine computer. If there's a problem, fault information is stored as "diagnostic trouble codes" (DTCs). There are two kinds of DTCs, "current", which identify faults existing at the time of the diagnostic session, and "history," which identify faults which do not exist during the session but did exist at some time in the previous 99 ('90-'91) or 49 ('92-'95) ignition cycles. There are two ways to retrieve DTCs: 1) using a scan tester, a hand-held computer dedicated to automotive diagnostic work and which is more common than the ABS Tester or 2) using either "flash code" diagnostics or "CCM diagnostics".

 Scan testers communicate with the EBCM or EBTCM and can read DTCs, display ABS parameters in real time, display a limited history of conditions present when a DTC sets, record a "snapshot" of data for use in solving problems and take control of certain system functions for diagnostic purposes. A "scanner" is the best way to diagnose ABS/TCS problems. Some may scoff at professional-grade equipment for DIY work, but trust us–diagnosing ABS/TCS is easiest with a tester and, once you have it, you can, also, given the proper software, diagnose engine and body computer problems. Best scanners for C4 owners are the TECH1 or Mastertech, both manufactured by Bosch Diagnostics or that company's predecessor Vetronix Corporation. The "T1" was the GM dealer tester from 1985 to 1997, but is now out of production and no longer supported. The Mastertech is Bosch's high-end aftermarket tester which shares all T1 functions and has significant additional abilities. Both, when equipped with chassis software, support '90-'95 ABS/TCS. New Mastertechs are available from Bosch. Both are available on the used market. Other testers support C4 ABS, such as units from Mac Tools, but we use a Mastertech because Bosch's relationship with GM makes its equipment the best developed for diagnosing Corvette computer systems. There are more-affordable, "consumer-grade" testers on the market but none support ABS/ASR diagnostics on a ZR-1.

"PC Scan Tool", is a software-based scan tester published by EASE Diagnostics which runs on a personal computer and supports '90-'96 ABS. We evaluated PC Scan Tool on an Acer 5672 notebook PC running Windows XP Home and found it very good good for C4 ABS/TCS diagnosis. EASE PC Scan Tool is a great value if you already own a laptop and want a full suite of Corvette diagnostic software which supports not only ABS/ASR but also the engine and body computers.

No Tester? Watch the Flashes

If you have a '90 or '91 but no scan tester, enable flash code diagnostics, by jumping pin H of the diagnostic link connector (DLC) to pin A or a chassis ground and then keying-up. The Service ABS light will come on for 4-sec., then start blinking two digit DTCs. Each digit is signified by half-second-apart flashes. One second separates the digits. Codes are flashed three times with a 3-sec. pause between. Each sequence is bracketed by code 12, signifying the sequence's start and finish. Say you have DTC21, a right front wheel sensor fault. The light flashes once, then twice (12), three times in succession. Next, it flashes twice, then once (21), repeating that three times. Finally, it flashes code 12, again, three times in succession. Numerically, it's: 12-12-12–21-21-21–12-12-12. A quirk of flash code diagnostics is five current codes which cannot be read so, if the Service ABS light is on, but you only get code 12; you need a scanner.

 Use CCM (Central Control Module) diagnostics if you're working on a '92-'95 and lack a tester. To put the CCM in diagnostic mode, with a '92 or '93 having a 12-pin DLC, jump pin-G to pin-A or a chassis ground or, on '94-'95 16-pin DLCs, jump pin-12 to pin-4, pin-5 or a chassis ground, then key-up. The IP display will show fault codes and the module in which they are stored. ABS/TCS is module nine. DTCs show on the instrument panel display where vehicle speed is normally seen. An extensive discussion of CCM diagnostics is in the Service Manual for the model year in question.

To make flash-code and CCM diagnostics work, you've got to jumper across DLC pins. In most C4s, it's tough to see the diagnostic connector well enough to count pins. Corvette Central has the solution: grounding plugs which snap onto the DLC. We've used a set of these plugs '94-'95 (PN 304800) and they are a great idea. You simply push one on the DLC, then key-up–way easier than fussing with jumper wires. A set of them (PN 304821) is, also, available for '90-'93, 12-pin DLCs.

What to do with DTCs once you have them? Start with the FSM or, for '90 or '91, its Section 8A Supplement. Troubleshooting information is arranged by code and presented in a flow-chart or "trouble-tree" form where an action is specified followed by a question. The next action is based on the answer. You move through the flow chart, step-by-step until you resolve the problem. When attempting to solve an ABS problem, keep in mind that the majority of automotive electronics faults are caused by wires and connections, so always look for damaged wires, loose fuses, faulty connectors, relays not properly seated, corroded contacts and that sort of thing. A common C4 ABS problem is a loose chassis ground connection. In a coupe, the ABS ground is on the left B-pillar and, in a convertible, it's on the right side of the cargo area. Always check them.

            Some solutions require a scan tester, in which case, if you're working without one, you'll have to either acquire the equipment or take the car to a service facility which has it. Some DTCs require a digital multimeter (DMM) which is used to measure voltage, resistance and other electrical parameters and should be a part of any serious DIY’s tool box. We prefer the Fluke 88V, which is designed specifically for automotive work, or the Fluke 87, the GM dealer DMM (PN J-39200-A). They're built to service trade standards and they have a record feature which can be useful when troubleshooting elusive, intermittent ABS faults. There are less expensive DMMs than the Flukes–the Actron CP7677 is a good one–but whatever the price range, keep in mind a key DMM property: impedance. Do not use a DMM with less than 10 megohm input impedance for automotive service work.

Other special tools some DTC solutions may require are: for the "Breakout Box" (PN J-39700). Each connects between the controller and the wiring

harness to allow easy testing of circuits, some of which may need to be energized during testing. The later box, also, requires an adapter, J 39700-10. In some situations, you can achieve similar tests by carefully connecting the DMM to harness connector pins, however, in other situations, you cannot perform the required test without the breakout box and, in any event, the box is always easier to work with. When your diagnosis leads you to these tools, take the car to a service facility which has one or, if you're a well-heeled DIY wanting either of these tools, order them from SPX Kent-Moore, the supplier of GM service tools.   

 According to David Fulcher, a Chevrolet dealer service technician well-known to CAC members, told us, "In the rear, the exposed sensors tend to accumulate metal filings from the rear brakes. After a while that can start to knock out the signal. The early ('86-'89) front units had a problem with tips breaking off if you hit a curb or similar object. That is one of the reasons I believe that GM moved the sensor into the hub assembly." Early style front wheel sensors and all rear sensors are inserted into holes in the suspension knuckles. If you're directed to replace a sensor, you may find it "frozen" due to galvanic corrosion which was a problem with early, metal-bodied sensors. The prescribed method of removal is to first, apply a good penetrate, such as "Sili-Kroil", available from the Eastwood Company, let it sit several hours, then apply a modest amount of heat to the surrounding structure using a propane torch or similar device. Start by rotating the sensor to loosen it and eventually it will come out. The heat usually ruins the sensor but if it's bad to begin with, so what.  

For replacement sensors, '90 fronts are no longer available from GM, however, NAPA, one of the better sources for C4 ABS hardware, sells replacements. The '91-'95 front sensor is part of the front hub and that assembly, along with rear sensors for '90-'95, are still available from GM parts sources such as Tom Henry Racing. On '92-up cars with traction control, occasionally, there can be a problem with the throttle cable system adjustment. Anytime you're troubleshooting a complaint of poor engine performance, check the adjustment of the all the throttle cables. Instructions are in the Service Manual.

                                   Parts Sources

With the first of the C4 antilock systems now 21 years old, we're happy to report that their major components are, generally, quite reliable. If an EBCM or EBTCM fails, it's usually due to negligence or abuse such as: water damage or voltage spikes caused by incorrect jumper cable connections, grounding of the battery or use of a charger/booster to start the car. Another threat to EBCMs, or any on-board electronics for that matter, is welding anything–a roll bar/cage, exhaust hangers and brackets, suspension modifications or whatever–to the car's structure. Welding causes damaging voltage spikes in the car's electronics, the extent of which depends on where a welding ground is attached. Welding can fry an ABS controller even with both battery cables disconnected. To be safe–especially, considering the cost of ABS controllers and that some are no longer available–disconnect the ABS/TCS wiring harness from EBCM or EBTCM before welding.

New modules for MY90-'95 are available from GM Parts sources such as Corvette specialist, Tom Henry Racing. Remanufactured units are available from

NAPA stores or its web site, www.napaonline.com. NAPA remanufactures the '90-'91 and '92-'94/'95 ZR-1 controllers. Prices start at about $500 for remanufactured controllers and go to $1500 for new units. Other system electronics, such as relays, wheel sensors, wires and connectors, are a little more likely to experience trouble, but are still pretty reliable and can be purchased from GM Parts sources or the aftermarket. The lateral acceleration accelerometer ('90-'93 and '94-'95) occasionally fail and all these have been discontinued by GM. The only sources for these parts are new-old-stock at Chevrolet dealers or serviceable used pieces from Corvette parts recyclers or salvage yards.

Brake pressure modulators have excellent long-term durability provided the brake fluid gets flushed occasionally. If you have to replace a BPM, new '92-'95 units can be had from GM Parts sources. BPM's are not field-serviceable, but NAPA currently remanufactures the '90-'91 and plans to add a remanufactured '92-'95 BPM. Prices for BPMs range from 500-1000 bucks for reman and go well over a thousand for new units.

Considering those prices, it's wise to enhance BPM life with the following procedures. First: when you change brake pads, never push brake caliper pistons back without opening the caliper bleed screws. Failing to do that forces old brake fluid to back-flow through the BPM. That fluid will be contaminated with dirt, corrosion or moisture and, because fluid back flow is not filtered, contamination could lodge in a solenoid valve and, eventually, cause the valve to stick. The solenoid valves are normally-open, however after an ABS application, in the very rare case a valve sticks in the pressure hold or decrease positions, base brake system performance may degrade. The symptoms would be the service light on and the brakes pulling to one side.

Secondly, we suggest you flush your brake

system, per the Service Manual, every two years. Occasional flushes keep the contaminant level low and ABS operating properly, as well as improving base brake system durability. At minimum, use a brake fluid that meets the DOT-3 specification. If you drive your car hard, use a premium brake fluid which meets DOT-4 or DOT-5.1 such as Motul "DOT5.1" fluid. Do not use silicone-based fluids which meet DOT-5.0. They are not compatible with antilock brake systems and will cause inconsistent and unsafe ABS operation.

What about "Why"

We've covered what makes up a C4's antilock braking system, when it works, how it works, what to do when it doesn't work, but we haven't talked much about why ABS is such a great feature. Recall that a tire generates maximum braking traction when it exhibits negative slip but is not fully-locked-up. Since ABS allows 99.5% of Corvette drivers to "threshold brake" more consistently, in an ABS-equipped car, you're going to be able to brake harder when driving aggressively. You're, also, going to be able to have maximum braking on surfaces with poor traction when, otherwise, threshold braking would be difficult if not impossible. Additionally, with a '92-'96's traction control, on slick surfaces, you're going to be less liable to get the rear wheels spinning.

 A tire under optimum braking is still capable of significant lateral traction, whereas one that's locked-up is incapable of any "lat" at all. Since ABS can only control the brakes, it gives priority to cornering (lateral acceleration) first. Hence, you can still steer the car while in full antilock braking. In addition, the front wheels have antilock braking on an individual basis, allowing safe, controlled stops on "split-mu" surfaces were one tire is on a surface that is more slick than the other.

Bottom Line: In a panic stop, instead of the wheels locking and steering control being lost, ABS provides maximum braking while enabling the driver to maintain steering control and that, for most of us, is the greatest single advantage of antilock brakes!

Thanks: The following people contributed to this article:

Chris Petris, technical writer and owner of the Corvette Clinic; David Fulcher, dealer service technician; Scott Allman, former Lead Engineer for Corvette ride and handling at GM and currently Principle Engineer, Vehicle Dynamics Group, Harley-Davidson Motor Company; Jack Gillies, Brake System Integration Engineer, GM High-Performance Vehicle Operations (now retired); Marc Haibeck at Haibeck Automotive Technology; Stan Lorence, Parts Department Manager at Tom Henry Chevrolet/Tom Henry Racing and Brad Thatcher, Service Program Manager, Performance Cars at GM.  Lastly, this CAC article was adapted from a two-part series published in Corvette Enthusiast magazine in its November and December 2007 issues. The CAC would like to thank CE Editor, Andy Bolig, for his support in allowing us to repost this article.

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