ABS light erratic, no ‘trouble code’ in car computer

Hey there, time traveller!
This article was published 27/09/2002 (8477 days ago), so information in it may no longer be current.

QUESTION — I have a ’93 Buick LeSabre with 173,000 km. Recently my anti-lock brake light (ABS) came on for a few minutes, then turned off. A week later, it came on again. I took it to the service station. The diagnostic scanner gave no “code”, but the alternator voltage was fluctuating so they replaced it. A week later, the light came on again with no code. The service station checked all wheel sensors and replaced the ABS module. The light was still coming on. A GM dealer suggested I disconnect the remote starter, because it taps into the brakes. This week, the ABS light turned off… usually it’s always on. Any ideas?

ANSWER — There are several faults that will keep the ABS light on, but most of these should set a trouble code in the computer, including a problem with the brake switch circuit. Most generic scan tools do not have the capability to read ABS system codes, so you may need to go to a GM dealer. I am assuming there are no codes in the computer.

The light can be turned on without any codes being stored three ways. First, if the wire from the instrument panel ABS indicator light to the electronic brake control module (EBCM) has a short to ground, the light will come on. This can be tested by disconnecting the EBCM, and disconnecting the ABS main power relay located in the underhood fuse/relay centre. There is a legend on the relay cover, but identify the correct relay by wire colours: red, tan, dark blue, and brown with white stripe wiring.

If the light is still on, then the warning light ground circuit is shorted to ground. This short could happen in the ABS pump relay, pressure modulator valve, or the wiring going to any of these devices. Removing the ABS diode connector (white connector with two wires) will disconnect the pump relay and modulator from the circuit, but the diode connector is up under the dash on the passenger side above the ABS computer and not easy to find.

That was the hard test. The easy solution could be a faulty ABS main power relay or electrical connector. The system is designed so that if the ABS power is interrupted, the ABS light gets a ground through the main power relay to warn the driver the system is inactive. Check the relay connector and try changing the relay.

Finally, the ABS computer could be grounding the circuit as it would normally if there was a problem, but no codes are stored. I had this happen to me on a 1993 Oldsmobile, where the light would not go out and the computer was the problem. I replaced the computer and it still wouldn’t go out. A third computer fixed the car, and I never did find out what caused the first two to act up. Replacing the computer again is a last resort, so try the other tests first.

QUESTION — I have a 1992 Nissan Maxima car with 190,000 km on it. When I first start it and drive for a while, the car is bucking and seems to lose power. The RPM almost drops to zero but the car never stalls. If I “tramp” on the accelerator, it surges and the RPM is erratic. After driving for a period of time, the car operates normally. Not sure what could cause this — water in the gas, fuel pump starting to fail, or… ? Please advise as to what you may think the cause is.

ANSWER — I would test the fuel pressure first. All these symptoms can be caused by low pressure. If the pressure is good, then the problem is likely with one of the input sensors to the engine computer. Either the throttle position sensor and mass airflow sensor could be the problem. The throttle position sensor wears as mileage accumulates, so this is often replaced on higher-mileage vehicles. I have had a few problems with contaminated mass air lead flow sensors on high-mileage vehicles too. The sensor filaments get coated over time. Fortunately, Nissan has made these sensors easy to access on the vehicle and relatively economical to buy. There may also be a code stored in the engine computer that would lead you to one of these sensors first, but many times there is none.

Jim Kerr is an experienced mechanic, instructor of automotive technology and freelance journalist. You can e-mail questions to Jim at the address below.

kerr.jim@sk.sympatico.ca