By Core Brake Parts Tech Team — Circleville, OH warehouse
An ABS light that stays on in a 1991-1993 Mitsubishi 3000GT almost always traces back to one of four things: a wheel speed sensor with a weak or missing signal, a damaged sensor wire or connector, low brake fluid from a leak, or a fault in the hydraulic unit, pump motor relay or ABS control unit. The factory service manual gives a clear order for checking them, and most of the time the fix is a sensor, a harness repair or a leaking brake hose rather than the ABS computer itself.
The good news: when the ABS light is on, the car still has normal power-assisted brakes. What you lose is the anti-lock function in a panic stop. The bad news: on a car this age, the same problem that turns on the ABS light can also be hiding a fluid leak, so it is worth diagnosing now rather than living with the light.
How the 3000GT ABS Light Is Supposed to Behave
Before chasing a fault, confirm the light is actually misbehaving. The 1991 factory service manual describes a self-check at every start. On the all-wheel-drive cars, the ABS light comes on when the key goes to ON, flashes about four times in roughly one second while the valve relay checks itself when you crank the engine, repeats that sequence when the key returns to ON, and then goes out. You may hear the valve relay click in time with the flashes. That is normal.
The manual separates the problems into three patterns, and each points somewhere different:
| What the light does | What it usually means |
|---|---|
| Never comes on, even at key-on | Burnt-out warning bulb, blown fuse, or an open circuit between the combination meter and the ABS unit. The system may be faulted without telling you. |
| Stays on after the engine starts | The ABS control unit has found a fault and shut ABS off. A trouble code is stored. |
| Comes on while driving, goes out after a key cycle | An intermittent signal, most often a wheel speed sensor wire that opens up when the suspension moves. |
The factory flowchart also has an end point many owners never reach: after a test drive, if self-diagnosis shows no trouble codes and no stored history, all ABS functions are considered normal. Do not replace the ABS control unit until the sensors and wiring have been checked.
Wheel Speed Sensors: The Most Common Cause
Each wheel has a magnetic speed sensor that reads a toothed sensor rotor turning with the hub. The first five trouble codes in the manual are all speed-signal faults:
- Code 11 - front right sensor open
- Code 12 - front left sensor open
- Code 13 - rear right sensor open
- Code 14 - rear left sensor open
- Code 15 - vehicle speed sensor signal abnormal
The manual sets these when the signal is missing, shorted, or too weak when you start to drive or while driving. Its list of causes is worth reading because only some of them are the sensor itself:
- A failed sensor (open coil or weak output)
- Too much gap between the sensor pole piece and the rotor teeth. The factory specification is 0.2 to 0.7 mm (.008 to .028 in.).
- A hub with excessive runout, which makes the gap change as the wheel turns
- Missing, cracked or damaged teeth on the sensor rotor
- A broken wire or connector in the harness
- A sensor that is not mounted correctly, which is common after a hub, knuckle or axle job
- Electrical noise interfering with the signal
The manual's test is simple enough for a DIYer with a meter: with the ABS unit's harness disconnected and the wheel off the ground, spin the wheel by hand at about half a turn to one turn per second and measure the sensor output. The factory figure is 70 mV or more on a circuit tester, or about 100 mV peak on an oscilloscope. Low output means either the gap is too large or the sensor is weak. Compare all four wheels; the odd one out is your suspect.
One detail from the manual explains many "ghost" ABS lights on these cars: the sensor cable moves with the suspension. A wire can test fine in the driveway and open up over bumps. If the light only comes on over rough roads, inspect the sensor leads where they flex near the strut and knuckle, and look for chafing against brackets and old repairs with electrical tape.
Low Brake Fluid and the Front Hose Recall
Low fluid will light the red brake warning lamp, and a leak often gets noticed at the same time as an ABS complaint. On 3000GTs built between August 1, 1990 and November 4, 1993, there is a specific reason to look: NHTSA recall 96V-143 covered front wheel-side brake hoses that could crack under full steering lock and full suspension travel, leaking fluid and reducing braking. The remedy was revised left and right front hoses. Roughly 10,000 cars were covered.
Even if the recall work was done decades ago, those hoses are now very old rubber. Turn the wheels lock to lock and look at the front hoses for cracks, bulges and wet spots. Replacement brake hoses are inexpensive insurance on a car this age, and if you are redoing the whole system, stainless brake lines are an option.
Fluid condition matters too. The factory service manual calls for DOT 3 brake fluid and warns against mixing fluids. Glycol-based DOT 3 is hygroscopic: brake-system maker Tilton notes it can absorb 1 to 2 percent water a year, and that a DOT 3 fluid's minimum dry boiling point of 401 degrees F drops to a minimum wet boiling point of 284 degrees F once it is saturated. Water in the fluid also corrodes the valves in the ABS hydraulic unit. If the fluid in the reservoir is dark, flush it. The manual also notes that on ABS cars you should keep the filter fitted in the reservoir when adding fluid, and bleed the brake lines with the engine running in the order the manual shows.
Hydraulic Unit, Pump Motor and ABS Control Unit Faults
If the codes point away from the sensors, the next suspects are in the hydraulic unit circuit. Code 52 in the manual, for example, is a motor relay fault. It sets when the pump motor relay does not work, the motor will not run, the motor ground is disconnected, or the motor runs continuously. The manual's checks are a relay resistance test, confirming the pump motor ground strap is connected, and checking for battery voltage at the hydraulic unit connector. A loose or corroded motor ground is a cheap fix that is easy to miss.
The manual also lists symptoms that point to the hydraulic side rather than the electrical side: the car pulling to one side under braking, low braking force, or weak ABS action. These can come from a clogged line or a solenoid valve sticking inside the hydraulic unit, which old fluid encourages.
Replacing the ABS control unit is the last step in every factory flowchart, not the first. The manual repeatedly directs the technician to check power at the unit's ignition feed, confirm the grounds, check harness continuity, and erase the codes and retest. Only if every input checks good and the fault returns is the control unit condemned.
How to Read the Codes and Work the Problem
The factory procedure uses the Mitsubishi MUT scan tool through the diagnosis connector to read ABS codes, force the hydraulic unit to operate, and confirm each wheel responds. A shop with a tool that covers early-1990s Mitsubishi ABS can do the same. Without one, you can still work the likely causes in order:
- Confirm the light's pattern at key-on and start (never on, always on, or intermittent).
- Check the fluid level and look for leaks, starting with the front hoses.
- Inspect all four sensor leads and connectors, especially where they flex with the suspension.
- Pull each sensor, clean the tip and the rotor teeth, and look for damaged teeth.
- Measure sensor output at the ABS unit connector with the wheel spinning by hand.
- Check fuses, the pump motor relay and the hydraulic unit ground.
- Erase codes, test drive, and recheck before replacing anything expensive.
What to Replace While You Are In There
Several brake jobs put the ABS sensors at risk. Removing a hub, knuckle or rear axle shaft means handling the sensor and its rotor, and a sensor that is not seated at the right gap will set a code on the first drive. If you are replacing pads and rotors anyway, it is a good time to inspect sensor wiring and hoses.
The manual's wear limits for this car are specific: the front pads have wear indicators that squeal when the pad reaches 2 mm, and the rear pads have a 2.0 mm replacement limit against a 10.0 mm new thickness. If your car is due, our 1991-1993 Mitsubishi 3000GT front and rear brake kit covers pads and rotors on both axles, and you can browse more brake pads and brake rotors. If a caliper is sticking or leaking, see our brake calipers.
Difficulty: checking fluid, hoses and sensor wiring is an easy driveway job. Measuring sensor output needs a multimeter and the wiring diagram. Hydraulic unit and control unit diagnosis is best left to a shop with a compatible scan tool, and any work that opens the ABS hydraulic circuit needs a correct bleed afterward.
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Shop Brake Parts →Frequently Asked Questions
Is it safe to drive a 3000GT with the ABS light on?
The normal brakes still work, but anti-lock is off, so the wheels can lock in a hard stop. If the red brake warning light is also on, or the pedal feels soft or low, stop driving and check for a fluid leak before going further.
Why does my ABS light only come on over bumps?
The factory manual points out that the wheel speed sensor cables move with the suspension. A cracked wire or loose connector can open up over rough roads and test fine when the car is parked. Inspect the leads where they flex.
Can new brake pads or rotors turn on the ABS light?
Pads and rotors themselves do not, but the work around them can. A sensor wire pulled or pinched during the job, a sensor knocked out of position, or a damaged sensor rotor after hub or axle work are common causes.
What brake fluid does a 1991-1993 3000GT use?
The factory service manual specifies DOT 3 and warns against mixing brake fluid types. Use fresh fluid from a sealed container.
Do I need to replace the ABS computer?
Rarely as a first step. The factory procedure checks sensors, wiring, fuses, relays and grounds first, then erases the codes and retests. The control unit is replaced only after every input checks good and the fault comes back.
Sources
- 1991 Mitsubishi 3000GT Service Manual, ABS troubleshooting (AWD) - normal ABS warning light self-check and fault patterns
- 1991 Mitsubishi 3000GT Service Manual, codes 11-15 - wheel speed sensor codes and their causes
- 1991 Mitsubishi 3000GT Service Manual, wheel speed sensor output - 70 mV / 100 mV output test
- 1991 Mitsubishi 3000GT Service Manual, waveform troubleshooting - sensor cable movement with suspension, causes of low output
- 1991 Mitsubishi 3000GT Service Manual, specifications - 0.2-0.7 mm sensor-to-rotor clearance
- 1991 Mitsubishi 3000GT Service Manual, post-test-drive check - no codes and no stored history means ABS is normal
- 1991 Mitsubishi 3000GT Service Manual, code 52 - pump motor relay fault checks
- 1991 Mitsubishi 3000GT Service Manual, bleeding and pads - DOT 3 fluid, reservoir filter on ABS cars, 2 mm pad wear indicator
- 1991 Mitsubishi 3000GT Service Manual, rear pads - 10.0 mm new and 2.0 mm limit for rear pads
- NHTSA recall 96V-143 (via Justia) - front brake hose cracking, production dates and remedy
- Tilton Engineering: Brake Fluid - DOT 3 boiling points and moisture absorption
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