Power Steering
Power steering has split into two worlds: hydraulic systems with pumps and fluid, and electric systems with torque sensors and software.
- Low or wrong fluid is the most common cause. Always check level and confirm correct fluid type before replacing components.
- EPS uses a torque sensor (usually in the column) to detect driver input and modulate the assist motor's output.
- EPS torque sensors typically require a zero-point calibration relearn after replacement, done with a scan tool.
You need the diagnostic habits for both — checking fluid before condemning pumps on one side, and running scan-tool calibrations on the other.
Hydraulic Systems: Fluid First, Always
When a hydraulic power steering system whines and the steering goes heavy, the first check is always the fluid — both level and type. Low fluid is the single most common cause of pump whine and hard steering: the pump pulls in air, cavitates, and howls, while assist fades. Wrong fluid is nearly as common a culprit, because the wrong chemistry can swell seals, foam, or fail to lubricate the pump. Verify the level, verify you're looking at the manufacturer-specified fluid, and find where any missing fluid went (a leak that emptied the reservoir will empty it again) before you spend the customer's money on a pump or rack. Replacing components before checking fluid is the classic rookie sequence — components rarely fix a fluid problem, and fluid often fixes a 'component' problem.
Electric Power Steering: Torque Sensors and Relearns
Electric power steering (EPS) replaces the pump and hydraulics with an electric assist motor on the column or rack. The key input that makes the whole system work is the steering torque sensor, usually mounted in the steering column (sometimes at the rack). It measures how hard the driver is twisting the wheel, and the control module uses that signal — along with vehicle speed — to command how much assist the motor delivers. No torque signal, no assist; a skewed torque signal, and the car pulls or assists unevenly.
Because the torque sensor's zero point is everything, replacing an EPS module or torque sensor is not plug-and-play. A calibration or relearn procedure with a scan tool is typically required — most systems need a torque sensor zero-point calibration so the module knows what 'no driver input' looks like. Skip the relearn and the customer gets uneven assist, a pull to one side, or a steering warning lamp. Treat every EPS component replacement like a job that ends at the scan tool, the same way every strut job ends at the alignment rack.
| Item | Typical spec | Notes |
|---|---|---|
| Idle pressure, wheels straight | Roughly 50-150 psi | High idle pressure = restriction in the system |
| Relief pressure at full lock | Commonly 1,000-1,500 psi | Test momentarily — never hold full lock more than a few seconds |
| Fluid specification | Manufacturer-specific: ATF, PSF, or CHF synthetics | Wrong chemistry swells seals and foams |
| Whine after service | Air in the system | Re-bleed before condemning the pump |
| EPS component replacement | Torque sensor zero-point relearn required | Scan tool procedure; skipping it causes pull or uneven assist |
Pressure specs vary by system — verify against service data and use the correct gauge adapters.
- Check fluid level and identify the fluid before anything else. Low fluid makes the pump cavitate and howl while assist fades, and wrong fluid foams and attacks seals — both are cheaper than any component.
- If fluid is low, find where it went: rack boots (squeeze them), pump shaft seal, hose crimps, and the cooler. A refill without the leak found is a refill you will do again.
- Inspect the belt and tensioner — a glazed, slipping belt drops assist under load and gets misdiagnosed as a dying pump.
- Bleed the system lock-to-lock per procedure. A whine that persists after bleeding means air is still there — check the suction side for a leak drawing air in, because pressure-side leaks drip and suction-side leaks aerate.
- Pressure-test with a gauge: read idle pressure, then relief pressure at momentary full lock. Low relief pressure means a worn pump; healthy pump pressure with heavy steering means the rack is bypassing internally.
- On EPS, scan for codes, watch torque sensor data for a skewed zero point, and run the relearn procedures — the scan tool is the pressure gauge of electric steering.
- Replacing the pump before checking fluid level, fluid type, and bleed state — components rarely fix a fluid problem, and fluid often fixes a 'component' problem.
- Topping off with whatever ATF is on the shelf. The wrong chemistry swells seals and foams, and the slow failure never gets connected back to that one courtesy top-off.
- Holding the wheel at full lock during pressure testing or bleeding for more than a few seconds — the pump sits at relief pressure and cooks itself.
- Skipping the torque sensor relearn after EPS work — the module's idea of 'no driver input' is skewed, and the customer gets uneven assist or a steady pull.
You install a new steering rack and now the pump whines. The old rack never whined. Is the new pump-and-rack combination faulty?
Almost certainly not — the system is full of air from being opened. Air makes the pump cavitate, and cavitation whines. Repeat the bleed procedure, check the suction line connections for leaks pulling air in, and let aerated fluid settle before re-testing. Condemning parts for an incomplete bleed is a classic misstep.
Pump pressure tests healthy at relief, but the steering is heavy in both directions. Where is the problem?
Inside the rack. The pump is delivering; the rack's internal seals are bypassing fluid past the piston instead of turning it into assist. Heavy steering with good pump pressure is the rack's signature — reseal or replace the rack, not the pump.
After a steering column replacement on an EPS car, the vehicle pulls lightly right with no codes. What procedure was likely skipped?
The torque sensor zero-point calibration. The new column's sensor reports a slight twist as 'driver input,' so the module quietly adds assist in that direction and the car pulls. Run the relearn with the scan tool so the module learns what hands-off actually looks like.
You install a new steering rack and now the pump whines. The old rack never whined. Is the new pump-and-rack combination faulty?
Missed one? The reasoning above comes straight from the SUS exam bank, so this is the standard you will be held to.