Faulty alternator voltage regulator (internal or external); Poor or corroded earth connections or battery cable terminals causing wrong voltage sensing; Wiring fault or poor connection in the field control circuit between the PCM and alternator; Failing or internally shorted battery giving incorrect system voltage feedback; PCM driver fault controlling the alternator field; Outdated PCM calibration where a manufacturer bulletin lists it as a known cause.

P1594

Charging System Voltage Too High.

Plymouth
Powertrain
Manufacturer code
Medium severity

How this code is built

P: SystemPowertrain
1: Defined byThe manufacturer
5: AreaVehicle speed, idle control and auxiliary inputs
94: FaultThe specific fault in that area

What it means

P1594 on a Plymouth means the powertrain control module has seen the charging system voltage rise above its upper limit. The module monitors battery or system voltage and sets the code when it stays too high, which points to over-charging by the alternator or its regulation. Exact thresholds and regulation strategy vary by model and engine.

Symptoms

Battery or charging warning light on, or check engine light on
Headlights and interior lights unusually bright or flickering
Bulbs blowing prematurely
Battery hot, swollen or smelling of sulphur, or needing frequent topping up if serviceable
Voltmeter reading high if fitted
Erratic behaviour of electrical accessories or instruments

Possible causes

Faulty alternator voltage regulator (internal or external)likely
Poor or corroded earth connections or battery cable terminals causing wrong voltage sensinglikely
Wiring fault or poor connection in the field control circuit between the PCM and alternatorpossible
Failing or internally shorted battery giving incorrect system voltage feedbackpossible
PCM driver fault controlling the alternator fieldrare
Outdated PCM calibration where a manufacturer bulletin lists it as a known causerare

How to diagnose

Read stored and pending codes and freeze frame data, noting voltage, engine speed and load when the code set
Inspect battery terminals, earth straps and alternator wiring and connectors for corrosion, looseness or damage
Test battery condition and state of charge, and check for signs of overheating or swelling
Measure battery voltage at idle and at raised engine speed with accessories on and off, and compare with the specification for the model
Check voltage drop on the positive and earth charging circuits to rule out poor connections
Test the alternator field control circuit for continuity, shorts and correct PCM command, following the model wiring diagram
Check for manufacturer service bulletins or calibration updates for the vehicle and engine
Check the alternator output and regulator operation using the model-specific procedure

How to fix

Replace the alternator or voltage regulator if output is above specification
Clean and tighten battery terminals and earth connections, and repair or replace damaged cables
Repair wiring or connector faults in the alternator field control circuit
Replace the battery if it is faulty or damaged by over-charging
Update the PCM calibration only if a manufacturer bulletin identifies it as a cause
Replace the PCM only after all wiring and charging components are proven good

Can I keep driving?

Medium severity

Short journeys are usually possible, but sustained over-voltage can damage the battery and electronics, so get it checked soon. Stop driving if the battery is hot, swollen or smelling strongly, or if lights are blowing.

Guide written by AI from the code's definition. Confirm with a diagnosis before replacing parts.