P1638 what is alternator terminal F

DTC P1638
The powertrain control module (PCM) uses the generator field duty cycle signal circuit to monitor the duty cycle of the generator. The generator field duty cycle signal circuit connects to the high side of the field winding in the generator. A pulse width modulated (PWM) high side driver in the voltage regulator turns the field winding ON and OFF. The PCM uses the PWM signal input to determine the generator load on the engine. This allows the PCM to adjust the idle speed to compensate for high electrical loads.
The PCM monitors the state of the generator field duty cycle signal circuit. When the key is in the RUN position and the engine is OFF, the PCM should detect a duty cycle near 0 percent. However, when the engine is running, the duty cycle should be between 5-100 percent. The PCM monitors the PWM signal using a key ON test and a RUN test. During the tests, if the PCM detects an out of range PWM signal, DTC P1638 will set. When the DTC sets, the PCM will send a class 2 serial data message to the IPC to illuminate the charge indicator.

Diagnostic Aids
• DTCs P1637 and P1638 may set at the same time depending on the failure.
• Use a scan tool in order to diagnosis the generator circuits. The scan tool should display Inactive for the L terminal and 10-40 percent for the F terminal when the ignition is ON. When the engine is operating, the display should indicate that the L terminal is Active and the F terminal is more than 10 percent.
• A generator fault, such as a shorted output diode, malfunctioning regulator, open or shorted rotor, or open sense lead or battery feed may cause a low battery charge. Refer to Charging System Test for further information and generator diagnosis.

What make , model an year vehicle ?
Generator L Terminal P1637? / Possible alternator problem The powertrain control module (PCM) uses the generator turn on signal circuit to control the load of the generator on the engine. A high side driver in the PCM applies a voltage to the voltage regulator. This signals the voltage regulator to turn the field circuit ON and OFF. When the PCM turns ON the high side driver, the voltage regulator turns ON the field circuit. When the PCM turns OFF the high side driver, the voltage regulator turns OFF the field circuit. Strongly suggest you take your vehicle to a ASE certified repair shop or the dealer .

If these codes are being only set during start I would expect a failure of the battery. If the voltage goes much below 9 VDC the computers (EMS) can become erratic and codes and limp home modes are a common result of defective and discharged batteries!



Careful having the battery tested. Any arcing etc at the battery can be the kiss of death for the engine computer. Better to remove terminal to check. Or test the crank voltage with multi-meter.

Both wires are red. 10 ga wire runs from Battery + terminal to Batt.
terminal on alternator.
Smaller wire routes 2 different ways.
On V8, runs through cluster to L terminal on alternator.
On V6 runs through cluster to PCM terminal 16B-- comes out from
terminal 35B to L terminal on alternator

Alternator F terminal circuit. Check your plug-in in the Alternator case. The field wire is not connecting. You could have a bad supply wire from the Ignition switch or a Relay.

Check with Autozone.com and register your vehicle for free wiring diagrams and fusebox layouts. Some GM vehicles use Relays or fuses in the charging circuit.

P1638 Instrument to ECM , CAN (control area network) short circuit failure,this is either the instrument panel at fault or the ECM.Basically its a open wire. between ECM and instrument panel

I assume a new regulator has pased QC and is OK. Is your volt meter accurate? Just a tip - we do not test alternator by removing terminal. That burns the alternator.

Are you getting the 12.1 v from alternator at alternator end or at the battery terminal? Check the connection between the + terminal on alternator and the battery. That wire might be disconnected at the starter motor.

CODE P1637 - GENERATOR L TERMINAL CIRCUIT MALFUNCTION CONDITIONS. CODE P1638 - GENERATOR F TERMINAL CIRCUIT MALFUNCTION CONDITIONS. POSSIBLE CAUSES FAULTY GENERATOR WHICH IS ALTERNATOR.CHECK GENERATOR POWER FEED WIRES AND CHECK GENERATOR VOLTAGE REGULATOR WIRES FOR LOOSENESS AND DAMAGE. MAKE SURE BATTERY VOLTS ARE 12.6 VOLTS. IF BATTERY WEAK DUE BAD GENERATOR VECHICLE WONT START.

Fault codes are listed in the technical handbook, no good to you as it's a Jaguar service item unless you can get a copy of JTIS from ebay etc.


Fault Code:- P1638 Indicates a Power-train fault


The fault is generally due to a problem in the Computer & Output Circuit area


Fault Description: CAN ECM / INST network malfunction

Action of ECM:- none


After clearing the DTC, complete the Ignition ON 10 seconds
Check for a reoccurrence of fault DTC.

Possible Fault:- CAN open circuit fault - INST to ECM CAN short circuit fault INST failure ECM failure D