'91 XJ MIL Code 41

"An open or shorted condition detected in the generator field control circuit."

How would you guys go about diagnosing this? Its the girlfriend's XJ, and she has had sporadic dead batteries lately. Not dead while driving, dead in the morning. I'm curious if this has something to do with it, or if it was just a fluke. Her gauge cluster volt meter always reads high, like border of the red, sometimes ventures slightly into, but when I checked it with a real multimeter it only reads 13.8 volts which is right on the money.

She said the CEL came on for one trip, but is no longer lit up and the only two codes the CEL flashed were 12 and 41.

I just went out and ran it and its still charging normally, so if its actually an issue its intermittent. :gonnablow

I have seen 41 quite a number of times and if memory serves me, it was almost always a failing alternator.

Older Model Computer Codes (OBDI) - IMPORTANT! Codes may be different for newer vehicles starting circa 1996.
11 No ignition reference signal detected during cranking OR timing belt skipped one or more teeth; OR loss of either camshaft or crankshaft position sensor.
12 Battery or computer recently disconnected
13 MAP sensor or vacuum line may not be working
14 MAP sensor voltage below .16V or over 4.96V
15 No speed/distance sensor signal
16 Loss of battery voltage detected with engine running
17 Engine stays cool too long (bad thermostat or coolant sensor?)
OR
17 (1985 turbo only): knock sensor circuit
21 Oxygen sensor signal doesn't change (stays at 4.3-4.5V). Probably bad oxygen sensor
22 Coolant sensor signal out of range - May have been disconnected to set timing
23 Incoming air temperature sensor may be bad
24 Throttle position sensor over 4.96V
25 Automatic Idle Speed (AIS) motor driver circuit shorted or target idle not reached
26 Peak injector circuit voltage has not been reached (need to check computer signals, voltage reg, injectors)
27 Injector circuit isn't switching when it's told to (TBI)
OR (MPI) injector circuit #1 not switching right
OR (turbo) injector circuit #2 not switching right
OR (all 1990-) injector output driver not responding
- check computer, connections
31 Bad evaporator purge solenoid circuit or driver
32 (1984 only) power loss/limited lamp or circuit
OR
32 EGR gases not working (1988) - check vacuum, valve
OR
32 (1990-92) computer didn't see change in air/'fuel ratio when EGR activated - check valve, vacuum lines, and EGR electrical
33 Air conditioning clutch relay circuit open or shorted (may be in the wide-open-throttle cutoff circuit or normal on early 80's models if you don't have air conditioning)
34 (1984-86) EGR solenoid circuit shorted or open
OR
34 (1987-1991) speed control shorted or open
35 Cooling fan relay circuit open or shorted
OR
35 idle switch motor fault - check connections
36 (turbo only) Wastegate control circuit open or shorted
37 Shift indicator light failure, 5-speed
OR
part throttle lock/unlock solenoid driver circuit (87-89)
OR
solenoid coil circuit (85-87 Turbo only)
OR
Trans temperature sensor voltage low (1995 and on)
41 Alternator field control circuit open or shorted
42 Automatic shutdown relay circuit open or shorted
OR
42 Fuel pump relay control circuit
OR
42 Fuel level unit - no change over miles
OR
42 Z1 voltage missing when autoshutdown circuit energized (The Z1 voltage is the voltage of the circuits fed by the autoshutdown relay. This typically includes fuel pump and switched-battery feed to the ignition coil)
43 Peak primary coil current not achieved with max dwell time
OR
43 Cylinder misfire
OR
43 Problem in power module to logic module interface
44 No FJ2 voltage present at logic board
OR
44 Logic module self-diagnostics indicate problem
OR
44 Battery temperature out of range
45 Turbo boost limit exceeded (engine was shut down by logic module)
46 Battery voltage too high during charging or charging system voltage too low
47 Battery voltage too low and alternator output too low
51 Oxygen sensor stuck at lean position (this may be tripped by a bad MAP sensor system causing a rich condition. If you get hot rough idle and stalling, especially on deceleration, accompanied by flooded engine and difficulty restarting, that can be a bad MAP sensor causing the O2 sensor to try to compensate. If you get poor cold driveability, stumbling and bucking, and acceptable warm driving with poor gas mileage (a drop of 10 mpg or more), that is usually the O2 sensor.
OR
51 Internal logic module fault ('84 turbo only).
52 Oxygen sensor stuck at rich position
OR
52 Internal logic module fault ('84 turbo only)
53 Logic module internal problem
54 No sync pickup signal during engine rotation (turbo only)
OR
54 Internal logic module fault ('84 turbo only) - or camshaft sensor/distributor timing
55 End of codes
61 "Baro" sensor open or shorted
62 EMR mileage cannot be stored in EEPROM
62 PCM failure SRI mile not stored
63 Controller cannot write to EEPROM
64 Catalytic converter efficiency failure
65 Power steering switch failure
66 No CCD Message From TCM and/or No CCD Message From BCM


i have had code 41 two times, both times, the alt was bad.
make sure and get one with a lifetime warranty, it pays for itself when you replace it a couple years later