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[jeepmodel]

The jeep was only putting out about 13 when the battery was tested so thought it was the alternator.  Alternator has been off and tested but theres nothing wrong with it.

Theres 1amp drain on one of the fuses and 6.7 on fuse 16 which appears to be for the undoor light .. which incidentally isnt working!  The alternator centre suggested this could be something to do with the ECU as the controller is bundled in with this??  Car is a 1998 XJ with 4.0 engine ..

Anyone any thoughts on this or ideas that are worth looking at? thanks

[gazjeep]

Might not be whats wrong with yours but my old 97 XJ managed to drain the battery with a dicky door light plunger switch arcing on the rear drivers door.

Hope its that simple for you 

[jeepmodel]

Will ensure we give it a go :-)  its not just the battery drain it's also the fact that the battery isnt fully charging.  It's only showing 13.2 or 13.3 when it should be over 14.  The alternator is fine so something is wrong somewhere!

[JamesH]

The electric aerial can commonly be a drain. There is a battery temperature sensor under the battery. Other than that I can't really help much.

[Trailerguy]

6.7amp drain is large, assume it flattens its battery over night? if its the same as mine the interior lights are on a timer (or go off when you put the ignition on) not sure but id suggest theres a comfort control module somewhere which could be the fault? are they/you testing it once its all shut down? also what does the alternator drop too with a load on? (headlights on full, fogs rear window & blowers etc) 

[Jonny Jeep]

OBDII systems with computers controlling alternators don't usually output over 14V. Typical voltage would be up to 13.8V. You'd only really expect over 14V if the battery is pretty cold.

6.7A is a huge load. Fuse 16 in the PDC is for the courtesy, cargo and under hood lamps, transmission control module on autos, radio and cluster for memory retention, visor vanity lamp, compass, overhead module.
The electric aerial gets power from fuse 17.

[jeepmodel]

Definitely 6.7 on Meter and definitely fuse 16. Definitely a big load being pulled. Even at 13.8 its nowhere near that ... It was only pulling 13.6 under revs and 13.2 or 13.3 at idle.

[Dave69]

i assume the drain was tested with the engine off.

Its a simple but tedious process to find the potential cause as it involves connecting and disconnecting wires to find where.

Simple explanation of process whoever is interested

http://www.wikihow.com/Find-a-Parasitic-Battery-Drain

[Jonny Jeep]

One thing you could try for increasing the alternator voltage, is to increase the resistance the PCM sees from the battery temp sensor. I've seen mentioned several times on ausjeepoffroad about adding a resistance inline or just replacing the BTS with a higher fixed resistance. That makes the PCM thinks the battery is colder and it will up the alternator output voltage as cold batteries can take more charge.

FSM states. At room temperature of 25° C (75-80° F), an ohmmeter reading of 9,000 to 11,000 ohms should be observed. If reading is above or below the specification, replace the sensor.

Can't find the threads at the moment on ausjeep but I think they increase the resistance up to about 20k to get around 14.3V for charging AGM batteries. I'm not sure what voltage the system would charge at with the sensor unplugged but that would probably trip a code for the BTS out of range.

[jeepmodel]

We agreed today we hadn't even heard of a battery temp sensor! Thanks for the input. Jeep is home for now and have pulled fuse 16 to see if it starts in the morning. Borrowee battery booster pack at the ready! :-)

[jeepmodel]

Fuse 16 pulled and car started this morning. What should the alternator be on a 98 xj 4.0? What should the battery be charging at minimum at idle? Anyone know?

Does anyone have any wiring diagrams that illustrate what fuse 16 does in full?

There a 1amp drain on another fuse but need to get this one sussed first. Any help appreciated.

[Jonny Jeep]

 Everything I listed in reply #5 is on fuse 16 circuit. I don't have a 98 XJ FSM. I have 97 nd 99 FSMs. There are some differences between them so I'm not sure which would be the best match for a 98. Off out xrinking very soon so will check back tomorrow.

[Dave69]

battery charge is dependant upon the state of the battery and the electrical draw,

fast idle i would expected to see around 14v and at idle anything between 12 and 14. that with no electric turned on

[gordy]

Jeep model, this site has free factory service manuals including XJ's although won't work on I pads, it's abobe. 

http://www.jeep4x4center.com/knowledge-base/index.htm

[Jonny Jeep]

There's only one manual on that site to cover the facelift XJ and it's from 2000 so there are differences in some of the wiring.

Can you have a look in the PDC for me and see if you have a 25A fuse in either fuse 18 or fuse 19 positions. That will determine which of the 97 and 99 FSMs should match your wiring. That fuse is for the circuits powered via the ASD relay. In the online (2000) FSM the fuse used for that purpose is fuse 8 30A, so you can see the importance of identifying the relevant FSM.

[jeepmodel]

Ill take a couple of photos tomorrow and post them in. Thanks for the help so far :-)