I have read through a bunch of threads and I'm at a loss as to what to do. Bought a 1999 XJ8 a few days ago (74K miles) and drove it home with no issues. 2 days later the driver side front window won't go down. Did a hard reset, and problem resolved until I turned the car off and back on again. Replaced the window master control with a supposedly reliable used one. No dice. Checked the battery and charging system with extensive testing using a multimeter. No problem there. Took apart the window switch and cleaned it. Still not any better.

I'm not sure what to do at this point. It's not a battery or charging issue. I've done several hard resets which will get the window working again for 1 or 2 drives/outings, then it will crap out again.

 

Can anybody help me? Same problem again today. Disconnected battery cables and did a reset. Switches worked fine for a few stop and go errands then crapped out again. /:[

 

You need to monitor the voltage at the window lift motor itself & see what is happening when the window fails to move-either the voltage is not getting to the motor from the control unit (control unit, supply fuse or relay problem) , or the voltage is getting to the motor but the motor is not turning (motor problem).

The door trim panel will need to come off, along with the protective cover behind to access the connectors. The motor plug only has 2 connections on it so it should be easy to check if the supply voltage is getting to it.

Once the window fails to move, do a resistance check at the window motor end of the plug to check for continuity in the motor-it should be a few ohms.

If the motor seems alright, then the next thing is to check the main supply to the door/window control module-this comes into the module on a thick brown/red wire to pin 1 of plug DD10 on the control module plug.

You coudl well have a wiring or connector problem inside the door-sometimes water gets inside & corrodes the electrical connectors, so it would be worth disconnecting the various plugs, sockets & giving them a clean-or at least 'working' them a few times by repeatedly connecting & disconnecting them. Sometimes this can remove light contact corrosion.

Check the wires that pass through the movable rubber protective tube between the door & the body frame-repeated movement over the years may have fractured the wires inside.