1989 xj40 battery drain

My son has just bought a 1989 4.0l xj40 , we are trying to sort out a few things , but the one that is giving us the most trouble is that the battery drains over night . I have read a few threads and no closer to knowing what i’m doing , i don’t have any electrical nowse at all . Can someone tell me where to start , short of going to an auto electrician . thanks Mark

Do you have a digital multi meter?
If you have you need to remove the positive lead from the battery and insert the DMM between the battery post and the positive lead. Start off with the meter in the amps range it takes a while for the systems to go to ‘sleep’ so wait for the reading to stabilise.
Once that has happened you need to start pulling fuses one at a time to see if one fuse drops the current draw down. That’s the circuit you need to investigate. You might need to open the front doors and trip the catches to fool the computers into thinking the car is closed up, that way you can lock the doors to see if that is a cause of the drain.

Thanks Robin , will give that a go on the weekend and see what comes up . Will report what we find . Mark

You can also do the test using the negative side. Either will work, but by using the negative side, there is no risk of accidental shorting out.

Good point, thinking back thats probably what I did, much less likelihood of blowing something up.

Should it be mentioned that when performing this test you do NOT want to have the key in the ‘run’ or ‘accessory’ positions? Best practice is to not even have the key in the ignition.

Thanks Rog. Will see what we find.

Thanks Mike , will take that advice , don’t want to be chasing another problem .