I (like MANY others) did the 87-90 power door lock relay upgrade. My once dead power locks now SLAM!!! back and forth freely on all doors plus the rear hatch.
I'll see if I can find the instructions and post them back here.
Here it is courtesy of Greg Smith from the NAXJA and JU boards.
Greg's Power Door Lock Fix.
(The fix should work for pre 91s, thereafter the wiring is different)
After 5 or more yrs of battling the reluctant power door locks on my 89 Jeep Cherokee, I believe I have finally won this battle. My problem was low voltage which prevented my hatch and driver's side rear door from always unlocking. In very cold weather the low voltage is not strong enough to lock or unlock the very cold and stiff lock mechanisms all the doors and hatch. Now lately even the driver's front door wouldn't unlock in warm weather. The cause was volts were less than 8 at the rear hatch and rear driver's side door and about 8.5 volts at all the other doors except the passenger front door lock motor which gets over 10 volts. Apparently this problem is not as frequent on post 90s due to the wiring change that first appeared on the 91s. Redoing the wiring to match the 91s is possible but more difficult and time consuming.
I have the factory service manual, so I have been able to trace everything. I have keyless entry w. remote, which has two relays in the passenger side kick panel. Power goes to the master switch on driver's door and to the passenger door switch. Power also goes to the keyless/remote relays. I had 12.4 volts going into all those relays and switches. The ground path is through the driver's side master switch then to the body ground. All pwr and the ground flows through the passenger side switch to lock or unlock the door locks regardless of whether you use either of the door switches or the remote/keyless/relays (i.e., the remote signal triggers the relays).
I was only getting 10.5 volts coming out of the front door passenger switch measured at the wire harness connector in the passenger side kick panel where the wires come out of the door. Note, inside the door there is a splice to that door's lock pulling off some volts. By the time that 10.5 volts makes it across the car to the driver's side kick panel wire harness connectors it was only 8.5 volts. Along the way the lock and unlock wires hit splices under front carpet under the driver's side for the driver's front door, driver's rear door/hatch and passenger side rear door. I checked the splices and they were clean and tight. The voltage to the rear hatch and driver's side rear door drops even more at the final splice to the rear hatch which near the driver's side rear door.
Solution: I used new relays and sockets that I bought from radio shack online (see link below) (they were not available at local radio shack stores). I wired them into the lock and unlock wires in passenger kick panel and let me tell you those lock motors slammed open and closed so hard the fillings in my teeth rattled. I measured volts at the driver's side kick panel (after the splices under the carpet) and it was now 10.5 (up from 8.5) volts - so I juiced up the voltage to give the lock motors more power. I tested all the switches, wires and relays pre my factory service manual and all appeared to be fine. I suspect that, since the wires are all the same gauge going through the splices to the lock motors, not enough volts were provided through the system to each motor to work the lock mechanisms when they get old/cold and stiff.
This is how the wiring system from the factory works. Coming out of the passenger door switch the lock wire is light green and the unlock wire is tan and they stay that color all the way to all the lock motors. The lock and unlock wires function as power and ground alternately. So when you move the switch to the lock position volts go through the lgreen wire to all lock motors and the tan wire functions as the ground path back from the lock motors through the passenger side switch then to the master switch and then to the body ground. When hitting unlock the reverse is true tan is pwr and lgreen is the ground path. All of that is helpful information to know in understanding the system.
Here is how I did the wiring: I used two 12v relays just like the ones used for remote keyless entry. They have 5 blades: 86 is trigger pwr from the passenger door switch; 85 is ground for the relay trigger side; 87 is the new pwr source; 30 is the path to the lock and 87a is to ground (substituting for the ground through the driver door switch). I cut the lgreen and tan wire about 5 inches forward of the wire harness connector that connects the wires to the wires coming out of the passenger frnt dr. In this example I attached the tan unlock wire coming from the passenger front door to 86 (red socket wire) and 85 (purple wire) to ground; 30 (yellow) is attached to the tan wire going to the door locks (the wire going in the wire harness going to the driver's side not the wire going to the passenger door); and 87a (white) is to ground. I used the new 10 gauge pwr line I had previously run to the battery/starter relay post with a 30 amp inline fuse. You can take off pwr from a splice on the red pwr wires in the wire harness going to the relays (there is a thick red wire in the harness at the splice). (Note the socket wire colors are valid for the radio shack sockets I was using - you need to verify which of your socket wire colors correspond to the relay blade numbers.)
The key to making this work was 87a to ground. When the lgreen or tan wire are not in pwr mode they provide a ground path through the driver's side switch to the body ground. With my new relays I substituted a new body ground which is necessary to allow the relays to increase voltage and still allow the system to work. The locks now all slam open and closed even in very cold weather. The repair should take about 45 minutes. I now know of a few people who have used my fix on their Cherokees with great success. Hope this helps someone else! However, anyone using my fix must do so at their own risk and liability. Greg
2394 Relay and 2396 Socket Link: http://www.radioshack.com/category....07%5F000&Page=1
The radio shack link in my article no longer works and they do not appear to sell the needed items. So I have provided below a new link to another company that has the 5 pin/blade 12 relay and sockets needed and they are cheaper than the radio shack ones! I sent Jay my article and wire diagrams and hope he will report back whether the fix was successful for him.
Note that my writeup refers to radio shack socket wire colors so if you use different ones you need to adjust to the colors provied to match up to the correct pin/blades numbered on the relays. http://www.partsexpress.com/pe/showp...ectGroup_ID=31
I ordered # 075, and 070...