I've done this myself easily with 100% success. Just follow the directions carefully and you should have your locks slamming open and closed. Here's Greg's writeup and a .jpg diagram:
Power Door Lock Fix for Reluctant Locks on Pre 1991 Jeep Cherokees
(for post 90 models with a similar problem I woud check the relays in the passenger side kick panel)
The Problem:
Some (usually not all) of the power door locks stop working, or more commonly work inconsistently on the lock and/or unlock position. The problem usually starts when the Jeep gets older and the locks become stiffer. Cold weather seems to cause the most problems. The rear hatch and rear doors seem to the most troublesome since they are on the end of the circuit and experience the largest voltage drop.
The Cause:
The cause is a design flaw which causes a loss of 2 volts. As the Jeep gets older and the locks become stiffer and the connections develop some resistance the amount of voltage that gets through the system is no longer enough to work the power locks near the end of the circuit. With the pre 1991 wiring all the power for the power door locks passes through the passenger door lock switch regardless of whether you trigger that switch, the driver’s side door switch or the remote keyless opener (if you have it). Power leaves that switch going to the door locks through the tan wire for unlock and the light green wire for lock.
The power goes from the passenger door switch to a splice in that door for the power lock in that door and then goes out the door and into the kick panel in front of the door on the passenger side. By the time the power gets to the kick panel it has lost about 2 volts. I replaced my passenger side switch suspecting a bad switch but still lost 2 volts out of the switch. I then did a full continuity and resistance test on the switch per the factory service manual and found no problems. I have never figured out why this happens but apparently Jeep could not either since they changed the wiring starting in 1991 to compensate for this problem. (Note the wiring change can be done like the post 1990 models but the fix I provide below is a lot easier in my opinion.) After the now lower voltage gets to the passenger side kick panel it goes under the carpet and over to the driver’s side and hits a master splice under the front floor carpet for the wires going to the driver’s side front and rear doors and the passenger side rear door. I unwrapped the tape around the splices and checked all the splices and they were clean and tight. Then I measured the voltage at the remaining power locks. By the time the voltage gets through the master splice is loses another 2+ volts and is not enough to power the locks consistently when they become stiff from old age or cold weather.
The Fix/Repair:
First make sure you are getting around 12 volts of power to your passenger side door switch directly or when you trigger the lock or unlock switch from the driver’s side or by the remote. You can test the voltage with a multi-meter at the wire harness connector in the passenger-side kick panel where the dark green, white and red wires go into the front door.
Assuming you have a full voltage supply to the passenger side switch, the fix is to install two relays in the passenger-side kick panel on the lock and unlock wires coming from the passenger side switch. The relays will increase voltage back up to 12 volts to compensate for the loss caused by the passenger side switch. With the voltage increase provided by the relays, the voltage at the remaining power door locks is increased by approximately 2 volts, enough to power all the locks, even in cold weather. This of course assumes that no wiring problems exist from the splices to the door locks. If you still have a problem at a particular door lock measure the voltage and make sure it is over 9 volts. With my fix it should be closer to 10 and perhaps 9 at the rear hatch.
The relays to use are 5 blade, 12 volt, 30 amp minimum rated relays with blades marked 85, 86, 87, 87a and 30. These relays are the same as those used for the keyless remote relays found in the passenger-side kick panel so you may find them cheap at the bone yard. You can also find them in automotive electric supply stores. Look in the section that has lighting upgrade supplies. For an easier install it is also best to buy wiring sockets that fit the relays. Here is a link to partsexpress.com an online supplier that has relays and sockets at reasonable prices. I used 2 bosch relays # 330-070 and 2 sockets # 330-075 (although 1 dual socket 330-078 could be used I believe).
http://www.partsexpress.com/webpage....oup_ID=31&SO=2
Here is how I did the wiring: I used the two 12v relays. They have 5 blades: 86 is trigger pwr from the passenger dr switch; 85 is ground; 87 is the new pwr source; 30 is the path to the lock and 87a is to ground (in this application). I cut the Lgreen and tan wire about 5 inches forward (toward the front of the Jeep) 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 frnt door to 86 and 85 to ground; 30 is attached to the tan wire going to the door locks; and 87a 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 going into the passenger door instead. (The same approach is used for the lgreen wire using the other new relay for for the lock circuit.) The key to making this work was 87a to ground. When the lgreen or tan wire are not in pwr mode they provide ground path to ground through the driver’s side switch. With my new relays I substituted a new body ground instead. As a result I increased voltage and created new better grounds. The locks now all slam open and closed even in very cold weather. Hope this helps.
