Power door locks, did some investigating

I have an 89 Cherokee pioneer with power door locks only.

For some reason or another, my locks just dont get enough power to turn over. So I investigated today:

With every door connected, only the front locks work, and barely. The back locks dont work at all. AS SOON as I disconnect one lock, my tailgate lock works, and my front locks work very good. It doesn't matter what lock I disconnect to try this. If I disconnect two locks, then my back left locks works very good. Seems like they're not getting enough juice aye?

So I looked in the wiring diagrams in my haynes manual, and there is an always-hot red wire that goes from the battery to the front switches. I tested this, and it got 12+ volts. It seems good. I tested the voltage at a door, and it only got 10 volts. However the interesting thing is, if I test the unlock side of the connector, and it gives me 10 volts, the lock side of the connector gets 2 volts.

I started up the engine and my doors suddenly got 11.21 volts, and, with two locks disconnected, my back left lock worked completley, although slow.


So to recap:
Each connected lock works better if other locks are disconnected.
Each door is getting <10.5 volts when I hit the switch, and the opposite connector is getting <2.5 volts.
The hot wire is getting >12 volts.
The ground at the driver's side, with the batt disconnected, is getting <1 ohm to the neg batt cable.



So my questions, is each door SUPPOSED to get 12 volts each? I'm sure if each of my doors got 12 volts each + the amps that go along, they would work perfectly. Anyone have an idea what's up?

Sounds like an interesting project. I have no doubt it should be 12 volts, with the engin e running it should be 13 or more volts if you don't have the A/C, blower and headlights hogging all the alternator juice.

You either have a bad connection on the way to the door lock switches, or multiple bad connections at the switches, or maybe a chasis ground problem keeping the current from flowing. I am guessing it is a bit of both.

I have the same problem with my 87 door locks, but I have not tried to fix them yet.

I suppose the only way to test this would be to ask someone with working door locks whether or not he/she receives 12 volts at the lock itself. Anyone willing to try for me/the good of naxja? I would really like to know.

Ecomike said:

Sounds like an interesting project. I have no doubt it should be 12 volts, with the engin e running it should be 13 or more volts if you don't have the A/C, blower and headlights hogging all the alternator juice.

You either have a bad connection on the way to the door lock switches, or multiple bad connections at the switches, or maybe a chasis ground problem keeping the current from flowing. I am guessing it is a bit of both.

I have the same problem with my 87 door locks, but I have not tried to fix them yet.

Click to expand...

You are supossed to get 12 volts to the locks. Thats what mine has.

mattbred said:

I suppose the only way to test this would be to ask someone with working door locks whether or not he/she receives 12 volts at the lock itself. Anyone willing to try for me/the good of naxja? I would really like to know.

Click to expand...

The static voltage at the door switch (no load condition) should be the same as the battery voltage. When the loaded lock switch is activated and the lock solenoid is trying to move, you should have 0 voltage on the ground leg of the solenoid, but a drop to 10 volts on the supply side may be normal, but I doubt it.

Just had a thought, is it dropping from 10 volts to 2 volts across the switch???? If so the switches are toast, I would think. There should be no volatge drop across the switch when it is turned on.

I re-read your post, is the 10/2 volt reading when the lock switch is pressed? If so there is a ground problem for sure, and it may be at the lock solenoid itself. I am not sure how those guys are grounded, but it may be right at the lock solenoid to the door chasis???? They would do that to reduce wire costs, but then the ground would depend on door hinges for a good ground, which is BS. I have heard the door lock problem is common. Anyway try and follow the wire to the solenoid, then follow the second wire from the solenoid to ground (if there is a second wire, if not then the case of the electric solenoid is grounded to the chasis through its mounting hardware.

Update, I'm getting a voltage drop from lock to lock.

I tested and my front right lock got 10.48 volts whereas my back left lock got 9.55 volts.

I'll try and explain this better Ecomike:

Each lock motor itself has two wires going into it. One wire is hot when you hit lock, and one wire is hot when you hit unlock. What I'm noticing is that when I hit unlock, the unlock wire will get 10.48 volts, yet the lock wire will also be getting 2-2.5 volts.

When I first bought my 96 Cherokee about two years ago, I also had a problem with the electric locks not working. They seemed to exhibit the same type of thing your seeing. They would attempt to cycle (cycle = lock or unlock, it didn't matter which). There seemed to be not enough power to make them work. They work fine now, with the occasional exception of the left rear (it is a four door). And the solution was not anything involving the wiring or voltage levels. It was dirty lock mechanisms. I took out each lock mechanism and thoroughly cleaned each one using brake cleaner. Then I sprayed each with WD-40 and lubed every moving part with a white grease spray. They've worked fine for over a year.

Ha, awesome. Makes sense.

I do believe the problem is in the actual switch itself; perhaps it builds up resistance over time.

I'll do one more test tomorrow, and if I can't get it goin, I'll do the relay trick.

You need to do the door lock mod. It works great!!!!! In short you wire up two relays in the passenger kick panel, bypassing the factory wiring harness. Search around and find the write up. Your locks will slam open and closed when completed.

Can you use the red wire? How do you splice it?

I'm not sure what power sources are readily available on your '89. On my '85 I ran a separately fused 10 gauge wire from the distribution point on the starter relay to be sure that I had as much capacity as possible for the new relays. My right rear lock is still balky and requires cleaning and lubricating from time to time. The others have now worked flawlessly for several years since I did the mod.

I guess you could try tapping into the red power wire as suggested by Greg and see if you get good locking action. Either clean the insulation off a spot and solder a lead or use one of the solderless connectors you can get at Walmart or wherever. I guess these are reliable enough, at least for low amperage small wire connections. If this doesn't produce reliable locking, you can always go back and run a new power lead.

I bought a couple Bosch 12v 30a relays, except instead of an 87 and an 87a, there's just two 87's. I tested it and there is zero resistance between both 87's, they are directly connected to eachother. I imagine this wont work since the power source will be constantly grounded, shorting out the battery..

I guess I need the one with 87 and 87a instead. damnit.

Aha, napa had the right relays. Awesome.

What's the best way to route a positive cable in, if I dont want to split the red one? I dont know how to even get a wire through the door, nevermind past the firewall and attached to the battery.

Ok I split the red wire and it's just not big enough. It's a 14 gauge and just doesn't supply the voltage/current. It all works, yet I just need a hot wire in from the battery somehow.

Well I ran a wire straight from the fuse box on the top there, there's 3 seperate prongs that hold 12 volts. The door locks STILL only got 10-10.5 volts when I engage the relays. Also, after using it 4 times, I suddenly got NO power from the fuse panel at all. I then put it into acc and tried it, and it worked the same for a few minutes and now all of a sudden NO power anymore. What is going on?

Did you check the fuses, probably overloaded the fuse in the fuse box?

Aha! All the top fuses, the dome light fuse, and the radio, are all interconnected. So if the dome light fuse blows (which it did, because I took too much current from it's circuit) then it stops the radio, dome lights, and the top fuse-clips. Glad I figured this out!

Also, anyone know how to bring a wire through the firewall into the engine bay? I succesfully got it through the door and am almost done with this mod.

To feed various wires on my '93 I drilled a hole on the driver's side an inch or so to the right of where the throttle cable goes through the firewall and about an inch above where it goes through (facing the firewall from the front of the truck). This puts the hole about about equidistant between the throttle cable and the edge of the brake booster and about two inches below the sheet metal ridge.

There may be a better way to get wires and whatnot through but this is an easy place to drill and it is easy to feed and retrieve. I know others have posted on this subject from time to time but I don't remember their solutions. Perhaps someone will chime in.

I recently added after-market power locks and remote (with their own fuses) to the '93 and used the contacts at the top of the interior fuse block for the various positive feeds. No problems with fuses blowing for several months but the new actuators may draw less current and there are only three of them for the 2 door. I didn't know these were fused through some of the other circuits but am glad to learn this for when one of them inevitably blows.