Ok, to start, 18 and 24 are not fuses, these are relays.
#18 locks both rear doors, and 24 unlocks both rear doors.

If neither function operates, the relays are probably not the problem.

The fuse you should check is #21 in the rear fuse panel, it provides power to lock/unlock the doors via the relays.
It also, by the way, provides the power to operate the relays, when the BCM commands a relay operation.

If the fuse is ok, you need it check if the relays are operating.
Have someone operate the lock function and feel relay 18 with your fingers.
An operating relay will provide a movement you can pickup by touch.

Repeat for unlock and relay 24.

If the relays don't operate, come back and we can go from there.

 

RayVoy, Thanks for the quick reply. Double checked fuse #21, its OK.
Had someone operate the lock function and did feel clicked movement on relay #18 and #24, but no response on rear door lock operation.
Ready to go from there.
Thanks Again,
Heyken

 

Next Step:
There are (4) wires coming from the rear Fuse and relay Block there are (2) Tan and (2) Gray wires, one Tan an one Gray to each door lock solenoid. When one of the relays are energized 12v is applied to send voltage to the solenoids. The opposite polarity is applied when the other relay is energized.

The easiest repair would be if one relay is bad internally and it would affect both functions on both doors. Try swapping each relay one at a time for another same part # relay in the box, not with each other.

Not sure of the accessibility at the Fuse Block, but try to check for voltage across the wires when Lock or Unlock is commanded. If you have voltage at the wires near the Fuse Block then the wiring to the doors is damaged or both solenoids are bad.

Because it affects both doors, I would say the problem is most likely in the Fuse Block somewhere.

 

Ok heyken, bromanjr has provided the wiring after the relays.

Like him (?), if the relays are working I find it hard to believe there is an electrical problem between the relays and the lock solenoids.

If both doors stopped working at the same time, it must be something common to lock, unlock and to both doors.

The only things that are common, would be the +12v through fuse #21, and the ground on a lead to the rear fuse block.

The next step is metering, do you own, have access to, a digital multimeter?

ps, just clarify for me, that the problem occurred with both doors at the same time.

 

RayVoy - (him=yes), and now that you mention it, there is a ground that is important here, It comes from the Relays in the Fuse Block to Ground G302 (on lower left B pillar).

But if there is 12v (actual system voltage) across the two wires at the Fuse Block during operation, then the Ground is good. The OP also reported that the Relays will click when lock or unlock is activated, so the fuse must be good as reported. Another reason to check for 12v because a bad connection somewhere may allow he relays to click but not supply enough power for the door lock solenoids.

I still think one of the 2 relays has a bad connection internally.

 

(him=yes), I still think one of the 2 relays has a bad connection internally.

Haha, thanks. You may be right about one of the relays, I don't think it's an individual wire.

Will get back with you on your question concerning about if both doors stopped working at the same time.
Thanks again,
Heyken

My reason for asking, is that this could be a problem inside the doors..........but only if it did not happen at the same time.

What I'm trying to say, is it possible that one door stopped working and then (sometime later) the 2nd door stopped.

If that could possibly be the case, we must consider the internal solenoids and the door wiring.
We also must consider a mechanical (rust maybe) problem with the doors.

However, if both doors stopped working together, then the problem is in the relay area.

We could try a relay swap.
Identify the 2 relays we have been talking about (they should be the same), then look around on the fuse board for the same relay.
If you find a match, pull the lock relay and try the 3rd relay and test the locks.
If no change, pull the 3rd relay and reinstall the lock relay.

Repeat above with the unlock relay.

 

WOW! Thanks, Rayvoy/Bromanjr,
I do have access to a borrowed meter. The rear door problem was apparent when I bought the tb. FYI, I know the tb was sideswiped on the passenger's side a year ago or so (could this be suspect of wiring damage?) and was repaired by a bodyshop. ASOP, I will find out if the rear door lock problems occured after the sideswipe incident or if both doors quit at the same time. Meantime, I will follow your instructions on checking the relay integrity, and voltage output to the solenoids.
Will get back with you on your question concerning about if both doors stopped working at the same time.
Thanks again,
Heyken

 

Rear door lock problems

Thanks Rayvoy for getting back with me on the rear door issue. I just found out from the previous owner that the passenger side sideswipe happened 4 years ago and did not affect the rear door operation at that time. He also said the rear door lock/unlock problem occured less that a year ago and seemed to happen to both rear doors at the same time. However I'm inclined to agree with you that maybe one door could of experienced problems before the other, especially since the lock/unlock noise echos inside the car on all the doors and liftgate.
Ready to go from here.
heyken

 

especially since the lock/unlock noise echos inside the car on all the doors and liftgate.
Ready to go from here.
heyken

Ok, if you can hear it inside the doors (and you may have told us that in your 1st post :duh: ), that pretty well rules out the relays.

Did the previous owner use the vehicle for something odd?
Something that may have required the doors to be left locked (no, you talked to him and he would have told you).

Hey, this isn't as simple as the "keep children from opening the door" slide being in the wrong position?
No, I just read the first post and you said they are disengaged (I assume that mean in the free to open position?)

You probably need to take the cover off the inside of a door. You may have a rod disconnected.

 

Rear door lock problems

Rayvoy, I may have mislead you in describing the lock/unlock noise, I have not heard any noise from the rear door lock commands. Only the front driver/passenger and liftgate noises. Sorry about all the confusion. FYI, the car was not involved in any special service, (no children) just to and from school/work and so on. I plan on trying the relay swap tomorrow and will get back with you on my findings. Like the linkage concept also.
Heyken

 

Rear door lock problems

Rayvoy, Back again, FYI, I took the liberty to replace the rear door lock/unlock relays (minimum expense) and still no response on the rear door operation. I am now seriously thinking about exploring the linkage aspect on the solenoid, however how easy is it to remove the interior door panels (looks complicated) for this observation.
Won't give up yet.
Heyken

 

I haven't taken any of our door panels off (many other GM vehicles, yes); usually, there is one, or two, screws in the handle/armrest area and the the main panel is held with push pins.

Start at the bottom of the door and gently pry outward, the pins should let go (just held with spring tension and friction).
Start at the bottom and work up both sides.
When the panel is loose, it will lift upward and off.

 

Rear door lock problems

Rayvoy,
Thanks for getting back with me on the interior door panel removal. I think I have found where the fastners are hidden in the armrest area. Appears that at the bottom of the armrest structure they are covered by a popout strip.
Anyway, for now I must put this project on the back burner because of the outdoor temperture is freezing in the low teens and since the panels are plastic, they may fracture in the removal process.
In the mtce records I found in 2004, the liftgate lock module had to be replaced by GM dealer costing around $450.00. WOW! I remain Hopeful for a DIY on this one. You have been a great help and I really appreciate your time and support working with me. Will get back with you in the near future.
Best Regards,
Heyken

 

:thumbsup:

 

Rear door lock problems



Hey Rayvoy,
Got er done! Still patting myself on the back. Thanks for your help. FYI, here is what I did. Lubed the latches
with synthetic oil and had to replace the right side rear door lock actuator ($108.00 for the part) from a GM DEaler.
It was quite a job, since the door interior was hardly wide enough to make a fist, and the linkage had to be temporarly disingaged along with the exterior and interior door handles to even see what I had to. Almost felt like I was rebuilding a watch, Lol.
Best Regards,
Heyken :hail:

 

2004 Trailblazer Rear Doors Lock Problem

Again thanks again for all the help on the door lock problems, and as you already know I successfully replaced the rear door passenger side actuator lock by following your instructions. It now appears that my Trailblazer still has some gremlins in the other door locks. FYI, I had to replace the driver's door lock actuator (Wow $219.00 GM only sells the combined lock and actuator assembled together for the driver's door). Now all the door locks seem to be working.
However I don't know if I booted out the gremlins yet. In any event, I'm ready to kick butt since I've got you on my side of the court.
I share with you a note of Interest on May's Consumers Report 2011, concerning the best and worst cars 2000 to 2011 to buy and this wasn't good to hear: The 2004 trailblazer wasn't on the best list. Anyway I like it, think its got a hell of a L6 engine, and I enjoy driving it
Best Regards,
Ken:grouphug:

 

Thanks for the update Ken, good luck :thumbsup: