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Discussion Starter #1
Rear liftgate will not lock. Does not work with either interior lock or key fob. Also, no longer sounds horn when doors locked. Will default to locked position when battery is disconnected and reconnected. Honda started troubleshooting, replaced actuator (no joy) and rear junction box (no joy). They seem to be hen pecking their way through the troubleshooting guide. Has anyone else ever had this issue?
 

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Ah, the good ol' dealer parts cannon. No diagnostics and all guessing.

Shouldn't be too hard to figure out with a wiring diagram and a little testing.

Not sure why they would replace the rear junction box since the lock/unlock signal comes from the driver's under dash fuse/relay box, called a MICU. The tailgate latch switch feeds into the rear MICU junction box. This would be a simple circuit to diagnose. Take it to any good independent shop and they should be able to give you a diagnosis.
 

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Rear liftgate will not lock. Does not work with either interior lock or key fob. Also, no longer sounds horn when doors locked. Will default to locked position when battery is disconnected and reconnected. Honda started troubleshooting, replaced actuator (no joy) and rear junction box (no joy). They seem to be hen pecking their way through the troubleshooting guide. Has anyone else ever had this issue?
Damn how much did you pay them suckers so far?
 

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Discussion Starter #4
Ah, the good ol' dealer parts cannon. No diagnostics and all guessing.

Shouldn't be too hard to figure out with a wiring diagram and a little testing.

Not sure why they would replace the rear junction box since the lock/unlock signal comes from the driver's under dash fuse/relay box, called a MICU. The tailgate latch switch feeds into the rear MICU junction box. This would be a simple circuit to diagnose. Take it to any good independent shop and they should be able to give you a diagnosis.
Anyway I can get the diagram and do some testing myself?
 

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Power tail gate or non-power tailgate?
 

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I don't have the actual diagram that shows all the connectors but this should give you enough to figure out where the problem lies. Not great resolution but the manual won't let me print to a pdf so screenshots is the best I can do.

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I'd check the following first:

1. Check for power at the actuator when you lock/unlock with a test light.
2. Then check for power at the wiring to the actuator coming out of the driver's fuse box.
3. Then check the switch inputs to the rear junction box for the latch switch and/or the lock switch.
 

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Discussion Starter #8
I'd check the following first:

1. Check for power at the actuator when you lock/unlock with a test light.
2. Then check for power at the wiring to the actuator coming out of the driver's fuse box.
3. Then check the switch inputs to the rear junction box for the latch switch and/or the lock switch.
John, Thanks for the input and diagrams. I forgot to mention it is a touring model, so does have power tailgate. Not sure if that changes the diagrams. But, will give this a shot.
 

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Well, I noticed those diagrams all show "without power tailgate" so I was wondering about that. That changes the troubleshooting significantly and the diagrams to use.

When I look at the diagram for the power tailgate I don't even see a lock actuator. My guess is everything is done electrically so there's no real need for a mechanical lock that needs to be actuated. My guess is something is just preventing it from being able to be opened with any of the switches and there is no mechanical release on the Touring models. I've never worked on mine so I'm not that familiar with it. Here are the separate diagrams for the power tailgate.

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Discussion Starter #10
Thanks. The Tailgate opens and closes fine under power. It just will not lock. Is there not a diagram with the lock actuator for the rear tailgate? My guess was a problem with the multiplex unit, but I cannot find any real information on that unit. Since everything else works, that would just be the next logical place to look. I appreciate the assistance, but is there a resource I can browse through for this data? I hate to continue bothering you.
 

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I'm getting it from my factory service manual. I don't have the electrical troubleshooting manual for the 2006, and though I have it for the 2007-2010, that is in paper version only.

Near as I can tell, there is no power lock actuator in the power tailgate. That is evident from the second set of diagrams I posted above that cover the power tailgate. I just went out to my 08 Touring which has a matching circuit to the 06 Touring power tailgate and confirmed there is no lock actuator that I can hear when I lock and unlock the doors. So, what that tells me is that when the van is locked the various switch inputs (tailgate outside switch, inside switch, dash switch and key fobs) to the power tailgate control unit are disabled by the control unit. What I can't tell for certain is how the control unit knows the vehicle is locked. The only thing I can see is that it there is a connection between the full latch switch in the closer unit and the rear MICU junction box, which communicates with the driver's under dash MICU. It looks like a communication line to me so that could explain how it knows the vehicle is locked. I don't know what the dealer replaced already but my first suspicion would be the closer unit with the latch switches in it. It looks a lot like the sliding door latches that so commonly experience failed microswitches.

A dealer level scan tool that can see the position of all those switches would be very helpful. It appears to me that possibly the control unit is not getting an input that tells it the tailgate is fully shut. I'm just coming up with possible theories to be checked....that's not a definitive diagnosis.
 

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And you're not bothering me. I haven't really studied this circuit up to now so I'm learning, as well, just from studying the diagrams. Knowing how switch inputs to a computer work is key, though. The control unit will likely supply a voltage to each of those switches. Then each switch will ground that voltage out, which tells the computer the switch has been pushed. Pushing any of the switches, i.e. the inside power tailgate switch, outside power tailgate switch, and dash switch, all ground the same switch input at the control unit.

The various latch switches will do the same thing except they all have their own switch input at the control unit. The switches just pull those inputs to ground and that's how the control unit knows the switch was activated.
 

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Discussion Starter #13
Well, as long as I am leading to greater automotive knowledge. I think I understand what you are saying about the actuator. While it has an actuator to latch and unlatch the tailgate, nothing seems to happen when you lock the vehicle. I don't know that I have ever heard a noise back there myself. As far as moving forward, is there anything that shows the driver under dash MICU better, so I would know where to test? Also, the fuse for the door locks is on the passenger side fuse box. Is there not a MICU associated with that fuse? Seems odd to have the fuse on the passenger side, but the MICU on the drivers side. I would love to solve this problem as I use this vehicle to travel quite a bit.
 

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The MICU is Honda's fancy name for a smart fuse box. Multiplex integrated control unit.

Here are the actual diagrams from my 07-09 ETM manual. The circuit appears to be the same on the 06.

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The actual door lock circuit has nothing to do with locking the power tailgate other than sending a signal to the rear MICU to tell the power tailgate control unit not to open the rear hatch via any switch. It appears as if there is no mechanical lock.

So, go with the clues you have. The tailgate is always able to be opened. The horn does not sound when the doors are locked with the key fob. What does it mean if the doors are locked and there is no horn? Does the security system actually arm when you lock the vehicle? You should see the Secuirty Indicator LED on the dash slow flash a few times then flash quickly every second or two after that

Any other clues that you've seen?
 

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It could be difficult to diagnose this without a scan tool that can read the switch inputs from the latch (tailgate closer unit.) I went out and looked at mine with my Foxwell NT510 tool. When closed all the switches are in the OFF position with exception of the Ratchet switch, which says ON when closed.

Tailgate close:
Neutral switch: Off
Release switch: Off
Full latch switch: Off
Ratchet switch: On
Half latch switch: Off

Tailgate open:
Neutral switch: Off
Release switch: On
Full latch switch: On
Ratchet switch: Off
Half latch switch: On
 

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Here's a good diagnostic question that might give us a clue. If you put the rear cargo light switch in the "door" position and close the tailgate does that light go out?
 

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I did find a diagram that draws a little better how the lock signal gets to the rear MICU. The B+ door lock signal goes out via B-CAN line from the driver's side under dash MICU/Fuse/Relay box, out to the rear MICU junction box which communicates that to the power tailgate control unit.

Contrary to my initial comments in this thread, it could get a little tricky to diagnose this one since there is no electro-mechanical lock mechanism. It's all communication and computer switch inputs.
 

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Discussion Starter #19
Thanks. I just spent some time going through all the controls. Here's what information I can provide. There are no indications the tailgate is not closed. The rear cargo light goes out when in door position. When the doors are locked with the key fob, no horn/lights sound or blink. The security system does not arm. In fact, the little red/amber light never comes on, even when you first roll the key and all the dashboard lights light up. You can always open the tailgate with the inside switch or key fob, although it does de-activate when the transmission is any gear other than P. The auto-lock feature was set to the speed setting, but the car always seemed to be trying to lock the doors as you changed speed in traffic. I just changed it to "shift from P" and it at least does not sound like it is always trying to re-lock the doors.

While it is probably not related, there are some lights on the instrument panel that appear to be out. The parking sensor light on the left side does not provide a green indicator when active and no lights at night with parking lights active. Although the parking sensor is currently active and working fine otherwise. The upper half of the climate control system are not illuminated either. Can't say any of them are related, but I do believe they have been out for about the same amount of time. I attached pics of the dashboard lights from this evening if that helps.

With respect to the scan tool, I can ask my regular mechanic on Wednesday to pull up those codes. I know his tool provides this information and he is always willing to run codes for me. He does bigger jobs like timing belts, but he is just a small shop, so he prefers not to do these types of diagnostics. Anyway, if it is helpful let me know which screens to pull and I can try to capture pics on Wednesday.

I appreciate the assistance, this problem has been troublesome.
 

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The full latch switch appears to be responsible for turning on and off the cargo light when the tailgate is opened and closed so that is likely working. I'm guessing that's the "actuator" they replaced. Also, you said they replaced the rear junction box? On the surface it seems that the tailgate control unit is not getting word from the driver's MICU that the doors have been locked. So, either that signal is not going out from the MICU or the tailgate control unit is not receiving it. I don't know a lot about the communication that goes on over the B-CAN line as I've never looked at it like I have a CAN line. My questions are, does the tailgate control unit receive the lock signal and then let the driver's MICU know that it received it and that's what allows the vehicle to arm the security system or if merely the fact that the MICU sent the lock signal is enough.

To me, the problem is likely in the driver's MICU or the tailgate control unit. Since the dealer already replaced the latch assembly (actuator) and the rear junction box that's really all that's left other than the wiring in between. The front driver's MICU has been known to get wet when water leaks from the roof seam so that might be something to check. The power tailgate control unit is located in the left D-pillar up toward the top.

Another thing I noted was that you said the doors want to lock all the time when driving when set to the speed setting. I think that triggers at 9mph, if I recall correctly, but I'm not sure where that input comes from. From what I've read the speed input to the gauges comes from the PCM through the Multiplex integrated control system, another nail in the coffin for an issue in the driver's side MICU. The speedometer works, OK, I assume?

Another thing I'm noticing, looking at the B-CAN communication circuit, is that there is one wire from the driver's MICU going back to the rear junction box (another MICU) and it talks with the power seat control unit and the sliding door units so if there are no communication issues with either of those I don't suspect a communication issue between the MICUs. That's a nail in the coffin for an issue with the tailgate control unit.

Bottom line, I'm not sure. I'm not sure how an issue with the power tailgate control unit would cause the locks to constantly try to lock when driving. That sounds like an issue with the MICU. Not sure how helpful this is, just more thinking out loud.
 
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