Electric trouble-shooting with the central lock system

Mitsubishi Delica L400 production commenced in 1994 -- After much anticipation, the L400 arrived on Canadian Soil in 2009!
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Joined: Sat Jul 18, 2020 12:31 pm
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Vehicle: 94 Space Gear
Location: Vancouevr

Electric trouble-shooting with the central lock system

Post by gavin_tomo » Sat Jul 18, 2020 12:56 pm

Hello! This is probably a question for the more electrically minded of mechanics, but maybe some of you that have been tinkering around repairing old PCBs might be able to help.

A while ago there was a battery short that damaged a few circuits in my 94 Space Gear and I've been identifying problem spots one by one and fixing them. My last issue has been quite elusive but I think I've narrowed it down to one of three possibilities.

Basically the problem is that the central lock doesn't work, either from the driver's side door control or the fob. The wireless fob I am using is attached to an after market alarm system installed underneath the steering that was apparently configured to work on top of the keyless system that is built in to this L400. It was working perfectly well (although with an obnoxiously loud alarm) before the battery short, so I know it works, and I have done some diagnostics on the alarm unit itself and have eliminated it as a vector for the problem.

That lead me to three pieces (three PCBs actually) that are involved. First, the electronics on the button itself (in the driver's side door). I've taken it apart, resoldered some traces and as far as I can tell the function of the button is fine. The only thing I haven't done is actually check if there is a signal coming from the button by tapping the wire, but given the circuitry is working fine I'd rather leave wire tapping as a last resort.

The second piece is the lock control unit / relay under the steering. It took me a while to find this piece, and a whole day to pull it out, but I fixed a trace on that board as well, and it seems to be working fine as well. I actually thought I fixed it when I plugged the unit back there was the sound of the central locks activating (not in one place but on all the locks) but none of the locks actually open or close and it wasn't really the sound of the system locking or unlocking, but rather closer to the sound you would expect if all of the actuators were fully functional. Since all I was doing was reconnecting the unit/relay, I didn't really expect the actuators to lock/unlock the system, so I imagine that's normal, but it might point to a problem with all of the actuators. Would be really strange if it was ALL of the actuators not working at the same time though, so I move on for now.

The third piece is the keyless entry receiver (MB954865), which is located near the back of the van in the wall on the drivers side. There is one switch on the side of the receiver labeled FIX, with SET I and SET II, and it is currently on set II I believe.

Now my understanding of electronics is mostly in PCBs so what I don't really understand is how the whole system is put together. My assumption is that the door button sends a wired signal to the keyless receiver, which wires a signal back to the actuators? I'm not sure where the relay fits into that though. If anyone could help me trace the signal from button -> relay -> actuators I think I might be able to eliminate some more variables here.

My next step may be to take out the wireless receiver and see if I can find any blown connections inside, but if possible I would like to remove it from the equation because that's a lot more involved than repairing a relay. Another possibility is inside the door lock control unit. I repaired the unit itself but there are two relays INSIDE the unit that are black boxed. They can receive current but they might not be fully functional so I could open them up and give them a look.

Sorry for the long post and thanks for any advice!


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