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New Relay on Junction Box

New Relay on Junction Box

New Relay on Junction Box

Car model: Toyota UZJ100 RHD. (right hand drive version makes an absolutely nightmare to reach and replace some of its most essential relays).

The central door lock relay fails in this car. You can hear it click but it doesn't lock doors. It only works fine when the weather gets cold.

Door unlocking (unlock relay) works fine. It is only the locking relay that gives problems.

1 - I tested first all the door lock actuators to make sure they are fine.
2 - I also checked wire continuity from the junction-box to the door lock actuators. Door lock wires are fine.
3 - I also tested door lock wires at the junction-box with a 12V tester. Door lock wires get power when relay works fine (doors are locked fine). No power when it fails. You can still hear the relay click but no power sent to the actuators.

The relay energizes/activates itself when it gets an earth signal from the Body control ECU. I had a a second Body-ECU I could test. The second ECU showed the same relay behaviour (you hear the click but no power).

Good a simple and easy fix. I need to replace the door lock relay.

Well not so easy. This car is the RHD (right hand drive) version. The relay is located in the hidden side of the junction-box under the glove box. To reach this relay in the RHD version is an absolute nightmare. There is no space to detach the junction box as it is mostly hidden by the A/C pump. You would have to remove the whole dashboard. Worst design ever.

Plan B:
Get the wiring diagrams, identify the wires involved in the door locking circuit, disable the old relay (unplugging the ECU ground signal will disable it) and fit a new relay that could do that job.

I've never done such a fix (hack) around a junction box wiring. Any good advice from experienced guys is welcome. What would be your preferred fix or solution if you were in my situation.
Thanks in advance.

My images:

Image A is a very simplified diagram of the central door lock in this car:

Image B is the EWD of the door lock/unlock relays at the junction box:

Image C is an image of the Junction box and the connections to the new relay that I have in mind:

Image D is a picture from the Junction Box wiring:

Thank you in advance,


RE: New Relay on Junction Box

Thank you TugboatEng.

That's a good point. In fact I tried to get a relay with a diode across the coil but what I got is a relay with a resistor across the coil.
Is a relay with a resistor acroos the coil good enough to prevent high voltage spikes reaching the ECU? I hope so. That's what they are designed for, aren't they?

This is the micro relay I got:

The original relay from Toyota is also a 5 pin micro relay but I assume it is a basic one since the EWD (https://ibb.co/VWrQr8b) shows no diode or resistor across the coil (???)
I assume the EWDs should show those critical details or am I too naive(?)

Thank you

RE: New Relay on Junction Box

No, it's not common for wiring diagrams to show the actual internal circuits of an item such as a relay.

While Toyota's relay may or may not have a diode, consideration was likely taken as to what the maximum voltage spike would be. When adding an aftermarket diode with unknown characteristics, it's best to CYA.

A user here provided a very good explanation of the use of a resistor and/or diode:


RE: New Relay on Junction Box

Thanks TugboatEng,
You are right!. I just ordered a new relay with a flyback diode. Just in case ...

By the way, when connecting the cables from the relay to those on the JuntionBox (https://ibb.co/9t71cWv) is it considered acceptable to:
1 - Unplug/Detach from the Junction Box the cable you want to connect to.
2 - Remove its faston terminal.
2 - Crimp both cables (the one you've just detached and the one you wish to connect from the relay https://ibb.co/zPXgDV3) with a new faston terminal and plug it back into the JunctionBox.

These 2 cables crimped together in a single faston terminal to connect them ... is this considered a messy solution or is it somehow acceptable? is there a better/proper way to do it?

Thank you.

RE: New Relay on Junction Box

I would personally go with something a bit more robust such as a Deutsch connector.

RE: New Relay on Junction Box

Thank you TugboatEng for all your good tips.

Yes, it makes sense. I'm going to buy some good splitters and terminals and fit things properly.


RE: New Relay on Junction Box

Eventually this weekend I had sometime for this fix.

I ordered a new relay that I received a while ago.
I fitted it as planned (although I changed a little detail from my drawing. Pin 30 to 2N connection is wrong in the drawing as it would give a short).

I connected all and ... it worked like a charm!

Perfect: locking, unlocking, remote lock/unlock, all lock switches, all doors activator work fine and sound strong and healthy. Great!!!

THANK YOU for all your tips, help and advice.
If someone has the very same symptoms I had, please make sure your relay connections are fine before you swap your lock activators or any other part.
On a LHD vehicle the test is very simple. Just remove the relay and test its connectors with a battery and e.g. a light tester. If it is a RHD then you can try this fix.

My new diode protected micro-relay:

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