Faulty backdoor latch fixed and failure analysis

December 28, 2018

The above is the faulty hinge before cleaning.

Faulty hinge after soaking in a solution of citric acid to remove rust, grease built up and dirt.

Failure Analysis – how did the component fail?

Originally, there are two identical assymmetrical hinges, L and R. When I remove the back plastic cover and metal cover tray. The L hinge had been disconnected from the pull level as the plastic guide had broken loose. Hence the backdoor was held close by only the R hinge.

I suspect the L hinge was deliberately rendered inactive by the last mechanic who opened up this section. Why was it not put back in the correct way is unknown. This panel was damaged due to a rear accident five years ago.

Hinge is constructed out of mild stamped steel without hardening heat treatment except the spring assembly.

Since only one hinge was holding up the back door all this while, it would be subjected to load stress since without the sister hinge working, this one working hinge would be a pivot point.

Failure is inevitable.

How did it fail?

The hinge mechanism is similar to trigger assembly on a standard firearm. A is the catch that when closed to a static U pin will in turn lock B thus preventing the rocker from moving.

The rocker @ B can only be released if a lever mechanism @ C is activated.

Observations:

(1) Lever assembly was bent.

(2) lever for rocker mechanism was below lever C.

Causation:

Load stress caused the metal to eventually lose its shape due to metal fatigue. This was inevitable as only one hinge was operational.

Solution:

I shaped all the critical points to what I believe to be the original position with a hammer and vice.

How to prevent failure in future?

Two hinge mechanism MUST be operational for the backdoor to open and lock reliably. In the event one fails. The backdoor is supposed to jam shut. This is supposed to be the fail safe according to Toyota. It should not open. In this case the back door continued to open and lock as if two hinges were working as they were supposed too. Hence no warning till catasthropic failure.

Since only one hinge was bearing all the stress of load. And with the other hinge no operating. This one operational hinge was the pivot point. Since the construction of the mechanism is only sheet metal, it is very likely that under moderate to heavy load stress, it will lose it shape and all the tolerances for the critical points will run. When this happens the backdoor will revert to its fail safe and lock up and fail to open normally.

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