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Custom JST & Molex Wire Harness: What a Factory Actually Checks Before You Order

Here is a scene we see every few weeks. An engineer emails us a drawing, then follows up an hour later because his line is down. The supplier before us shipped a batch of connectors that were "JST-compatible" but turned out to be anything but. Pins would not seat. Housings cracked. His PCB was already sitting in the wave solder machine. We have built JST and Molex harnesses for ten years now, for kiosk terminals, battery packs, energy storage cabinets, 3D printers, you name it. This guide is basically everything we learned the hard way, so you do not have to. In this guide: JST and Molex series, with the numbers that actually matter (pitch, current, wire size) JST vs Molex: an honest comparison, because neither brand wins everywhere How a custom JST wire harness gets built, step by step, no shortcuts Wire, stripping, crimping, and the tests that catch a bad crimp before it ships Where these harnesses actually fail in the field, and what to do about it A spec template you can copy into your next RFQ Table of Contents

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