If your OBS 7.3 (’94.5–’97) has random misses, contribution codes, rough cold starts, or the classic “it runs better when I wiggle the valve cover plug”… you’re not imagining it. The under-valve-cover harness (UVCH) and valve cover gasket pass-through connectors live in harsh environments (heat + oil), and they eventually wear out to the point of increased resistance, intermittent issues, and dead cylinders.

 

Why do the Super Duty UVCH mod?

OBS 7.3’s use two connectors per valve cover (more connections = more failure points). The ’99–’03 Super Duty style goes to one connector per cover with more robust terminals—especially where glow plug current matters. Result: fewer gremlins and easier future service.

 

Benefits you’ll notice (assuming your harness’ are in need of some love)

• More consistent cold starts (glow plug circuit stops being sketchy).
• Fewer random misfires/contribution codes tied to harness/connector issues.
• Cleaner wiring / simpler diagnosis (one connector per cover).

 

How it’s done

1. Pull valve covers, remove OBS UVCH + gasket.
2. Install ’99–’03 valve cover gaskets (they’re the electrical pass-through, not just “rubber”).
3. Install ’99–’03 UVCH (one per head) and reconnect injectors + glow plugs.
4. Splice your truck-side harness to the ’99–’03 single-connector pigtails (OBS has two plugs per cover, SD has one).

NOTE: Do not rely on wire colors when splicing. We've found that different harness manufacturers used different colors, so they may not always match. You can use the below diagrams to help, and always check continuity with a multi-meter (or the like) to be sure. Be sure you are confident in your wiring abilities before beginning this project. 

Related parts

• Valve Cover Gasket ’99–’03: https://www.riffraffdiesel.com/valve-cover-gasket-99-03/
• Under Valve Cover Harness ’99–’03: https://www.riffraffdiesel.com/under-valve-cover-harness-99-03/
• Valve Cover Gasket Connector Pigtail ’99–’03 (the “adapter” piece): https://www.riffraffdiesel.com/valve-cover-gasket-connector-pigtail-99-03/
• Valve Cover Harness Connector Repair Kit ’99–’03 (if yours is melted/damaged): https://www.riffraffdiesel.com/valve-cover-harness-connector-repair-kit-99-03/
• Motorcraft Glowplug ZD-30: https://www.riffraffdiesel.com/glowplug-zd-30-94-03-7-3l/
• Glow Plug Relay (GPR): https://www.riffraffdiesel.com/glow-plug-relay-gpr/

 

 

 

Disclaimer: The information provided on this blog is for informational purposes only. We share our knowledge and experience, but we are not liable for any damages, injuries, or losses that may occur as a result of using this information. Situations are rarely cut and dry in the automotive world. Your situation will likely be somewhat different than what we describe here. Use your best judgment and always consult a qualified professional for automotive repairs and modifications. Your safety is your responsibility.