F-150 R203 Relay Failures Revisited

Back in 2013 I wrote about how my 2004 F-150 stranded me with an R203 relay failure. Since then I haven’t gotten any closer to a cause or solution. I’ve periodically replaced the relay as needed. But I’ve got a few more observations to add.

  • The DTC codes reported by the in-dash diagnostics (Engineering Mode) have meaningless meanings. D900 is just a general communication fault, not helpful, like “syntax error”.  D950 is apparently an instrument cluster issue. Both make sense. No power to the PCM will tend to inhibit communications. No PCM data means the instrument cluster will be data-starved despite being powered up. Duh.
  • When running, the R203 relay runs HOT. You can hardly keep a finger on it.
  • It’s not necessary to replace the R203 relay as soon as it balks. In the beginning, simply removing and replacing it will often get it working again. After a while, stronger ‘persuasion’ is needed. I’ve gotten good at pulling the relay, giving it a couple of raps on the pillar, and jamming it home – even in the dark. Eventually the relay requires replacement.
  • The part of the relay that fails is energizing circuit. A completely failed relay seems physically distorted by the heat over time. I haven’t been able to correlate this to long trips – we don’t use the truck that much – but it wouldn’t surprise me. A cross-country trip involving all-day use might be troublesome. Conclusion: there’s a high current draw on that circuit somewhere. I doubt it’s the relay itself, too many parts have been in there. Maybe the fuse panel itself?
  • Ambient heat worsens the problem. In winter (like Florida has a ‘winter’, right?) the failure doesn’t happen as often. Leaving the kickpanel off helps extend life.  I’ve considered sandwiching the relay with Peltier devices, maybe something like this. Yeah, that’s fixing the symptom and not the cause, but WTF.

That’s about all I’ve got.

Maybe someone out there has the Ford shop manual pages for the fuse/relay panel? And/or the wiring diagram page(s) for the PCM power relay circuit? (2004 Ford F-150, SuperCrew, 5.4L V8, automatic transmission.)

Thanks, you great, big, wonderful Internet, you.

Go ahead and say it.