We recently worked on a Western Star 4900 that had been converted from an MBE 4000 to a Detroit Diesel Series 60 — a solid upgrade, but one that came with its share of electrical mods.
After the swap, everything ran fine, except one of the things: the fan override switch in the cab didn’t do anything. No fan engagement, no ECU response.
We had already remapped the entire engine wiring harness for the Detroit, but the fan control logic wasn’t behaving. The reason? On the Series 60, fan override is an Input/Output (I/O) function defined inside the ECM configuration, and just plugging in the wires doesn’t mean the ECM knows what to do with them.
The fix required reprogramming the ECM to accept the override input and assign a proper output to the fan solenoid. Once those settings were correctly mapped using DRS (or DDRS for DDEC IV+ ECUs), the manual fan switch started working exactly as expected — no jumper wires, no guessing, just solid OEM-style integration.
When you swap engines, it’s not just a mechanical job — the brains of the system need to be updated too. Rewiring is half the battle. The rest is smart configuration.
