WAIT-to-Start Crank Lockout — Design Note (Deferred)¶

Why it was deferred¶

An automatic lockout was evaluated in depth and set aside as not worth the complexity for this build:

• Skipping the wait is harmless. On a common-rail R2.8 the grid heater is an ECM-managed cold-start aid; cranking during preheat doesn't damage anything — worst case is a marginally harder fire. The WAIT lamp is advisory, not a protection interlock.
• It's a 3–5 second wait, once per cold start (per Grid Heater), and watching the lamp is the manufacturer's documented diesel procedure.
• Every automatic option adds disproportionate cost — parts, wiring, and/or coupling the start path to another module — to automate something a glance handles.
• Direction of travel. Critical systems (TCU, iBooster) are being moved off the PMU; adding the crank lockout onto the PMU runs against that.

The requirement (if revived)¶

Block the crank signal while the WAIT lamp is on, ANDed with the PBS-I crank command, without loading ECM Pin 35:

• WAIT signal: ECM Pin 35 (yellow), sink-circuit lamp topology — active LOW (~0 V when WAIT on, ~13 V off).¹
• Constraint: the Pin 35 lamp-driver sink-current rating is not published in any public Cummins crate document, so a design must not lean on it. A relay coil on Pin 35 (~160 mA) was the original concern tracked as issue #117.²
• AND logic: crank only when the PBS-I is commanding START and WAIT is off.
• Fail-safe: any fault must default to crank-enabled (engine still starts).

Options evaluated¶

Recommended implementation, if revived¶

A relay-based gate where the PMU is only the sensor, never in the crank path. This was fully worked out and is the design to resurrect:

PMU In 4  ← ECM Pin 35 (WAIT lamp)        high-Z digital input → ~µA on Pin 35
PMU Out 24 → gate-relay coil+             (from +12, NOT Pin 35); coil− to cabin ground
relay NC   in series:  PBS-I PURPLE START → relay COM → NC → firewall Pin 15 → Cole Hersee coil+

• PMU logic: IF In4_WaitToStart == LOW THEN Out24 = ON (block) ELSE OFF (pass).
• Behavior: WAIT on → PMU energizes relay → NC opens → crank blocked. WAIT off → relay drops → NC closed → start passes.
• Zero Pin 35 load: the PMU senses with a high-impedance input; the relay coil is driven from the PMU output, so the coil current never touches Pin 35 — a standard ~150 mA relay is fine.
• Block-only: the PMU drives the relay coil, not the starter solenoid, so it can only interrupt an already-authorized PBS-I crank, never command one.
• Fail-safe, no bypass needed: dead/unpowered PMU, failed Out 24, or open coil → relay de-energized → NC closed → PBS-I cranks. A dead PMU also stops the cooling fan, so the start path is never the limiting failure.

Proposed I/O (both currently spare): PMU In 4 (WAIT sense) and Out 24 (gate-relay coil, ~0.16 A). See PMU Inputs / PMU Outputs.

Parts:

• Gate relay: any standard 12 V SPDT (changeover) automotive relay wired SPST-NC; NC contact carries only the ~0.69 A Cole Hersee coil. Mount cabin, adjacent to the PBS-I.
• (Optional limp-home, not required) a manual bypass across the relay's COM↔NC — a latching illuminated push-button such as the Blue Sea 4160 (build standard) glowing when engaged, or simply a spare relay / a COM-NC jumpered "bypass plug" in the glovebox. The NC topology already self-enables on the common faults, so this is belt-and-suspenders.

Why not the simpler-looking alternatives¶

• No off-the-shelf "WAIT-gate" module exists. Plug-in SSRs don't come in a form that does a fail-safe series AND of two 12 V signals, and low-coil-current relays only exist as PCB parts. Anything that gets Pin 35 to ~zero load while staying a finished, plug-in product ends up being the PMU (sensor) + relay (gate) above.
• It can't live in the BODY PDU. The Bussmann LR-2 uses ISO 280 micro relays (~150 mA coils → re-creates #117), and it's a high-side power panel, not a series-signal-gate host.

Sources¶

Related Documentation¶

• Starter System - The crank circuit this would gate (sole inbound link to this note)
• Keyless Ignition - PBS-I architecture and the current watch-the-lamp cold-start sequence
• Grid Heater System - R2.8 preheat duty cycle (3–5 s typical)
• HDX Control - ECM Pin 35 WAIT signal and its active-low sourcing