The Short Answer

If your ARB compressor won't start on a Grenadier, the most common cause is a wiring fault — a blown fuse, an improperly wired relay, or connecting to the wrong wire in the factory pre-wired circuit. If it starts then stops, it's almost always the thermal cutout tripping from poor ventilation under the rear seat. If it runs but inflates slowly, voltage drop from undersized wire is strangling output. Here's how to diagnose and fix each failure mode.

Quick Diagnostic — ARB Compressor on INEOS Grenadier
  • Won't start at all: Check fuse F113 (interior electrical centre), verify relay trigger wire is ignition-switched (yellow), and confirm motor wires go to brown (+12V) not yellow.
  • Starts then cuts out: Thermal cutout. Move the compressor or add a ventilation gap — under the rear seat has almost no airflow.
  • Slow inflation / low pressure: Voltage drop. Measure battery voltage at the compressor terminals under load — anything below 11.8V at the compressor means the wiring is undersized or the connection is resistive.
  • DTP port won't power it: Factory DTP EXT1/EXT2 ports are rated 25A switched. The ARB twin (CKMTP12) draws up to 32A peak. It exceeds the DTP port limit — wire direct to battery instead.
  • Switch light on constant / compressor pulses: Switch wiring error. The switch trigger wire (middle terminal on the ARB switch) must go to ignition-switched +12V, not motor +12V.
  • Connector corroding / water ingress: Anderson connectors are not IP-rated. If mounted in an exposed location under the seat or near the door sill, seal or relocate.

Understanding the Grenadier's Compressor Power Circuit

The INEOS Grenadier includes a factory-wired 40A compressor circuit on non-US, high-load specification models. Under the rear seat on the right (passenger) side, you'll find three loose tails: a brown wire (+12V permanently live, fused at F113 for 40A), a yellow wire (ignition-switched +12V, signal use only), and a black wire (chassis earth). The brown wire is your motor power source. The yellow wire is the relay trigger — it activates the relay coil only, it cannot carry 32A of compressor current.

US-spec Grenadiers do not have these pre-wired tails. US owners need to source power directly from the main battery, a busbar, or the spare 80A fuse slot beside the starter battery — a route confirmed by multiple forum members on TheIneosForum as the cleanest solution on non-high-load equipped vehicles.

For lower-draw compressors (single-piston ARB CKMP12, ~17A peak), the Grenadier's DTP EXT ports are a tempting tap point. The EXT1 and EXT2 ports on the roof each carry 25A switched — workable for a single compressor used intermittently, but a tight fit. The twin (CKMTP12) at 32A peak exceeds this limit and must go direct to battery. DVA's DTP-06-2S to Anderson SB50 cable makes a clean EXT port tap for single-compressor setups, with an Anderson SB50 output your compressor can quick-connect to.

Fault 1: Compressor Won't Start

This is almost always electrical. Work through this sequence before assuming the compressor itself is faulty:

Step 1 — Check fuse F113

On high-load spec Grenadiers, the compressor circuit is protected by fuse F113 in the interior electrical centre. If F113 is blown, the brown wire has no power. Replace it with a 40A blade fuse. If it blows again immediately, there's a short in the harness.

Step 2 — Confirm relay wiring

The ARB relay (or any 40A relay you've fitted) has four terminals: 85 (relay coil trigger), 86 (coil ground), 30 (motor +12V in), and 87 (motor +12V out to compressor). Common mistakes:

  • Connecting motor power (terminal 30) to the yellow ignition wire — yellow is a signal wire, not a power wire. The relay coil has negligible draw, but the motor can't run off it.
  • Leaving terminal 86 ungrounded — the relay never closes.
  • Using the relay's coil trigger as the compressor power wire directly — bypasses the relay entirely and puts 17-32A through a signal circuit.

One forum member on TheIneosForum described installing a covered switch (to prevent accidental activation) and deliberately bypassing the ignition wire to allow the compressor to run with the engine off — a valid mod, but it requires the relay trigger to come from a permanently-live source, not the yellow ignition wire.

Step 3 — Test voltage at the relay output under crank

With a multimeter on terminal 87 (relay output), turn on your switch and measure voltage. You should see battery voltage (12.6–13.8V). If you see less than 11V, check the ground connection — a resistive ground is the second most common cause of "compressor won't start." The chassis has plenty of spare earth lugs near the battery box; use one.

Fault 2: Compressor Starts, Then Cuts Out

The ARB CKMP12 and CKMTP12 both have thermal protection that cuts the motor if the head temperature exceeds safe limits. Under the rear seat, there's almost no ventilation. Forum member marko noted on TheIneosForum's fixed air compressor thread: "I wonder if the compressor is going to get enough ventilation mounted under the rear seat cushion?" — a concern that proved accurate for many owners who tried it.

Fixes in order of effectiveness:

  1. Leave the seat cushion slightly propped during use — adds a few centimetres of air gap. Not elegant, but works.
  2. Relocate to the wheel arch cavity (left side, between inner and outer body). dcpu on TheIneosForum identified the left-hand wheel arch/cavity between inner and outer body sides as a better-ventilated location with accessible fuse tails.
  3. Run it with the engine on and doors open to maximise ambient airflow through the cabin. Compressor duty cycle matters: inflating four tyres from 20 PSI to 40 PSI takes approximately 8–12 minutes for the twin; single-piston setups take 15–20 minutes. That's enough to trip the thermal cutout in a sealed under-seat enclosure.

Fault 3: Slow Inflation / Low Pressure Output

If the compressor is running but taking twice as long as expected to inflate a tyre — or stopping at lower-than-target pressure — voltage drop is the likely cause. The ARB CKMTP12 twin loses meaningful output when supply voltage drops below 11.5V at the compressor terminals. At 10.5V it may not build enough pressure to seat a bead.

Measure voltage at the compressor motor terminals (not at the battery) with the compressor running under load. If you're seeing more than 0.5V drop from battery terminal to compressor terminal, the wiring is undersized or has a resistive connection.

Minimum wire gauge for ARB compressors:

  • CKMP12 single (~17A): 6 AWG (8mm²) for runs over 2 metres
  • CKMTP12 twin (~32A peak): 4 AWG (25mm²) for runs up to 3 metres

The stock ARB wiring harness is undersized for runs longer than 1 metre to a rear-of-vehicle mounting point. Many Grenadier owners who mounted under the rear seat with a long run to the battery reported slow inflation — extending with same-gauge wire compounds the issue. Run proper gauge wire from the power source to the relay, and from the relay to the compressor.

Fault 4: DTP Port Trips or Won't Power the Compressor

The Grenadier's factory DTP EXT system uses DTP-06-2S roof connectors, with EXT1 and EXT2 rated at 25A switched. The ARB single (CKMP12) draws ~17A average and up to 25A at startup — a marginal fit at best. The ARB twin (CKMTP12) draws up to 32A peak, which exceeds the EXT port rating and will trip the circuit protection.

If you want to use the DTP system for a single compressor, DVA's DTP-06-2S to Anderson SB50 cable adapts the roof EXT port to a standard 50A Anderson connector — the same connector used on most portable compressor kits. This is a clean, weatherproof solution for a single ARB compressor used intermittently. For twin setups or extended use, direct battery wiring is the correct route.

The DVA PowerVault™ Utility Hub R2 adds a dedicated Anderson SB50 output port plus USB and IP68 power outputs to the Grenadier's exterior, all powered via the DTP system. For owners who want a clean plug-in point without running new wires through the body, the PowerVault routes EXT port power to accessible connectors that a compressor can tap into directly.

Fault 5: Switch Light Stays On Constant / Compressor Activates Briefly Then Stops

This is a switch wiring error. The ARB illuminated switch has three left-side terminals: top (negative/black), middle (motor +12V), and bottom (ignition trigger/red). Forum members on TheIneosForum's ARB twin wiring thread worked through this explicitly:

"I first tried top black, middle purple, and bottom red on the left side. The switch's light stayed on constant and the compressor switched on for a second that way."

— forum member, TheIneosForum, "Wiring ARB twin compressor," Sep 2024

The correct wiring (left side of switch, from top): black (ground), red (motor +12V feed), purple (ignition trigger). Multiple members confirmed this sequence. If the switch light stays illuminated with the compressor off, the ignition trigger (purple) is receiving battery voltage permanently rather than ignition-switched voltage.

Fault 6: Connector Corrosion / Water Ingress

Anderson connectors are not IP-rated. One forum member who completed the single-compressor install described his approach directly: he used Anderson connectors but placed them "as high as possible" since they weren't waterproof, noting it was a compromise. If your Andersons are mounted low in the doorway area or near the sill where water runs, corrosion builds on the blade contacts within a season.

Solutions:

  • Move the connector to a higher location — above the seat line is significantly drier than below it.
  • Switch to Deutsch DT connectors — the DT series is IP67-rated and common in marine and automotive installations. The DTP connectors already used in the Grenadier's roof circuit are IP67.
  • Use dielectric grease on the Anderson blades — reduces but doesn't eliminate moisture ingress.

What DVA Makes for ARB Compressor Setups

DVA builds cables specifically for the Grenadier's DTP power architecture — so you can tap the factory EXT system without cutting into factory wiring:

Wiring Reference: ARB Compressor Circuit Summary

Model Peak Draw Wire Gauge (3m run) Fuse DTP EXT Compatible?
CKMP12 Single ~25A peak, ~17A avg 6 AWG (8mm²) 40A Marginal (EXT1/2 only, intermittent use)
CKMTP12 Twin ~32A peak, ~28A avg 4 AWG (25mm²) 80A (two 40A inline to each motor) No — exceeds 25A EXT limit

Sources

Get the Right Power Cable for Your Grenadier Compressor

DVA builds DTP-native power adapters so you can tap the Grenadier's factory EXT ports without cutting wires. All cables are weatherproof and designed for the Grenadier's architecture.