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Liquid cooling loops

Bleed airlocks, find leaks, and prove flow before you condemn cooled cables.

Liquid cooling loops

When you use this / what it is not

Use this when high-power DC cables or power modules derate, overtemp, report flow/pressure faults, leak, or lose performance after cable swaps. Audience: techs who will handle glycol mixes and quick-connects on cooled gun systems.

This is not:

  • A chemistry lab for inventing coolant recipes
  • HVAC certification
  • Permission to run dry “for a minute” to test pumps
  • An excuse to ignore OEM coolant part numbers (wrong fluid eats seals)

Air-cooled cabinets still exist — confirm the stall actually has a liquid loop before you bleed anything.

How cooling faults present

Display / LEDs

  • Overtemperature, flow error, coolant level, pump fault
  • Power derate banners long before Faulted
  • Pump LED off / flashing on power module or CDU (coolant distribution)

Physical

  • Puddles under holster, oily/glycol smell, sticky residue on quick-connects
  • Cable abnormally hot near handle at modest kW
  • Gurgling, cavitation noise from pump
  • Collapsed hose, kinked loom after reinstall

OCPP / CSMS

  • SuspendedEVSE or Charging at low kW with thermal vendor codes
  • Faulted: flow sensor, pump, overtemp
  • Seasonal pattern: fine in March, dead in July

Decision tree

Branch A — Leak visible

  1. Power down / LOTO as required; contain spill (glycol environmental rules).
  2. Identify source: handle, mid-cable, cabinet QD, pump fittings, cold plate.
  3. Do not top off forever — fix the fitting.
  4. After repair: fill, bleed, pressure/flow check per OEM.

Branch B — No leak, flow fault

  1. Check level in reservoir / sight glass.
  2. Listen/feel pump; check pump power and PWM/enable.
  3. Inspect for pinched line after recent cable change.
  4. Flow sensor spoof/failure: OEM may show flow=0 with pump clearly running — sensor/path.

Branch C — Airlock after cable swap (very common)

  1. New cable installed, immediate flow errors or loud pump.
  2. Bleed at OEM valves; invert/manipulate cable per procedure.
  3. Run pump with service mode if available until bubbles stop.
  4. Retest at moderate kW before releasing to public peak.

Branch D — Overtemp with flow OK

  1. Radiator/fan pack clogged with lint/cottonwood.
  2. High ambient + sun + full power.
  3. Wrong glycol mix (too much water → corrosion; too much glycol → poor heat transfer).
  4. Module cold plate issue — escalate.

Branch E — Intermittent only at high kW

  1. Marginal flow / partial clog.
  2. Sensor hysteresis.
  3. Shared CDU feeding multiple guns — capacity short when all run.

Branch F — False sensor

  1. Service UI shows nonsense temps (−40 / +200).
  2. Reseat connectors; replace sensor per OEM.
  3. Do not ignore — derate logic may zero power or may fail unsafe if defeated (never defeat).

Ranked causes

  1. Airlock after cable or QD service — #1 after maintenance. Field note: bleed discipline > parts shotgun.
  2. Low coolant level / microleak — Field note: UV dye if OEM allows.
  3. Pinched quick-connect / half-latched QD — Field note: click-click both sides; tug test.
  4. Failed pump — Field note: voltage present, no vibration → pump; no voltage → enable circuit.
  5. Clogged heat exchanger / fans — Field note: vacuum/brush; don’t pressure-wash electronics.
  6. Flow sensor failure — Field note: compare inferred heat vs reported flow.
  7. Wrong fluid / contaminated fluid — Field note: oily sludge → flush per OEM.
  8. Kinked external cable loom — Field note: holster routing.
  9. Shared CDU undersized — Field note: multi-gun simultaneous test.
  10. Temp sensor drift — Field note: IR vs reported (approximation only).

Step procedures

Procedure 1 — Identify loop topology

Tools: OEM diagram if available, flashlight.

  1. Single-gun integral vs remote CDU vs module cold plates vs cable-only loop.
  2. Sketch in ticket: reservoir → pump → cable → return. Expected: You know which QD belongs to which gun.

Procedure 2 — Visual leak survey

  1. Holster, strain relief, cabinet floor, drip tray.
  2. Paper towel wipe on fittings to find wetness.
  3. Photo everything before cleanup. Expected: Source localized or confirmed dry.

Procedure 3 — Level and fluid check (LOTO if opening reservoir in live cabinet per OEM)

  1. Cool system if OEM requires cool-touch.
  2. Level between MIN/MAX.
  3. Note fluid color/clarity; milky = water contamination sometimes.
  4. Refill only with OEM-specified mix; record lot. Expected: Level stable after run (if drops → leak).

Procedure 4 — Bleed / de-air

  1. Follow OEM valve order — wrong order reintroduces air.
  2. Run service pump mode.
  3. Cycle cable through gentle loops to move bubbles (don’t kink).
  4. Recheck level. Expected: Flow sensor OK; quieter pump; stable temps under load.

Procedure 5 — Flow / pressure verification

Tools: OEM service screens; inline gauges only if OEM fitted.

  1. Record flow setpoint vs actual if displayed.
  2. Record pump RPM/PWM command.
  3. If flow=0 and pump commanded 100% → clog, air, seized pump, or open impeller. Expected: Numbers inside OEM band.

Procedure 6 — Load validation

  1. Known-good EV, target significant kW for ≥5–10 minutes.
  2. Watch handle temp and module temp trends.
  3. Confirm no SuspendedEVSE from thermal. Expected: Sustained power appropriate to ambient.

Safety / LOTO

  • Glycol is slippery and environmentally regulated — contain and dispose properly.
  • Hot coolant burns; pressurized systems can spray — cool down, wear eye protection.
  • Never open HV cold plate loops without full electrical LOTO — conductive fluid + HV is nightmare fuel.
  • Do not substitute automotive “whatever is in the truck” coolant.
  • QD depressurization: know if the system holds pressure when off.

Cable swap discipline

Before disconnect:

  1. Mark supply vs return.
  2. Cap open fittings immediately.
  3. Catch spillage. After connect:
  4. Tug test QDs.
  5. Fill/bleed.
  6. Service-mode pump.
  7. Short load test. Skipping bleed is the fastest way to a comeback and a cooked cable.

Shared CDU notes

When one CDU cools two+ guns:

  • One gun flow fault may be steering valve / balance issue
  • Simultaneous max sessions expose undersized pumps
  • Isolating one loop incorrectly can starve the other

Test guns alone and together before closing.

When to escalate

OEM: cold plate leak inside module, persistent sensor faults after harness check, CDU control board, repeated airlocks with intact bleed procedure.

Environmental/facility: large spills, drain path to storm sewer.

Engineering: chronic undersized cooling on multi-stall buildouts.

Do not escalate “needs new cable” until bleed and level are proven. OEMs notice.

Field worksheet

CheckValue
Loop type
Level
Leak source
Pump commanded / actual
Flow reading
Bleed performed
Fluid type/lot
Load test kW / minutes
Temps start/end

Winter vs summer

  • Winter: thick fluid, weak pumps, false flow faults on cold start — allow warm-up if OEM says.
  • Summer: real thermal limits; fans matter as much as pumps.
  • Spring cottonwood: clog season for exchangers.

Closeout

State whether you bled the loop, what fluid you added, and the sustained kW result. “Cleared flow fault” without a load test is an incomplete close. EOF

Extended bleed failures — why air comes back

If you bled yesterday and flow fault returns today:

  1. Microleak introducing air on the suction side of the pump (worst).
  2. Reservoir cap vent issues.
  3. QD not fully latched — sucks air under pump vacuum.
  4. Vertical loops trapping pockets at handle — need OEM-specific manipulation.

Suction-side leaks are sneaky: dry floor, chronic air. Soap test fittings under slight pressure if OEM procedure allows.

Coolant mix field reality

OEM specifies ratio for freeze and boil performance. Roadside “50/50 whatever” causes:

  • Gel in winter
  • Corrosion in summer
  • Seal swell

If the reservoir looks rusty or oily, plan a flush — that may be a longer outage; tell ops upfront.

Pump current as a diagnostic

If service UI shows pump current:

  • High current + low flow → clog or seized bearings starting
  • Low current + low flow → cavitation/air or pump not coupled
  • Normal current + flow=0 reported → sensor/path electronics

Write the triad: command, current, flow.

Cable loom routing mistakes after swap

  • Bend radius too tight at holster
  • Twisted supply/return
  • Weight hanging on QD
  • Zip ties crushing soft hose

Fix routing before condemning the new cable.

Environmental spill kit

Carry or request:

  • Absorbent pads
  • Disposal bags labeled per site rules
  • Gloves compatible with glycol
  • Drain covers if near storm drains

Report significant spills to facility — do not hose into parking lot drains.

Multi-gun CDU balancing

After repairing gun A flow, verify gun B. Steering valves and check valves fail partially. Simultaneous load test is mandatory on shared CDUs.

Expanded closeout

Record fluid added (type, approximate volume, lot), bleed steps, flow numbers, and sustained kW test. Note if another gun on the CDU was validated.

Pressure vs flow mental model

Techs mix these up:

  • Pressure high + flow low: restriction downstream, kink, clogged gun passages.
  • Pressure low + flow low: pump weak, air, low level, bypass open.
  • Pressure OK + flow sensor 0: sensor or harness.
  • Everything OK + overtemp: radiator/fan side or undersized design at ambient.

Write pressure and flow when the UI offers both. One number alone lies.

Commissioning a new cooled gun (full sequence)

  1. Confirm part number cooled vs uncooled.
  2. Cap and catch fluid on removal of old gun.
  3. Mate QDs fully; tug test; route loom.
  4. Fill to mark with OEM fluid.
  5. Bleed per valve order.
  6. Service pump until bubble-free.
  7. Inspect for leaks under pump pressure.
  8. Idle sensors sanity check.
  9. Load test moderate kW, then near-rated if ambient allows.
  10. Document lot codes and volumes.

Skipping 5–7 is the comeback loop.

Fan pack service without drowning the cabinet

Use brush/vacuum. If washing the radiator exterior, shield electronics, disconnect power as required, and dry fully. Cottonwood season deserves a calendar reminder in depot maintenance, not only EVSE break/fix.

Module cold plate vs cable loop

Some faults are module-only loops with a healthy cable loop (or reverse). Identify which pump runs when you command service modes. Replacing a cable will not fix a module plate clog. Topology sketch in the ticket prevents wrong parts.

Final field reminders (cooling)

  • Airlock after QD service is more common than failed pumps — bleed before parts shotgun.
  • Capture command / pump current / flow together when UI allows.
  • Shared CDUs need alone and together load tests.
  • Wrong fluid is a slow failure mode; record lots.
  • Hot glycol sprays — eye PPE, depressurize per OEM.
  • Overtemp with good flow → clean heat exchange path and check ambient/sun load.

Sight-glass interpretation

  • Clear green/pink/blue per OEM color: normal.
  • Cloudy: possible water contamination or air emulsion — recheck after pump stop.
  • Rust tint: corrosion; plan flush.
  • Oil sheen: abnormal; escalate fluid analysis if OEM supports it.
  • Empty glass with “level OK” in UI: sensor lie — trust eyes, then LOTO investigate.

Log color and clarity every cooling call. Trends matter across visits.

Site record prompt

Before leaving, note ambient conditions, recent firmware, and whether any other trade was on site the same day. Correlate those facts with the failure window. Many “mystery” tickets dissolve once concurrent work is written down.

Character of evidence required: timestamps, chargeBoxId, connectorId, and exact vendor strings — not paraphrases.

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