Purpose and overview (theory)
- The heater core is a small radiator inside the vehicle/cab HVAC box. Hot engine coolant is routed through it; the HVAC blower forces air across the heater-core fins and that warmed air is sent into the cab. Think of it like a miniature radiator or a tea infuser: hot (coolant) flows through tubes while a stream of air picks up that heat.
- Why repair is needed: leaks let coolant into the cabin and lower system coolant volume; clogs/restrictions or airlocks prevent hot coolant flow so the heater produces little or no heat. Leaks cause coolant smell, wet carpets, fogged windows and engine overheating risk. Clogs reduce cabin comfort and can indicate corrosion inside the cooling system.
- What can go wrong: heater-core leaks (tube/fins corrosion, broken end-tanks, damaged solder joints), internal blockage from corrosion/sediment, heater hoses split or collapse, failed heater control valve or blend door causing no flow or no warm air, air trapped in the system, or HVAC box damage during removal.

Components — detailed descriptions
- Heater core (matrix)
- Tubes: small-diameter coolant passages, usually brazed to end tanks. Hot coolant flows in through the inlet and out through the outlet.
- Fins: thin metal sheets between tubes that increase air contact area for heat transfer.
- End tanks/headers: the inlet/outlet manifolds that direct coolant through the tubes.
- Seals/solder/brazing: joints that can fail and cause leaks.
- Heater hoses
- Supply hose (hot, from engine/radiator/thermostat area) and return hose (goes back to radiator/water pump). Often rubber reinforced hose with clamps.
- Hose clamps / quick-connect fittings
- Worm-drive clamps, spring clamps or OEM quick-disconnects seal hoses to fittings.
- Heater control valve (if fitted)
- A valve that meters coolant flow through the heater core. Can be vacuum, cable, or electrically operated. If it fails closed, no heat even if the heater core is fine.
- HVAC housing / heater box
- Plastic or metal enclosure under the dash that contains the heater core and blend doors. Often secured with screws/bolts and sealed with gaskets to prevent leaks.
- Blower motor & resistor / fan
- Moves air through the HVAC box across the heater core. If the blower is dead you’ll get no warm air flow even if heater core is good.
- Blend door / actuator
- Directs airflow through heater core or bypass. A stuck/failed door can cause no warm air even when the core is hot.
- Drain/overflow / expansion tank / coolant reservoir
- Holds excess coolant and accepts coolant when system expands. Relevant for draining/refilling.
- Thermostat & water pump (related)
- Thermostat controls engine temp; if stuck open you may not get hot coolant. Water pump circulates coolant; weak flow reduces heat transfer.
- Cabin carpet / insulation / floor pans
- Areas that get wet if the heater core leaks; inspection and drying/replacement may be necessary.

Common symptoms and diagnosis
- Symptoms:
- Sweet antifreeze smell inside cab; fogging of windows.
- Damp or wet carpet (usually passenger side).
- Loss of coolant level without visible external leak.
- Little or no cabin heat while engine is up to temperature.
- Heater hoses show one hot and one cool (normal: both hot when flowing) or both warm/cool if no flow.
- Quick checks:
1. With engine warm and heater on maximum, carefully feel both heater hoses at firewall: both should be hot; if one is cold and one hot, likely restricted or blocked or valve closed.
2. Look for coolant under dash or on passenger floor.
3. Pressure-test cooling system to check for leaks (use a hand pump tester, compare to radiator cap rating).
4. Run engine until warm with heater on and use scan or feel for blower activity and blend-door movement.
5. Check heater control valve operation (manual/cable/vacuum/electric).

Tools & consumables
- Basic hand tools: screwdrivers, ratchet & metric sockets, pliers, trim removal tools.
- Hose clamp pliers or screwdriver for worm clamps.
- Container to catch coolant (large drain pan).
- Replacement heater core (OEM or equivalent).
- New heater hoses and hose clamps recommended.
- Replacement gaskets/seals and HVAC housing screws.
- Coolant to refill; disposable gloves; shop rags; eye protection.
- Vacuum coolant filler or funnel with bleed hose.
- Hand coolant pressure tester (recommended).
- Multimeter if testing electrical components (blower, valve).
- Torque wrench for any torque-specified bolts if provided by manufacturer.

Step-by-step repair (generic workshop-style; specific dash-removal steps vary by machine/installation)
Note: Many Perkins 403C/404C installations are in different vehicles/cabs. The heater core replacement steps below are detailed but generic — adapt to the cab’s trim layout. Do this with the engine cold.

1) Safety & preparation
- Park on level ground, set parking brake, chock wheels.
- Wear gloves/eye protection.
- Disconnect negative battery terminal (prevents stray fan/blower operation or shorting).
- Allow engine and coolant to cool completely.

2) Drain coolant to below heater core level
- Place drain pan under engine radiator drain or low point and drain enough coolant so the heater core won’t spill when hoses are disconnected. If there’s a local drain cock near the block or under the cab, use it. Save coolant if clean; otherwise dispose properly.
- Alternatively, you can pinch hoses and partially drain to firewall hose level.

3) Access the heater core
- Remove dash lower trim panels, glovebox, center console, and/or under-dash covers as required. Use trim tools to avoid breaking clips.
- On some cabs you must remove the blower motor or glovebox to reach the heater box cover.
- Remove screws/fasteners holding HVAC housing access panel. Take note of fastener locations for reinstallation.
- Disconnect HVAC ducts as needed; label hoses and wires for correct reassembly.

4) Disconnect heater hoses at firewall
- From under the hood or inside engine bay, locate the two heater hoses going through the firewall to the heater core. Loosen clamps and carefully disconnect. Catch residual coolant.
- If hoses are old, cut and replace them; inspect fittings for corrosion.
- Plug hoses to prevent contamination and reduce spillage.

5) Remove heater core from HVAC housing
- Inside the cab, remove the HVAC housing access panel or unzip/remove insulation.
- Carefully free any wiring, actuators, linkages or sensors attached to the housing (label and photograph linkages for reassembly).
- Remove the heater core retaining clips or screws. The core may be mounted in a cradle or slide into a recess. Note orientation of inlet/outlet.
- Gently slide the core forward and out. Be mindful of sharp fins and brittle plastic housings.

6) Inspect and prepare new core
- Compare new core to old: inlet/outlet locations and size must match.
- Fit any provided seals, gaskets or O-rings to the new core. Replace HVAC housing seal/gasket if deteriorated.
- Replace heater hoses and clamps if needed.

7) Bench-test new core (recommended)
- Before installation, pressure-test the new core at system pressure (generally around 15 psi / ~1.0 bar — match your system spec) to check for leaks. Submerge in water and pressurize to verify no bubbles. Do not exceed the normal system pressure.

8) Install new core
- Slide the new core into the heater box in the same orientation as removed. Ensure seals seat properly, and that the inlet/outlet tubes protrude through the firewall correctly.
- Refasten any retaining clips and replace the HVAC housing cover and gaskets.

9) Reconnect heater hoses and controls
- Reinstall hoses to the inlet/outlet, using new clamps or OEM quick couplers. Ensure hoses are routed without kinks and away from sharp edges or heat sources.
- Reconnect any blower wiring, actuators, ducts, and sensors.
- Reinstall dash panels, glovebox, and all removed trim.

10) Refill coolant & bleed air
- Refill with the correct type and concentration of coolant to the specified level. Use distilled water mixed per manufacturer spec if diluting concentrate.
- Bleed air from the cooling system: common method — start engine with heater set to maximum hot and blower on high, open bleed screw(s) if fitted, and keep adding coolant until no air bubbles come out and heater hoses are both hot. Alternatively use a vacuum-fill tool to remove air for a more reliable fill.
- Tighten radiator cap when no air remains and top up expansion tank.

11) Leak and function checks
- Run engine to operating temperature, monitor for leaks at heater hose connections and around the heater core area under dash.
- Confirm heater now produces hot air and both supply and return heater hoses are hot.
- Check coolant level again after a short drive/operation and top up if necessary.
- Dispose of old coolant responsibly.

Special notes, tips & troubleshooting
- If heater still cold after replacement: check heater control valve (stuck closed), blend door actuator, blower motor, thermostat (stuck open) or air trapped in the system (re-bleed).
- If coolant is leaking into cabin after replacement: inspect firewall seals and HVAC housing gasket; check for missing or damaged drain tube (many heater boxes have a small drain); inspect seams in the heater core and firewall connection for proper sealing.
- Temporary fixes (stop-leak) can block heater core tubes and ruin the core; avoid unless emergency temporary fix.
- Corrosion and electrolysis: if the heater core failed from corrosion, inspect the whole cooling system and consider flushing and replacing corroded components; test for stray electrical currents in the cooling system if repeated corrosion occurs.
- Replacing the blower motor or blend door actuator often requires similar disassembly—inspect them while the dash is open.
- Keep track of small parts and make notes/photos of wire/duct routing — reassembly is easier with reference photos.

Safety reminders (short)
- Work only when engine cold. Hot coolant causes severe burns.
- Catch and dispose of coolant properly — it is toxic and attractive to animals.
- Support dash parts so they aren’t damaged when fasteners are removed.
- Use proper PPE and avoid breathing dust if HVAC insulation is disturbed.

Expected time and difficulty
- Difficulty: moderate to high — depends on cab layout. Access can be simple in some machines but requires complete dash removal in others.
- Time: 2–6 hours typical for experienced tech; more if dash removal is complex or access limited.

Quick summary
- Heater core = small radiator inside HVAC box. Repair needed for leaks or blockages. Procedure: drain coolant, remove dash/HVAC access, disconnect hoses at firewall, remove old core, test and install new core, reconnect hoses, refill and bleed coolant, check for leaks and proper operation. Replace associated hoses, seals and inspect blower and blend doors while you’re in there.
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Perkins 400 series ARD animation ARD.

PERKINS ENGINE ON BACK OF TRUCK 404D OLD FORGE POWER EQUIPMENT, INC. 5 BROOKSIDE ROAD CARBONDALE PA 18407 570-471-3069 WWW.

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