- Purpose and quick overview
- The coolant/thermal switch on Perkins 4.107 / 4.108 / 4.99 engines is an electrical switch screwed into the coolant jacket or thermostat housing that closes or opens at a set temperature to drive a warning lamp, fan relay, or gauge sender. The steps below cover how to locate, test, remove and replace it safely.

- Safety first
- Work only on a cold engine; hot coolant can cause severe burns.
- Wear safety glasses and nitrile or mechanic gloves to protect from coolant and grime.
- Have a catch pan for coolant and rags to hand; clean up spills and dispose of used coolant in accordance with local rules.
- Disconnect the battery negative terminal if you will be working on electrical connectors for extended periods or if there’s a risk of shorting.

- Tools you will need (each tool described and how to use it)
- Combination spanner set (open-end + box-end)
- Use to hold or turn hex-headed fittings and switch bodies. Choose the correctly sized spanner that fits the thermal switch hex so you don’t round the flats. Box-end gives better grip.
- Deep socket set with ratchet and extension
- A deep 1/2" or 3/8" drive socket that fits the switch hex can make removal easier, especially if the switch sits recessed. Use a ratchet to apply steady torque; an extension lets you reach awkward positions.
- Multimeter (digital, with continuity or ohms function)
- Use to test whether the switch opens or closes electrical continuity as the engine heats. Set to continuity/beeper or low-ohms range to check switching.
- Small flat-blade screwdriver and small pick or thin awl
- Use to release the wiring connector clip and to help remove old sealant or gasket remnants from the thread or seat.
- Coolant drain pan (shallow)
- Place under the engine to catch coolant when you back the switch out. Prevents spills and makes cleanup easier.
- Funnel and coolant container for refill
- Use for topping up or refilling the cooling system after the job.
- Torque wrench (recommended)
- If you have one, use it to tighten the replacement switch to the correct torque. Prevents over- or under-tightening. If you do not have one, tighten carefully by hand and then a further 1/8–1/4 turn with a spanner.
- PTFE tape or suitable thread sealant (if specified for the replacement part)
- Some switch types use a sealing washer; others use pipe thread sealant. Use the correct sealing method for the part you buy (see replacement part notes below).
- Clean rags and brake cleaner or degreaser
- Use to clean the thread area and sensor mounting surface before installing the new switch.
- Heat gun or pot of hot water (optional, for bench testing only)
- If you want to test a removed switch by heating it to activation temperature, carefully heat it in hot water and monitor continuity. Do not use an open flame.

- Locating the thermal switch
- Find the switch generally screwed into the cylinder head, thermostat housing, or a coolant gallery on the side of the block — look for a small hex-headed sensor with one or two electrical spade terminals and a wiring connector.
- On these Perkins engines it is commonly near the thermostat housing or upper water outlet; follow the coolant hoses from the radiator to the head to locate the housing.

- How to test the thermal switch in situ (without removal)
- Ensure engine is cool and parked safely; remove the electrical connector from the switch by releasing the clip with a small screwdriver if needed.
- Set the multimeter to continuity or ohms.
- Probe the switch terminals: one probe on the switch spade and the other on the body/ground (if it is a single-terminal switch it will close to body ground), or across both spades if two-terminals.
- With the cold engine (or after immersing the switch area in cold), note the multimeter reading — typically open (no continuity) or very high resistance below the switching temperature.
- Warm the engine slowly to the operating range (or carefully pour warm water into thermostat housing area) and watch the meter. The switch should change state (beep/close or go to low resistance) at its rated temperature.
- If the switch never changes state, or is intermittent, the switch is faulty and should be replaced.

- How to remove the thermal switch
- Make sure the engine is cool and the area is clean.
- Place the drain pan beneath the switch.
- Disconnect the wiring connector fully.
- Use the correct spanner or deep socket on the hex of the switch and turn counter-clockwise. Expect a small amount of coolant to leak as the sensor is backed out.
- Once loose, pull the switch out; collect any dripping coolant in the pan and wipe the housing seat clean with a rag and a little degreaser.
- Inspect the thread and seat for damage or corrosion.

- How to bench-test an removed thermal switch (optional, careful)
- Secure the switch so the terminals are accessible and safe from shorting.
- Use the multimeter as above to note initial state.
- Place the sensing end in hot water (not boiling) and heat slowly while monitoring continuity; when the switch reaches its cut-in temperature it should change state.
- If it does not switch, replace it.

- When part replacement is required and why
- Replace the switch if:
- It fails the continuity/temperature test (doesn’t change state at operating temperature).
- It is physically corroded, cracked, leaking, or the electrical connector is brittle/broken.
- It is causing fan, gauge, or warning lamp failure and testing shows it’s at fault.
- Replacement ensures correct activation temperature and reliable cooling-fan or warning operation, preventing overheating or false warnings.

- What replacement part to buy
- Buy a thermal switch specified for your engine application — OEM Perkins part if available, or a reputable aftermarket equivalent (Bosch-compatible or equivalent) that matches:
- Thread type and size (match original thread to avoid leaks/damage).
- Activation temperature (match original spec for fan/on/off operation).
- Electrical terminal type (single spade, twin spade, or connector style).
- If unsure, take the old switch to a parts supplier and match by physical dimensions and electrical connector, or reference the engine parts manual for the exact Perkins part number.

- How to install the replacement switch
- Clean the thread and mating surface in the engine.
- If the replacement uses a sealing washer, fit the washer as supplied. If it uses pipe thread sealant, apply a small amount recommended for coolant environments (or a wrap of PTFE tape applied correctly to the male threads, avoiding the sensing tip) — follow the new switch manufacturer’s instructions.
- Hand-start the new switch into the thread to avoid cross-threading.
- Tighten with a spanner or socket until snug; if you have a torque wrench follow the manufacturer torque spec; if not, tighten to a firm hand-tight plus a small additional turn (avoid over-tightening — roughly 10–20 Nm typical for small sensors, but accept manufacturer spec is best).
- Reconnect the electrical connector.
- Refill coolant if needed, bleed air from the system (open bleed screws, run engine with cap off until thermostat opens, top up), and check for leaks around the switch.
- Run engine to operating temperature and re-test switch operation with the multimeter or by observing fan/indicator operation.

- Extra tools you might need and why
- Thread chaser or tap (only if threads are damaged)
- Used to clean and restore thread form in the head. Use only if you know how; otherwise see a machinist/garage.
- Replacement sealing washer or switch-specific sealing kit
- Ensures coolant-tight seal without using incorrect materials.
- Anti-seize compound (sparingly, not on sealing face)
- Helps future removal but avoid contaminating sealing surfaces.
- Service manual or parts diagram
- Confirms correct part number, thread size, and torque specs for your specific engine variant.

- Final checks
- Confirm no leaks after engine reaches operating temperature.
- Confirm fan or warning lamp now operates correctly when engine reaches the specified temperature.
- Dispose of old switch and used coolant responsibly.

- Quick troubleshooting notes
- If new switch also fails, check wiring, grounding, wiring connector, and the fan relay/indicator circuit.
- A corroded ground or bad relay can mimic a failed switch; use the multimeter to check supply and ground on the switch wiring.

- Common replacement part guidance (examples)
- Look for an OEM Perkins coolant/thermal switch for 4.107 / 4.108 / 4.99 or an equivalent sensor by a reputable manufacturer matching thread and temperature spec. Bring the old part to a parts supplier to match exactly.

- Final safety reminder
- Do not work on a hot cooling system. Take care when handling coolant and electrical connectors.

(End of instructions)
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