Note: Those Perkins 4‑cylinder engines (4.212, 4.236, 4.248, 4.2482, T4.236, T4.38) are diesel. They use glow plugs (low‑voltage heating elements) and a glow‑plug harness/lead set, not high‑tension spark plug leads. Below is a concise, ordered procedure for inspecting, testing and replacing glow‑plug wires/leads with the underlying theory and how the repair fixes faults.

Key facts (theory)
- Glow plugs are resistive/heating elements powered at battery voltage. The harness must deliver sufficient current with minimal voltage drop. Faulty leads cause excessive resistance or open circuits → insufficient current → weak/short heat-up → hard cold starting and white/grey smoke on start.
- Insulation breakdown or cracked boots allows leakage/arcing to engine ground → loss of current to the glow plug and possible electrical noise; corrosion on terminals increases contact resistance.
- Correct routing and secure connections prevent mechanical chafing and heat damage (heat raises resistance and ages insulation).

Preparation (safety, identification)
1. Safety: disconnect battery negative. Wear gloves and eye protection.
2. Identify cylinders: Perkins inline‑4 numbering is 1–4 from the front (pulley/fan) to the rear (flywheel). Note where each lead connects: controller/relay → individual glow plugs. Mark or photograph routing so replacement follows same order.

Ordered procedure with theory (concise)
1. Visual inspection
- Remove harness cover/clips. Inspect each lead and boot for cracks, brittle insulation, burns, oil contamination, corrosion at terminals.
- Theory: visible damage indicates paths for leakage or high contact resistance; oil/heat accelerates insulation failure.

2. Check continuity and resistance
- With battery disconnected, remove one end of the lead from the glow plug or relay so you measure the wire itself. Use a multimeter: check continuity and DC resistance between the two ends.
- Theory: a good low‑voltage lead should be near short/low resistance (almost continuity). High or infinite resistance means internal break or corrosion causing insufficient current to the glow plug.

3. Check for insulation leakage/shorts
- With one lead end disconnected, measure resistance from the conductor to engine block (ground). It should be open/very high. Any measurable resistance indicates insulation leakage.
- Theory: leakage shunts current away from the glow plug, reducing heating and possibly producing arcing.

4. Inspect termination points
- Remove boots and inspect terminals (spade/ring/ferrule). Clean corrosion with contact cleaner and light brush. Bent or pitted terminals should be replaced.
- Theory: poor metal‑to‑metal contact increases contact resistance → voltage drop → less current through glow plug.

5. Replace suspect leads
- Order correct replacement leads/harness for the exact engine model. If making new leads, use correct conductor gauge and heat‑resistant, oil‑resistant insulation and high‑temp boots.
- Remove old lead: pull the boot squarely from the glow plug (avoid twisting), and from the relay/connector.
- Fit new lead: ensure terminals/ferrules are correct and secure. Seat boots fully over terminals. Route leads the same way, away from exhaust, sharp edges and moving parts; use clips to secure.
- Theory: a low‑resistance, well‑terminated and properly routed lead restores full current to the glow plug and reduces mechanical/thermal stresses that cause repeated failure.

6. Reassemble and test electrically
- Reconnect battery negative. Energize the glow circuit (turn key to glow/ON) and check that the glow indicator behaves normally and that relay clicks. With a clamp ammeter you can measure current per glow plug if needed — consistent current across cylinders indicates uniform resistance.
- Theory: confirming current flow shows the new leads deliver adequate current; equal currents mean the plugs and leads are matched.

7. Functional (starting) test
- Start engine cold. Observe starting time, smoke color, and idle smoothness. A corrected harness should yield quicker starts, less white smoke and smoother idle during warm‑up.
- Theory: restored current to glow plugs yields correct preheat temperature so fuel ignites reliably on compression; reduced smoke indicates proper combustion.

How the repair fixes common faults (brief)
- Symptom: hard cold starts, long cranking, lots of white smoke on start.
Repair effect: replacing high‑resistance/open leads restores low voltage drop and full current to glow plugs so they reach proper temperature; fuel ignites reliably and combustion stabilizes sooner.
- Symptom: intermittent starting or mis‑starting, or engine runs rough until warm.
Repair effect: fixing insulation breakdown or corroded terminals removes leakage and arcing paths, ensuring all plugs heat consistently and eliminating cylinder‑to‑cylinder imbalance.
- Symptom: electrical faults, blown fuses or relay issues.
Repair effect: replacing shorted or chafed leads eliminates shorts to ground and prevents excessive current draw that damages control components.

Quick acceptance checks after repair
- Resistance low and similar across new leads; no continuity to chassis ground.
- Glow indicator and relay operate normally; measured current per plug within expected range.
- Engine starts faster with less white smoke and runs smoothly through warm‑up.

Done.
rteeqp73

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