Goal: replace (relay) the starter solenoid/relay on a Massey Ferguson MF135 (M148) and understand why the fix works. Steps are in order; each step includes the underlying theory and what the test/repair proves.

1) Recognize symptoms and the theory
- Common symptoms: single click when key is turned, no crank, slow crank, or starter spins but engine doesn’t turn over.
- Theory: the starter solenoid is an electromagnetic switch. A small “ignition” voltage (from the key) energizes a coil that pulls a plunger/contacts closed, connecting the battery positive to the starter motor with a low-resistance path. If the coil, plunger, or contacts fail (open coil, stuck plunger, burned contacts), the starter won’t get full battery current even though the battery and ignition switch may be OK. High resistance in cable connections or grounds produces the same effect because it reduces current and the magnetic field.

2) Safety and prep
- Always disconnect the battery negative before touching heavy cables. Theory: prevents short circuits when removing/attaching thick battery leads; protects you and electrical components.
- Have a multimeter, basic spanners, wire brush, and replacement solenoid/relay ready.

3) Locate the solenoid/relay and do a visual inspection
- Location on MF135: typically mounted on the starter or inner wing/bulkhead area; it’s the cylindrical/squared unit with two big terminals for battery/starter and a small terminal (ignition).
- Visual check: look for loose or corroded battery cable lugs, melted insulation, oil contamination, or cracked housing.
- Theory: corroded/loose large terminals add series resistance -> voltage drop when high current is demanded -> weak magnetic field or insufficient voltage to the starter.

4) Verify battery and heavy-circuit voltages
- Measure battery voltage (resting) — ~12.6 V is a healthy charged battery.
- With helper turning key to START, measure voltage at the solenoid battery terminal and at the solenoid output terminal to the starter. Theory: during cranking the battery terminal should remain near battery voltage; the solenoid output terminal should also rise to roughly battery voltage when the solenoid closes. If the battery terminal drops significantly under load, the battery or cable connections are suspect. If the small “S” terminal gets ignition voltage but the output terminal stays low, the solenoid contacts did not close.

5) Check the small “control” circuit
- Measure voltage at the small (ignition) terminal on the solenoid while turning the key to START. You should see ~12 V.
- Theory: if there is no voltage here, the problem may be the ignition switch, wiring, neutral/clutch safety switch — not the solenoid. If voltage is present and solenoid does not click/close, the solenoid coil or mechanical linkage is at fault.

6) Bench/functional test of the solenoid (if removed)
- Remove solenoid after disconnecting battery negative. Label wires so you can reconnect correctly.
- Bench test: apply 12 V to the small terminal and ground the body; you should hear a positive click and see continuity between the two large terminals when engaged. Use an ohmmeter to check coil resistance (typically a few ohms to a few tens of ohms depending on design) and continuity across main contacts when energized.
- Theory: a healthy coil draws current and produces magnetic field to pull contacts closed; clicking confirms coil and plunger movement. No click -> open coil or stuck mechanism. Click but no continuity across main contacts -> burnt/eroded contacts.

7) Replace the solenoid/relay (ordered removal and fit)
- Disconnect negative battery terminal. Remove small ignition wire (label or photograph), then remove the two heavy battery/starter cables from the big terminals. Unbolt the unit from its mount.
- Fit new solenoid in same orientation. Clean cable lugs and ensure good metal-to-metal contact (wire brush if necessary). Reattach heavy cable to the correct big terminals (battery positive to the battery terminal; output to starter on the other big terminal). Connect the small control wire to the small terminal. Tighten securely. Reconnect battery negative.
- Theory: the replacement restores a low-resistance, low-impedance path from battery to starter when commanded. A new coil produces proper magnetic force to close the contacts, and new contacts present low resistance so full battery current reaches the starter motor.

8) Confirm operation and diagnose remaining issues
- Turn key to START: you should hear the solenoid pull and the starter motor should spin crisply; engine should crank normally. Measure voltage at starter terminal during crank — near battery voltage.
- If starter still cranks slowly or there is no improvement: check battery capacity (load test), ground strap between engine and chassis, heavy cable condition, and the starter motor itself (bench test or direct-jump the starter to confirm).
- Theory: replacement of solenoid fixes coil/contact/plunger failures and poor contact resistance. If the core problem was battery capacity, poor ground or starter windings, replacing the solenoid won’t cure those; measurements above isolate which system failed.

9) Why this repair fixes the fault (summary)
- Fault types fixed by replacing solenoid: open coil (no pull), frozen/stuck plunger (no contact), burnt/eroded contacts (high resistance), loose terminal posts (voltage drop). Replacing the solenoid restores a correctly functioning electromagnetic switch: the control circuit (small terminal) reliably actuates the coil, the plunger moves freely, and the main contacts close with low resistance so full battery current flows to the starter. That restores the required torque/current to crank the engine.

10) Additional notes (short)
- Always ensure heavy cables and engine/chassis grounds are clean and tight — they are frequent causes of starter problems.
- If direct-jumping the starter (battery positive to starter terminal) spins the starter normally, but the new solenoid still didn’t fix the problem, focus on ignition/start circuit wiring or safety interlocks.

End.
rteeqp73

DIY Leaf Bagger | Ride on Lawnmower Hack In this video I show you (after a few attempts) how to make your own DIY leaf bagger for a ride on lawnmower. Materials used ...

DIY Leaf Bagger | Ride on Lawnmower Hack In this video I show you (after a few attempts) how to make your own DIY leaf bagger for a ride on lawnmower. Materials used ...

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