You turn the key and… nothing. Wiggle it, switch off and on a couple of times, and the car finally wakes up. It is one of the most common electrical niggles on an older MG — and one of the most satisfying to cure, because the fault is almost always a tired ignition switch or its connections, and the fix is usually a few minutes with a screwdriver and some contact cleaner. Here is how to diagnose it properly and put it right, before it strands you at a show.
The symptom
The tell-tale is intermittency: the key turns as normal, but the ignition does not always come alive — no fuel-pump tick, no warning light, no start — and jiggling the key, or switching off and on a few times, brings it back. It comes and goes, and it usually worsens over weeks rather than failing outright. That "have to wiggle it" behaviour is the signature of a contact that is only making some of the time.
Why it happens
The ignition switch feeds the ignition circuit — coil and fuel pump — through a shared live wire. Anywhere along that path where contact goes high-resistance or intermittent, the symptom appears. Three causes account for almost every case:
| Cause | What's going on |
|---|---|
| Loose or corroded connectors | The spade or bullet connectors on the back of the switch work loose or tarnish, so the circuit makes only intermittently. The commonest cause — and the easiest to cure. |
| Worn internal contacts | Inside the switch, the contacts oxidise or wear so the switch itself makes poor contact. Cured by cleaning and regreasing, or by replacement. |
| A worn key or barrel | The key no longer turns the mechanism fully home, and jiggling helps it catch. Often shows as a sloppy key, or a switch loose in the dash. |
Diagnose it with a multimeter
Five minutes settles it. Set a meter to DC volts, put the black probe on a good bare-metal earth and the red probe on the switch's output terminal — the wire that leaves the switch to feed the coil and fuel pump (white on Lucas wiring).
- Key on, engine off: you should read a steady 12 volts or so.
- Now wiggle the key. If the reading flickers or drops out, the switch or its connections are the fault — confirmed.
- Check the feed (the wire *into* the switch) reads a steady 12 volts throughout. Feed steady but output dropping means the switch or its own connectors, not the wiring upstream.
This is a positive-earth car, so if the probes are the wrong way round the meter simply shows a minus sign — just read the number.
The fix, cheapest first
1. Clean and tighten the connectors. Pull each spade off the back of the switch, clean the terminals with a little fine abrasive or contact cleaner, and gently pinch the female connectors so they grip firmly again. A smear of dielectric or petroleum grease keeps corrosion out. Push them back on snugly, and make sure the switch body itself is tight in the dash — a switch loose in its hole invites the very wiggle you are trying to stop. Reconnect and retest; this alone cures a great many cases.
2. Clean the shared circuit. If the connectors are sound but the fault remains, clean every connection along the shared ignition (white) wire — the fuse terminal, the coil, the fuel pump and the earths. A poor joint anywhere on that circuit mimics a bad switch.
3. Address the switch itself. If the meter still shows a dropout at the switch with good connections, the internal contacts are worn. Some switches can be dismantled, cleaned and regreased with a switch-rated dielectric compound and reassembled; others are sealed and are simply replaced.
If you replace it
Buy from a proper T-Series specialist rather than the cheapest option you can find. A well-known trap is that reproduction switches vary in quality and can be faulty from new or fail within weeks — so test the new switch with your meter before and after fitting. Photograph or label the wiring before you unplug it, and keep the old switch as a reference.
Don't ignore it
An intermittent ignition switch is a benign, cheap fault to fix — but it is exactly the sort of thing that leaves you stranded in a car park just when you want to drive home, and a switch making poor contact can warm up in use. Caught early it is a five-minute job; left alone it only gets worse. If the fault sits on the ignition and charging side of the car more generally, the Ignition and Charging guide is a good companion read.
*Written for the T-Series Archive. Diagnostic approach based on long-established Lucas switch practice; confirm wire colours and terminals against the wiring diagram for your car.*