1. Battery and Connections:
* Battery Voltage: Check the battery voltage with a voltmeter. It should be around 12.6 volts when fully charged. A low voltage will prevent the starter from engaging.
* Clean Connections: Clean the battery terminals and cables thoroughly. Corrosion can significantly impede current flow. Use a wire brush and baking soda/water solution to clean them, then apply a corrosion inhibitor. Check all connections at the starter motor itself as well.
* Cable Condition: Inspect the battery cables for damage, fraying, or corrosion along their length. A bad cable can have high resistance and prevent sufficient current from reaching the starter.
2. Starter Motor:
* Starter Relay (Solenoid): This is a common culprit. The relay is activated by the ignition switch, and it's what physically engages the starter motor. You may be able to tap the relay with a wrench or screwdriver while someone tries to crank the engine – a temporary engagement might confirm a bad relay. Relays are relatively inexpensive and easy to replace.
* Starter Motor Itself: If the relay seems good, the starter motor itself might be faulty. This requires more advanced diagnostic testing, potentially needing to be bench-tested or replaced. A failing starter will sometimes make a clicking noise, but may not function at all.
* Starter Motor Wiring: Check the wiring harness connections leading to the starter motor for any loose or corroded connections.
3. Ignition System (Even though you replaced the switch):
* Ignition Switch Wiring: Although you replaced the switch, ensure that all the wiring connections to the new switch are secure and correctly connected. A bad connection within the harness *before* the switch would still cause problems.
* Neutral Safety Switch (Automatic Transmission): If your van has an automatic transmission, the neutral safety switch prevents starting unless the transmission is in Park or Neutral. A faulty switch could prevent the starter from engaging. Check the transmission linkage for proper adjustment.
4. Other Potential Issues:
* Fuses and Circuit Breakers: Check the relevant fuses and circuit breakers in the fuse box for the starter circuit. A blown fuse is an easy fix.
* Wiring Harness: A fault within the wiring harness itself – damage, short circuit or broken wire – is possible, but more difficult to diagnose. Visual inspection is recommended.
* Grounds: Poor grounding can severely impede current flow. Look for loose ground straps or connections. Clean and tighten all ground points you find.
Testing Procedure:
The most useful test is to directly test voltage at the starter motor's solenoid while someone attempts to start the engine. You'll need a helper and a voltmeter with the positive lead connected to the solenoid's large power terminal, and the negative lead to a clean ground point on the engine block. If voltage is present and the starter doesn't engage, the starter itself is likely the problem. If no voltage is present, work your way back through the circuit (relay, ignition switch wiring etc.)
Important Note: Working on the electrical system of your vehicle involves working with potentially high currents and voltages. If you are not comfortable with auto electrical work, take it to a qualified mechanic. Improper work can lead to injury or damage to your vehicle.
