TEST 2: Making Sure Fuse #18 Has Power
Fuse #18 (5 Amps), of the under-dash fuse panel, gets battery power from the headlight switch.
Specifically, it gets power when you turn on the headlight switch to the ‘park-lamps on’ and ‘headlights on’ position.
In this test step we're gonna' make sure there's battery power available at the fuse when we turn on the park-lamps and/or headlights.
NOTE: You don't have to remove the #18 fuse to check for power, since the test steps below indicate testing it with it in place (in the under-dash fuse panel).
OK, here are the test steps:
Place your multimeter in Volts DC mode.
Turn the headlight switch to the ‘headlights on’ position.
NOTE: The headlight switch must be connected to its connectors.
Touch one of two metal terminals that stick out from above the #18 fuse itself with the red multimeter test lead.
Ground the black multimeter test lead to a clean metal surface. Or, use a battery jump start cable to ground the black test lead to the battery negative post.
Your multimeter should read 10 to 12 Volts DC if battery power is present.
Turn off the headlights when done with the test.
Let's interpret your test results:
CASE 1: Battery power IS NOT present. This usually means that the headlight switch itself is defective, since it's the one that feeds battery power to the #18 fuse when you turn the the switch to the ‘park-lamps on’ or ‘headlights on’ position.
Your next step is to test the headlight switch itself. Specifically, you need to test the park-lamps circuit of the headlight switch, since this is the circuit that feeds battery power to the #18 fuse.
This tutorial, on testing a ‘no park-lamps problem’ will help you to this end: Testing A No Park-Lamps Problem (1997-1998 Ford F150).
CASE 2: 10 to 12 Volts are present. This is the correct and expected test result and confirms that the headlight switch is feeding battery power to fuse #18.
We can also conclude that the dimmer switch is getting battery power (since it gets it from the #18 fuse).
Now that we know that the dimmer switch is getting power from fuse #18, the next step is to bypass the dimmer switch function altogether. For this test go to: TEST 3: Bypassing The Instrument Panel (IP) Dimmer Switch.
TEST 3: Bypassing The Instrument Panel (IP) Dimmer Switch
In this test section we're gonna' jumper female terminals 5 and 6 of the grey and black headlight switch connectors.
Specifically, we're gonna' use a 5 Amp fused jumper wire to jumper the tan with white stripe (TAN/WHT) wire (of the grey connector) to the light blue with red stripe (LT BLU/RED) wire (of the black connector).
This procedure will bypass the IP illumination lamps dimmer switch and fuse #18 and apply battery power to the instrument panel illumination lamps.
Once the fused jumper wire is installed, and if the circuit is OK, the instrument panel illumination lamps should come on at full illumination.
If the instrument panel illumination lamps do come on, then we can usually conclude that the headlight switch dimmer function is defective and the headlight switch needs to be replaced.
IMPORTANT: You must use a 5 Amp fused jumper wire to jumper the TAN/WHT and LT BLU/RED wires of the headlight switch connectors. You can see an example of a fused jumper wire here: How To Make A 5 Amp Fused Jumper Wire.
OK, this is what you need to do:
Locate female terminals #5 and #6 of the grey and black headlight switch connector (see photo above).
Using a 5 Amps fused jumper wire, jumper terminals 5 and 6.
The instrument panel illumination lamps should come on as soon as you jumper the two terminals of the headlight switch's connector.
NOTE: The park-lamps should come on whether you have the ignition switch key in the ON or OFF position.
Remove the jumper wire once you have confirmed that the IP lights came on.
Let's take a look at what your test results mean:
CASE 1: The instrument panel illumination lamps turned on. This is the correct and expected test result.
This usually means that the headlight switch dimmer function is defective and that there aren't any electrical wiring problems between the connectors and the instrument panel.
There's one more test to do (before concluding the dimmer switch is defective) and it's to make sure that the headlight switch's dimmer function has Ground. For this test go to: TEST 4: Testing The Dimmer Switch's Ground.
CASE 2: The instrument panel illumination lamps DID NOT turn on. This usually indicates a problem in the wiring between the connector and the park-lamps.
To be a bit more specific, there's probably an open-circuit in the LT BLU/RED wire that's stopping the voltage from reaching the bulbs.
Your next step is to check the continuity of the LT BLU/RED wire between the headlight switch connector and the park-lamps with a wiring diagram. Repair any open-circuit found.
CASE 3: The 5 Amp fuse on your fused jumper wire blew. This test result confirms that there's a short to Ground in the IP illumination lamps circuit (wires). You can also conclude that the dimmer switch function (inside the headlight switch) is not the cause of the problem.
For more info on this particular problem, take a look at the section: Fuse #18 Keeps Blowing.