Light Control/Dimmer/FOGLIGHT Switch Circuit: LS400

Turn ignition off. Disconnect 18-pin combination switch connector. Using an ohmmeter, check continuity between connector (harness side) terminal No. 16 (White/Black wire) and ground. Also, check continuity between connector (harness side) terminal No. 17 (White/Black wire) and ground. If continuity exists at both terminals, go to next step. If continuity does not exist at either terminal, repair wire harness between combination switch and ground connection. Retest system operation.
Using an ohmmeter, check continuity between connector (harness side) terminals No. 11 (Blue wire) and No. 18 (Blue wire). If continuity exists, go to next step. If continuity does not exist, repair wire harness between combination switch. Retest system operation.

NOTE: Light control switch is controlled by the body control Electronic Control Unit (ECU). For circuit testing procedure, see BODY CONTROL SYSTEMS - LS400 article. Proceed with step 2) for dimmer/foglight switch circuit testing.

Turn ignition off. Reconnect 18-pin combination switch connector. Turn ignition on. Using DVOM, measure voltage by backprobing rear of combination switch connector between the following terminals and ground with dimmer/foglight switch in specified position. See DIMMER/FOGLIGHT SWITCH CIRCUIT VOLTAGE (LS400) table. See Figure. If voltages are not as specified, repair wiring harness or replace components as necessary. If voltages are as specified, check dimmer/foglight switch. See COMBINATION SWITCH under COMPONENT TESTS. Replace combination switch as necessary. Retest system operation.

DIMMER/FOGLIGHT SWITCH CIRCUIT VOLTAGE (LS400)

Check Voltage Between Ground & Terminal No.	Switch Position	Voltage
8	HEAD & Low Beam	Zero
8	Flash Or HEAD & High Beam	Battery
9	Any Position	Battery
11	HEAD, Low Beam & Fog Light Switch ON	Zero
11	HEAD, High Beam Or Flash & Fog Light Switch ON	Battery
12	OFF Or TAIL	Zero
12	HEAD	Battery
18	HEAD & High Beam Or Flash	Zero
18	HEAD & Low Beam	Battery