1987gmc pickup headlights will not dim changed

. The illumination is controlled by the BCM - body control module .
Interior Lamps Dimming
This group includes lamps which may dim. This group may use a combination of vacuum fluorescent (VF) illumination and incandescent lamps.
• HVAC control module, head assembly
• Rear HVAC control module, head assembly
• Radio
• Rear seat Audio
• The instrument panel cluster (IPC)
• The PRNDL lamp, with the exception of the current gear select position
• Power window switches
• Various switches
When the ignition switch is turned to the ON position, the VF display, radio, turns ON at maximum brightness. When the park lamps are ON, all incandescent back lighting turn ON at the dimming level indicated by the instrument panel (I/P) dimmer switch. At the same time all VF displays dim to match the indicated dimming level. When the headlamp switch is placed in the PARK position, the park lamp supply voltage circuit provides an input to the body control module (BCM). The BCM then supplies voltage to the I/P dimmer switch through the I/P dimming lamps control circuit. The setting of the I/P dimmer switch determines the amount of voltage that the I/P dimmer switch supplies to the BCM through the I/P dimming lamps low reference circuit. The BCM then sends a pulse width modulation (PWM) voltage to all the interior lamps. All the VF and incandescent back lighting lamps are provided a specific voltage and are then grounded. When the headlight switch is turned to the park lamp or headlamp position, all incandescent back lighting turn ON at the dimming level indicated by the I/P dimmer switch. When the I/P dimmer switch is moved from MIN to MAX, all VF displays, as well as all incandescent back lighting respond from minimum intensity to maximum brightness in response to the I/P dimmer switch.


Do you know what DTC'S - diagnostic trouble codes are ? Codes set in the BCM
DTC B2610 00: Passenger Compartment Dimming 1 Circuit
DTC B2615 00: Passenger Compartment Dimming 2 Circuit
DTC B2652 00: Passenger Compartment Dimming 3
Circuit/System Description
The instrument panel (I/P) lamp dimmer switch is used to increase and decrease the brightness of the interior backlighting components. The I/P dimmer switch provides a voltage signal that will increase as the brightness of the lights are increased and decrease as the brightness of the lights are decreased. The body control module (BCM) provides a low reference signal and a B+ voltage reference to the I/P dimmer switch. When the I/P dimmer switch is placed in the desired position, the dimmed voltage setting is applied from the I/P dimmer switch through the I/P dimmer switch signal circuit to the BCM. The BCM interprets the signal and applies the dimmed voltage to the I/P lamps supply voltage circuit and the interior backlighting components dim to the requested level. Ground for the I/P dimmer switch is provided at G201.

sunroof fuse 1 20amp / power windows - circuit breaker 25 amp
both of these are located in the instrument panel fuse box .

That is a common problem, the switch needs changing if the high and low beams do work, the only problem is that it's an expensive part to replace, try a local wrecking yard they should have a bunch of suitable replacements at a reduced price.

try checking dimmer switch, if bad replace , if it has any adjustment check this, most will click when going from dim to high

sounds like you have a bad panel dimmer control. try turning the control from bright to dim very fast a few times then slowly, do this in a dark area or at night with the headlights on, you should see a flicker of light occasionally. If you don't see any light then it could be either the headlight switch or the dimmer control, if you DO see any light, then it's your dimmer

Should have changed the dimmer switch....

If the headlights were not changed at the same time the dim one may be about to go out. If you change the dim one and not both of them the other one will most likely get dim while the other is still bright

some times when the dimmer switch on the floor goes bad the headlights will cut out. Try replacing the dimmer switch and the headlights should come back on. My 84 dodge had that exact problem.

Check the charging system first,make sure it is charging 13.65 volts,to 14.25 volts at the battery,with the headlights on,and the ac on high blower motor.

You need a new headlight switch most likley. Try turning the knob to dim and brighten quickly and push in and out on the headlight between parking and headlights.

Skinny, I think your on the right track. Disconnect one of your low beam head lights, turn the headlights on and to low beam, you should have voltage on the "tan"wire at the headlight. If you do not, go to the dimmer switch under the steering column and check the same wire. If it still has no voltage, replace the dimmer switch.
Good luck and let me know.
Regards,