No tailights on 88 F-250 even with new switch.

Should that read the brake lights and turn signals work but the tail lights don't?

They are on different fuses.

Auto lamps or manual ?
The headlamps may be turned on 2 different ways. First, when the driver places the headlamp switch in the ON position, for manual operation. Second, with automatic lamp control ALC, and the headlamp switch placed in the OFF position.
The LH HDLP and RH HDLP fuse in the fuse block - underhood, supply battery positive voltage to both low and high beam lamps at all times. For manual operation , when the headlamp switch is in the ON position, the headlamp switch supplies ground directly to the headlamps and to the body control module (BCM) on the headlamp low or high beam signal circuit, depending on the position of the headlamp dimmer switch. The BCM uses this signal to determine if the conditions are present to illuminate the daytime running lamps or headlamps.
For ALC operation, the BCM receives a signal from the ambient light sensor indicating a low or high ambient (outside) light level. The ambient light sensor is a light sensitive transistor that varies its voltage signal to the BCM in response to changes to the outside (ambient) light level. Under the correct conditions the BCM grounds the headlamp relay control circuit energizing the headlamp control relay. The now closed relay supplies ground through the headlamp switch to the headlamps and the BCM on the headlamp low or high beam signal circuit, depending on the position of the headlamp dimmer switch. When the BCM grounds the headlamp relay control circuit, it also grounds the park lamp relay control circuit illuminating the park lamps. The headlamp relay is grounded at G203.
If the engine stalls with the headlamps in the ALC mode under low light conditions, the headlamps will be controlled OFF during cranking.
The conditions necessary for the BCM to activate the ACL mode are as follows:
?€¢
The ignition switch is in the RUN position.

?€¢
The headlamps are in the OFF position.

?€¢
The park brake is released.

?€¢
The BCM is receiving a low light condition from the ambient light sensor, if equipped with RPO T 82.

Doesn't have a headlamp control module . Has a headlamp relay , located in the instrument panel fuse box.
Headlamps
The headlamps may be turned on 2 different ways. First, when the driver places the headlamp switch in the ON position, for manual operation. Second, with the headlamp switch placed in the OFF position activating automatic lamp control (ALC).
The LH HDLP and RH HDLP fuse in the fuse block - underhood, supply battery positive voltage to both low and high beam lamps at all times. For manual operation, when the headlamp switch is in the ON position, the headlamp switch supplies ground directly to the headlamps and to the body control module (BCM) on the headlamp low or high beam signal circuit, depending on the position of the headlamp dimmer switch. The BCM uses this signal to determine if the conditions are present to illuminate the daytime running lamps or park lamps.
For ALC operation, the BCM receives a signal from the ambient light sensor indicating a low or high ambient, outside, light level. Under low light conditions the BCM grounds the headlamp relay control circuit energizing the headlamp control relay. The now closed relay supplies ground through the headlamp switch to the headlamps and the BCM on the headlamp low or high beam signal circuit, depending on the position of the headlamp dimmer switch. When the BCM grounds the headlamp relay control circuit it also grounds the park lamp relay control circuit illuminating the park lamps. The headlamp relay is grounded at G203.
If the engine stalls with the headlamps in the ALC mode under low light conditions, the headlamps will be controlled OFF during cranking.
The conditions necessary for the BCM to activate the ALC mode are as follows:
• The ignition switch is in the RUN position.
• The headlamps are in the OFF position.
• The park brake is released. If the park brake is applied after the ignition key is placed in the RUN position the ALC is not effected.

Headlamps
The headlamps may be turned on 2 different ways. First, when the driver places the headlamp switch in the ON position, for manual operation. Second, with automatic lamp control ALC, and the headlamp switch placed in the OFF position.
The LH HDLP and RH HDLP fuse in the fuse block - underhood, supply battery positive voltage to both low and high beam lamps at all times. For manual operation , when the headlamp switch is in the ON position, the headlamp switch supplies ground directly to the headlamps and to the body control module (BCM) on the headlamp low or high beam signal circuit, depending on the position of the headlamp dimmer switch. The BCM uses this signal to determine if the conditions are present to illuminate the daytime running lamps or headlamps.
For ALC operation, the BCM receives a signal from the ambient light sensor indicating a low or high ambient (outside) light level. The ambient light sensor is a light sensitive transistor that varies its voltage signal to the BCM in response to changes to the outside (ambient) light level. Under the correct conditions the BCM grounds the headlamp relay control circuit energizing the headlamp control relay. The now closed relay supplies ground through the headlamp switch to the headlamps and the BCM on the headlamp low or high beam signal circuit, depending on the position of the headlamp dimmer switch. When the BCM grounds the headlamp relay control circuit, it also grounds the park lamp relay control circuit illuminating the park lamps. The headlamp relay is grounded at G203.
If the engine stalls with the headlamps in the ALC mode under low light conditions, the headlamps will be controlled OFF during cranking.
The conditions necessary for the BCM to activate the ACL mode are as follows:
• The ignition switch is in the RUN position.
• The headlamps are in the OFF position.
• The park brake is released.
• The BCM is receiving a low light condition from the ambient light sensor, if equipped with RPO T 82.


G-203
G203
At the lower center of the I/P, attached to the upper front side of the I/P fuse block

The problem is your not familiar with electronic's on modern automobiles . Module controlled systems . BCM - body control modules .
The park lamps consist of the park, tail, marker and license lamps. These lamps are illuminated any time the headlamp switch is in the PARK, ON position, or anytime the body control module (BCM) illuminates the headlamps. The PARK fuse in the fuse block - instrument panel (I/P) supplies voltage to both the coil and switch input side of the park lamp relay. The BCM controls the park lamp relay by grounding the park lamp control circuit energizing the relay.

PCB Relays
This vehicle is equipped with the following PCB mounted relays. High Beam, Low Beam, DRL, Fog Lamp, Park Lamp, Stop Lamp, and Horn relay. They are directly soldered to the underhood fuseblock circuit board. A failure of one of these relays will require the whole underhood fuseblock to be replaced.

Your best bet have a qualified repair shop diagnose the problem . Could have DTC'S - diagnostic trouble codes stored in the BCM For circuit faults .
Park Lamp Switch On
B257A 00
B2057A 00
B257A 00
--
When the headlamp switch is in the OFF position, the body control module (BCM) receives a ground signal through the headlamp switch OFF circuit and an open signal on the park lamp ON circuit. When the headlamp switch is in the PARK or HEAD position, the BCM receives an open signal through the headlamp switch OFF circuit and a ground signal through the park lamp ON circuit.

DTC B2585 00: Park Lamp Control Circuit - Domestic

Well Jeni there are a few thing's that could be wrong , the headlamp switch , the BCM -body control module -a electronic control module . There are relays involved also ,but don't think there at fault . To be sure you need to do some electrical test to know for sure ! I suggest you take it to a ASE certified repair shop that has the knowledge , tools an service info - wiring diagrams etc.. ! Oh and a scan tool that can extract DTC'S from the BCM there could be a diagnostic trouble code pointing to the problem !
DTC B257A 00: Headlamp Switch Input Signal Mismatch
Diagnostic Fault Information

Circuit

Short to Ground

Open/High Resistance

Short to Voltage

Signal Performance
Headlamp Switch Off Signal
B257A 00
3
--
--
Park Lamp Switch On Signal
5
4
--
--
Headlamp Switch On Signal
B257A 00
2
--
--
Headlamp Ground
--
2, 3, 4
2
--

1. Headlamps Always On

2. Headlamps Inoperative From Switch

3. Unable To Turn OFF Automatic Headlamps

4. Park Lamps Inoperative From Switch

5. Park Lamps Always On
When the headlamp switch is in the OFF position, the body control module (BCM) receives a ground signal through the headlamp switch OFF circuit and an open signal on the park lamp ON circuit and headlamps ON circuit. When the headlamp switch is in the PARK or HEAD position, the BCM receives an open signal through the headlamp switch OFF circuit and a ground signal through the park lamp ON circuit or headlamp ON circuit.
Circuit/System Testing

1. Ignition OFF, disconnect the harness connector at the headlamp switch.
2. Verify that a test lamp illuminates between the ground circuit terminal 8 and B+.
?‡'
If the test lamp does not illuminate, test the ground circuit for an open/high resistance.

3. Ignition ON, verify the scan tool Auto Hdlp. Disable Sw. parameter is Inactive.
?‡'
If not the specified value, test the signal circuit terminal 3 for a short to ground. If the circuit tests normal, replace the BCM.

4. Verify the scan tool Headlamp Switch parameter is Inactive.
?‡'
If not the specified value, test the signal circuit terminal 1 for a short to ground. If the circuit tests normal, replace the BCM.

5. Verify the scan tool Parklamp Switch parameter is Inactive.
?‡'
If not the specified value, test the signal circuit terminal 2 for a short to ground. If the circuit tests normal, replace the BCM.

6. If all circuits test normal, test or replace the headlamp switch.

If the BCM needs to be replaced , it has to be programmed . The dealer is pretty much the only place to have that done at ! There are other places ,you have to look in the phone book or inquire at local repair shops . Some towns ,city's have mobile repair guy's that do flash programming of modules etc... Good luck hope I gave you some insight on the problem ! I tell people these thing's an it's like they don't believe me , I see they repost the same question . I started working as a Technician in 1984 at a Chevy dealer ship an have worked at Cadillac , Pontiac , GMC dealer ships ! I see all to often people trying to fix these type of problems by just replacing parts not knowing what they are doing an wasting there money an later bring it to the shop anyway ! So I hope you make a good decision an take to a qualified repair person or shop !

There are several reasons why your headlamps an running lights don't work , the headlamp switch is a good one ,plus relays an the DIM - dash integrated module an associated wiring ! Here is how the headlamps work , right out of Buick factory service info !
The dash integration module (DIM) provides battery positive voltage to the signal circuits of the headlamp switch. The signal circuits include the headlamp switch headlamps on signal, the flash to pass switch signal and the high beam input. With the headlamp switch in the ON position, a ground path is available for the headlamp switch headlamps on signal circuit of the DIM through the headlamp switch. The DIM provides ground to either the headlamps high beam relay control circuit or the headlamps low beam relay control circuit of the low beam circuit. The position of the headlamp dimmer switch determines which relay control circuit has ground. The DIM supplies battery positive voltage to the relay control circuits, if the headlamps are necessary. The fuse block - underhood supplies battery positive voltage to both relay switch circuits. When the low beam relay control circuit is energized, current flow is to both low beam fuses and to the low beam headlamps. The headlamps have ground at G103 and at G401. When the headlamp high beam relay control circuit is energized, current flow is to both high beam fuses and to the high beam headlamps. The high beam circuit is divided into a left and right side circuit. The current flow of the left high beam is from the fuse to the left high beam lamp. From the lamp, the current flows to the normally-closed contact of the DRL relay to G103. The current flow of the right high beam is from the fuse to the right high beam lamp. From the lamp, the current flow is to ground at G401. When the headlamp dimmer switch lever is pulled toward the driver, the flash-to-pass switch closes. This grounds the DIM flash to pass switch signal circuit. In response to this input, the DIM energizes the HDLP HI BM relay. Both high beams remain on until the driver releases the switch handle. If the low beam headlamps were on during a flash to pass operation, the low beams remain on. The headlights may be switched to high beam with the opposite movement of the headlamp dimmer switch lever.
By running lights , do you mean park lamps or day time running lamps ?
Park, Tail, License and Marker Lamps
The park lamps, including the license lamps, are turned on when the headlamp switch is placed in the PARK or ON position or anytime the headlights are requested. The fuse block - rear supplies battery positive voltage to the switch contacts of the Park LP relay. When the headlamp switch is in PARK or in ON, the park lamp switch on signal circuit of the DIM has a ground path. The DIM provides battery positive voltage to the control circuit of the Park LP relay. When the park lamps and/or the tail lamps are ON, the DIM energizes the park lamp relay control circuit. This closes the switch contacts. Current flows from the Park LP relay to the LH PARK LP and the RH PARK LP fuses. From the fuse, the current flows to the park lamps. The left park lamps are grounded at G103, G401 and G402. The right park lamps and the right tail lamps are grounded at G103 and G401. The DIMR fuse supplies battery positive voltage to the license lamps and to the DIM instrument panel lamp fuse supply voltage - 1 circuit, for interior lamps dimming. The license lamps are grounded at G401.
The best advice I can give is that you take this to a ASE certified professional repair facility unless your a qualified automotive electric repair person !
Diagnostic Starting Point - Lighting Systems
Begin the system diagnosis with the Diagnostic System Check - Lighting Systems . The Diagnostic System Check will provide the following information:
?€¢
The identification of the control modules which command the system

?€¢
The ability of the control modules to communicate through the serial data circuit

?€¢
The identification of any stored diagnostic trouble codes (DTCs) and their status

The use of the Diagnostic System Check will identify the correct procedure for diagnosing the system and where the procedure is located.

The

ITS THE SWITCH ITSELF.

I have the same problem few days ago, and still didn't find any solution. Can any one please help?????18

What relay's ?
The instrument cluster controls the headlamp systems that include daytime running lamps (DRL). The system allows the headlamps to operate at a reduced intensity when all of the following conditions apply:
• The engine is running.
• The transaxle is not in the PARK position.
• The normal headlamps are not on.
The instrument cluster turns the DRL system on by grounding CKT 592, thereby energizing the DRL relay. This action allows a reduced voltage of 6 volts to flow across the headlamps. If the vehicle has the optional fog lamps, the fog lamp relay will also be energized. The park lamps will turn on and the fog lamps will operate normally. When the headlamp switch is turned to the ON position, the instrument cluster allows normal headlamp operation.
The instrument cluster also has an input from the park brake. If the park brake is in the ON position, the DRL will remain disabled until the park brake is placed in the OFF position. On vehicles with an automatic transaxle, the instrument cluster will disable the DRL until the transaxle has been taken out of the PARK position.

Headlamps Inoperative - High Beams
Step
Action
Value(s)
Yes
No
1
Did you perform the Daytime Running Lamps (DRL) System Check?
--
YES - Go to Step 2
NO - Go to Daytime Running Lights (DRL) System Check
2
Disconnect the turn/hazard headlamp switch connector C1.
Connect the fused jumper between terminal B of the turn/hazard headlamp switch assembly harness connector C1 and ground.
Do the high beam headlamps operate?
--
YES - Go to Step 3
NO - Go to Step 4
3
Replace the turn/hazard headlamp switch assembly.
YES - Did you complete the repair?
--
NO - Go to Daytime Running Lights (DRL) System Check
--
4
Repair the open in CKT 1200 between the turn/hazard headlamp switch assembly. Refer to Wiring Repairs in Wiring Systems.
Did you complete the repair?
--
Go to Daytime Running Lights (DRL) System Check
--