1300L Military Blackout Wiring - Brake Lights

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AbramSearcy

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Sep 5, 2024
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The brake light circuit on my 1990 1300L seems to have a severe short to ground inside the system. It emerged last spring, and for my purposes (local, extremely rural driving), I used it without them. The service manual illustrations depict a straightforward circuit: a power supply connects to a brake light switch, which then leads to two bulbs in the brake lights that are grounded. The instructions seems to lack the military wiring. The first inquiry is if the brake lights are regulated by the rotary headlight switch on the dashboard, which has a pull-out tang for blackout settings.

My 419 SEE is equipped with the distinct US military blackout box switch used in several US military vehicles for an extended period. It is linked to a relay rack on the other side of the firewall. Does the 1300L blackout system include a similar configuration?

I believe the simplest and least elegant solution may be to install a new wire from the rear brake lights to the brake light switch, given the many possible grounding spots in the circuit. One thing I have considered is to check for ground at the massive multi conductor harness passthrough on the firewall but was extremely cautious to attempt to take that connection apart as it may open up a bag of worms. It would reduce the length of my wire run based on which side of the connection is grounded.

Does the group possess any insights?
 
Excessively lengthy.

I possess a schematic that delineates the military light switch assignments, specifying which positions activate or deactivate certain lights.
Regrettably, I am unable to locate it at this time.

However, naturally, the taillights and brake lights would be off during blackout mode. They would be too luminous for blackout circumstances.

The military vehicles are equipped with tiny circular tail lights for blackout operations.

The primary light switch has circuitry that disables the tail and brake lights when activated.
This is the first inquiry; however, it would simultaneously disable both the brake and tail lights, as well as the headlights, suggesting that the issue lies elsewhere.

The multi-plug connection via the firewall is designed to be disassembled, and Tarn-X or a comparable cleaner may be used to clean the contacts.

The object is concealed and difficult to access, requiring navigation around several other items in the vicinity.

I would generally refrain from intervening until there is unequivocal evidence that it is the issue.

M-B reengineered the bulkhead fitting in the 427 - 437 series, indicating potential for improvement.

I would examine the dashboard switch, verifying the continuity of all wiring and connections.

However, if the blackout switch were malfunctioning, I would anticipate that all lights would be simultaneously impacted: headlights, tail lights, brake lights, and marker lights.

The wiring to the taillights might short out directly in the taillights. I’ve had to repair both of mine, and the insulation was worn / cut where the wire was twisted around one of the metal separators inside the light casing.

They may also create a short circuit inside the curved tube that constitutes the lamp mount. The wiring should not be exposed, since it is encased inside a sheath within the tube; nonetheless, vibrations and friction may potentially abrade the sheath, exposing the wires.
Mine exhibited evidence of such, and historically it has been a low mileage, pavement driven (mostly) example.

The military taillights have been enhanced from the civilian models. I don’t know about the internals, but the lens itself is fastened on with nearly twice as many screws. I would still suspect damaged insulation inside the light.

I would examine the bulbs and inspect the brake light sockets for shorts. The short circuit might occur within an inch of the bulb.

Then the wiring from the light to the junction box on the frame rail (driver’s side, forward of the axle a little).
Subsequently, from the box to the switch.

The pressure switch may also malfunction. Simple to verify before to accessing the taillights, and cost-effective to replace.

The updated sequence for verification is as follows:

Brake light pressure switch
Brake light socket components of tail lamp
Electrical connections between the lights and the brake light switch
Dashboard rotary switch
 
Thank you for the prompt response. The suggestion about the brake light switch is commendable. I have had this 1300 for some years and have not undertaken any actions that would likely disrupt the wiring from front to rear.
 
The civilian tail lights are there only due to our location in that vicinity.

Lens secured by two screws.

The galvanized internal partitions are equipped with plastic guards.

These would likely inhibit the edges from damaging the wires and also obstruct light leakage between the zones.

The 4 or 6 of these taillights I have taken apart merely had bare metal in there - I assume the plastic components came around later.

The galvanized metal stamping functions as the ground plate for all bulbs, allowing any positive wire to short-circuit to it if the insulation is compromised
 
The military variants.

The lens is secured with six screws, three times more than the inferior civilian models.

The housing is metallic, however the civilian variant has a plastic housing.

The first photograph shows what seems to be a blackout tail light component inside the housing.


Conversely, the internal components seem to remain mostly unchanged.
 
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