Triumphs Only, Inc. - Triumph Technical Service Bulletin

Preliminary Information1974 Vehicle Seat Belt Interlock System - TR-6 - Spitfire


NOTE: There is a time delay feature incorporated into the seat belt interlock system. This feature will allow the starter to be operated under any seat conditions (fastened or unfastened) after the drivers seat has been vacated. Therefore, it is necessary to have the engine shut off and the driver's seat vacated prior to performing the following test procedure.

Operational Test Procedure of Smith's Seat Belt Interlock System

1. Sit on driver's seat with belt unlatched, door closed and transmission in neutral.

2. Turn ignition switch to start position. Starter should not operate, seat belt warning light and buzzer should operate.

3. With ignition switch in "on" position (Pos. II), put transmission in any gear. Seat belt warning light and buzzer should operate.

4. Latch driver's seat belt, put transmission in neutral, press firmly on passenger's seat cushion. Starter should not operate and warning buzzer and light should operate until the key is released.

5. With driver's belt still latched, transmission in neutral, operate the starter switch, starter should operate, seat belt warning light and buzzer should not operate.

6. With transmission in any gear, press passenger's seat cushion. Warning light and buzzer should operate.

Smith's Seat Belt Interlock System

The Smith's seat belt interlock system is designed to act as a seat belt warning and prevent engine start if the sit-buckle-start sequence is not followed. It will also give a visual and audible warning if transmission is placed in a forward gear and the driver or passenger do not have their seat belts fastened.

Start Prevention

The engine starting system will not operate if either:

a) A seated driver does not have the driver's seat belt in use.

b) A seated passenger does not have the passenger's seat belt in use.

The engine starting system will operate only if the following conditions have been met.

a) The driver must sit and subsequently buckle his or her seat belt.

b) A passenger must sit and subsequently buckle his or her seat belt.

c) The stopped engine may be restarted without restriction from the seat belt interlock provided the driver has not left the driver's seat or in the event of an engine stall, the ignition swtch has not been switched to OFF

The seat belt interlock system will not affect the vehicle operation when the engine is running because it is part of the starting circuit only and does not influence engine ignition. The start system will operate if both front seats are vacated.

Operation of the starting system is dependent on the seat belt control module. Removal of the control module from the system will prevent operation of the starting system.

Testing Guide for Smith's Seat Belt Interlock Control Unit

The seat belt interlock system is made up of a control unit and associating wiring and switches. In the event of a malfunction, it will first be necessary to determine if the fault lies in the automobile wiring and switches or within the control unit. The simplest method of determining a malfunction in the control unit is by substitution with a new one.

Important - Check switches and wiring first to avoid damaging a new control unit if circuit faults are present.

Test Equipment for Switches and Wiring

1. 12 volt 2.2 watt lamp with insulated flying leads 6" long, the ends of which should be stripped back of insulation for 1/2 inch. Note: It is important that this lamp rating not be exceeded to avoid burning out the fuses in the circuit.

2. A six inch piece of insulated wire (jumper wire).

TR-6 Smith's Seat Belt Interlock System Component Locations

1. Driver's Seat Switch

2. Driver's Belt Switch

3. Passenger's Seat Switch

4. Passenger's Belt Switch

5. "Seat Belt" Warning Light

6. Transmission Switch

7. Control Unit and Buzzer (Located Under Dashboard)

Wiring Harness Plug Pin Numbering Scheme

Test Procedures

Function Connect Lamp Connect Jumper Wire Effect Fault
Supply from battery 4 - 9 --- Lamp On If not, check line fuse
Driver's belt switch 4 - 6 --- Lamp - On(unfastened) Off (Fastened) If not, check fuse wiring and switch
Driver's seat switch 4 - 5 --- Lamp - On (Seated) Off (Unseated) If not, check fuse wiring and switch
Passenger belt switch 4 - 3 --- Lamp - On (Unfastened) Off (Fastened) If not, check fuse wiring and switch
Passenger seat switch 4 - 2 --- Lamp - On (Seated) Off (Unseated) If not, check fuse wiring and switch
Transmission switch ignition "ON" 7 - 9 --- Lamp - On (In gear) Off (Neutral) If not, check ignition switch, fuse, and transmission switch and wiring. If light is on in neutral, transmision switch is bad.
Start Switch 11-9 --- Lamp - On (In position III) Off (In position 0, I, or II) If not, check wiring and switch
Now Substitute Jumper Wire for Test Bulb
Start Relay --- 12 - 4 Starting motor operates If not, check relay, relay wiring, starter or starter wiring for defect
Seat belt warning lamp --- 8 - 9 Warning lamp on If not, check fuse, wiring, or warning lamp bulb
Warning buzzer and door switch --- Plug harness into module Warning on key in switch Driver's door open Buzzer off key in switch. Door closed. If not, fault is in driver's door switch or wiring. If buzzer operates, fault is in driver's door switch or wiring.

Questions That May Arise About the System -- and the Answers to Them

Q - What if one places a fairly heavy package on the passenger front seat?

Q - Does the driver have to get in before the front seat passenger?

Q - If the engine stalls, will one have to go through the sequence again?

Q - Suppose the driver or passenger raises himself off the seat in reaching for change at a toll booth. Will the engine stop?

Q - What happens if the seat belts are left fastened at all times?

Q - Suppose something goes wrong with the system; how can I start the car?


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