Seat Belt Starter Interlock System & Wiring Explained - 1974 GM, Ford & Chrysler

Federal Motor Vehicle Safety Standard # 208 requires that the safety belt system and all 1974 passenger cars include a starter interlock system which will not allow the engine to be started until the driver and right seat passenger fasten their seatbelts after they are seated.
The process to start your vehicle:
- Enter vehicle
- Sit
- Buckle
- Start
If either person buckles first, then sits on the seats the engine will not start. This is the basic requirement of the seatbelt interlock.
Background:
In the early 1970s, in an effort to get people to fasten their seatbelts, the concept of a Seat Belt Interlock System arose. By definition, an interlock is an electrical-circuit arrangement that prevents a second operation from taking place until the first operation is completed. In the case of the seatbelt interlock system, the car cannot be started unless the seatbelts are fastened. But it goes beyond that. The seatbelts must be fastened within a specific sequence. For example, the driver (and front seat passenger) must get into the car, close the door(s), sit down, and then fasten the seat belts. If the seat belts are not fastened, or if they are fastened in an incorrect order, the car will not start.
In 1974 model year, the Seat Belt Starter Interlock System was incorporated into every automobile to comply with Motor Vehicle Safety Standard 208.
The following components were used for this system:
- An electronic control (logic) module (transistorized) that tracked the series of events.
- (2) or more pressure sensitive front seat switches, a starter relay, a manual bypass relay (in case of a system failure and to make it easier for mechanics working on a car, a manual bypass button is located under the hood. Pushing this button allows 1 free start without needing to fasten the seat belt. Each additional free start requires that the button be pushed again).
- (2) or more seat belt buckle switches.
- Door Jamb Switches
Although the warning buzzer and warning light were used to notify the driver to fasten his seat belt, they were not incorporated into the interlock system.
Soon after its implementation, frustrated and indignant citizens bombarded their Congressmen to complain about the interlock device, and in early 1975, representatives quickly changed the law. The new law only required that a light and buzzer that would activate until the seatbelts were fastened; doing away with the troublesome starter interlock system.
What Vehicles Incorporated This Interlock System?
All 1974, and early-to-mid 1975 vehicles.
Can I Bypass Or Disable This System?
From our experience, there is no documentation published on how to bypass or disable this system (most likely due to liability reasons). We can not offer any assistance in this area.
Can I Repair This System?
Unless you're lucky enough to obtain a special tester, which connects to the seat belt/starter interlock system at several points, for most people, the answer is no.
Can I Eliminate This System?
If you have a 1974 vehicle the answer is NO. If you have a 1975 GM vehicle, it is possible. However, because the interlock system was integrated into several wiring harnesses, you will usually need to replace the Dash, Engine and Rear Body Harnesses with harnesses made for 'WITHOUT seatbelt interlock system'.
NOTE: If you've made the decision to completely rewire your 1975 vehicle, we strongly recommend that you purchase your wiring configured for 'WITHOUT seat belt interlock system'. Doing so will allow you to eliminate all of the electrical components that are part of this system; components that may eventually fail, are no longer made, and are hard to find.
Can I Find Replacement Parts?
Lectric Limited offers exact reproduction wiring harnesses for 1974 GM vehicles with the interlock system. We also offer wiring harnesses for 1975 GM vehicles with or without the interlock system. However, the other components of the system (logic module, sensors, bypass device, and relays) have long been discontinued and are difficult to find. We do not know where to obtain these components but suggest a junk yard, swap meet, Ebay, or a car club member.
How Do I Identify If My Vehicle Has/had This System?
All 1974 vehicles had the seatbelt/starter interlock system. As for 1975 vehicles, unless you are the original owner or know the history of the vehicle, unknowing wiring modifications by previous owners or repair shops can make this identification difficult. We suggest trying to locate the specific components of the interlock system within the vehicle.
- Logic Module and Sensors often located behind or under the seats. (in Corvettes, it's located behind the seats)
- Starter Interlock Relay mounted in console or under the dash. (in Corvettes, it's mounted in the console)
- Manual Interlock Bypass Relay mounted on firewall or on the fender well.
www.LECTRICLIMITED.com

[@wilsonle61]

I remember this system in my Mom's 74 Pinto. It had so many problems the feds had to repeal its use in the 75s and later.

[@megalomotors]

The music, the announcer, the Monaco hardtop, and the film's groovy graphics scream the 1974 of my youth!

[@jonathanmorrisey5771]

That's actually a '74 Plymouth GranSedan.

[@megalomotors]

@@jonathanmorrisey5771 You are correct, good catch! By '74 they had both become Plodges with only a few trim pieces cluing one in on whether the car was Plymouth or Dodge.

[@briansd2772]

Thank you! Now I know what that red button on my Trans Am's driver side inner fender does and how/when to use it! Yes my system still works.

[@hunglomein8047]

Thank you for your video on the proper use of Dialectic grease. Seriously, I used to just goop it in the end of the boot.. I'm ritardid

[@vickyzhang4137]

Good quality

[@traptown5087]

Lectric Limited

[@chrishowell4241]

I want to own this patten I had a dream about this and it should be in every car today do anybody listen to this show in 2020