1974 Corvette Seat Belt Interlock Problem

[Charles E.]

I am preparing my 1974 L-48 TH400 for flight judging, so disconnecting the interlock system is not an option for me at this point.
I would like to have the system function correctly to maximize my operational judging points.
My problem is this:

when out of the car, I can reach in and start the car.

when seated in the car with the seat belt unfastened, the car will not start and the fasten seat belt light illuminates and the buzzer sounds.

after fastening the seat belt, the car will not start and the fasten seat belt light illuminates and the buzzer sounds.
I exit the car, and with the key switch on, I push the start button on the firewall, return to the car buckle my seat belt and the car starts.
When I turn off the car, and try to restart it - it will not start and I have to repeat the steps above.

In other words, I have to press the start button on the firewall each time I attempt to start the car with my seat belt fastened.

Has anyone experienced this situation and been able to correct it?

Thanks
Chuck

[Joe L.]

Re: 1974 Corvette Seat Belt Interlock Problem

I am preparing my 1974 L-48 TH400 for flight judging, so disconnecting the interlock system is not an option for me at this point.
I would like to have the system function correctly to maximize my operational judging points.
My problem is this:

when out of the car, I can reach in and start the car.

when seated in the car with the seat belt unfastened, the car will not start and the fasten seat belt light illuminates and the buzzer sounds.

after fastening the seat belt, the car will not start and the fasten seat belt light illuminates and the buzzer sounds.
I exit the car, and with the key switch on, I push the start button on the firewall, return to the car buckle my seat belt and the car starts.
When I turn off the car, and try to restart it - it will not start and I have to repeat the steps above.

In other words, I have to press the start button on the firewall each time I attempt to start the car with my seat belt fastened.

Has anyone experienced this situation and been able to correct it?

Thanks
Chuck

Chuck------

First off, I'll say I have never dealt with this problem, at all. However, I THINK your problem relates to some disfunction with the "logic module" which is the "brains" of the system. This module was GM #9736651 but it's long-since GM discontinued. If you need one, I don't think you'll have much problem finding an NOS example. The vast majority of folks disabled the interlock system and, thus, had no need for this part. The system was only used for 1974-75.

[Charles E.]

Re: 1974 Corvette Seat Belt Interlock Problem

just checked Ebay - they are out there NOS under $50.
I am going to run sequential diagnostics tomorrow out of the repair manual - see if that pinpoints anything.

[Charles E.]

Re: 1974 Corvette Seat Belt Interlock Problem

I have isolated the problem. It is a poor connection in the latch portion of the driver side seat belt.

If I depress the GM logo portion of the seat belt while buckled, it makes a connection - I hear a double click in the dash - and the car will start.
Also if I wiggle around with the retractor portion of the seat belt while attached, I hear the same clicking in the dash and the car will start.
After starting and the car is running the buzzer and light will go off - and when I depress the logo portion or wiggle the retractor portion - it stops.

now I just have to figure out how to improve the connection - hopefully without having to remove the seat and console and possibly have to find and replace the latch portion of the seat belt.

[Jimmy P.]

Re: 1974 Corvette Seat Belt Interlock Problem

I have isolated the problem. It is a poor connection in the latch portion of the driver side seat belt.

If I depress the GM logo portion of the seat belt while buckled, it makes a connection - I hear a double click in the dash - and the car will start.
Also if I wiggle around with the retractor portion of the seat belt while attached, I hear the same clicking in the dash and the car will start.
After starting and the car is running the buzzer and light will go off - and when I depress the logo portion or wiggle the retractor portion - it stops.

now I just have to figure out how to improve the connection - hopefully without having to remove the seat and console and possibly have to find and replace the latch portion of the seat belt.

Charles I doubt if you'll be able to correct your problem without taking your seat out. It's only four bolts ( if they come loose easily)! Then you'll need to remove the black retractor cover under the seat to get to the retractor/ seatbelt wiring. You won't need to remove the console.
Good luck. Keep us posted!

[Joe L.]

Re: 1974 Corvette Seat Belt Interlock Problem

I have isolated the problem. It is a poor connection in the latch portion of the driver side seat belt.

If I depress the GM logo portion of the seat belt while buckled, it makes a connection - I hear a double click in the dash - and the car will start.
Also if I wiggle around with the retractor portion of the seat belt while attached, I hear the same clicking in the dash and the car will start.
After starting and the car is running the buzzer and light will go off - and when I depress the logo portion or wiggle the retractor portion - it stops.

now I just have to figure out how to improve the connection - hopefully without having to remove the seat and console and possibly have to find and replace the latch portion of the seat belt.

Charles------


I believe the activating switch for the seat belt interlock system is within the buckle portion of the seat belt (i.e. the end with the GM logo). I don't think there are any electrical components in the tongue end. So, I'd say there must be some problem with the switch. As far as I can tell, the switch was only serviced as part of the buckle assembly although it's POSSIBLE it was once serviced under GM #6273358 which was discontinued in 1980. So, you'll probably need to disassemble your buckle and see if you can repair the switch.

Addendum

I did confirm that the switch was serviced separately and it was GM #6273358. It includes the switch, itself, which is contained within the buckle, a "spine" which extended down into the buckle boot, and a pigtail with connector.

[Charles E.]

Re: 1974 Corvette Seat Belt Interlock Problem

I have been playing with the seat belt for a while - several hours over the last few days and now it seems to be working properly. I guess maybe the contacts had just become a bit "rusty" due to lack of use over the last several years. Repeated buckling and unbuckling seems to have revived the contacts.....
And now I am also the proud owner of a NOS logic module GM #9736651 that I bought on eBay for $40.00 !!
Maybe I can use it on my next '74 project car.

[Robert R.]

Re: 1974 Corvette Seat Belt Interlock Problem

Charles------


I believe the activating switch for the seat belt interlock system is within the buckle portion of the seat belt (i.e. the end with the GM logo). I don't think there are any electrical components in the tongue end. So, I'd say there must be some problem with the switch. As far as I can tell, the switch was only serviced as part of the buckle assembly although it's POSSIBLE it was once serviced under GM #6273358 which was discontinued in 1980. So, you'll probably need to disassemble your buckle and see if you can repair the switch.

Addendum

I did confirm that the switch was serviced separately and it was GM #6273358. It includes the switch, itself, which is contained within the buckle, a "spine" which extended down into the buckle boot, and a pigtail with connector.

Joe,
I am having the same issue as Chuck with my '74 seat belt interlock. I narrowed it down to the switch within the buckle end and I am able to start the car if I wiggle the tongue end in the buckle. I would prefer to replace only the buckle assembly which would include the leads that go to the electrical connection under the seat. After checking the major Corvette parts suppliers and eBay, apparently the buckle assembly is only serviced with the entire seat belt assembly which includes the shoulder harness. Are you aware of a source to purchase just the buckle with electrical leads and, if not, are you aware of a procedure to repair the switch within the buckle?

Thanks for any advise,
Bob

[Joe L.]

Re: 1974 Corvette Seat Belt Interlock Problem

Joe,
I am having the same issue as Chuck with my '74 seat belt interlock. I narrowed it down to the switch within the buckle end and I am able to start the car if I wiggle the tongue end in the buckle. I would prefer to replace only the buckle assembly which would include the leads that go to the electrical connection under the seat. After checking the major Corvette parts suppliers and eBay, apparently the buckle assembly is only serviced with the entire seat belt assembly which includes the shoulder harness. Are you aware of a source to purchase just the buckle with electrical leads and, if not, are you aware of a procedure to repair the switch within the buckle?

Thanks for any advise,
Bob

Bob------

I don't know of either. However, I really think that your problem lies in the switch I previously mentioned rather than the buckle, itself. Unfortunately, I don't think that anyone reproduces this switch. I expect that the vast majority of folks with a car using this switch are vastly more inclined to disable and eliminate this system than those wanting to repair/restore it. That fact would not bode well for a reproduction being available.

[Robert R.]

Re: 1974 Corvette Seat Belt Interlock Problem

Bob------

I don't know of either. However, I really think that your problem lies in the switch I previously mentioned rather than the buckle, itself. Unfortunately, I don't think that anyone reproduces this switch. I expect that the vast majority of folks with a car using this switch are vastly more inclined to disable and eliminate this system than those wanting to repair/restore it. That fact would not bode well for a reproduction being available.

Thanks Joe. I appreciate your response. I think I will disable it since I have no visions of having the car judged again. It was last judged National Top Flight at Seven Springs in 1994 and has been a sweet driver since.

[Joe L.]

Re: 1974 Corvette Seat Belt Interlock Problem

Thanks Joe. I appreciate your response. I think I will disable it since I have no visions of having the car judged again. It was last judged National Top Flight at Seven Springs in 1994 and has been a sweet driver since.

Robert-----


In 1974 I got a brand new 1974 Vega station wagon for a work car. It had the same system that your car has (as did many 1974 GM products). I disabled it the first day I got the car. I did use my seatbelts, of course, but without the annoying "interlock".

[Robert R.]

Re: 1974 Corvette Seat Belt Interlock Problem

Robert-----


In 1974 I got a brand new 1974 Vega station wagon for a work car. It had the same system that your car has (as did many 1974 GM products). I disabled it the first day I got the car. I did use my seatbelts, of course, but without the annoying "interlock".

I should have done the same thing. Here's a funny story. Shortly after I bought the car in '74, my wife went to the grocery store and placed grocery bags on the passenger seat. She called me at work, crying, and said the car won't start. Someone gave her a ride home with the groceries and after I got off work I went to see what was wrong with the car. Of course it started. When I got it home I asked if she had any weight on the passenger seat. I had to explain the system to her and if she placed any weight on the seat she needed to fasten the seat belt.