My car is a ‘74 with the seat belt interrupt circuit. All the years I’ve had it I’ve just had the system bypassed with the yellow wires connected together. But after restoring most of my interior (including seats and the ‘Fasten Seat Belt’ light. I thought it would be neat to reinstall the relay (while keeping the yellow wires spliced together. So I picked up a relay from Mark Heard (thanks Mark!) and installed it. A perhaps important side note is that when I had my seats redone by Autobahn Interiors, they replaced the seat bottom foam and did reinstall the seat bottom sensors. Now I have the following issues. The light doesn’t come on when someone is in the drivers seat, whether the seat belt is connected or not. On the passenger side the light comes on whe NOBODY is in the seat but turns off when I connect the belt. In looking at the diagram I suspect the seat bottom sensors. They may not have been put in the ideal place when the seat cushions were re-foamed. Or something might be wrong with the sensors themselves. My question is, exactly how do the seat bottom sensors work?

Sorry I’m advance for the long post but wanted to give as much info as possible. Shameless interior pic for good measure.

Any chance the receivers are switched right and left? I just went through this and of course I installed on the wrong side first. After swapping them, they both work as expected.

Oh, absolutely that could be the issue. In looking at the wiring diagram I was just focused on where the fault may lie. It’s possible that the wires to the receivers are switched. I pulled both receivers, stripped them, cleaned everything up, installed new ‘PRESS’ stickers and even put on new heat shrink. But didn’t think about the wiring. I’ll definitely take a look tomorrow and report back.

Looked at the wires last night. I don’t think they can be interchanged between seats. They’re grouped together in the loom heading to each seat. Picture of the drivers side seat area below.

So that leaves my original question. How do the seat sensors work and how can I test them?

The receivers themselves are right and left. The driver's side works just like your passenger side is workingj (on all the time until the belt is latched. If you switch the actual receivers side to side, at least your drivers side will work properly. You still might have a problem with the passenger seat switch or something else. I can't remember exactly how passenger side behaved when I had the receivers on the wrong side. You can unplug the switch and test for connectivity across the plug contacts. It should close to zero resistance when you push or sit on the seat enough to bend the switch. If it doesn't, that's your other problem.

I discovered something new today testing all my electrical. The fasten seatbelt light switches off when the e-brake lever is pulled. Something to keep in mind when testing.

I agree with swap the seat belt receivers. I had the same problem when I was restoring the "fasten seat belt light" to the car. It was on for the passenger side and shut off when the passenger seat belt was fastened. After swapping the receivers, things worked as normal, or at least how I wanted them to! haha.

I just float test them.

I remove them from the car and throw them in the lake to see if they float.


I always wear my seat belt. Saved my butt a couple weeks ago.

So I don't need an electronic nag annoying me.


Clay

I’ll try switching them. But when I disassembled them it appears that all they are is two wires in the receiver. When the metal tab of the seat is inserted into the receiver the metal tab completes the circuit. It looks that way in the wiring diagram as well. But I will check it!

Yeah, I wear the seat belt all the time, too. I don’t need the light to remind me, I just thought, what the heck, it’s something that came with the car, why not get it to work as intended.

I thought the same thing until I put them in backwards And I'm with you on having them working. I pulled the buzzer (a.k.a. the German lullaby), but I think the light is kinda cool.

This evening I did what I should have done at the beginning. Before I started checking I cleaned up the contacts on both seat cushions and both seat belt receivers. Reattached and no change in my issue.

I put the DVM to it and checked continuity and resistance and it turns out both seat belt receivers are the same. They gave the same readings when checked for continuity. Then I checked the seat cushion sensors for resistance. The drivers side readings didn’t change when I applied pressure to the seat cushion. On the passenger side cushion the resistance dropped when I pushed my hand into the cushion. So it looks like the sensor in the drivers seat is bad. To confirm I swapped the cushions from side to side so the working cushion is now on the drivers side. Works as advertised. When I start the car the light is off (because the handbrake is engaged). When I release the handbrake the ‘fasten seat belt’ light illuminates. Then once I connect the seat belt the light turns off.

Just kind of documenting this all here in case someone needs the info later.

OK, now I get it. I didn't realize the later cars with the interrupt have sensors in both seats. My 73 only has it in the passenger seat, hence the left and right receiver issue. Makes sense now.

I just made it simple. I removed all the seat belt interlock stuff from the car.

I don't need it. I always wear my seat belt no matter what.

I do too but am getting a kick out of making all of the little stuff on my car work like they did when it was new. I don't know why it makes me happy to see that light come on when I sit in the passenger seat, it just does

I understand. If it makes you happy, do it.

Brent,

It’s odd that you have it on a ‘73. Even more odd is that only one seat has it. I’m pretty sure that the ‘73’s didn’t have the seat belt interlock system at all...on either seat. I thought it started in ‘74 and ran through the end of production in ‘76. I’ve owned a ‘74, a ‘75 and a ‘76 and they all had it (well, the wiring for it was there but everyone of them no longer had the relay). I’ve had two ‘73 models and neither one of them had it.

The 1972-73 has a Seatbelt Warning System - 4-pole 'Fasten Seatbelt' light, seatbelt switches, passenger-only seatpad switch, round buzzer at fusepanel

The 1974-76 has a Seatbelt Interlock System - 2-pole 'Fasten Seatbelt' light, seatbelt switches, seatpad switches, interlock/cube buzzer relays

I've been considering using the Fasten Seatbelt light for a different indicator and was wondering if (on my 74) it's simply + and - connection....I'm guessing that's the case.

Wow, had no idea. I don’t remember the light in the ‘73, or any other hardware. Of course they were my second and third 914’s and that was late ‘80’s.

To clarify, my 73 has the warning light and buzzer as Jeff said, but not the lockout. For some reason I had a brain fart that your 74 would be wired different.

No worries!