1974 Seat Belt Interlock Questions Please help


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Dec 6, 2019
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lakeland florida
1974 Dodge Monaco 4 dr.
Had been running well for years, yesterday the "Fasten Safety Belt" light suddenly decides to come on and now the car wont crank. If I jumper the starter solenoid, car will start, however, if I turn the key, starter will not work. So.... I have replaced: starter solenoid, neutral safety switch then hit the web and per Nacho's recommendation, I jumpered the two yellow wires on the emergency start relay under the hood. Still no start via the key/ignition switch. I have ordered an orange module box and a new emergency start relay which should arrive in a week or so, however, I would like to delete this interlock system completely. I can not find the interlock module! Where is this module located in a 1974 Monaco? Please do not reply "under the dash". Where under the dash? Is it under the carpet between the two front seats? Is there a wiring solution to disable this system other than jumpering the two yellow wires, which hasn't worked on my car yet.
Please help!!!

What you have not mentioned is if the belts were fastened and you still have a no start condition. The only sensors for the interlock were under the front seats (green and grey wires) and front belts. Sometimes if you put cargo on the front seat, it would trip the switch because the system sensed weight (like a person) but no seat belt fastened on that side. I sold my 74 FSM with my Imperial so I no longer can access diagrams.

But there has got to be a way...
There is an article on allpar.com by Daniel Stern entitled The return of the seatbelt interlock: crazy rule or money saver?

About halfway down the page it shows a black connector plug which, once disconnected, defeats the system.

Interesting article, hope it answers your question.
The bypass button is on a box under the hood between the brake booster and fender on the drivers side as pictured above. Press it and it bypasses the interlock for one start.
The bypass button is on a box under the hood between the brake booster and fender on the drivers side as pictured above. Press it and it bypasses the interlock for one start.
I found that switch, no problem and went ahead and jumped the yellow wires per Nacho's instruction. Still no starter. I pushed the button and no start either. I am looking for someone to point me to the interlock module, not the by pass switch. I looked quickly under the dash near the gauge cluster, but could not find it. Is it under the carpet, or in my case, the rubber flooring under the front seats? Anyone found this thing?
Unless the bypass switch is also shot I’d have thought pushing the button would eliminate an issue with the control box from the equation, but who knows.

I’ll see if I can find the actual box on mine. It’s 100% not under the carpet because I have no carpet and it’s no laying on the floor.

Thanks for the info, I was going to remove the seats today to look for it! Let me know where you find it, I've got a green or red module on the way, can't remember which one I bought, to teplace mine, just in case.
I had a few minutes this morning to poke around. I couldn’t find the location in the service manual, so I went looking on the car. I couldn’t find it under the dash on mine, but for the most part things are fully assembled. I’ll go look harder this afternoon.

Unless your bypass switch is also faulty I still think pressing it would put this module outside of the problem path.
I just received a new starter bypass switch and will be installing it tonight to test that theory. I am worried that the module has failed to a point that it is not letting any voltage through even when a bypass is used. My car is in very good shape, and I can tell thet it hasn't been worked on much at all, so why that module would fail is a bit of a mystery. My car also sleeps inside a dry, clean garage. Only is driven on dry, sunny days, except one day this year during a race, but it lives a very sheltered life. Why this system would all of a sudden freak out is beyond me. I am glad it decided to fail now and no during a race!

This might help you work some bypass tests or other troubleshooting if you haven’t seen it already.

At about 12min in the my show a test box that has the cable vaguely going towards the middle dash area by the glovebox.
In the off chance you haven’t found the module yet, I did. It is mounted on a bracket near where the brake booster bolts come through the firewall. Mine is blue, or this is actually the wrong box. I regret to inform you that even viewing this thing required some uncomfortable maneuvers.