'74 seat belt interlock bypass - documenting bypass and questions

T

tdskip

True Classic
Have both a question but also trying to document for future searches so links and pictures come up in one place.

The seat belt interlock, unique to 1974 and very early 1975 cars, have two (?) relays and controller tucked under the passenger side dash as pictured here.



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Here is a picture of the controller.



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Here is a picture of the one of the relays.



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There is supposed to be a second one, correct?

Bypass instructions from DaytonTom:

[FONT=Trebuchet MS, Verdana, Tahoma, Arial]"To by-pass the seat belt system disconnect the large white and the large red connectors between the two relays. On the white connector attached to the starter relay, this will have the FEMALE connections in it, use a short piece of wire, crimp a male spade on each end, and use that to jumper from the blue wire with white stripe over to the light blue wire with red stripe. (these will be 2 corners on the same side of the white connector.)[/FONT]"
 
Update - one relay does look to have been removed as there are mounting point holes were it would have mounted.

I am getting power at the red wire solenoid control wire when I turn the ignition key to start.

That suggests that a bypass has already been done, right?
 
My 74 had the system bypassed, and contained only one relay (reset relay) and the control unit per your pics. The second relay in the system that's related to the interlock system is the buzzer relay, and it's on the drivers side of the car near the buzzer itself.

Schematic of Interlock box (wire colors seem to not match however):


Another version:


There are two red wires going to the reset relay... One with a secondary lead connected to it, and one that's just a single red wire. To bypass all one has to do is make a connection between the two large red wires... This in essence makes a direct connection between the ignition switch and the starter solenoid.

Instead I did the connection one better by utilizing the relay as a load bypass relay, putting the red ignition switch source wire as the relay energizer and a new wire direct from the battery supplying the solenoid current. A new ground lead for terminal 86 needs to be made, and after that you're golden :)

Original bypass relay wiring per factory manual:


Utilizing reset relay as a load bypass relay to improve current flow to solenoid and to relieve ignition switch loads:


Finished product has the interlock control unit removed off the car, the two plugs connected to it zip tied up and away, and the wires to the starter reset relay look like this:
 
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:)

Oh and one more thing... the radiator fan relay, carb fan relay, buzzer relay, rear window defogger relay, and the starter relay are all the same part (p/n 4254643). So there are a few spares on the car for testing should you ever suspect the starter relay to have failed.
 
zonker said:
There are two red wires going to the reset relay... One with a secondary lead connected to it, and one that's just a single red wire. To bypass all one has to do is make a connection between the two large red wires... This in essence makes a direct connection between the ignition switch and the starter solenoid.
Click to expand...

Zonker - may be a dumb question but once the red wires are connected they do not go through the relay any longer since the idea is to bypass the relay, correct?
 
Yes, connecting the two large red wires together at the relay bypasses all the seat belt related relays and control unit.
 
zonker said:
Yes, connecting the two large red wires together at the relay bypasses all the seat belt related relays and control unit.
Click to expand...

And at that point there is only one wire connected to the relay, which is the interlock input wire, which doesn't do anything at this point since it is the sole input into the relay. Right?
 
There are four wires connected to the reset relay, the two large red wires, the smaller orange with white wire which trunks off the ign side red wire, and a blue wire with white stripe, which grounds out once the control box allows it to. so in other words if you're not inclined to add a load bypass circuit, then another way to make the system bypass the interlock control unit is just to ground out the blue white wire to chassis.
 
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