1975 Plymouth duster brake relay question?

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flathead31coupe

flathead31coupe

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On my driver side interfender there looks to be some kind of a relay that has a reset button of some kind. The wire from the proportioning valve goes up to this relay. Does anyone know what it is or what it does? Thanks!
 
It's the relay with the two bolts holding it down with a little. Looks like a reset button on it. The wire from the switch on a proportioning valve plugs into that

PXL_20230705_221243350.jpg
 
I believe that's a "maintenance minder"- illuminates the idiot light when maintenance is due. Kind of a timer thing, the button is a reset.
 
So what I did was take the black wire from the idiot light out of the bulkhead connector and connected it to the wire going down to the switch on the proportioning valve.
 
It might be that you have an off model and it may be the 1974 seat belt interlock. To find out, pull the connector and see if the the key will operate the starter. If that happens, you need to bypass it. Should have two yellowish wires, which you need to jumper together. They are the start wire going from the key to the start relay. That box, when tripped (if seat belt) opens that circuit

This was a Federally mandated "one year only" deal in 74, and EVERYBODY hated it.
 
flathead31coupe said:
One of the wires go down to the switch on the proportioning valve. Something to do with the brake light in the dash
Click to expand...

Based on the spaghetti I see, I would be careful about trusting that switch is original and/or wired correctly.

I agree with 67Dart273, looks like the seatbelt interlock switch. In '74 those weren't wired into the proportioning valve switch circuit though, so no idea what is going on.

Can you post a close up picture of the plug going into that relay?
 
It’s a seat belt interlock, same exact piece that was on my ‘74. Supposed to be one year only but if you’ve got an early ‘75 then that might be what happened.

And that rats nest of wiring needs to be dealt with, that’s just asking for a fire. Not sure who’s been wiring that thing, but I’d be looking for a new engine harness.
 
What is the build date.
Can be found on door decal or left inner fender body tag.
 
Here's a few pictures of the one that was on my '74 Duster. And no, I didn't put the wire nuts there, and yes, they were dealt with before the car hit the road. These pictures were taken right after I bought it, when I had the original /6 out, before I swapped the small block in and fixed some of the questionable things some of the PO's had done. It is in fact a reset button that resets the relay if you trip the interlock mechanism.



IMG_1103.JPG


IMG_1102.JPG


The seat belt interlocks were a pain in the ***. Mine were disconnected when I bough the car, but even with the seat belt switches disconnected the interlock can still kill the ignition if any of the original interlock electronics fail. The reset relay shown here later went bad, and when it did it disabled the ignition. I bypassed it later, but even then there are other components in the system that can come back and haunt you.

With as chopped up as your harness looks, I would look for a complete '73 engine harness.
 
That's what the wiring harness looks like if you unwrap it. That's been unwrapped so I can trace down wiring and find out where things go. Kind of like that when I got it but everything works and it runs. I was just questioning that relay. I didn't think the outside wire by itself went from that relay down to the proportioning valve. So where it came out of the number m spot on the middle bulkhead connector that goes to the dash brake light. I just tied on to that and ran it down to the switch on the proportion valve. I left everything else. Stay the same on the relay
 
flathead31coupe said:
That's what the wiring harness looks like if you unwrap it. That's been unwrapped so I can trace down wiring and find out where things go. Kind of like that when I got it but everything works and it runs. I was just questioning that relay. I didn't think the outside wire by itself went from that relay down to the proportioning valve. So where it came out of the number m spot on the middle bulkhead connector that goes to the dash brake light. I just tied on to that and ran it down to the switch on the proportion valve. I left everything else. Stay the same on the relay
Click to expand...

Yes, I am fully aware of what a wire harness looks like if you unwrap it. What you've got there is not good, but hey, it's your car so do whatever.

It sounds like you're talking about the black wire then. That's the power feed wire, just because one end is connected to the brake pressure switch does not indicate that all of the items connected to it are for the brakes. You will find a black wire connected to almost every switch in the car, again, it's typically the power feed to the switch.



74DartFullharness.jpg


And as @67Dart273 said, your best course of action with that seat belt interlock relay is to bypass it, because it does not serve any purpose but it can and will disable your ignition if the relay or anything else in the interlock system fails.
 
I agree with you 100% but while looking at the wire for the idiot light on the dash for the brake, it does go straight to the proportioning valve. That's where it is now. And I appreciate all the help. I am going to get to work on that harness with a replacement as well
 
My car is a 75 and it has that plug under the hood but, it has never had the relay in it for as long as I have had it. I always wondered what that plug went to.
 
Here's how the wire to the brake prop works. That is part of the differential pressure piston in case one end of the system springs a leak,the difference in pressure will push the piston to one end, and that actuates the switch to ground. That will light the "BRAKE" lamp on the dash. In parallel with that switch is the switch on the parking brake which functions same way. When you pull the handle, the switch grounds--just like a courtesy lamp door switch, and lights the BRAKE lamp.
 
In the 1975 model year, there were two engine wiring harnesses.
Chrysler part #3843129 was used up to 10-14-74 and it had the interlock system.
Chrysler part #3843461 was used after 10-14-74 and did not have the interlock system. That was the reason for asking for the build date of the car.
There are 3 steps to permanently disable the system.
1. Reach under the seat and unplug the sensor.
2. Unplug the green or blue interlock module under the dash.
3. Splice together the two yellow wires going to the interlock relay under the hood.
If you don't want to cut and splice the yellow wires you can make a jumper cable. Take a 12 gauge yellow wire and crimp a male terminal on both ends, then plug the jumper cable into the interlock connector plug. This connects the yellow wires together instead of cut and splice.
 
As others have commented, that is a mess of wiring. Hopefully you carry a fire extinguisher in your car? Disabling that seat belt interlock is a start but do consider buying a new harness soon. What a dogs breakfast that looks like!

If your Duster still has the fender tag, it will tell you the scheduled production date of it. The fender tag is a ~2x3” piece of metal screwed to the top of the inner fender past the windshield washer reservoir in your photo towards the front of the car. It’s a 3 digit (maybe starts with an A with two numbers or just three digits) to the right of the code that has “000”. Given your 75 has the seatbelt interlock/wiring harness, it would have been built in August (8) September (9) or October (A) 1974.
 
Last edited:
townsend said:
In the 1975 model year, there were two engine wiring harnesses.
Chrysler part #3843129 was used up to 10-14-74 and it had the interlock system.
Chrysler part #3843461 was used after 10-14-74 and did not have the interlock system. That was the reason for asking for the build date of the car.
There are 3 steps to permanently disable the system.
1. Reach under the seat and unplug the sensor.
2. Unplug the green or blue interlock module under the dash.
3. Splice together the two yellow wires going to the interlock relay under the hood.
If you don't want to cut and splice the yellow wires you can make a jumper cable. Take a 12 gauge yellow wire and crimp a male terminal on both ends, then plug the jumper cable into the interlock connector plug. This connects the yellow wires together instead of cut and splice.
Click to expand...

I pulled the bulkhead connector from the dash side and swapped the 2 yellow wires in it to bypass mine. This effectively connects the starter switch directly to the starter relay. The OEM wiring has the yellow wire split and go to the interlock module under the dash and through the bulkhead connector to the emergency start relay next to the power brake booster, resulting in no direct connection to the starter relay. There is in addition yellow wire from the interlock module through the bulkhead connector to the starter relay. So by swapping the yellow wires in the bulkhead connector, it looks completely stock but there is absolutely no way the module can cut the starter circuit.

I believe the wires could be swapped in the engine side harness as well. I had my dash harness out so I did it there while I was dinking with other changes.
 
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