Better Beaters - Building Cars and Daily Drivers

 Doing a little more work on the Plymouth. Part Two

My hand drawn wiring diagram.

As I mentioned in my last post, I had experienced a brake problem and was expecting to find a noticeable leak in the system. I did not find any leaking, however the problem could have been caused by an internal leak in the master cylinder.

I had parked the car in the garage after I got back from that drive, and became busy with other matters. So it sat for almost three months. After I replaced the battery and backed it out of the garage, I decided to drive it around the neighborhood a bit. On this drive I didn't have a problem with the brakes. But when I got back and checked the brake lamps they were really dim. Not good. 

To check the operation of the stop lamps, I disconnected the wires at the switch, and connected them together with a jumper wire. I could clearly see the CHMSL light up, and I went around the back and saw the trunk mounted brake lamp brightly lit. I left the jumper on for a few minutes and felt the brake switch, it did feel a bit warm. 

I needed to do a test to see if there was an internal leak inside the master cylinder. This would also show if the switch was heating up. 

I decided that I would fully apply the brake pedal for a full five minutes. I would find out if the pedal would sink towards the floor over time. I could also check the brake switch and see if it was heating up. I got my stop watch and pushed hard on the pedal. Five minutes later it had not budged a bit, Then I quickly got out and felt the stop light switch. It was pretty hot! I wondered if I'd burnt it out. 

I needed to check it.

First I disconnected the battery, then I connected my multi meter to the terminals of the brake switch, it was set to indicate continuity. I pressed on the brake pedal and heard the buzzer sound, so the switch still worked. Then I re-connected the battery and connected the input power terminal and hooked up my multimeter to measure voltage. I used a couple of long jumper wires so that I could position the meter where I could see it while I was in the car pressing on the brake pedal. The meter indicated a bit over six volts. I figured that it was probably enough to switch the relay system that I was going to add.

I had ordered a couple of relays earlier this Summer, so I already had them on hand. I went to a couple of local Auto parts stores and bought wire, connectors, and a terminal block. Wire has gotten to be pretty expensive lately.

This is how I set up the relays for the headlamps on my '70 Mustang.
It worked like a charm.

Then I looked online for info on how to hook them up. I had previously used relays on the headlamp wiring of my '66 Mustang. So I was familiar with how to hook them up, but it had been awhile. I drew a diagram showing how to hook up the wires to the relay, and it also showed how the wires were going to be run in the car. 

I put this diagram in my Plymouth's owner's manual for later reference. 

Brake lamps usually have constant power on most cars, so I decided to hook the circuit to the hot side of the starter solenoid. I would run a 10 gauge wire through the firewall to a terminal block under the dash. Then I would run a 12 gauge wire to the trunk compartment where I would install the relay and connect it to the original brake light, as well as connect it to the wires of the CHMSL that I'd added. 

The second wire will be in place if I need it.
Maybe to connect the heater that I might install later.

I added a 30 amp fuse to the power feed then ran the wire to the terminal block that I attached to the radio box. I also added an additional 10 gauge wire so that I had two wires going to the terminal block, but the extra wire was not connected at either end. I might want to wire up a heater in the future so it would be in place if needed. 

Plenty of room inside the trunk to work comfortably.

Afterwards I went back and tidied up the job and put some
sheathing around the new wires.

By placing the relay in the trunk I only had to run a single wire from the junction block. If I had wired it under the hood, I'd have to run at least two wires. I added an inline 20 amp fuse before the relay, and I ran a new wire from the relay to the original brake lamp. I had thought that the wire that had been used when it was rewired was a bit thin for the job.

The additional circuit was run independently from the existing harness. I did not want to start messing with the existing wiring. It might not be a perfect job, but it has worked fine for many, many, years. 

The existing brake light switch worked out quite well and with direct power, the brake lamps were really bright! I felt that these would be easy to notice, and that would add a lot of safety.

Much better!

Now, I'll turn my attention to the inoperative ratchet mechanism on the hand brake lever.