Controls 1: Going from switches to PLC controls

The original car did come with powered just-about-everything, but it was done with technology and components available 60 years ago. The engineer who designed it all did an amazing job, but there was only so much for him to work with.

So, the existing wiring harness is pretty limited in functions that it controls. It’s also all dried out, cracked and rusty, so I really don’t want to spend the time to clean it all up and then splice into it in a hundred-and-one places because it’ll never be super-reliable. There are a lot of small changes to the way the car will now work, and upgrading the wiring harness that’s there just seems more trouble than it’s worth.

Instead, I’m going with a brand-new 12V-powered PLC. Finding one of those was actually a lot harder than I expected. There’s PLC’s aplenty, but they’re not cheap (inc programming software, spending $3k-$10k is totally normal) and pretty much all of them want either 24V power or an AC-mains line. There are some small units out there that do take 12V, but they’re really tiny in capacity and feature-set and not exactly expandable (unless you want to spend the weekend soldering the pieces together).

I finally came across the BRX PLC line from in Atlanta (no relationship / sponsoring etc). It’s very configurable, the feature-set is totally adequate for what I’m doing, and the cost came in under $1,500 all said and done with all of the specialty modules included. That’s $15 per input/output (switches and sensors, driving gauges and lights and relays), and compared to most fancy PLC’s, that is super-reasonable.

I have not found any specs on this unit as far as g-force/vibration tolerance, so that will be the big discovery once we’re bouncing down the road. I’m in a Cadillac (at 6,500 lbs) so it shouldn’t bounce much, but there will be some and it could make the relay-outputs jitter. We’ll find out, and if I need to use additional automotive relays I can.

As for operating temperatures, this thing will ride in the trunk in a decently ventilated “box”, so it should not get much over 100F and the unit should be able to handle that. There is a water-cooling circuit going to the rear pack that runs right under the PLC, so I could splice into that if I needed to create some additional cooling capacity.

The idea of this unit is to

  • control all 12V-run accessories, just like an original ignition switch, turning the motor-controllers, AC-compressor and DC/DC converter that run off of the traction pack on and off
  • Track the charge-level of the battery pack by measuring the pack voltage (up to 350V), the charge/discharge current (up to 800A) and the battery temp. Between these three I should have a decent indication of % charge that the PLC can display on the original fuel-gauge
  • re-use the all-original instruments and control them directly (the speedo will run through a small electrical motor with speed-control)
  • run the wipers with long interval/short interval/all-time/high-speed settings, and add a one-swipe function to the sprayer button
  • add powered door-locks that react to the speed (lock at 10 mph) and the gear shifter (unlock when going into park)

Standard lights on the car included the famous tail-fin flame lights. They’re cool in the styling and all that, but those 2.5″ lights were all that lights up on the back of the car at night. They’re small, they’re dim, and they serve(d) as tail lights, brake lights and indicators. To me (matter of taste) they under-state too much and don’t show off the scale of the car.

So, I am adding LED back-lighting to the large red reflectors (kind-of like the RH picture) to create larger, more visible tail lights. I’m also turning the back-up lights into combined white / yellow bulbs so I can use those as the main indicators. Both of these functions can be enabled/disabled through the PLC to make everything go back to original (same in the front for two-tone indicators and DRL), in case somebody insists, but for me a little more light in the back is a good thing.