Pulse Width Modulated (PWM) Dimmer Control Upgrade

[Captain Obvious]

Most vehicles have long since changed away from a potentiometer based instrument dimmer to a Pulse Width Modulated (PWM) based electronic dimmer controller and I've been working on retrofitting one of these newer style electronic dimmer controllers into my 280Z.

I poked around the junkyard looking for one that would mechanically fit in the original potentiometer location and settled on one from a late eighties to early nineties Toyota Camry. The main criteria needed are short stubby mounting depth and rotate control instead of a thumbwheel so I could mount it in the original dimmer pot location. This is the dimmer out of a 91 Toyota Camry:

 

The biggest hurdle with this retrofit is that the shaft is too short, and because of that, this became mostly a mechanical project instead of an electrical one. I took the dimmer module apart and took the control potentiometer out:

 

Mounted the pot in a "specially designed fixture" to be able to modify the shaft. I wanted to turn it down a little to fit inside an extendo-shaft. Here's the pot mounted in the fixture and the rough cut brass extendo-shaft at the bottom:

I turned the pot nub down a little and bored the end of the extendo to fit onto the pot nub. After the rest of the machining is done I'll epoxy the extendo to the original pot nub:

:

 

Then for the other end of the extendo, I machined some splines to fit the original dimmer knob:

 

 

Here's the completed dimmer next to the original dimmer pot. I glued the shaft together and everything looks great. I didn't mount it in the dash yet, but I did connect it up and it works great. I probably won't mount it in the dash until the off season. I'm going to pull my dash out for some HVAC work and it'll be a lot easier to swap the original pot out with the dash out of the car. Here's the completed dimmer:

[Zed Head]

 

Most vehicles have long since changed away from a potentiometer based instrument dimmer to a Pulse Width Modulated (PWM) based electronic dimmer controller and I've been working on retrofitting one of these newer style electronic dimmer controllers into my 280Z.

I poked around the junkyard looking for one that would mechanically fit in the original potentiometer location and settled on one from a late eighties to early nineties Toyota Camry. The main criteria needed are short stubby mounting depth and rotate control instead of a thumbwheel so I could mount it in the original dimmer pot location. This is the dimmer out of a 91 Toyota Camry:

 

 

That looks pretty cool, but................why?  Does the current (no pun...) system waste that much energy or are you just running out of things to make better?  PWM implies some microcircuitry.  Is there a computer in that little box?  Pretty far out.

 

If I could get my potentiometer out I'd be on-board with it.  It's way up in there though.

 

You seem to be making a lot of brass extendo devices...

[SteveJ]

Many LEDs need PWM instead of pots for brightness control.

[Patcon]

That little stirring feeling I have deep inside may be machinists lathe envy...

[SteveJ]

I couldn't resist. I bought a couple of cheap PWMs via Amazon.

 

Dammit, Bruce. You're going to get me in the doghouse because every time I see you experimenting, it makes me want to spend money to experiment on my own. It's all YOUR fault.

[Captain Obvious]

SteveJ nailed the "why".... I'm messing around with LED lighting for the dash and the original resistance based pot style dimmer doesn't work properly. The original dimmer pot is 10 Ohms and has to be somewhat "matched" to the impedance of the original incandescent bulbs in order for it to work properly. When you go to LED's, the impedance is so radically different that the dimmer function doesn't work effectively anymore. Besides... It's cool?

The PWM controller does contain electronics, but I doubt there's anything programmable in there. It's just a fixed frequency oscillator that they use to drive a variable pulse width output to a power transistor. You turn the knob and it varies the pulse width from 0% ON up to 100% ON.

 

"You seem to be making a lot of brass extendo devices..."

 

Haha!  Because I can. 

[Captain Obvious]

Steve,

 

Haha!! Yes, I'll take the blame. I'm glad to shunt the heat since it's harder to feel the impact when I'm so far away. You're one of the people who are partially to blame for this anyway. Showing off your LEDs in Memphis and all.

 

So, about the electronics... The PWM controller is typically a three wire device:

 

First connection gets connected to a source that goes hot when the bulbs are on.

Second connection is the interrupted PWM output which gets connected to the low side of all the bulbs you want to light.

Third connection gets connected to ground.

 

Problem is that the original pot is a two wire device, so you need to run a new third wire to the PWM controller. Or....

 

Through a little electronic Tomfoolery I modified the PWM controller circuit inside the box to make it a two wire device instead of a three wire device. I'll get into the details if anyone wants, but I basically traded a small bit of PWM adjustment at the upper end for the ability to eliminate the third rail. So instead of operating from (0% to 100%) mine now operates from (0% to ~98%). I don't miss that upper 2% of "full brightness" duty cycle. And in the unlikely event that you DO miss that upper 2% of brightness, you can switch to brighter bulbs or something.

 

Only other trade off is that I'll need to keep one incandescent bulb somewhere in the system or bury a lowish value resistor in place of a bulb somewhere in the system. So you either need to add the third wire (which is also easy) or you can modify the controller to use two wires instead of three. I decided to modify the controller but either way would work.

[SteveJ]

The PWMs that I ordered are 2 in and 2 out. I was thinking that the path to ground from the dimmer would be the negative in. The wire from the dash lights to the dimmer would be the negative out. The combo switch would be the positive in, and the wire going to the positive side of the dash lights would be the positive out.

[Captain Obvious]

Hmmm... On those pre-canned four wire dimmers, it looks like they're actually DRIVING the output terminals instead of just pulling one of them to ground through a transistor. Two of the connections are easy:

 

Negative in would connect to ground. Easiest place to pick this up would be the black wire at the dimmer connector.

Negative out would connect to the low side of all the bulbs. Easiest place to pick this up would be the red/blue wire at the dimmer connector.

 

But the other two connections are a little problematic... You would have to break the original connection between the combo switch and all the dimmable bulbs to insert the module between the two.

 

Hate to suggest that you spend more $$ if you're already in the doghouse, but can't you just pick up a low side switching PWM controller like I used? Something that shares the same low side and just drags the bulbs to ground through a transistor? Would certainly be easier wiring! You'd make up the cost in time spent with your head underneath the dash!

 

You want me to pick one up at the yard and send it to ya? My treat!

[Captain Obvious]

One other thought...

 

Maybe they're simply tying the positive out right to the positive in on the PCB... It might be a simple PWM to ground through a transistor and the only reason they included a separate terminal for positive out was to simplify the wiring for people who didn't want to put two wires under the same terminal screw?

 

Without having one in hand to do a little reverse engineering I can't tell.

[SteveJ]

I looked at the wiring diagram for the 74. I do believe you're right that I may have a problem. The dash harness splice that goes off to the parking lights appears to be downstream of the dash lights. 

 

I did notice this one little oddity when I was scanning the wiring diagram:

 

Well, the Janglish is what I noticed today. Another interesting thing is that the lamp in the glove box is powered off the dash light circuit. I discovered that a couple of months back when I put an LED bulb in the glove box lamp socket.

[Captain Obvious]

Hahaha!!! Nice Janglish! I'd not noticed that before. Thanks for pointing that out. Reminds me of something I often see at my local Asian supermarket... They sell "Beef For Stir Fly". I've been tempted to bring it to their attention, but don't want to accidently risk an incident.

 

Yeah, unless that four wire PWM does just tie the two positives together for ease of wiring, grafting that thing into the Z wiring would be a pain. You still need the green/white to go to the parking lights, but want to separate it from just the dimmed lights while leaving the un-dimmed ones alone. Let's hope it's just a tie point for ease of wiring.