Modifying your Nissan 300zx digital dash to use modern sensors

The digital dashboard of the 1984 Nissan 300zx looks amazing. Why would you change this out for either some analog dash or something modern like a big emotionless screen. This has many segment VFDs, and they can be of variable brightness. The trouble is certain sensors they need are not super easy to find. For a friend’s project car we traced out how to easily modify the dash to take 5v logic level signals so we wouldn’t have to spoof the old junk.

The first one is rather uneventful, the vacuum sensor that indicates how much power the car’s making. With about 5 pounds of boost added to this engine the original sensor would just peg the scale to the max at nothing approaching full throttle. We dug into the diagnostic manual and simulated the voltage going in to the dash to see if we could simply make that signal with an analog voltage. That worked very well because the original sensor was not a variable resistance, but a variable voltage. This is something a teensy on the new CAN bus can do, output a voltage relative to the amount of boost currently being detected and scale that to the range of the gauge.

The next relatively easy ones are the temperature gauge and the oil pressure gauge. I traced those connections back to those four high wattage resistors you can see on the left. Each sensor uses one of those resistors as a pull up to an internal 5v reference, the sensors are a variable resistance to ground. By lifting one leg of each of the resistors we care about I can convert the input from a variable resistance to a variable voltage. Removing the half of the voltage divider in the dash and now sending a signal that’s between 0 and 5 volts. We did this for the two identical green 43 ohm resistors and it works great. The nice thing about this mod is that if you install the original sensor there it just reads 0 volts, it doesn’t read correctly but no damage is done to the dash.

The tricky signal is the tach signal, expecting a high voltage pulse from the distributor every time a cylinder fires. I traced the signal from the pin on the outside to this resistor between these two hybrid modules, R11. From there it goes into the hybrid module…

This is not actually that crazy hard to trace as long as you take for granted those ‘LC’ devices are some sort of transistor and I don’t care what the signal is doing at any given point I just want to follow it. Eventually it leaves the hybrid module, goes through ZD5 that zener diode and onto some surface mount stuff on the back of the board.

From the zener diode it goes through R92 and to pin 8 on one of the microcontrollers. We set up a function generator outputting 12v at 3600hz or so through a relay coil to generate spikes, a capacitor to suppress some noise, and a series resistor to protect the dash input. Once we were sure the dash was reading our simulated tach signal I measured pin 8 and sure enough there was a series of high going pulses capped at around 5v. I moved R92 to break connection with the signal conditioning circuitry and did the same on the top with R11, then I connected the pin on the dash connector straight through to the microcontroller pin through a 100 ohm current limiting resistor. I intend to replace that wire with an opto-isolator based around the dashboard’s local 5v reference at some point but for now the connection works. The down side is that serious damage could occur to the microcontroller if you run a stock tach signal into this pin now. The opto-isolator will protect the processor, but in a sort of sacrificial way.

All 4 resistors I moved are still there and can be reconnected if someone wants this dash more stock, so it’s a completely reversible mod. Until I come back with the opto-isolator circuit I leave you with this custom VFD that they had installed for the speedo:

The speedometer only goes to 399, you mean to tell me this thing can’t go faster than 399 miles per hour? No, it can’t.

EDIT: here’s the optoisolator circuit I put together. It’s super simple to make on some protoboard, you should reference the right side 5v to the dash (See how I labeled it?) and the ground also to the logic ground in the dash. You can get all 3 pins straight from the microcontroller to be sure it’s the right power and ground busses. You can use the car’s local ground and just run your 5v tach signal to the original pin in the dash or you can run both the tach signal and reference ground together on a separate connector. Remember, to drive an optoisolator all you’re doing is lighting up an LED, it’s not complicated.