Miata Temp Gauge Linearization Process

(or How to make a 97 Miata Temperature Gauge work right)

Written and conceived by Wallyman

 

This is how I took a mildly useless gauge and turned it into something that actually gives you useful information. This information is being presented so that other Miata owners may modify their cars as I have done. USUAL DISCLAIMER: Use this info at your own risk, just because it works for me doesn't mean it will for you, research application and compatibility before taking tools to your car. If this modification summons aliens to your house, not my fault! :)

Wallyman

The stock Miata temperature gauge is fine for normal people. Being a guy that runs a turbo modified Miata, I'm far from the norm. The car gets hot, reaaaaaally hot when driven hard for any length of time, and I like to frequent the racetracks on occasion. So, the temperature gauge that serves normal folks well does not do so good for a modified car that relies on accurate and quick temperature monitoring.

 

STOCK: The stock gauge needle moves in a fashion similar to this:

Stock Temp Gauge Volts vs. Temp

Basically, if the car is warm and anywhere between 180 and 235 degrees, the needle will point up, and your average driver is happy as a clam. The only time the needle flinches is during warm up and when something evil has begun (or completed). When the needle starts to move towards "hot", the car is actually already very hot (240 degrees), and should be checked immediately to avoid possible damage. This is all fine and dandy for a unmodified car, but the forced induction (turbo and supercharged) Miata has a tendency to get hot in a hurry, so by the time the gauge wiggles, you are opening the wallet to replace a bunch of expensive parts.

 

MODIFIED: The modified gauge is just what most old-school guys/gals would expect. If it is half way to hot, the needle reads half way.. if it is hot, it reads . Rather commonsensical, don't ya think? Of course, a wiggling gauge is scary to normal people! :)

Modified Temp Gauge Volts vs. Temp

 

Seems like such a simple concept, but for the sake of not having to answer a ton of "Why does my temperature needle move?" questions, I assume that Mazda made it the way they did. And that's fine, but not for me... so off to the electronics store we go!

 

The tools you need:

  • Solder iron
  • Solder
  • Solder Sucker, or wick
  • Small needle nose pliers
  • Small wire cutters, preferably side cutter (nippers)
  • Drill
  • Small drill bit (???? 1/16th inch?)
  • Patience
  • Steady Hands

The parts you need:

82 ohm, 5 watt resistor
55 ohm, 5 watt resistor
16 ohm, 5 watt resistor
150 ohm, 1 watt resistor - "COLD" Calibration Resistor - equivalent to about 150 degree Fahrenheit NO IMAGE
20 ohm, 1 watt resistor - "HOT" Calibration Resistor - equivalent to about 270 degrees Fahrenheit NO IMAGE
2" piece of small wire, 22 AWG is fine (I used a piece of cut off resistor leg)
Heat Shrink tubing/electrical tape (tube must fit over 16 ohm resistor) NO IMAGE

 

The modifications are as follows...

  • Remove Cluster
    • Remove 2 screws on Kick Panel below the steering column
    • Remove the 4 screws that hold the steering column/lock cylinder cover in place (see picture 1a)
    • Remove the 2 screws in the bottom of the cluster shroud
    • Remove the 4 screws that hold the cluster in (see picture 1b)
    • Unhook the 2 large wiring harnesses on the back of the cluster (see picture 1c)
    • Unhook small wiring harness on the back of the cluster
    • Unhook the speedometer cable (push down on the raised bar on the top of the connector while pulling out gently on the cluster) (see picture 1c)
    • Take the cluster to the workbench
Remove Column Screws

Remove Cluster Mounting Screws

Disconnect Cluster Connections

  • Disassemble the Cluster
    • CAUTION - Do not touch the gauge faces, fingerprint oils will not come off!
    • CAUTION - Do not remove the needles, or bump them. You don't need to mess with the needles at all for this modification.
    • CAUTION - Don't let anything get inside the clear plastic cover once it is removed. Don't touch the inside of it. Don't scratch it!
    • Remove the clear plastic cover
      • Press in the black tabs while gently pulling out on the clear assembly. Start at one end, applying pressure while pushing them in, and it should work fine. (see picture 2a)
      • If you are having trouble with it clicking itself back together, stick a small screwdriver in between the cluster and the clear piece as you work your way around.
      • Make sure the wiring harness comes out through the hole in the casing, you will probably have to pull the circuit trace part back a little to feed it through the hole. Bending it won't hurt the traces, so move it as needed. Make a note of how it was routed so it is reassembled the same. (see picture 2b)
Remove Clear Cluster Cover

Pull Wiring Harness Out

  • Remove the Temperature Gauge Assembly
    • Remove the 3 screws on the back side that are holding the gauge in (see picture 3a). Gauge may fall out, so be mindful of it coming loose.
    • Pull gauge out the front of the cluster, set cluster aside (make sure the faces don't get touched!)
  • Modify the Temperature Gauge
    • Remove the 82 ohm resistor (see picture 4a)
      • Heat the solder on the back (see picture 4b)
      • Suck/wick hot solder away
      • Bend legs over on resistor so it will pull out
      • Pull out resistor from front side
  • Install the 82 ohm, 5 watt resistor
    • Bend legs over on the new resitor
    • Insert legs into holes from the top side. Make sure to leave it up off the board so it isn't touching anything and air can get around it on all sides.
    • Bend over ends of tails on back side
    • Solder points on the board on the back side
    • Nip off the tails of the wires you just soldered
    • Bend the resistor as needed to maintain air gap on all sides.
  • Remove the Zener Diode (see picture 6a)
    • Heat the solder on the back (see picture 6b)
    • Suck/wick hot solder away
    • Bend legs over on zener diode so it will pull out
    • Pull out zener diode from front side
  • Using the drill and a very small bit, enlarge the power side hole of where the Zener Diode was (see picture 7a)
  • Drill small hole in ground trace, just above existing diode solder location (see picture 8a)