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Old 08-29-2017, 12:55 PM   #1 (permalink)
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1990 (NA) Miata MPGuino step-by-step installation (VSS = RSW from cluster)



This thread will document the installation of an MPGuino custom fuel economy gauge in a first gen 1990 (NA) Mazda Miata MX-5. (I couldn't find a comprehensive DIY out there.)

Since it's pre-OBD-II, the 1990-1995 Miatas can't use one of the many plug-and-play fuel economy computers available (eg. ScanGauge).

The good news is the installation of the MPGuino isn't terribly complicated. But you will need to be comfortable finding & splicing a few wires for the MPGuino's 4 input connections:

- 12V (DC)
- Ground
- Fuel injector
- VSS (vehicle speed sensor)

Miata "RSW" = VSS...

There's one small hiccup concerning the Miata's VSS signal.

Typically, a car's VSS can be accessed by splicing a wire at the ECU. But according to the interwebs, the Miata's ECU doesn't use a speed signal, so there's no wire to tap there.

[UPDATE: WAIT A MINUTE! According to my service manual wiring diagram, the RSW/VSS wire goes to the ECU, which removes the need to pull the cluster. Researching further... UPDATE #2: Lies! My Haynes service manual lied! There is no VSS wire at the ECU, despite what it says.]

Fortunately, the Miata's speedometer generates a speed signal for use by the (optional) cruise control. Pulses are generated by a simple reed switch, and are accessible via the "RSW" circuit (with handy screw tap) which is clearly labeled on the back of the cluster.

The signal from this RSW/VSS circuit needs a small modification so the MPGuino can use it. This will be documented below.

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Old 08-29-2017, 12:59 PM   #2 (permalink)
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12V + Source:

Note! You need an "always on" 12v source for the MPGuino to retain tank data.



I tapped into the "ROOM" circuit in the interior fuse block (under the driver's side of the dash, above the dead pedal). ROOM provides power to the radio (which is always hot so the radio retains presets & the clock function), interior lights, etc.



I made an "add a fuse" type of circuit (similar to above, but much more primitive looking!) to tap into the ROOM fuse slot so the 12v MPGuino source is also fused in case of crossed wires.


Ground:



I just used a ring terminal connector and put it on a random screw that went into metal under the dash.


Injector Wire:

I tapped into the wire at the ECU, which is behind a kick panel in the passenger side footwell:



(The rust in this area has since been cleaned up & the metal repainted!)


Note! A Haynes repair manual came with my car, and while searching for an injector wire to tap I discovered it has a misleading wiring schematic / pinout for the ECU.



So, be careful believing what you read!

Search for "ECU pinout" on your favourite search engine to find the wires you need. Then TEST the wire to make sure it's actually doing what you think it's supposed to be doing.

You'll know you've got the injector signal at the MPGuino when you can see the Gallons per Hour value working when the engine is running.

VSS Signal:

Again with the lies!



My manual shows in a schematic that there's a VSS signal available on one of the wires at the ECU. There. Is. Not. (Not on my 1990 Miata, anyway.)

Instead, you have to pull the instrument cluster to access the "RSW" signal trace on the back of it.

RSW probably stands for "reed switch", which runs off the speedometer cable and is there to be used as a signal for the optional cruise control.

Note: be careful when removing the hood/bezel from around the cluster. If you don't pull straight back, the clip pillars that connect it to the dash will snap off. Ask me how I know this.

Once the hood/bezel is off, look down through the windshield and you'll see the speedometer cable is connected to the cluster with a locking tab (white cylindrical plastic piece). Figure out how to squeeze the tab to unlock it from the cluster. You have to release the speedo cable to be able to pull the cluster:



Pic below shows the RSW (VSS) trace clearly labeled on the back of the cluster:



You can attach a little ring terminal under the screw on the cluster at the end of the RSW trace, or follow the trace to the wiring plug and splice into the appropriate wire:



Ideally, if someone found out where that green wire/red stripe (6th from the bottom in the above pic) went, it might save having to take the cluster out to access the RSW/VSS signal.

However you do it, once you've spliced into the RSW circuit, you now have the last piece of the puzzle to calculate MPG!

VSS Signal Strength: Pull Up Resistor

Unless your car has factory cruise control and you tap the RSW circuit there, you'll need to modify the RSW/VSS signal with a pull up resistor so that the MPGuino can detect and use it.



Search for "Pull-up resistor" if you want a tutorial about what adding this simple circuit does. (I had to!)

Calibrating the MPGuino - Initial Settings

Lastly, you need to tell the MPGuino how much fuel your injector squirts and how many pulses your VSS sends per mile to get accurate MPG readings.

Thanks to AntiochOG for sending me his initial settings. Here's a starting point:
  • MicroSec/Gallon: 0250300000
  • VSS Pulses/Mile: 8200
I've since refined these settings for better accuracy. (Each car will vary depending on a bunch of things like tire size, state of tune, etc.)

---

Final note: you may need to "de-bounce" the speed signal, depending on how clean it is. In my car, it does bounce around quite a bit once you get above 25 mph or so. Meaning, the speed display on the MPGuino jumps around (sometimes by a lot), even though your speed is steady.

CoyoteX started a thread about doing this for the MPGuino on his Metro: How to Debounce a Geo Metro VSS
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Old 09-26-2017, 03:18 PM   #3 (permalink)
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(Post #2 now updated with installation details!)

---

Here's what the MPGuino looks like with some useful data on one of its many screens/pages:



This screen shows:
  • IM000.00 = Instant MPG is 0 (because the engine is off)
  • S000.00 = Speed is 0 (because the car isn't moving)
  • TM049.92 = Tank MPG is 49.92
  • D039.33 = Tank Distance (since last reset) is 39.33 miles
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Old 09-26-2017, 03:23 PM   #4 (permalink)
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unfortunately... MPGuino reset probs

Unfortunately, I'm having problems with my MPGuino: it keeps randomly resetting, so I haven't had any useful "tank" data.

Sometimes it will go many miles between resets (as in the pic above). Other times it will reset multiple times in the course of a couple of miles.
Yet other times it will completely freeze, and I have to physically unplug it to reset it.

It's still very useful on an individual trip basis, but not having the ability to save data over a complete tank makes it harder to calibrate (plus it's just mildly annoying).

I keep meaning to examine the board for soldering problems, but haven't gotten around to it yet.
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Old 09-27-2017, 03:20 AM   #5 (permalink)
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Dope. I think I'll have to pick one up myself.
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Old 09-27-2017, 09:00 AM   #6 (permalink)
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Definitely worth doing! Even with the reset problem mine has, it's incredibly useful.
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Old 09-27-2017, 11:46 AM   #7 (permalink)
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any idea if the reed switch puts out a ground signal or a +12v signal?

I'm thinking ground, which I wasn't expecting the MPGuino to appreciate, so I'm pleasantly surprised if it is.
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Old 09-27-2017, 12:36 PM   #8 (permalink)
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It's a switch to ground.

I edited post #2 since you last read it. I forgot to include the part about it needing a pull up circuit, if the car doesn't have factory cruise.
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Old 09-27-2017, 09:05 PM   #9 (permalink)
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UTComp is the way to go these days:

https://www.reveltronics.com/en/prod...omputer-utcomp

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