Megasquirt 3 -First engine startup


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Intro - Sensor calibration - Sensor Testing - Base Settings - Getting RPM - Cranking timing - Start


Before attemption to start the engine for the first time it is very important to take a stepwise approach to preparation. This will give you confidence in your work and confirm each item is operating correctly.
Wiring up the Megasquirt, turning the key and hoping is NOT the right approach - don't do it !
This page assumes that you have already assembled and powered on your Megasquirt. See the Quick Start page first.

Sensor calibration

The Megasquirt-3 uses internal calibration tables to convert the resistance/voltage from the input sensors into a real world number. It is necessary to setup this calibration so the inputs read correctly. The built-in calibrations for coolant temperature (CLT) and air temperature (MAT) are for GM sensors, so if you have GM sensors you are safe to leave these alone. All installs should set the calibration for the AFR (air fuel ratio) table that is used for the oxygen sensor (lambda sensor) whether narrowband or wideband.

1. Apply power to the Megasquirt, but do not attempt to start the engine.

2. Unlock the calibrations (This lock/unlock is to ensure the calibration isn't changed by accident.)

3. Open the Set Thermistor Tables dialogue, then set the CLT calibration (if needed) Pick the sensor type and enter the bias resistor value. This is R7 on the mainboard and is usually 2490. For a custom sensor, overtype three measured temperature and resistance pairs. (Don't guess, measure.) For greatest accuracy the bias resistor should be close to the sensor resistance at a 'warm' temperature.
Write to controller.

4. Set the MAT calibration in the same way (if needed.) The mainboard bias resistor is R4.
Write to controller. Close the dialogue.

5. Calibrate the AFR table. Either pick a preset or look up the voltage/AFR pairs from your controller manual.
Write to controller.

Note! You should also set your Project Properties for narrowband/wideband and configure the settings on the 'EGO control' page.

6. Re-lock the calibrations and Burn.

7. Calibrate TPS. With the throttle closed, "Get Current" on the closed throttle. Then fully open the throttle and "Get Current" on the open throttle. Close.

8. Calibrate the MAP/Baro. Typically chose from one of the preset values.

9. It is not usually needed to calibrate the battery voltage. Do not alter these values from the defaults unless you have confirmed the actual voltage reading is inaccurate.

Sensor Testing

This screenshot shows a typical TunerStudio dash with all sensor inputs shown. To change any gauges Right-Click on them and pick a different gauge. The ones we need here are under the Sensor Inputs categories.

If you have a Stim you should do this on the bench first (See Stim testing page for bench testing), but then repeat when installing on the engine.
Check each of the following:
Coolant temp - is it reasonable?
Manifold air temp - is it reasonable and within a few degrees of CLT?
Throttle - is it close to 0% with no throttle and close to 100% with full throttle
Fuel load - show be close to 100kPa at sea level, down to say 80kPa at high elevations.
Air:Fuel ratio - this won't read anything sensible with the engine off.
Engine speed - should be zero.
(Note! If RPM ever reads anything like 65000rpm it means YOU have made an error in your configuration settings. Open up Communications, MiniTerminal and turn the Megasquirt off then back on again. You'll get a message explaining what is wrong. Close miniterminal and fix the faulty settings. Then turn the Megasquirt off then back on again.)
Fuel pump - the fuel pump output on the Megasquirt is ground switching for a relay. When the Megasquirt is first turned on it will run for a couple of seconds. As soon as tach input pulses are seen the pump will turn on again. If the engine stalls, the pump is turned off.

Base Settings

The Megasquirt comes loaded with a base set of settings, however it is essential that you configure some of the key parameters before trying to start. The most critical are the settings on:

Engine and Sequential Settings (more here)
Number of cylinders, injectors, injector size (ReqFuel), batch/sequential, how your injectors are wired

Injector dead-time/PWM
Defines high-z (~14 ohm) vs. low-z injectors (~2.5 ohm) and dead-time parameters (more to come)

Ignition Settings / Wheel decoder (more to come)
Defines the tach input type (i.e. wheel pattern) how it is wired, what type of spark output you have and how it is wired.

General Settings
For many installs, these can safely be left alone for the first start, but important settings like alpha-N vs. Speed Density and secondary table load-blending are controlled here.

It is worthwhile reviewing the other menus too, but to begin with be sure that all advanced options like launch and nitrous are left switched off - get the basics right first.


This means that you have no tune loaded. This is ok if you are just having a look, but DO NOT try to start a tune like this.

Getting RPM

This is a critical step before attempting to start. Without a stable tach input and rpm nothing else will work right.
Before checking for rpm, you need to have wired up and configured the tach input for your engine. (i.e. crank and/or cam sensor inputs.) Take your time here as many users get stuck by skimming in this area.
The crank and cam inputs into the Megasquirt are used to determine engine position and speed and are the basis of the engine control. The ignition settings you choose tell Megasquirt what crank/cam pattern to expect. It then decodes the electrical pulses into speed and position.

Before starting this test, ensure that your injectors and coils are NOT going to receive power. Remove the fuses or undo the connectors. This is important to ensure there are no misfires or flooding during testing.
With TunerStudio connected and running, key on and crank the engine. If all is well you see "Synced" and a stable engine speed (rpm) number. If the pattern is noisy, incorrect or missing you might see "Synced" then "Not-synced" and/or rpms jumping up and down.
The tooth logger should also be used to confirm that the pattern is stable. Keep working on the crank/cam sensor inputs until you have stable rpm with no loss of sync.
See Tooth logger and Sync loss and the examples page. compare with the patterns you see on your engine.

Do not try to start the engine until you have good rpm.

Output test mode

Use the test mode to confirm injectors and coil(s) are functioning. Ensure the fuel pump is disconnected or you'll fill the cylinders with fuel risking engine damage.

Checking cranking timing

Having confirmed that the coils work as expected and with the fuel pump still disconnected, hook up a timing light/strobe to plug lead #1. Yes this is really needed - correct timing is very important.
Crank the engine and confirm that #1 timing is somewhere close. (You need to reconfirm timing is spot on once the engine is running.) If you are running wasted spark you need to ensure your strobe is compatible. "Dumb" timing lights are fine, some dial-back lights can be awkward. Use a "2-stroke" setting for wasted-spark. With a coil-on-plug pencil type coil you will need to remove the coil from the engine and install a short length of regular HT plug from the COP to the spark plug and possibly ground the COP if it was bolted on. Connect your timing light over this temporary plug lead. Read more here
While cranking, take a look at the fuel load (map) reading in TunerStudio, you should expect to see it drop slightly as the engine pulls some vacuum. If it stays stuck on atmospheric pressure (near 100kPa) then your map hose might be connected incorrectly.

Starting the engine

Now that you've confirmed sensor inputs, coil and injector outputs, have good rpm and cranking timing is close you are ready to start the engine.
Ensure the fuel pump is connected, battery is charged and all hoses are secure.
Cross fingers and try to start...
It is common to need to give the engine a little throttle to keep it alive on the first start. If all sounds well, then keep the engine running and warm it up. Check for oil pressure, coolant temperature etc.
If anything sounds wrong - stop and investigate.

Take your time! If things aren't working out, then take a breather and come back another day. Come to the forum and ask for help. When posting, please include full details of your install e.g. engine type, make, model, ignition setup. The forum is global and we might not be familiar with your engine, so just quoting an engine code is not sufficient. Don't use the forum to vent your frustration, keep it calm...

If you have a question, comment, or suggestion for this FAQ please post it on the forum.

No part of this manual may be reproduced or changed without written permission from James Murray, Ken Culver and Philip Ringwood.