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Installing a Megasquirt Fuel computer


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To give credit where it's due, Jim's (Parish) post on his megasquirt install can be found here.


Installing the megasquirt was easy -- to easy to be as frustrating as it turned out to be. There are a couple of gotchas that reared their ugly head during my install, so here's a post to hopefully let you avoid them.


First let me publically answer some questions, and I will post the how to in subsequent threads:


What's a megasquirt?


The simple answer is a DIY stand alone speed density fuel (and more) computer. Please see the megasquirt website for more detailed information about what it can and can't do.


So basically, its a DIY Accel DFI or Big Stuff 3, right?




Why use a megasquirt instead of a <insert other aftermarket computer here>?


I used it because it offers real time tuning, AFR targeting, and a slew of added features, plus I like the DIY nature of it. Also, it is an 'open source' development and is constantly being improved.


Why not use HPT's speed density tuning?


That was my original plan. But when Chris told me that my PCM used the 'problem' OS that broke their files, and the fact that he didn't know when it would be working, I decided to try the megasquirt. For $270 bucks, you can't lose.


Plus the Megasquirt comes with a 2.5bar MAP sensor, so that eliminated a potential headache for me. Add that to the $300 cost of the HPT 2bar SD format, the real time tuning of the megasquirt, and the MS just seemed like a better answer in the short run.


Will you ever go back to the PCM only method?


Jury's still out on this, but unless HPT is able to fix the file problems, no.


Will you put your MAF back on?


MAF is out. No reason to put it back -- just set the MAF codes to "2" and everything will work fine. Don't disable the codes, just turn off the lights, or the PCM will freak out on the timing.


Should/Can everyone use a megasquirt?


Can they? Yes, absolutely. Should they? No.


If you don't have a compelling reason to run a speed density tune, the MAF makes for a much less fragile configuration. Even if you *do* have a compelling reason to convert to speed density, MS may not be for you. There is a lot of experimentation, DIY, and problem solving that's involved. If you want a plug-n-chug solution, MS probably isn't it (although it certainly can be if you do your homework).


Megasquirt is (for the most part :banghead: ) an easy system for me -- I have a Ph.D. in computer science and understand what's happening with the little electrons and junk like that, and actually LIKE this kind of thing. If you don't like this kind of thing, don't do MS. It takes too much time and effort and you will hate the experience.


Ok, I installed my megasquirt and now am getting DTC 1515 - Electronic Throttle System Throttle Position Error (or something like that).  What the hell?


This was a tough one to figure out. Some of you may get this error, and others will not and I have no idea why for either case. But here's the fix: Add some resistance to the TPS wire that inputs into the Megasquirt -- I had to add 15k6 Ohms (15,600) to decrease the noise on the line. That fixed it. Add the resistors as close to the junction with the dark green (TPS1) wire as possible.



I will add more to this thread as questions come in.

Edited by TurbochargedBerserker (see edit history)
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Okay -- on to the nuts and bolts:


Installation Day



You have a megasquirt in an aluminum case

You have a wiring harness premade

You have a wideband o2 kit already installed (hardwired for full time use)

You have the correct sensors for the MS already installed and have the correct plug ends for them.


What you need:

A nice assortment of 1/2 volt resistors (or a multipack) from Radio Shack

A 10k Ohm 1/2 volt potentiometer

A soldering iron

A variety of connectors, taps, and tees

Electrical tape

16 gauge wire -- get 3-4 colors

Injector plugs (unless you are running stock type injectors, then you can use your stock plugs)

Side cutters

QUALITY pair of wire crimpers (this is one of the gotchas so PAY ATTENTION :seeya: )

A male to female serial cable

A vacuum tee

Vacuum line

4 Vacuum line connectors

Keyspan USB to Serial connection if your laptop doesn't have serial (another gotcha :seeya: ) -- it's expensive but the only I found that worked for me.

Voltmeter that can do resistance (ohms)


What you do:

1. Establish your grounds -- there are 6 or so grounds coming out of the megasquirt harness. These are COMMON grounds, so you can ground them all to the same place. I chose one of the studs to the door frame under the cab fuse panel.

2. Find a 12v power source that has CRANKING power and tap that to the megasquirt power line.

3. If you are flashing the firmware, connect the battery and flash now. You will need to install the boot jumper (this single time only), so remove the aluminum top of the MS case, and follow the instructions for installing the boot jumper and flashing the firmware.

4. Disconnect the battery and perform these steps in any order you want:

- seperate the harness wiring into "inside" and "outside" bundles (KEEP THE TACH WIRE IN THE INSIDE BUNDLE :seeya: )

- find the firewall bulkhead hole between the master cylinder and the fender skin (it looks like a small shifter boot with a wiring loom coming through it). The gromet is actually two pieces: a hard rubber donut and a soft boot like thingy. Push then into the cab and seperate them. You'll have to cut the boot part to allow the outside wires to pass through, and then reassemble and reseat the gromet (with the wires passing through of course :))

- wire your 5v WBO2 signal to the MS

- take apart your dash and pull out the gauge pack (you can leave it plugged in).

- Find the WHITE wire in the gauge plug and put in a tee with the MS tach wire

- Find another 12v CRANKING power source and tap it with enough wire to reach the MS tach wire up near the tee to the WHITE wire and put in a tee on the MS TACH WIRE. Once there, CUT the new power wire about 2-3 inches from the tee (** see the stepsbelow on creating a pull up circuit)

- cut off your existing injector plugs from the stock harness

- find the COMMON COLORED wires for your stock injector plugs (mine was PINK with a single ORANGE wire). These common colored wires are 12v CRANK POWER for the injectors. Ignore all those wires with the stripes -- they are the grounds from the stock PCM :)

- Find your INJECTOR CIRCUIT wires from your MS harness. There are two circuits, two wires each. Take a wire from each circuit to each bank of the engine. These are your new grounding circuits for the injectors. You will need to run two lines from each single line to two side-by-side injectors (unless you've figured a way to run an 8 injector truck with 4 injectors :)) and the power for each injector station to a new injector plug. MAKE SURE YOUR CONNECTIONS IN THE PLUGS AND AT EACH BUTT CONNECTOR ARE SOLID (gotcha :seeya: )

- run the temp sensor wire to the temp sensor and attach a suitable ground to the other side

- run the IAT sensor wire to the IAT sensor and attach a suitable ground

- Find the DARK GREEN wire from the TPS plug (passenger side on the throttle body) and tap that wire with the MS TPS wire

5. Create a pull up circuit on the tach wire. You've already run the 12v CRANKING power line up to the tach wire tee and have cut it 2-3 inches from the tee to the MS tach wire. Temporarily install the pot, testing it first to make sure its set at about 380 ohms. Pots get wired with one wire to the middle and one wire to one of the outside terminals -- solder your connections (I made short little leads and connected into the system with butt connectors). Attach the other terminal to the short 2-3 inch lead going to the MS tach tee. Attach the middle terminal to the other side.

6. install the vacuum line tee and run a vacuum line to your MS

7. plug in the serial cable and the wiring harness

8. mount your MS somewhere in the cab, out of the way and out of site (unless you like aluminum boxes :P)

9. connect your battery

10. turn the key to run and start up megatune

11. crank the truck while watching the megatune tach -- you should see 100-200 rpm . At the 380 ohm setting, your truck will probably try to start. If you don't see tach on the megatune display, barely move the pot (upwards) until you begin to see tach signal -- the range is huge 150 ohms to something like 7.2k for me. If you don't see tach signal, anywhere in the range, try making a minute move, crank, minute move, crank, etc. If you still don't ever see tach, make sure the 12v you tapped for the pull up circuit has CRANKING power.


If the truck cranks and you are seeing tach signal at the 380 setting, remove the pot and replace it with a 380 ohm resistor (or a 330 ohm + 47 ohm in SERIAL). Solder these connections and solder the resistors together if you run two in serial.


Move on to the Getting it to Run section :)

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Getting it to Run day:



You've verified you HAVE tach signal

You've verified your injectors are getting signal with a noid light


This is an all software day, so get out that laptop. THIS IS NOT INTENDED TO REPLACE READING THE MS MANUAL AND TUNING SECTION!!


First -- the LS1 tach signal is a 4 cylinder signal, so you'll need to set your constants page to 4 cylinder... Weird, huh?


Second -- there is a required fuel dialog -- USE it :) Here's a gotcha :seeya: ... Remember we just said set the thing to 4 cylinder? The number of cylinders is used in the required fuel calculator and is unchangable from the constants setting... Set it to 8, run the calculator, reset to 4, make sure the lower required fuel box reads what it used to (should be okay already but if it changed, manually adjust the required fuel until it matches).


Set the PW time to 1.0 and the duty cycle to 75% (this is from the MS manual)


On the enrichments page:

Set your cranking pulsewidth to 1.5, the 40* PW to 12.0, the 160* PW to 3.0


Set Accel to 1.1 and decel to .75

Set your accel enrichment bins to 1,2,6,8


On the VE page (note the menus at the TOP!!)

Import your VE or create one using the dialogue.


Try to start it. Getting it started is a function of tweaking the 160* pulsewidth and possibly the required fuel. It may start and not idle -- that's fine.


If it starts but won't idle, keep it running using the gas pedal and hold it at any rpm it will stay at. Open tuning>VE and adjust until you're at stoich -- it's realtime, but remember to hit the burn table when done. Do this recursively until you are at your idle point.


Once idle is at stoich, go to the cell directly above it (same rpm, lower MAP) and boost it up a bit higher than the idle cell (mine is set to give about 12.5 AFR) -- this is to help stop 'lean starvations' where the truck goes lean, drops the MAP goes leaner still, and dies.


Move on to regular tuning :)

Edited by TurbochargedBerserker (see edit history)
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