The Megasquirt Conversion

So here I was with my 1976 Eldorado, fresh paint, warmed over engine, all tricked out and ready to roll, but there was a little matter of the fuel injection. You see, this car was equipped with the fairly rare, first stateside electronic fuel injection. It used an open-loop analog computer, monitoring coolant temp, air temp, manifold pressure and RPM. Trouble is, it couldn't compensate for the changes we made to the 500, and even then, there's better technology out there readily adaptable to the plumbing and hardware under the hood. Enter the Megasquirt.

The Megasquirt is a DIY digital fuel injection computer you assemble yourself, and tune with the aid of a laptop in real-time. You can use your choice of injectors and sensors, the sky is the limit. So it's only natural to adapt the system to the early Cad setup. The intank and chassis mount high pressure pump were still used, the low-impedance injectors were a perfect fit with the Megasquirt's 'flyback board'. For the control freak, there's also the MegaView, a unit that displays running parameters and allows quick fuel mods with the push of a button. I'll get mine together when I find a nice VFD.

There's loads of info out there about the Megasquirt, so I won't waste time going over the same old. Do a Google search, or check out Geoff's setup, or this awesome page. Don't want the hassle of piecing together the electronic bits? Check out the guys at DIYAutoTune.

So now for the Cad- The existing port injection intake, fuel rail, injectors, plumbing and pumps stayed exactly as they were. The original system used two difficult to find temp sensors that were effective, but not the best route to go with this system (they had a positive temp coefficient). Instead, I went with GM #12146312 / (TSU81) sensors that are cheap off the shelf, and already configured for the MS software. If you prefer to walk the junkyarde to grab the needed connectors, check out any of the mid to late 80's GM cars and you'll come out ahead. For faster reacting air temp monitoring, there's an equivalent sensor to the #12146312 but it has an open tip (Camaro- GP Sorensen 779-3541). It's just as easy to find too. The TPS I found that fit my throttle body was GP Sorensen 779-19001. With an aluminum plate adaptor it fit like a charm. The MS uses the tach signal from the HEI dizzy so hooking it up was a no-brainer. You'll want to add a bung in the exhaust close to the engine to mount a generic single wire oxygen sensor. For tuning, I'm using a wide-band setup that allows me to monitor my exhaust stream for rich/lean conditions and adjust accordingly. It's the best tool you could ever want.

The peak-and-hold low-impedance stock injectors allow greater control of fuel metering, but it's harder on the electronics.You can either use a resistor pack to limit current, or use a flyback board and control it in software. The resistor pack works, but I consider it sloppy engineering.

My initial reaction after we got the setup semi-dialed in was an amazement at the increase in performance. The car all-out was quicker off the line and faster all through the power-band (and I'm comparing this to a compensated analog setup adjusted for stoich). Besides improved drivability, cold starts are a snap, there's no rough idling while the engine warms up- you've now got modern engine controls. Some folks have gotten away without a TPS, and it can be done, but you lose throttle tip-in response, especially if you have large bores in your TB.

Something else to consider is that by leaving the stock FI harness and sensors on the vehicle, you can revert to that system should you ever need to. Below is the fool-proof way I set up the Eldo's system so that I could change from digital to analog in a matter of minutes without so much as popping the hood.

The megasquirt harness is very self contained, it uses its own wiring for coolant temp, air temp, O2 and tach signal, plus a 5v reference and return for the TPS. It needs a ground, a switched 12v, a 12v battery source and of course outputs to the injector banks. Powering the fuel pumps is via a relay energized by the MS. There's a separate output that can be connected to a fast idle relay allowing you to power whatever fast idle heater or air valve you wish, however, the Cad system uses a fast-idle heater that requires power so long as the pumps are running, it regulates itself, thus I wired the fast-idle heater relay in parallel with the fuel pumps.

The wire coming from the fuel pumps is separate from the FI harness- by unplugging its connector on the inside of the firewall, it's isolated from the stock system. Ideally, if I had an identical mating connector on hand I would have used that to connect to the megasquirt system, but I didn't, so I hard wired the Megasquirt's fuel pump relay to it. If I had to revert to analog, pulling the Megasquirt's DB-37 connector would ensure only the analog ECU would have control over the pumps once it was re-connected. I also hard-wired a relay at this point that is energized whenever the pumps are running. This relay closes the negative side of the fast-idle heater circuit.

The original system used high-side drivers for the injectors, that is, their common was ground and they were fired by positive pulses. Most modern systems, including MS, use low side drivers. The injector's common side is tied to 12v and they're fired by completing the ground side of the circuit. To get around this problem, there were two black ground wires terminating at the front fender. One wire is the ground return for the computer, the other is the ground for the injectors and fast idle heater. Tracing the wires reveals which is which- the computer return makes its way all the way back to the ECU, the other splits in the looming. To ensure reliability I grounded the computer ground wire to the negative terminal of the battery using appropriate hardware- don't get sloppy! The other wire that is the injector/fast-idle common was connected to a length of wire and run back inside the car near the ECU- it now gets 12v when the system is running (job of the Main Relay).

But how to get switched 12v to activate the computer? Piece of Cake. There's a blk wire with yellow stripes that has its own connector and it connects to the analog harness up in the dash. I had a mating connector for this one, thus you unplug the wire from the analog harness and plug it in to the MS harness.

I tried to avoid as many permanent wiring alterations as possible, but I did have to cut the fast-idle wire (J on the blue connector) It was rerouted to the fast-idle relay energized by MS. This heater is not critical if I had to limp home with the other system.

I tied in to the 12v feed entering the firewall near the computer (A on the blue connector). This is the full 12v supply, if yours doesn't have a fuse holder, add one inline!

Now to gain control of the two injector banks- Bank 1 was a red wire (E on the black connector). I cut it, then rerouted it through a length of wire to the MS DB-37 connector. Bank 2 was a white wire (F on the blue connector). I cut it and rerouted it to the DB-37 also. Now you might be wondering, if I disconnect the DB-37 to limp home with the analog system, aren't the injector wires left unconnected!? This is where the "limp-home" bypass connector comes into play. It's designed to plug into the connector that would have plugged into the megasquirt, thus rerouting the circuits to the old analog connectors (see the diagram). See how pins 32-33 reconnect the white INJ1 wire back to the blue analog connector. Same for pins 34-35 on the DB-37.

Then why are pins 7-11 connected to 28 on the Limp-Home connector? This effectively converts the injector bank common back over to ground so the high-side drivers will work. Note 7 through 11 is ground right from the battery, 28 is injector common. Just be damn sure the 12v switched wire (blk w/ yellow stripes) for MS is disconnected or you'd have a nice short and blow your fuse (you did put that fuse in, didn't you?!).

So how do you switch over? Easy. Ignition Off. Swap the vac line from MS to analog ECU (referred to here on out as just ECU). Unplug DB-37 connector on MS and plug into Limp-Home connector. Swap 12v switched wire from MS to ECU. Plug Fuel Pump connector in to mating ECU connector. Plug the three colored analog harness connectors back into their mating connectors (passenger kick pad behind trash receptacle). I keep these unplugged for safe keeping. And that's it. You could also pull the three relays from their sockets for peace of mind, but it's not necessary. Have I lost you yet?

There's lots more info out there about the Megasquirt conversions, if you'd like to hear more or details you'd like to see, drop me a line.

Click Images for Larger Version


500 FI Cubes	Coolant sensor at rear of passenger head. Drilled & tapped hot water fitting.	Air Temp sensor- fits grommet on air cleaner	TPS & aluminum plate on TB.

3 Required sensors- MAT, CTS, TPS	Harness w/ relays and DB-37 connector.	Fuel Rail & Throttle Body	Driver's manifold- Wide Band O2 Sensor

Glove Box (limp home connector visible)	Eldo at the drags! MS Injected


425/500 CID EFI diagram	Quick sketch of Eldo conversion w/ relays