Fuel Pump wiring and Megasquirt

[merlinfe]

Hey Pelicans,

I've searched and been THOROUGHLY confused by the fuel pump wiring when it comes to Megasquirt 2. As far as I can tell you need to have a switched 12v to activate the fuel pump but I can't find which wire in the engine bay to connect that voltage to once the CIS components have been removed.

I have a March 1983 911SC and have read the Bentley, multiple wiring diagrams on the internet, and sooo many threads here on Pelican. Ugh.


Help a brother out and be specific please.


Thanks,

- Steve

[xbmwguy]

Steve if I remember correctly the ms2 sends a ground signal when the key is switched and the ecu gets power. I think I wired that wire ( ground) from the ecu to the ground wire in the frunk on the red fuel pump relay. I’m assuming you are running factory pumps

[Marwil]

I rewired my fuel pump relay as described in the paragraph and pics below. Use Tony Bitz's manual to know what wire to tap into in the engine compartment (I think it connects to the pink MS2 wire).

Reversing the fuel pump "logic" at the car's fuel pump relay appears to be the easiest thing to do. MS-II will pull ground to activate the fuel pump relay (FPR) so you need to switch the wiring at the FPR. I did this by first cutting the RPM limiter out of the circuit (MS has this feature built in). Then, you have to switch the wires going to 87 and 87a as well as move the "jumper" that goes from 87a to 86 to create a "jumper" from 87 to 86.

Original Fuel Pump Relay Wiring


Fuel Pump Relay Rewired for MS2


Hope this helps,
Mark

[timmy2]

The 2 wire “T” connector for CDI power and fuel pump relay safety switch is below the CDI.
Should be 2 reds on one terminal and a single brown/red or brown/black wire. The single wire is the wire you want to tie into for activating the fuel pump. You can reverse the relay logic as described above or use another “grounding relay” in line before tapping into the wire to unground the wire using the MSD signal the activate the relay

[pampadori]

The plug Timmy2 is talking about is white in color. WHen i wired my car i used the FP signal wire and ran it to a relay that switched the this wire in the white connector to ground. That then triggered the red relay in the frunk and turned on the FP.

Does that make sense? I used a relay to trip the factory relay. But instead of 12v i had it supply ground.

[3rd_gear_Ted]

Isn't this always the case; ECU's use negative logic to energize a circuit by "switching" the ground into the circuit via a relay ???

[pampadori]

Actually, many allow for lowside or highside output for fp. Mne does but I didn't know how to set it up when I was wiring my setup. Learning curve was steep...

[timmy2]

I believe you couldn’t just connect the MS fuel pump output directly to the brown/red to provide ground for the original FP relay. (To 85 not 86 as in photo, doesn’t matter to the coil)

It is in the white “T” connector at rear fuse panel as stated earlier. (T2k on diagram)

The problem is the fuel pump relay is in normal operation mode with the ground removed!

You need the MS output to open the ground to the FP relay.

Thus the need for a “grounding relay”

[xbmwguy]

Dennis your pic of wiring into the existing relay is far easier for the op to get it going that’s how I was trying to explain

[timmy2]

Originally Posted by xbmwguy

Dennis your pic of wiring into the existing relay is far easier for the op to get it going that’s how I was trying to explain

Just re-read my post and edited it to reflect Porsche’s use of reverse relay logic to run the fuel pump.
Ground to fuel pump relay must be open in normal operating mode.
Original Porsche terminal 85 is grounded when cranking to start, ungrounded once running

[merlinfe]

Thanks for all the information guys! I'll draw up some plans for the relay install this weekend. I plan on using a relay board with my Megasquirt installation so I'll probably just rewire the relay on the board itself for the fuel pump output terminal. Shouldn't be too difficult.

Thanks again everyone, much better understanding now!

- Steve