The stock fuel pump relay circuit works very well and everything is already wired in with the proper fuses etc. The only thing you have to make sure of is that you feed a tach signal to one wire in the engine bay. This can be a tach signal from the stock Bosch ignition, the Ignition out line from Megasquirt or the tach out from the MSD. Either way it saved a lot of wiring using the stock fuel relay and has proven to be reliable. I just needed to refresh my memory from over a year ago about the tach signal. The Tach signal also feeds the instrument cluster tachometer.
I pulled the majority of the stock ignition wiring to straighten things up. I am running a MSD6AL for ignition. This made the stock ceramic low resistance resistors mounted on the driverside fender surperflous. The MSD drives the coil directly and does not need any external resistance. Under the mounting point for the resistors on the driver side fender there is a 2 post terminal strip. On one of the posts is a green/yellow wire and a green wire. I believe the green/yellow is the tach signal for the dash tachometer. I believe the green wire is the tach signal for the fuel relay circuit. I believe the green/yellow wire from the Bosch ignition box was originally connected here. I just needed to reconnect it after my cleanup.
The fuel relay circuit is completely independent of the K-Jetronic CPU. I have my K-Jetronic module completely removed and the fuel relay circuit still functions as it should. My theory is that the tach signal detection is actually within the fuel pump relay itself but it has been quite a while since I looked at this schematic.
I am considering removing the bosch ignition box altogether to further simplify things. This would leave megasquirt reading the tach signal directly from the distributor (VR I believe). But for right now the Bosch box is doing a good job of conditioning the signal and with the MSD driving the coil (or megasquirt directly) there is really no load on the bosch box. By my thinking this means it should last a long time as the power transistors are switching a very small fraction of the current they were designed for.
Any way it is nice to have the old girl back on the road again! I drove her to work for the first time in about two years. She was running over a year ago and running well a few months ago but the rats nest wiring was pretty bad. Installing the MegaSquirt relay board made a huge difference in the cleanliness of the install. I also feel much much better about the reliability as everything is well laid out and tight.
I hope to post some pictures and commentary about the wiring on my website
later on this week. I will also post some gas mileage figures as they start adding up here in this forum.