This might not help a lot of people, but if it saves somebody the trouble I went through it is worth it.

Project is a 1988 EFI 300 into a 1974 f-250.

After obtaining the factory Ford diagrams for the 88 model, all the needed interfaces were pretty easy to marry into the 74 truck.

Except apparently one. The EFI harness requires A signal from the ignition circuit, that is hot in both run and start. This is to run the TFI coil and EEC-IV relay control. Looking at my haynes wiring diagram for the 1974. No such circuit is available....

I considered many options, separate switch, an additional relay etc etc.

Then I looked up parts interchangeability. 1970-1976, and 1977 only were listed for the dash mounted switches.

So I thought since the 1977 was an only year maybe it's switch circuitry was different. But the haynes manual showed it basicaly the same. Then I went to Fordification because I knew they had the real Ford diagrams for some years.

They didn't have the 1977 diagram so I looked at the 1976 for giggles and low and behold their diagram was different then haynes for the ign switch. The Ford diagram shows the red/green wire hot in both start and run.

Wait a minute didn't I just read that 70-76 switches were listed as interchangeable?

Sure enough I metered out my 74 switch and indeed have the proper circuit required.

Now Fords Diagrams for the 1974 match haynes manuals. My guess is that the
replacement ign switches were all made to satisfy the 1976 requirements and also satisfy the earlier ones. Or it was always that way and the diagrams were just easier to draw that way because the earlier models didn't use that circuitry.

In summary the red/green is hot in my after market replacement dash mounted switch in both start and run.

I decided to go ahead and add an additional relay, ran by this circuit. Since its also the alternator turn on etc. I wanted clean power to run my TFI and eec-iv relay.

Brigart

 

What kind of Gas mileage do you get now with the EFI? I have been considering putting a 300 EFI in my 1974 F350. Was it a fairly simple conversion?

 

Unfortunately I am still not done with this project.
My budget allotments from the better half come in spurts lol.

I dunno if I'd recommend the project to most people. But if you are the type like me that has to be always tinkering with something, then I'd say go for it.

The only step thats a real pita is using a factory AC truck.
The evaporator / heater box gets in the way of the 300's huge intake on the EFI model. The rest of the conversion is simple but time consuming.

My truck gets used as a real truck like 2% of the time. So the light duty Mazda tranny will be fine for me.

From what I have read that I actually believe, my goal of 17-18 on the highway unloaded should be obtainable.