Bringing this post back to life to explain all the issues that I figured out.
The ECM was bad. Turning the key on wasn't sending power to the fuel relay for the initial start prior to the oil pressure switch assuming the responsibility of powering the fuel system. This is why the previous shop...
I have family in town so won't have a chance to do any testing until Wed.
I had previously checked the ECM and there were no codes. The code for no faults kept repeating itself.
You got me. No schematic shows any other power source. Once primed by the fuel relay it looks like the pressure from the oil pump created by the cranking of the motor must be enough to close the circuit in the oil psi switch before the fuel pump gets any more power.
After checking the...
GeorgeB,
As I understand it, the oil psi switch and fuel relay are in a parallel circuit with either one being able to power the fuel pumps.
There is a 12v orange wire One the ECM B fuse with power all the time. Power is transferred to the tan/white wire at the fuel relay one of two ways. The...
I only checked A1, B1 and B2 at the ECM yesterday. Along with verifying the grounds on the A/B side of the ECM yesterday. When I get a chance I'm going to check the grounds on the C/D side of the ECM as well as the other terminals with the IGN on and while running.
What I think is odd is that...
With oil psi switch jumped there is no difference for the grn/wht wire at relay connector or ECM. 12v with ign on, 0v when ign at start or while running.
With oil psi switch connected and not jumped the grn/wht shows 12v at relay connector or ECM when on and 0v with ign at start or while...
As of my last testing the green/white wire goes from 12v with key on to 0v with key in start position. Just doesn't make sense when looking at the wiring diagrams. Shouldn't it be 0v with key on and 12v with key in start to close the relay and allow the orange wire to provide power to the pump...
With the ignition on the green and white wire has power, but no power when you turn the ignition to start. It is like this when cold or hot. The ground on the relay is good. I agree that it seems like the ECM isnt sending a signal, but the ECM B fuse is good. I checked the fault codes last night...
I hooked the old oil pressure switch to an air regulator this morning and I get continuity between the two terminals before reaching 5psi. Looks like I went through the hassle of replacing the oil pressure switch for nothing.
Went back and tested the relay again, with about 5Vs the circuit...
1987 TBI 305, when cold it'll start after turning over 5-10 times. You can see the oil pressure gauge start increasing and that's when it'll start. Once warm, the gauge doesn't increase and the truck won't start. Putting power to the test lead on the fuel relay starts it up on the 1st or 2nd...