Low voltage at ecm

[Jim Wright]

Have a 1989 ford ranger 2.9 V6. Got a no start. Starts and runs when spraying fuel into intake until gas runs out. Have 40lbs fuel pressure. Good spark to coil. After chasing my tail I've come to the conclusion that it is a ground side electrical issue. Injectors not opening. I have found that with the tfi connector disconnected and checking voltage at the conection all is good except I am only getting 4.6 volts at the ground. The wiring diagram shows it going to pin 16 on ecm. I checked voltage at that pin by back probing and still only 4.6 volts. Omed out wire from ecm connection to tfi connector and showed no resistance. Wire good. Checked all grounds with power probe and made sure i have good grounds at ecm. I thought bad ecm replaced with known good ecm and same thing low voltage at pin 16. So that's it. If anyone out there has some ideas on what direction I should go I am open to any and all suggestions. Thanks. Jim

 

[RonD]

Pin 16 on the PCM(EEC) is ignition ground from TFI, so floating ground basically
With PCM and TFI plugged in and key on does it show 4v?
And what about with key off?

PCM opens injector based on PIP and IDM signals from TFI

Do the injectors have 12volts key on?
From EEC Relay, just the one red wire feeds 12v to all 6 injectors

 

[Jim Wright]

Thank you for your post.
Key off 0 volts.
Key on from battery positive to ground on tfi 12 volts.
From tfi power in to tfi ground 12 volts.
Injectors 12 volts at red wire
All was back probed at tif connection with pcm and tfi connected. My problem is have voltage drop at injectors causing them not to open. Thats why I started chasing grounds. Based on what you told me and running the voltage tests looks like the tfi ground is good. Guess I will start looking at the injector wires to make sure it's not the problem. Ground side issues can be a pain in the neck. Thanks again. PS still looking for suggestions.

 

[RonD]

If the engine starts and runs briefly when you add gas/fuel manually(50/50 test) then Spark and spark timing are OK, so you have a good PIP and IDM signal at the TFI
But computer is not opening injectors, assuming fuel pressure as reported
So definitely computer issue

Does the 1989 have a Tach or CEL(check engine light)?
Tach should read about 200 rpm cranking speed
CEL will go OFF when cranking IF computer is seeing a good/valid timing signal from TFI

V6 Computer grounds 3 injectors at the same time, then the other 3 injectors next time, and back and forth, each RPM
So there are 2 injector ground wires from computer, pins 58 and 59

Hard to test for injector pulse, ground pulse, because it happens so fast, there are NOID lights made for testing injector pulses
An old analog Volt meter can sometimes work, its needle will flicker when computer grounds the injector plug for a few milliseconds